Fracture Non Union

[Biomechanics & Union] Page | 11

Fracture Biomechanics • Bone can be considered as a biphasic composite material, mineral as one phase, and

collagen and ground substance as the other • The combined substances are stronger for their weight than either substance alone • Cortical bone is stiffer than cancellous bone and more brittle, withstanding less strain before

failure than cancellous bone o Fracture occurs in cortical bone in vitro at strains of only 2% o Fracture occurs in cancellous bone in vitro at strains of > 75%

• Bone is VISCOELASTIC (= time dependent property where the deformation of the material is related to the rate of loading, hysteresis, creep, stress relaxation)

• Load deformation curve for bone compared to other materials = the elastic portion of the graph has a slight curve in bone.

• Bone stiffness compared to other materials:

Bone behaviour under various loading modes • Bone is ANISOTROPIC (i.e it has different mechanical properties when loaded along different

axes). This is ð structure of bone is dissimilar in the transverse and longitudinal directions • Adult cortical bone is stronger in compression than tension and weakest in shear. • Most fractures occur as a result of several loading modes 1- Tension

• At the microscopic level, the failure mechanism for bone loaded in tension is mainly debonding at the cement lines and pulling out of the osteons

• The type of fracture occurring in tension is a transverse fracture • Tension #s tend to occur in areas with a large proportion of cancellous bone eg calcaneum,

5th metatarsal 2- Compression

• At the microscopic level the failure mechanism for bone tissue in compression is mainly oblique cracking of the osteons

• The type of fracture that occurs in compression is an oblique fracture at an angle of 30 degrees as shear forces at this angle are responsible for the failure.

• There are few fractures which occur purely due to compression • These fractures tend to occur in the metaphyses of bones where there is more cancellous

bone which is weaker. 3- Bending

• In bending there is a combination of compression and tension. Tensile stresses and strains on one side of the neutral axis and compressive stresses and strains on the other side. Because bone is assymmetrical, the compressive and tensile stresses may not be equal

• Bending causes transverse fractures as failure on the tension side progresses transversely across the bone and the neutral axis shifts.

Three point bending- three forces act on a structure produce 2 equal moments, each being the product of one of the two peripheral forces and the distance to the axis of rotation (the point at which the middle force is applied. If loading continues to yield point assuming the structure is

22 | Page [Biomechanics & Union]

homogenous and symmetrical, it will break at the point of application of the middle force. Fracture begins on the tension side in adult bone as bone is weaker in tension than compression. Examples include skiboot fractures of the tibia. In immature bone it may fail by compression causing buckling

on the compression side Four point bending- Two force couples acting on a structure produce two equal moments. The magnitude of the bending moment is the same throughout the area between the two force couples. The structure will break at its weakest point between them. Eg a previous unhealed fracture.

4- Compression and bending combined

• A combination of fracture type occurs. Bending produces a transverse crack on the tensile side of the bone, compression causes an oblique fracture on the other side. Where they meet a butterfly segment results

5- Torsion

• A load is placed on a structure so that twisting occurs about an axis. A torque or moment is produced within the structure.

• Maximal shear stresses act in planes parallel and perpendicular to the neutral axis • Maximal tensile and compressive forces act on planes diagonal to the neutral axis • The fracture for a bone loaded in torsion is a spiral fracture. • It begins é failure in shear, with the formation of a crack parallel to neutral axis of the bone • Followed by failure in tension along the line of maximal tensile stress at a diagonal to the axis

6- Shear

• A structure subjected to shear loading deforms internally in an angular manner, right angles on a plane surface within the structure become obtuse or acute.

• Whenever a structure is subject to compressive or tensile loading, shear stress is also produced

• The value for the stiffness of a material under shear loading is known as the shear modulus, not elastic modulus

• Shear fractures tend to occur in cancellous bone eg. Femoral condyles, tibial plateau.


Bone strength Compression Strongest Tension Weak Shear Weakest Bone type Load type Elastic modulus (109 N/m2) Ultimate stress (106 N/m2 ) Cortical Compression

Tension Shear

15.1 - 19.7 11.4 - 19.1

156 - 212 107 - 146 73 - 82

Cancellous Compression Tension Shear

0.1 - 3 0.2 - 5

1.5 - 50 3 - 20 6.6

Influence of muscle activity & loading on stress distribution in bone 1- When bone is loaded in vivo, simultaneous contraction of surrounding muscles act to oppose

these loads, so that it can withstand higher loads. Wolff's Law (Julius Wolff, 1884)

• 'form follows function'. • Bone has the ability to adapt, by changing its size, shape, and structure, to the mechanical

demands placed on it. • Bone is laid down where needed and resorbed where not needed. • The remodelling may be either external (a change in the external shape of the bone) or

internal (a change in the porosity, mineral content, and density of bone).

Rate dependency in bone • Because bone is viscoelastic, its biomechanical behaviour varies with the rate of application

of forces • Bone is stiffer and more brittle and can sustain a higher load to failure when loads are

applied at higher rates [Graph] • Bone also stores more energy to failure before failure at high loading rates. When a bone

fractures the stored energy is released. At a low loading rate the energy can dissipate through formation of a single crack. At a high loading rate, the greater energy stored cannot be dissipated rapidly enough through a single crack and comminution and extensive soft tissue damage result


Fatigue fracture of bone • Caused by repeated applications of a load below the ultimate strength/stress of the bone • The fatigue process in living bone is affected by the amount of load, the number of

repetitions and the frequency of loading. Fatigue fracture only occurs when the rate of remodelling is outpaced by the fatigue process.

• Fatigue fractures tend to occur during continuous strenuous physical activity causing the muscles to fatigue and reduces their ability to contract and counteract the imposed loading.

Influence of bone geometry on biomechanical behaviour In tension and compression,

• The load to failure and stiffness are proportional to the cross sectional area of the bone In bending,

• The load to failure and the stiffness are proportional to the ‘area moment of inertia.’ This is a figure which takes into account the cross sectional area and the distribution of bone about the neutral axis.

• The area moment of inertia for a rectangular block= BxH3/ 12 (B = width H = height)

• Block III is more resistant to bend than block I and II. • Bones increase their area moment of inertia by distributing most of the bone tissue in the

periphery, away from the neutral axis • In bending, the load to failure and stiffness is also inversely proportional to the length of the

bone. The longer the bone is, the bigger the bending moment produced for the same force.

• For a tubular structure / cylinder the further the material is from the neutral axis, the stiffer the construct under a given loads = Second Moment of Area (I)

o Circle: I = [pi.r4] /4 (hollow: r= outer radius-inner rad.) o Bending Stiffness = E.I (where E is Youngs Modulus) o The region of a bone/nail with the smallest I is subjected to the largest deformation

under load & will fail first o Indirect bone healing (thick periosteum) -> incr. I -> incr. stiffness & strength.

In Torsion: • The load to failure and stiffness are proportional to the Polar Moment of Inertia(J) • This takes into account the cross sectional area and the distribution of bone tissue around

the neutral axis • J = [pi/2]x[Ro4-Ri4] = 2.I; T/ø = JG/L (T/ø= torsional stiffness, T= torque, ø= angle of twist, G=

shear modulus, L= length of shaft) In bone healing:

• Callus formation around the periphery of a fracture increases the Second Moment of Area (I) and the Polar Moment of Inertia(J) of a bone, thus maximising the strength and stiffness of the bone in bending and torsion during healing.

Bone remodelling

• Wolff’s law – Bone is laid down where needed and resorbed where not needed • Thus disuse leads to supperiosteal and periosteal bone resorption, reducing its stiffness and

strength. • Stress protection of bone- is a phenomenon whereby an implant, by sharing the imposed

load can cause resorption of the underlying/surrounding bone as this bone carries less load than normal.

• Bone hypertrophy can also occur at implant attachment sites, eg. Around screws. • Laying down of bone can occur as a result of strenuous exercise, or resorption can occur in

prolonged weightlessness or inactivity.


Strain Theory of Fracture Healing • The theory of interfragmentary strain hypothesis is that the type of tissue formed in a

healing gap depends on the strain that it experiences • If the strain is between:

o 10%-100% granulation tissue can be expected to form o 2%-10% fibrocartilage will form o < 2% bone will form

Effect of movement on bone healing Kenwright et al studied osteotomies with a gap of 3mm and subjected them to movement. They showed that when compared to a rigidly held osteotomy there was:

• Increased bone mineral content in the gap with movement of 0.5mm (16% strain) • Decreased bone mineral content in the gap with movement of 2.0mm (66% strain)

It is important to note that it is not compressive load but strain, whether compressive or tensile that increases bone mineralisation

Other Factors Affecting Bone Strength Effects of use and disuse Rubin and Laynon in an avian model (turkey ulna): Disuse

• 42 days without functional load decreased bone mineral content to 88% of normal • Bone is lost from the endosteal surface

Use • Controlled cyclical loading (as low as 36 cycles per day) produced a hypertrophic response

with an increase of between 140%-150% of normal bone mineral content • Bone is deposited on the periosteal surface

Effects of holes on bone strength

• The strength of bone is effected by the size and shape of holes • Holes with sharp corners will reduce the torsional strength of bone to a greater extent than

those with smooth edges due to the stress riser effect associated with sharp corners • 4 point bending strength decreased to 80% of normal for a hole diameter of 10% of the

diameter of the bone • Torsional strength is affected when the hole size is greater than 10% of the diameter of the

bone • 20% size hole would reduce the torsional strength to 67% of normal

Changes in bone associated with aging

• Progressive loss of bone density occurs with age • Young bone is more ductile /less brittle than older bone, so more strain before breakage is

allowed in young bone.


Fracture Healing 11-- HHEEMMAATTOOMMAA FFOORRMMAATTIIOONN 22-- IINNFFLLAAMMMMAATTOORRYY RREESSPPOONNSSEE ...................................................... WWIITTHHIINN 24-72 hours

• Injured tissues and platelets release vasoactive mediators, growth factors and other cytokines.

• These cytokines influence cell migration, proliferation, differentiation and matrix synthesis.

• Growth factors recruit fibroblasts, mesenchymal cells & osteoprogenitor cells to the fracture site.

• Macrophages, PMNs & mast cells (48hr) arrive at the fracture site to begin the process of removing the tissue debris.

Important cytokines in bone healing: BMP Osteoinductive, induces metaplasia of mesenchymal cells into osteoblasts

Target cell for BMP is the undifferentiated perivascular mesenchymal cell TGFβ ⊕ UMC to produce type II collagen and proteoglycans

⊕ osteoblasts to produce collagen PDGF Attracts inflammatory cells to the fracture site FGF ⊕ fibroblast proliferation IGF-II Stimulates type I collagen production, cartilage matrix synthesis and cellular proliferation IL-1 Attracts inflammatory cells to the fracture site IL-6 Attracts inflammatory cells to the fracture site

33-- RREEPPAARRAATTIIVVEE RREESSPPOONNSSEE .................................................................. WWIITTHHIINN 2 weeks

aa.. Vasoactive substances (Nitric Oxide & Endothelial Stimulating Angiogenesis Factor) cause neovascularisation & local vasodilation

bb.. Undifferentiated mesenchymal cells migrate to the fracture site and have the ability to form cells which in turn form cartilage, bone or fibrous tissue.

cc.. The fracture haematoma is organised and fibroblasts and chondroblasts appear between the bone ends and cartilage + Type II collagen are formed ((SSOOFFTT CCAALLLLUUSS))

dd.. Endochondral ossification takes place and the soft callus is turned Into ((HHAARRDD CCAALLLLUUSS)) ee.. The amount of callus formed is inversely ∝ to the amount of immobilisation of the fracture. • In fractures that are fixed with rigid compression plates there can be primary bone healing with little

or no visible callus formation. Types of callus: External (bridging) callus From the # haematoma endochondral ossification woven bone Periosteal callus from inner cambium layer intramembranous ossification woven bone Internal (medullary) callus Forms more slowly and occurs later 44-- RREEMMOODDEELLLLIINNGG:: – Middle of repair phase up to 7 years

• Remodelling of woven bone depends on mechanical forces applied (WWOOLLFFFF’’SS LLAAWW - 'form follows function') • Fracture healing is complete when there is repopulation of the medullary canal • Cortical bone

o Remodelling occurs by invasion of an osteoclast “cutting cone” which is then followed by osteoblasts which lay down new lamellar bone (osteon)

• Cancellous bone o Remodelling occurs on the surface of the trabeculae ώ causes trabeculae to become thicker


Bone Remodeling

The BMU remodeling sequence Phase Factors Description

1- Origination (+) PTH, IGF, IL-1, IL-6, PGE, calcitriol, TNF, NO (-) estrogen

After microdamage to the bone, following mechanical stress, following exposure to some cytokines, or at random, a BMU will originate. The lining cells become active and change from a pancake-like to a cuboidal shape.

2- Osteoclast recruitment

(+) RANK-ligand, M-CSF (-) osteoprotegerin (OPG), GM-CSF

Lining cells that have been activated by IL-1, PTH, calcitriol, etc (but not IL-6) will then secrete RANK-ligand, which may remain bound to the cell surface. Osteoblast precursors also secrete RANK-ligand. Pre-osteoclasts have membrane receptors called RANK. When RANK-ligand activates these receptors the cells fuse and differentiate into mature multinucleared osteoclasts which develop a ruffled border and resorb bone. Meanwhile, OPG is a free-floating decoy receptor, related to the TNF family, which can bind the RANK-ligand and prevent it from activating the RANK.

3- Resorption (+) Integrins, some interleukins, acidosis, vitamin A (-) estrogen, calcitonin, interferon, TGF, other interleukins, sFRP-1

The mature osteoclasts resorb bone. As the BMU wanders, new osteoclasts are continuously activated and then start resorption. At any one spot on the surface the resorption lasts about two weeks. The osteoclasts then undergo programmed cell death or apoptosis, which is delayed by estrogen deficiency.

4- Osteoblast recruitment

(+) Wnt, BMPs, IGF, FGFs, PDGFs, CSF, PTH, calcitriol, Runx2, GST-RANK-Ligand, TGF-beta (-) ? leptin

Osteoblasts are derived from marrow stromal cells, which can differentiate into either adipocytes or osteoblasts; the transcription factor Runx2 (previously named Cbfa1) is necessary for osteoblastic differentiation. Osteoblasts are probably attracted by bone-derived growth factors. Wnt-signalling and bone morphogenic proteins are important.

5- Osteoid formation

(+) TGF-beta, BMPs, IGF (-) FGFs, PDGFs, glucocorticoids

The active, secreting osteoblasts then make layers of osteoid and slowly refil the cavity. They also secrete growth factors, osteopontin, osteocalcin, and other proteins.

6- Mineralization (+) calcium, phosphate (-) pyrophosphate

When the osteoid is about 6 microns thick, it begins to mineralize. This process, also, is regulated by the osteoblasts.

7- Mineral maturation

Other ions For months after the cavity has been filled with bone, the crystals of mineral are packed more closely and the density of the new bone increases.

8- Quiescence The final osteoblasts turn into lining cells which participate in the minute-to-minute release of calcium from the bones. Some of the osteoblasts also turn into osteocytes which remain in the bone, connected by long cell processes which can sense mechanical stresses to the bones.



Factors influencing bone healing Systemic Local Age Degree of local trauma &vascular injury Hormones Degree of bone loss Functional activity Local pathological condition Nerve function Type of bone fractured Nutrition Immobilization Drugs (NSAID) Infection Hormonal influences on bone healing Hormone Effect Mechanism Cortisone Decreased callus production Calcitonin Unknown TH/PTH Bone remodelling GH Increased callus volume Androgens Increased callus volume Type of immobilization and Healing Implant Type of Healing Cast Periosteal bridging callus + endochond ossification DCP Primary cortical healing (cutting cone) IMN Early ......... as cast

Late .......... Medullary callus External Fixator rigid ..... Periosteal Callus

rigid ..... Primary cortical healing Inadequate immobilization + adequate blood supply

Hypertrophic non union ( type II collagen)

Inadequate immobilization + Inadequate blood supply

Atrophic non union

Fracture displacement Oligotrophic nonunion Electricity and fracture healing

• Stress generated potentials serve as signals that modulate cellular activity. Piezoelectric effect and streaming potentials are examples of stress generated potentials

1. Piezoelectric effect: charges in tissues are changed secondary to mechanical forces 2. Streaming potentials: occur when electrically charged fluid is forced over a tissue (cell

membrane) with a fixed charge • Transmembrane potentials: generated by cellular metabolism • Fracture Healing

1. Direct current inflammatory response 2. Alternating current repair phase collagen synthesis and calcification 3. Pulsed Electro Magnetic Field remodeling & calcification of fibrocartilage

Ultrasound • Can decrease the time to clinical healing and radiological union


IMPLANTS FOR FRACTURE SURGERY 11.. BBOONNEE SSCCRREEWWSS

There are two types of screws = Machine screws & Wood screws. Bone screws are machine screws.

1. A wood screw is inserted into a small pilot hole. The screw threads compress the wood, which is less stiff than the screw, resulting in an elastic force.

2. A machine screw is inserted into a pre-drilled & pre-tapped hole. The screw itself deforms plastically when inserted into metal.

Screw Head

• = attachment for screwdriver • Countersink = conical area under head • Hexagonal head recess design is most popular because:

1. it avoids slippage of screwdriver & thus head distortion 2. it allows for better directional control during screw insertion 3. the torque is spread between 6 points of contact

Screw Shaft • = smooth link betw. head & thread. • The 'Run out' is the transitional area between shaft & thread. This is the area screws break.

Screw Thread • The standard orthopaedic screw has a single thread (more threads increase the rate of advancement,

but produces less compression for the same energy) • Core/root diameter = the narrowest diameter.

o The cube of the root diameter is proportional to the torsional strength of the screw. • Outer/thread diameter = across the maximum thread width.

o The larger the outer diameter the greater the resistance to screw pullout. • Pitch= the distance between adjacent threads.

o Cortical screws have small pitch & cancellous screws have large pitch o The stronger the bone the smaller the pitch

• Lead= the distance the screw advances with each turn. o The smaller the lead the greater the mechanical advantage of the screw. o Cortical screws have a smaller lead than cancellous screws

• Pitch & lead = incline of a ramp. A barrel travels a shorter distance on a steeper incline before it gets to the top, but it is harder to push it up the ramp.

• Thread design: o 'V' profile - produces shear + compression forces o Buttress profile - produces compression forces only o shear forces promote bone resorption, reducing pullout strength.

• Thread length: o Partially threaded screws are designed for lagging cancellous bone. o 80% of the screw's grip is determined by the thread on the near cortex & 20% on the

purchase at the far cortex.


Screw Tip 1. Blunt tip of self-tapping screw - cortical

• fluted to act as a cutting edge & transport bone chips away.

• the sharpness, number & geometry of flutes determines its effectiveness.

2. Blunt tip of non-self-tapping screw - cortical • the rounded tip allows for more accuracy &

direction into a pre-tapped hole. • More 'effective torque' is obtained from pre-

tapping -> increased interfragmentary compression.

3. Corkscrew tip - cancellous screw • compresses trabecular bone & produces

compression by overshooting the pre-drilled hole. 4. Trocar tip -

• doesn't have a flute, thus displaces bone as it advances.

SCREW INSERTION Drilling: Heat Generation:

1. Bone heated to >45ºC leads to osteocyte necrosis, deactivation of alkaline phosphatase & degradation of collagen-hydroxyapatite bone. This results in permanent alterations in the mechanical properties.

2. Causes: 1. dull drill bit - also causes crushing of bone & small local fractures. 2. Time 3. Thick bone 4. Excessive thrust & speed 5. Dry bone 6. No drill sleeve -> drill wandering

3. Good drilling practice: 1. straight, sharp drill bit with 3 flutes & cutting angle of >70o 2. Clean the tip frequently 3. start slowly & maintain the drilling angle 4. Use a drill sleeve 5. Simultaneous saline irrigation

Tapping:

1. Allows precision placement when placing screw obliquely (lag) 2. Less torque lost in overcoming friction at the bone-screw interface. 3. Less force required. = less likelihood of losing # position.

Self-Tapping Screws => quicker, less instruments, tight fit, same holding power as pre-tapped screw. Lag Screws:

• = involves placement of one or more screws across a fracture or osteotomy site to produce interfragmentary compression.

• achieved by overdrilling the near cortex. • The ideal position is perpendicular to line of fracture, but this does not provide axial or rotational

stability. Therefore, should try & use more than one screw with the other screw perpendicular to the long axis of the shaft.

• LLAAGG SSCCRREEWW EEXXEERRTTSS 33000000 NN IINNTTEERRFFRRAAGGMMEENNTTAARRYY EEVVEENN CCOOMMPPRREESSSSIIVVEE FFOORRCCEE FFRROOMM WWIITTHHIINN TTHHEE

FFRRAACCTTUURREE


22.. PPLLAATTEESS::

Benefits: • Anatomical reduction of the fracture with open techniques • Stability for early function of muscle-tendon units and joints

Disadvantages: • Risk of bone refracture after their removal • Stress protection and osteoporosis beneath a plate • Plate irritation

Types/ Techniques of Plates: 1) Compression Plate(DCP):

• Applied to the tensile surface; under compression tension within plate & compression on bone. • Compression produced by the DCP = 600 N, and not even (either on the compression side in prestressed

plates, or one the tension side in the contoured plates) • Fracture edges resorb after 72hrs stresses in plate & bone -> improved apposition. • Plate resists bending moment by its tension.

2) Neutralisation Plate (semitubular plate usually): • applied at right angles to the above. • If apposition is poor this arrangement is more rigid. • But screws are subject to bending & torsional forces. • Plate is centred at the neutral axis rather than the extreme fibre.

3) Buttress 4) Bridging 5) Tension-band 6) Double plates

• torsional rigidity. 7) LC-DCP (Titanium)

• less disturbance of periosteal blood supply, reduces bone resorption under plate • Prebending plates -> prevents gapping of cortex opp. to plate -> more uniform compression.

8) LCP locked Compression Plate: • Best for osteoporotic patients

AO PLATES & SCREWS SIZES BASIC LAG DCP CANCELLOUS Drill 3.2 & 4.5 3.2 3.2/4.5 Tap 4.5 4.5 6.5 Screw 4.5 4.5 cort. 6.5 spong. SMALL LAG DCP/Tub. CANCELLOUS Drill 2.5 & 3.5 2.5 2.5 Tap 3.5 3.5 4.0 Screw 3.5 3.5 4.0 MINI LAG Drill 1.5 & 2.0 OR 2.0 & 2.7 Tap 2.0 OR 2.7 Screw 2.0 OR 2.7 Max. Screw-Plate Angle:

• DCP = 25º in horizontal plane & 7º in transverse plane • Third Tubular = 50º •• DDCCPP EEXXEERRTTSS 660000 NN AAXXIIAALL UUNNEEVVEENN CCOOMMPPRREESSSSIIVVEE FFOORRCCEE

LAG COMPRESSION DCP COMPRESSION FORCE 3000 N 600 N (prestressing & eccentric fix) DISTRIBUTION EVEN FROM WITHIN # UNEVEN DIRECTION INTER-FRAGMENTARY AXIAL BEST SUIT SMALL POROUS BONE LARGE DENSE BONE


33.. IINNTTRRAAMMEEDDUULLLLAARRYY NNAAIILLSS

Nail Length: • The working length is defined as the length of a nail bet the

most proximal point of fixation in the distal fragment and the most distal point of fixation in the proximal fragment = the unsupported portion of nail between the bone fragments. More important than actual length

• Torsional stiffness 1/working length • Bending stiffness 1/working length2 • Femoral bow forces the nail to contact the medullary wall

working length torsional & bending stiffness Nail Diameter & Area moment of inertia: • = The principal factor that alters BBEENNDDIINNGG stiffness (and shape

also) • Distribution of material in cross section ((SSEECCOONNDD MMOOMMEENNTT OOFF AARREEAA)) is a crucial factor to

bending • Wider diameter hollow tube is stiffer than solid smaller dr tube é the same amount of

material. • This is why bones have a medullary canal • Tubes with a wall thickness/radius < 1/8 behave as curved sheets rather than tubes.

These thin-walled tubes are subject to buckling. (Bone is thick-walled). Slot: •• = The principal factor that alters TTOORRSSIIOONNAALL stiffness ((PPOOLLAARR MMOOMMEENNTT OOFF IINNEERRTTIIAA)) • Non-slotted nail is 40 times stiffer in Torsion. • A slot reduces torsional stiffness by 98% -> quicker healing with callus. • Disadvantages: torsional stress during insertion material around the screw holes Locking:

Dynamic Static Deformation control Bending & rotational Axial, bending, & rotational Load Sharing Bearing Used in Axially stable # Comminuted unstable #

Reaming Effects on Blood flow & union: Normally bone has centrifugal blood flow, from centre (endosteal) outwards. Callus is largely independent of endosteal blood supply. 1. Reaming periosteal bl flow. (not dependent on nutrient artery), -> endosteal bl flow. 2. 2wk: reaming markedly blood flow. 3. 6wks flow in cortical bone recovers to normal & periosteal blood flow is still high.

Reason= centrifugal flow is reversed. (law of compensation, Treute) 4. Reaming products = fibroblasts, bone, mesenchymal cells NBF. 5. union is quicker in reamed fractures (Hooper, 1991) Reamed vs Unreamed:

Reamed Unreamed Larger, stiffer nail Suitable for contaminated fractures

area of bone-nail contact stability of locking bolts Does not disturb the endosteal blood vessels

initial bone devascularization delayed union (due to excessive motion at # site). local pressure less pressure

microthrombi PE less microthrombi thermal necrosis nil

Compartment Pressures: • Compartment pressures: in bone (up to 300mmHg), but no change in compartmental pressure


Intramedullary Nails vs. Plates IM NAIL PLATE & SCREWS Load sharing Load Bearing

endosteal circ. periosteal circ. Indirect reduction Direct reduction Preserves soft tissue Destroys soft tissue Allows # motion Rigid fixation Early union-callus Slow union- no callus Rare anat. Reduction Frequent anat. Red. Failure at crossbolts Failure at plate For segmental #'s For intraarticular #'s For shaft #'s For juxtaarticular #'s


44.. EEXXTTEERRNNAALL FFIIXXAATTIIOONN::

Advantages: • Apply quickly • Technically easy to perform • Adjust later • Soft tissues not disturbed • Access to wounds • Joints can be mobilized • Can dynamize • Easy removal • Reconstruction surgery

Disadvantages: 1- Pin tract infection 2- Malunion 3- Patient compliance required Types: • Rod

1- Uniplanar 2- Biplanar

• Circular • Hybrid Factors affecting construct stiffness Useful for: 1- Any fracture 2- Bone transport 3- Limb lengthening 4- Angular correction 5- Soft tissue reconstruction 6- Contractures ILIZAROV EXTERNAL FIXATOR 1- wires= 1.5mm in adults & children; 1.8mm in adult femur. 2- wire types= smooth & olives (for stability/translation) 3- Insertion= Push-Drill-Tap 4- Aim for wires at 90deg. to each other & 4-5 wires per segment 5- Bring the ring to the wire- Not the wire to ring -Tether through muscles in joint extension 6- Wire Tension= 1.2mm-90kg; 1.5mm-110kg; 1.8mm-150kg 7- Focus = fracture / non-union site 8- Segments = bone fragments

Removal of Internal Fixation Devices:

Usually remove 12 to 18 months following insertion. There is a very high incidence of refracture and of neurological complication following removal

of forearm plates.


Implant Failure DDeeffiinniittiioonn::

• It is failure of an implant (Standard alloy) to satisfy the specific function for which it is implanted or inserted.

• In the past, there were different improper implants not of a quality good enough to withstand bone stresses.

• Nowadays, due to the evolution in metallurgy & biomechanics, we have (standard alloy) which is a metal , if inserted accurately & properly, will mostly achieve the aims of its application

TTyyppeess::

TThhiiss ddeeppeennddss oonn TTwwoo ffaaccttoorrss:: 1- Implant material choice

o Implant material o Implant design o Implant biochemical activity (inert or not)

2- Implant application: o Implant type size shape o Application technique.

TTyyppeess ooff iimmppllaanntt.. 1) Implants for fracture fixation e.g. Plating. 2) Arthroplasty. 3) Artificial ligaments. 4) Silastic implants e.g. Bead sadius & MP Joints.

BBiioommeecchhaanniiccss ooff iimmppllaanntt

YYOOUUNNGG’’SS MMOODDUULLUUSS OOFF EELLAASSTTIICCIITTYY :: (measure of stiffness) = stress / strain. MMOODDUULLUUSS OOFF RREESSIILLIIEENNCCEE:: energy/vol. a material can absorb éout yielding (=area below the elastic

curve) MMOODDUULLUUSS OOFF TTOOUUGGHHNNEESSSS:: energy/vol. a material can absorb till breakage (=area below the curve) AAMMOOUUNNTT OOFF DDEEFFLLEECCTTIIOONN = measure of rigidity or stiffness of implant. YYIIEELLDD SSTTRREESSSS:: the max stress a metal éstand éout plastic deformation UULLTTIIMMAATTEE TTEENNSSIILLEE SSTTRREENNGGTTHH = the max stress a metal éstand éout # é a single peek load FFAATTIIGGUUEE SSTTRREENNGGTTHH = the maximum cyclic load a metal éstand éout # é 107 cyclic loads. EENNDDUURRAANNCCEE ((Fatigue Limit)) = the cyclic load limit below fatigue will not occur FFAATTIIGGUUEE: failure 2ry to cyclic loading FFRRAACCTTUURREE: failure 2ry to bending stresses into > 2 parts BBUUCCKKLLIINNGG: failure 2ry to compression of a thin walled tube CCOORRRROOSSIIOONN: failure 2ry to electrochemical action WWEEAARR: failure 2ry to mechanical deterioration of solid surface CCRREEEEPP & deformation LLOOOOSSEENNIINNGG: failure 2ry to a biologic response of colonizing bacteria or wear particles (septic or aseptic)


Implant Failure 11.. CCOORRRROOSSIIOONN

•• MMAATTEERRIIAALL DDEETTEERRIIOORRAATTIIOONN ðð EELLEECCTTRROOCCHHEEMMIICCAALL AACCTTIIOONN • It requires a GGAALLVVAANNIICC CCEELLLL = 2 diff electrically conducting solids + conducting pathway +

electrolytes in-between • PPAASSSSIIVVAATTIIOONN is the formation of an oxide layer on the surface to prevent corrosion • Types:

1) GGAALLVVAANNIICC: between metals é different electrochemical potentials 2) FFRREETTTTIINNGG: surface breakdown 2ry to motion & loads between metal surfaces 3) CCRREEVVIICCEE: motion bet metals depassivate their surfaces 4) PPIITTTTIINNGG: surface abrasion galvanic corrosion 5) SSTTRREESSSS: load generated crack galvanic corrosion the crack, and so on 6) MMIICCRROO--BBIIOOLLOOGGIICC: micro-org secrete corrosive metabolite 7) IINNTTEERRGGRRAANNUULLAARR: corrosion at weld points & not the metal structure failure (weld decay)

• Corrosion can be minimised by o Choosing a corrosion resistant material o Treating the surface with a passivating layer prior to use o Not using combinations of metals in close proximity o Careful operating technique to reduce surface scratching o Using non modular implants.

22.. FFAATTIIGGUUEE-- •• PPRROOGGRREESSSSIIVVEE MMAATTEERRIIAALL DDEETTEERRIIOORRAATTIIOONN 22RRYY TTOO CCYYCCLLIICC SSTTRREESSSSEESS BBEELLOOWW TTHHEE UULLTTIIMMAATTEE TTEENNSSIILLEE SSTTRREESSSS CCAAUUSSIINNGG

CCRRAACCKK PPRROOPPAAGGAATTIIOONN.. • Crack usually starts at a SSTTRREESSSS RRIISSEERR:

o Scratch o Hole o Corner o Change in cross section o Fretting

• The stress concentration factor (ratio of maximum stress at the surface irregularity to the average stress in the same direction depends on the geometry of the surface. Stress at a large distal interlocking hole of an IM nail is < small hole, but the stress concentration factor is higher é the large hole because the surface area of the metal left in that plane will be less.

• SS--NN CCUURRVVEE relates stress applied to number of cycles to failure • EENNDDUURRAANNCCEE,, FFAATTIIGGUUEE LLIIMMIITT is the maximum cyclic loads below fatigue will not occur. However,

it is best to consider all orthopaedic implants as having no fatigue limit as there is the potential for damage during insertion, and the corrosive environment of the human body and the variability of the stresses applied are difficult to control.

• Reduction of fatigue failure can be achieved by o Appropriate design of implants, avoiding sudden changes in geometry o Surface treatments of implant, e.g. peening, polishing o fretting corrosion o Correct insertion of implants, e.g. avoiding distraction of fractures, so that bone heals

and can share the loads with the implant. o early WB until fracture is healing.

33-- BBUUCCKKLLIINNGG:: • sudden material deterioration 2ry to compression of a thin walled tube (diameter < 1/8 its length)


44.. WWEEAARR •• MMEECCHHAANNIICCAALL DDEETTEERRIIOORRAATTIIOONN OOFF SSOOLLIIDD SSUURRFFAACCEE • Types: (the main are the 1st two types)

1]. AABBRRAASSIIVVEE: the harder grooves the softer material 2]. AADDHHEESSIIVVEE: the softer material adheres on the harder surface 3]. FFAATTIIGGUUEE, in which repetitive loading subsurface delaminate lost from the surface 4]. TTHHIIRRDD--BBOODDYY WWEEAARR implies the retention of debris bet. sliding surfaces abrasive wear. 5]. BBAACCKK SSIIDDEE WWEEAARR: bet PE & the metal backing 6]. RRUUNN IINN WWEEAARR: is the accelerated wear that occur in the 1st few millions of cycling

• Effects of wear most predominant in joint prostheses. Particles produced by wear (metal/PE/PMMA) are phagocytosed by osteoclasts osteolysis loosening + material loss

55.. SSEEPPTTIICC LLOOOOSSEENNIINNGG RRAACCEE FFOORR SSUURRFFAACCEE TTHHEEOORRYY When a total joint prosthesis is placed into the

human body, the body's cells & bacteria (usually skin bacteria) hurry to get hold on the prosthesis surface &colonize.

If bacteria win, thet evolve the capability to adhere to surfaces for their survival, by secretion of a surface glycoprotien called GGLLYYCCOOCCAALLYYXX:

i. Very strong adhesive ii. Mask the bacterial antigens iii. Colonize inside this biofilm away from

immune system iv. Invite other types of bacteriae to trick the immune system v. When they adhere to the inert implant surface, bacteria are protected by the

antiphagocytic effect of biomaterial. All these powerful resistance 100-1000 times against AB & immune system.

MATERIALS USED IN FRACTURE FIXATION Stainless steel 1]. Stiff 2]. Cheap 3]. Ductile; so it is useful in contouring of plates and wires during operative procedures. 4]. Relatively inert 5]. Chromium passivate when dipped in nitric acid corrosion 6]. Can still undergo corrosion if carbon gets to the surface. 7]. Young’s modulus - 200 Pascals (10x that of bone) stress shielding bone resorption • Used in plates, screws, external fixators, I.M. nails. • Stainless Steel Composed of:

o Iron ................................................. 60% (cold forged or annealed to strength) o Chromium .................................... 20% (major corrosion protection after passivation) o Nickel ............................................. 15% (corrosion resistance) o Molybdenum ............................... 3% (protects against pitting corrosion) o Carbon .......................................... 0.03% ( stiffness) o Mg, Si, P, S .................................... 2%

Titanium and its alloys 1]. Inert 2]. Less stiff: less stress shielding & stress risers at the tip of the implant (modulus ≈ ½ of SS) 3]. More expensive than stainless steels 4]. More wear (not good for bearing surfaces) 5]. Less ductile < stainless steel, but ductile titanium alloys being produced • Used in plates, screws, I.M. nails, external fixators, & halos.


Adhesives • Not common in orthopaedics but potentially useful in small fragment fixation, controversial • Prerequisits:

i. Sufficient bond strength ii. Able to bond to moist surfaces iii. Permit healing across the bond line iv. Sterilizable.

Bone cement does not count as an adhesive. CCYYAANNOOAACCRRYYLLAATTEE:: has poor results FIBRIN: is the only suitable adhesive for fracture provided that it has an inherent stability or NWB Biodegradable polymers • Potential advantages

o Hardware removal not necessary, reducing morbidity and cost. o Stiffness of polymer decreases as stiffness of fracture callus increases. o Can be used in future for controlled release of antibiotics or stimulants to healing

• Requirements o Adequate mechanical stability o Sufficient strength over a sufficient period of time o Degradability into products those are not harmful.

• Examples o Polyglycolic acid o Polylactic acid o Copolymers

• Only about 1/20 the stiffness and strength of stainless steel • Used in ankle fractures with poor results • Used in phalangeal fractures with better results

Summary Of Implant Properties

Steel Titanium alloy Ceramic Composite Stiffness ++ + +++ + Hardness ++ + +++ + Corrosion Resistance + ++ +++ ++ Wear Resistance + - +++ + Ultimate Strength ++ + ++ ++ Yield Strength + +++ - + Ductility + ++ - + Cost ++ - -- + Perfect Material =

1]. Stiff .................................................... resist deformation 2]. Hard ................................................. resist surface abrasion 3]. Inert .................................................. resist corrosion 4]. Tough .............................................. resist breakage 5]. Ductile ............................................. able to deform before breakage 6]. Adapt to loading 7]. Regenerate (reduce failure) = a composite = Bone (a ceramic phase (calcium

hydroxyapatite), dispersed in a collagen-based matrix).


Fracture Non-Union Pseudoarthrosis

DDeeffiinniittiioonn:: • Arrest of bony fracture repair process, Short of osseous bridging of the defect between the

fracture fragments, where fibrous or cartilaginous tissue will interpose. • Pseudoarthrosis is the final status of non-union é formation of a synovial lining & joint fluid.

CCaauusseess ooff nnoonn uunniioonn:: General factors:

Age. Nutrition. Radiation Burns Hyperpara Drugs: anticoagulants, steroids

Local: 1- Biological:

[1]. Individual bone succeptibility: Scaphoid. Neck femur Lower 1/3 tibia. (no surrounding ms & depend on vessels)

[2]. Injury to: Soft tissue Vascular inj: severe injury, periosteal stripping, reaming poor revascularization

[3]. Infection Necrosis & bone devitalization bl. Supply. Osteolysis gaps Motion instability.

2- Mechanical: [4]. Improper fracture coaption (gap):

Loss of bone substance Soft tissue interposition Distraction, Displacement, or overriding

[5]. Insufficient immobilization: Moving fracture fragments.

[6]. Abnormal mechanics: Shearing, torsional & bending stresses counteract the biological repair process,

e.g. Vertical fr. Neck femur Shearing stresses.

Pathology non union: Stage I ..................................................................... ( 3-6 months) • Bone ends are covered by fibrocartilage & enclosed in a fibrous capsule • The centre of the callus shows:

1- Amorphous fibrinoid degeneration 2- Hyaline degeneration.

Stage II ....................................................................... (2 Years) • Bone ends become highly sclerotic • Mechanical disturbance of the fracture Cleavage of the amorphous area & formation of

extra-cellular fluid containing mucin. Stage III ...................................................................... (2-5 years): • Mature pseudo-arthrosis is formed:

1- Cavity filled with highly viscous fluid. 2- Lining synovial like membrane

• Callus osteogenesis never ceases, but never bridges the gap • Proximally it is saucered concave cavity to receive the rounded distal end • Over growth of bone around the bone ends. • Continued fibrinoid degeneration of callus



CCllaassssiiffiiccaattiioonn nnoonn uunniioonn::

I. AACCCCOORRDDIINNGG TTOO CCAALLLLUUSS FFOORRMMAATTIIOONN ((WWEEBBEERR)) A. Hypervascular (Hypertrophic)

11-- EELLEEPPHHAANNTT FFOOOOTT:: i) Rich in callus ii) Caused by:

• insecure fixation. • Premature W.B.

22-- HHOORRSSEE HHOOOOFF:: i) Poor in callus. ii) Caused by moderately unstable

plate & screw fixation. 33-- OOLLIIGGOOTTRROOPPHHIICC::

i) Absent callus ii) Caused by:

• Fracture displacement • Fragment distraction

B. Avascular (Atrophic). 1- TTOORRSSIIOONN WWEEDDGGEE: Intermediate fragment

vascularity unites to one end 2- CCOOMMMMIINNUUTTEEDD:

• One fragment, became necrotic • No callus formation • Usually complicated by plate break

3- DDEEFFEECCTT non-union: lost diaph fragment 4- AATTRROOPPHHIICC non-union:

• Lost diaphyseal fragment + atrophic ends • After sequestrectomy, tumor excision

II. AACCCCOORRDDIINNGG TTOO TTIIMMEE OORR DDEEGGRREEEE :: A. Delayed union: healing has not advanced at the

average rate for the site & type of fracture (usually 3-6 mo). It needs immobilization, osteoinduction, PEMF,….

B. Non-union: either é mobile gap or immobile gap C. Synovial pseudo-arthrosis.

III. AACCCCOORRDDIINNGG LLOOCCAATTIIOONN:: A. Diaphyseal. B. Metaphyseal C. Intra-articular

IV. AACCCCOORRDDIINNGG TTOO IINNFFEECCTTIIOONNSS.. A. Non-infected (felsitic). B. Infected (static)

1. Draining. 2. Non-draining (Dry) 3 months

V. CCLLIINNIICCAALL && RRAADDIIOOGGRRAAPPHHIICC PPAALLYY CCLLAASSSSIIFFIICCAATTIIOONN.. A. Type A (with bone loss < 1 cm)

1. A1 : mobile deformity. 2. A2 : stiff .

- A2.1 eout deformity - A2.2 é fixed deformity

B. Type B (with bone loss > 1 cm). 1. B1 : with bony defect. 2. B2 : with loss of bone length. 3. B3 : with both


DDiiaaggnnoossiiss ooff nnoonn uunniioonn::

A) History: 1. Mechanism of inj

(high or low energy) 2. History of infection 3. History of operation

4. Excessive traction. 5. Long immobilization. 6. Implant removal.

7. Other # & their healing 8. Skin grafts or muscle

transfers.

B) Clinical examination, (S.&S.): 1. Pain 2. Swelling 3. ROM 4. Tenderness 5. Colour charges.

6. Sinus 7. Limb vascularity. 8. Limp. 9. ms. Weakness.

10. joint pain, contraction. 11. Skin condition. 12. Limb sensations 13. Malrotation

C) Investigations : 1. X-rays (for both sides): AP, Lateral, Obliques (rt & lt according to type of non.)

o The entire bone in diaphyseal non-union. o Leg-length film in L.L. frs (shortening, rotation).

2. CT & Tomogram (AP, lat) , esp in metaph non- unions. 3. Arthrography or arthroscopy (to check state of cartilage in metaph non-unions). 4. Siniogram (M.blue) 5. Culture & Sensitivity test. 6. MRI. 7. EMG & nerve conduction test. 8. Arteriogram if limb circulation is doulotfull. 9. Tc99, Ga67, In111: hot zone = biologically active non-union. Cold zone = pseudoarthrosis.

1- Non-Operative Treatment Objectives

1. Union of the bone in a reasonable time. 2. Correction of shortening, angulation or notation. 3. Mobilization of the adjacent stiff joint(s). 4. Eradication of infection.

Modalities:

1- Functional cast bracing with weight-bearing (tibia). 2- Functional cast bracing after osteotomy of intact or united fibula. 3- Electric stimulation by: invasive, semi-invasive, non-invasive

Indications:

1- Gaps > 1 cm 2- Synovial pseudoarthrosis 3- Metaphyseal non-union 4- Difficult control of # motion; e.g. proximal femur & proximal humerus

Disadvantages

1. Does not correct shortening or malposition 2. Requires long POP NWB immobilize. stiffness, porosis & loss of function. 3. Usually does not suffice alone, so used as an adjuvant to operative treatment.

Principle:

Cathodal electrodes convert fibrous union to fibrocartilage endochondral ossification


2- Operative Principles

1. RREEDDUUCCTTIIOONN OOFF TTHHEE FFRRAAGGMMEENNTTSS : (provides axial compression with mechanical stability) . • When in good position, do not dissect the fibrous tissue surrounding the periosteum • Callus and fibrous tissue preserves the fragment's circulation they ossify ofter a

bridging graft unites with the fracture fragments . • Necrotic bone acts as a scaffold for union.

2. GGRRAAFFTTIINNGG BBOONNEE Induction of ostergenesi cortical . • Bridge gaps with bone graft:

o cancellous. o cortico-cancellous.

• Types: A. Onlay, sliding , inlay. B. Autogemnous, allograft. C. Vascularized, non – vascularized.

• Also, bone covering by skin of flaps is essential. 3. CCOORRRREECCTTIIOONN OOFF BBIIOOMMEECCHHAANNIICCAALL FFAACCTTOORRSS e.g. By osteotomy: Shearing , torsion or bending

stresses should be eliminated by e.g. McMurray medial osteotomy & Schanz Osteotomy. 4. SSTTAABBIILLIIZZIINNGG TTHHEE FFRRAAGGMMEENNTTSS , by a compressive device: e.g. plate & screws or Ilizarov

• External support should be for many months to guard against fatigue failure. 5. EERRAADDIICCAATTIIOONN OOFF IINNFFEECCTTIIOONN:

• Excision of non-union site. • Sequestrectomy.

66.. EEXXCCIISSIIOONN OOFF SSYYNNOOVVIIAALL PPSSEEUUDDOOAARRTTHHRROOSSIISS.. 77.. PPRROOSSTTHHEETTIICC RREEPPLLAACCEEMMEENNTT :: in Old patients . 8. AAMMPPUUTTAATTIIOONN:: When the anticipated results of ttt are inferior to that after amputation.

NNOOTTEESS

Operative rationale: The rationale for treatment of non-unions is to reverse the causative factors:

1]. If excess motion stable internal or external fixation. 2]. If there is a gap obliterating or diminishing the space by compression or bone grafting. 3]. IF there is poor blood supply.

• start early active exercise of adjacent joints. • Shingling & cancellous bone gr bone stim, induct. & revasc. • Drilling or petalling avasc. Cortices revascularization them.

N.B: SSHHIINNGGLLIINNGG: both sides of non union, by using sharp chisel to decorticate bone with fine asteopertosteal fragments attached to peritoneum and muscle , assuring their vascularity, and increasing surface area of fracture. This is usually followed by cancellous bone grating of the pocket between shingles and bone. PPrriinncciipplleess ooff ttrreeaattmmeenntt::

1]. Know the local pathology; non-union vs delayed–union, by history, examination, PXR & Tc 2]. Correct biomechanical factors e.g. Transposition osteotomy 3]. Provide stability: by internal or external fixation. 4]. BG 5]. Excessive synovial pseudoarthrosis "When Tc shows hot zone, with central cold zone". 6]. Bridge gaps 7]. Decortications "SSHHIINNGGLLIINNGG" procedure of Dunn, to elevate periosteum & ⊕ periosteal NBF 8]. Eradicate infection by

• Excision of non-unions • Antibiotics • Sequestrectomy

9]. Plan surgical approach to ensure skin covering.


NN..BB:: When large gaps are present.

OR When a shortened extremity requires lengthening prior to the above proc.

⇓ • Vascularized fibular, iliac or rib graft. (By microvasc. Anast.) • Continue with the external fixator till healing occurs. Encourage early joint motion. • Don't accept mal position or shortening. It is mandatory to achieve a final mechanically neutral

position of the limb. Unacceptable major shortening is corrected by preliminary lengthening with the Wagner apparatus before definitive fixation

• Lengthening of lesser degrees (up to l inch) is usually done as are procedure with the müller distractor, Wagner apparatus or external fixator rods in bilateral frame configuration, at the time of internal fixation.

TTrreeaattmmeenntt ooff ssppeecciiffiicc ttyyppeess ooff NNoonn--uunniioonn:: 1- Hypertrophic vital non-union (Elephant's foot callus):

1]. Non – displaced diaphyseal 2]. Corectable diaph. Non –unions.

⇓ a]. External fixators. b]. Closed I.M.N. (é reaming) + I.M. BG (through chest tube) ILN (if not Instability) c]. Open I.M.N. d]. Tension band plating.

• BG is not necessary, as hypertrophic callus provides > enough BG for healing. • Some prefer removing excess callus small fragments & use it as BG heal. Potent. • Some prefer shingling :

a]. surface area. b]. Induce local bone formation

• To control rotational instability either by: Lag screw fixation, Cerclage wire. • Before correction of deformity, insert k- wires in the proximal & distal fragments at the

exact angle & rotation to be corrected.

3]. Open displaced diaphyseal non-union a]. Shingling b]. Excise pseudarthrosis. c]. Mobilize the non-union. d]. Correct the deformity. e]. ORIF either by: Plate, T.band, I.M.N.

2- Atrophic Non-unions:

1]. Stable fixation (plates, lag screws, I.M.N…) 2]. Shingling (or decortications). 3]. Bone graft inserted between the shingled osteoperiosteal fragments & the cortex.

1,2,3 to reactivate the dormant bone healing" switch.

When the cortex is osteoporotic: 1]. With plate & screws petalling instead of shingling. 2]. With IMN no much reaming.

• Reinforce the screws with liquid PMMA bone cement, injected with a syringe intothe loose screw holes. Tighten! Screws only after setting cement.

• Avoid cement entesing into the fracture site. • Use cancellous bane graft liberally. • External hinged plaster post – operative is recommended.


3- Metaphyseal articular Non-unions : (The most difficult). 1]. Arthrotomy see articular surface, realign fragments, lyse adhesions, release contractures,

remove loose bodies or fragments, gently manipulate the joint. 2]. Reconstruct the articular surface (k.wires, screws, ). 3]. Attach the reconstructed articular Block to the metaph. Or. Shaft straight, blade, T,L,

spoon. Plates, with compr. 4]. Start early active motion after prelimin splinting, depend on ligam. Repair or release, or

immediately, via a C.P.M. machine. 5]. Weight bearing is late, with brace or hinged cast brace, when fr. Is uniting.

4- Synovial pseudarthrosis: (PXR, Clinical: motion at fr. Site, Tc: cold cleft) 1]. Reaming the medullary Cavity. 2]. Excision of pseudarthrosis tissue. 3]. Opening the medullary Canal. 4]. Fracture reduction. 5]. Internal fix plates, IMN. 6]. Shingling. 7]. Bone grafting in atrophic types or in presence of gaps.

5- Infected non- draining non-unions:

1]. If dry for at least months as non-infected, but they should be debrided of any potent infected fibrous as granulation tissue.

2]. Shingling & bone grafting if avascular bone is present. 3]. Excision of Sequestra. 4]. Internal fixation: Plates & screws / IMN with reaming. 5]. Proper antibiotics & triple antibiotics for irrig. Intra-oper.

6- Infected draining non-union:

1]. If hardware is still holding and giving stability to the fracture, leave it in situ. 2]. If hardware is loose and ineffective Remove it + :

a]. Incision & drainage. b]. Debridement. c]. Sequestrectomy. d]. Open packing or closed suction irrigatioin. e]. Antibiotic- Impregnated PMMA. Beads may be used to fill the dead space till healthy

granulation tissue develops (usually 1-3 wks.) f]. Healing the non-union:

• Tibia bypass fibula pro-tibia operatio, through posterolat app. At fr. Level or by a proximal & distal tibial. Bl. Synastosis using cancel bonegralf & screw fix.

• Ext fixation frames Unit or bil. g]. Following successful bypass. Bridging (usually months. Etadicate the infection.

aa.. Saucerization sequestrectomy. bb.. Radjcal cxcistion of infected sinuses. cc.. Excl of fibrurs & granul tissue with the addution of another bone PMMA

anthbiotic beads.


MMAANNAAGGEEMMEENNTT OOFF IINNFFEECCTTEEDD NNOONNUUNNIIOONN

1. Conventional treatment:

• The object is to convert an infected draining nonunion to one that has not drained for several months and then promote healing of the nonunion by bone grafting.

• Disadvantage: needs a long time (one or more years) and may lead to joint stiffness. • The skin requires three operations:

1]. Wound saucarization and debridement to provide a vascular bed. • Correct major overlap or displacement and attempt to fix the fracture: • Plates and screws usually lead to persistent drainage. • Pins in prox & distal fragments are incorporated in a cast may be used (less secure) • After 4-7 days when the granulation tissue covers the wound

2]. Split thickness skin graft. After 4 wks. 3]. Full thickness pedicled graft. .

• BG is delayed until the skin graft is stabilized. • Reconstructive operations are delayed until at least 6 mo till infection is gone

2. Active treatment:

• The object is to obtain early bone union and thus shorten the period of convalescence and preserve motion in the adjacent joints.

• This is done in the following steps: 1]. Restore bone continuity. This takes absolute priority over treatment of infection.

Expose the nonunion through the old scar and sinuses decorticate the ends of the bones forming small osteoperiosteal grafts (detached grafts are discarded).

2]. Remove all devitalized infected bone and soft tissue. 3]. Align the fragment and stabillze by an external fixator while applying compression

across the nonunion if possible. A plate may be used when drainage have stopped. 4]. Apply cancellous bone graft. 5]. Close as much of the wound as possible and apply suction. Give AB.

3. Ilizaroy method:

4. PEMF

EELLEECCTTRROO--SSTTIIMMUULLAATTIIOONN OOSSTTEEOOGGEENNEESSIISS Electricity and fracture healing

1]. PPIIEEZZOOEELLEECCTTRRIICC EEFFFFEECCTT: charges in tissues are changed secondary to mechanical forces, so the compression side has the negatively charged potentials & the tension side has he positively charges

2]. SSTTRREEAAMMIINNGG PPOOTTEENNTTIIAALLSS: occur as electrically charged fluid is forced over a cell membrane 3]. TTRRAANNSSMMEEMMBBRRAANNEE PPOOTTEENNTTIIAALLSS: generated by cellular metabolism

Fracture Healing 1]. DC (Direct Current) ...................................... inflammatory response (constant better than pulsed) 2]. AC (Alternating current) ............................ repair phase collagen synthesis and calcification 3]. PEMF (Pulsed Electro Magnetic Field) ........ remodeling & calcification of fibrocartilage

RREESSPPOONNSSEE OOFF BBOONNEE TTOO DDIIRREECCTT CCUURRRREENNTT::

1]. Bone forms at the cathode, whereas cell necrosis occurs around the anode. 2]. Resistance rapidly between the electrodes in current; and so further increase in the

voltage is required to keep the amperage at the optimum level. 3]. Electrically induced osteogenesis exhibits a dose –response curve:

A) Current < 5 μAmp .................... do not produce ostegenesis. B) Current = 5-20 μAmp ............... Produce amount of bone formation and. C) Current levels > 20 μAmp ....... Show NBF giving way to cell necrosis.

4]. Electricity # healing & NBF, but the cathodes must be at the fracture site. 5]. Reaction at cathode consumption of O2 hydroxyl radicals: 2 H2O + 4e - + O2 = 4 OH-.


RREESSPPOONNSSEE OOFF BBOONNEE TTOO EELLEECCTTRROOMMAAGGNNEETTIICC FFIIEELLDDSS::

• Pulsed electromagnetic fields (PEMF) induce electric potentials, The polarity of these potentials changes as the magnetic field & alternating current in the tissue.

• The PEMF – induced currents modify cell behavior in bone, cartilage and other tissues, Thus, by properly programming the electrical events about the mesenchymal cells, a sequence of histologic changes can be induced. For example, the calcium contont of chondrocytes can be

or , cAMP, collagen & proteoglycans can be modified & DNA synthesis can be changed.

MMEECCHHAANNIISSMM OOFF EELLEECCTTRRIICCAALLLLYY IINNDDUUCCEEDD OOSSTTEEOOGGEENNEESSIISS::

1]. Cathode local O2 consumption relative hypoxia NBF. It is known that bone follows a predominantly anaerobic metabolic pathway.

2]. The electric impulses realign the collagen molecules initiating calcification. 3]. cAMP by electrical stimulation has also been suggested.

Methods Used For Application Of Electricity: Invasive Semi-Invasive Non Invasive Idea • Totally implantable electrical

stimulator of three parts:

1. Power unit DC of 20 μamp regardless of bone tissue resistance Changes. Some use a pulsed direct current freq of 20 Hz.

2. 1-4 titanium cathodes to be implanted in the nonunion site whether it is a single or multiple fracture sites.

3. One anode that is placed in the soft tissues adjacent to the generator.

• Insertion of cathodes indirectly into nonunion

1. The power source. 2. The cathode is a Teflon

coated stainless steel k-wires percutaneously.

3. The anode.

• PEMF induce electric potentials ώ change polarity as the magnetic field and , ώ AC in the tissues.

• Pair of coils mounted on

the surface of the cast. They should be // to each other & centered over the # site

Results >85% within 12-36 weeks

Cons 1. It is portable é minimal postop discomfort.

2. Short hospital stay. 3. No pt coop needed

1- High success rate, 2- No need for operations. 3- infection 4- postop pain. 5- Portable

1- Can be used in OM. 2- No surgery needed 3- No risk of infection

Pros 1. Need minor op for insert & removal

2. Not é acute OM.

1- Patient remains NWB for 3 mo to cathode break

2- Not é acute OM. 3- Not é motion at # site 4- Pin tract infection, 5- cathodes breaking 6- Recurrence of the OM. 7- Cathode dislodgement.

1- It is not portable. 2- Should be used daily for

at least 10 hours, 3- Prolonged NWB POP.


BBoonnee GGrraafftt Definition • Replacing missing bone, adding to existing bone, or stimulation of the existing bone to

produce adequate structural and functional support

Classification: IInnddiiccaattiioonnss 1. To provide structural stability (cortical bone best) 2. To provide linkage, i.e. replace missing bone:

Congenital deficiencies Traumatic deficiencies: best to be applied at the compression side Infections after debridement: e.g. ca sulphate granules Tumors after excisions

3. To stimulate osteogenesis and bone healing Non united fractures Osteoporotic fractures Revision surgeries Spine fusions (TCP) & Kyphoplasty (ceramic cement)

Contraindications: 1. Infections: (can use ca sulphate bone substitutes)

Wound infection Open fractures

2. Non-viable surrounding bone not capable of supporting and anchoring the implant 3. Bone disorders that hinders BG incorporation

Inflammatory bone disease. Metabolic bone disease é altered calcium metabolism. Immunologic abnormalities. Systemic disorders é poor wound healing over the implant site.

TTyyppeess aaccccoorrddiinngg ttoo ssttrruuccttuurree 11.. Cortical:

• Strong immediate structural support • Slow incorporation initially weakens the graft • ~ 50% weaker than normal bone from 6 wks - 6 mo returns to normal 1-2 y after

22.. Cancellous: • Revascularised more quickly • Osteoblasts lay NBF down on old trabeculae which is later remodelled • Only surface cells remain viable by diffusion.

33.. Cortico-cancellous: • Produce both structural stability & quick incorporation

44.. Osteochondral • For tumour surgery • Osteochondral graft survival enhanced by immersion in glycerol

55.. Bone marrow & bioactive inductive substances: • BMAT; has osteoprogenitor cells • BMP; impregnated ceramics


Types According to Source 1. Autografts

• From the same person, most still dies • No immunogenicity • Highest osteogenic and osteoinductive capacity • Revascularized more quickly than allograft • Donor site morbidity (20%) with hematoma, pain, fracture, wound infection • Limited supply • Best reserved for area of large bone loss or irradiated tissues • No resorption at either ends of BG, segment heals as a fracture

2. Isograft:

• Same as allograft but from genetically identical twin not immunogenic

3. Allograft • Donor bone from another person • No donor site morbidity • Large amounts available • Not osteogenic • Incorporation:

o Qualitatively similar to that for autografts o Delayed (μß ð collagen alteration after irradiation) o Less extensive o Biologically inferior

• Immunological response and less reliable incorporation • Infection 10% 80% clinical failure • Transmission of HIV, Hep B, Hep C

4. Xenograft

• From a different species i.e. porcine, bovine • Similar to allograft bone after freezing and irradiation.

5. Synthetic Grafts / Ceramics (see later) 1]. Allograft matrix 2]. Polylactic & Polyglycolic matrix 3]. Hyaluronic acid 4]. Collagen 5]. Ceramics


Types according to fuction 11-- OOSSTTEEOOIINNDDUUCCTTIIVVEE:: = BBIIOO--AACCTTIIVVEE PPOOLLYY--PPEEPPTTIIDDEESS TTHHAATT BBOONNEE FFOORRMMAATTIIOONN

I. Bone Morphogenetic Proteins (BMP.s): • Recruit & progenitor cells of osteoblast lineage • bone collagen synthesis.

II. Insulin-Like Growth Factor (IGF): • It plays a critical role in growth, whether it plays a role in bone healing is less certain.

III. Platelet-Derived Growth Factor (PDGF): • Potent mitogen for UMC • DNA synthesis, cell replication, and production of collagen

IV. Transforming Growth Factor-ß (TGF-ß): • Mesenchymal cell growth and differentiation • Collagen synthesis • fibroblasts and macrophages chemotaxis • TGF-ß osteoinductive activities of BMP.s.

V. Fibroblast Growth Factors (FGF.s): • Mesenchymal cell growth and differentiation • The most studied members are aFGF and bFGF

22-- OOSSTTEEOOGGEENNIICC:: = AACCTTIIVVEE CCEELLLLSS CCAAPPAABBLLEE OOFF BBOONNEE PPRROODDUUCCTTIIOONN

I. Unfractionated Fresh Bone Marrow: BBMMAATT • Harvested from the iliac crest and immediately transplanted to skeletal repair • Simple procedure that is inexpensive and can be done on an outpatient basis. • Limited source of osteoprogenitor cells • Complications of harvesting

II. Connective Tissue Progenitors: • Able to replication without differentiation, & has multilineage developmental

potential. • C.T. progenitors are expanded in number without undergoing differentiation.

III. Differentiated Osteoblasts & Chondrocytes • Difficult to obtain > osteoprogenitor cells & has limited capacity for proliferation • Mature osteoblasts could be generated from culture expanded progenitor cells

IV. Genetically Modified Cells: • In this new technique, gene therapy is used for treatment of bone cells, using a

delivery vehicle to transmit the genetic material coding for osteoinductive stimuli

33-- OOSSTTEEOOCCOONNDDUUCCTTIIVVEE (scaffolds) = materials that attachment, migration, and distribution

of cells responsible for bone-healing

II.. AALLLLOOGGRRAAFFTT BBOONNEE MMAATTRRIICCEESS:: • Although allograft bone lacks any viable cells that might contribute to NBF • Allograft matrix is highly OOSSTTEEOOCCOONNDDUUCCTTIIVVEE é some osteoinductive properties. • Drawbacks; less satisfactory results < autograft, disease transfer, & immunogenic reaction

IIII.. CCOOLLLLAAGGEENN:: (delivery system) • Collagen is conductive to bone formation • Surface contains sites for deposition of mineral • Binds the non-collagen proteins, which provides sites for cell attachment

IIIIII.. HHYYAALLUURROONNAANN:: • Hyaluronan is not osteoconductive, but it is useful tissue engineering substrate

IIVV.. PPOOLLYYLLAACCTTIICC AANNDD PPOOLLYYGGLLYYCCOOLLIICC PPOOLLYYMMEERRSS:: • Degradable polymers have little osteoconductive potential • Highly biocompatible so it is used also a successful substrate in tissue engineering


VV.. CCEERRAAMMIICC MMAATTRRIICCEESS:: A. HA from corals: = HHYYDDRROOXXYYAAPPAATTIITTEE::

• Derived from coral ca carbonate, PPOORRIITTEESS as cortical bone & GGEENNIIPPOORRAA as cancellous • Slowly resorbed & low porosity

B. Calcium Sulfate Matrices: • Can be used in presence of infection, & is the cheapest • Two forms, with or without AB.

C. Tricalcium phosphate: • The porosity ≈ 3355%%, with pores ranging from 110000--330000 μμMM. • Greater solubility >HA, and as a result implants are reabsorbed more rapidly.

D. Injectable Ceramic Cements : These Injectable cements are usually composed of α-TCP, dicalcium and tetra calcium phosphate monoxide. • Cements can be injected into # sites or bone defects

E. Ultraporous β-tricalcium Phosphate: • A newly developed β-TCP é higher porosity & faster resorption. • Larger surface area is exposed to cells and nutrients. • Ultraporous β-TCP seeded é autologous BM could act as autograft

CCOOLLLLEECCTTIIOONN OOFF DDOONNOORR BBOONNEE (Femoral Heads) • Blood tests (HIV, Hep B, VDRL, Rhesus) • Swabs are taken form cut site & acetabulum • Head placed in 2 sterile bags, sterile container & un-sterile bag • 2.5 MRad of γ radiation • Stored in (-70°C) ultra cold freezer

Preservation & transplantation: 1- FFRREESSHH - requires no preservation. No test for disease or sterility. There is immune response.

The application of fresh allograft is limited to joint resurfacing. 2- FFRROOZZEENN < (-60°C) enzyme degradation immunogenicity + intact mech. Properties 3- LLYYOOPPHHIILLIIZZEEDD ((Freeze-Dried)):

• Removing water + vacuum packing + freezing + storage up to 5y • antigenicity • Osteoconductive only • Biomechanical alteration on rehydration.

4- IIRRRRAADDIIAATTEEDD:: • Powerful sterilizing method • antigenicity • Biomechanical alteration

GGrraafftt HHeeaalliinngg

SSTTAAGGEE DDEESSCCRRIIPPTTIIOONN 1- Haemorrhage 2- Inflammation Chemotaxis stimulated by necrotic debris 3- Revascularisation 4- Creeping Substitution

1]. Osteoblast differentiation ............... 2]. Osteoinduction ................................... 3]. Osteoconduction ..............................

Replacement of necrotic host tissue by donor NBF along the invasive host Bl. v v. From Precursors Osteoblast and clast function New bone formation over a scaffold

5- Remodelling Continues for years


Factors adversely affecting healing 1- General:

o Mal nutrition o Debility o Extreme ages o Drugs: NSAID’s, diphosphonates

2- Local: o Severe soft tissue laceration & devitalization o Vascularity o Infection o Foreign Material

Immunogenicity In general bone and cartilage ................. weakly immunogenic Fresh allografts .............................................. most immunogenic Freeze dried (lyophilized) ................................... least immunogenic; but BMP is depleted + low structural

integrity Irradiation ....................................................... alter its structural strength

Advantages of grafts: 1]. Decrease cost: by seeking new definitive treatments (e.g. Osteoarthritis). 2]. Solve many reconstructive problems 3]. Good results as regard management of delayed and non union 4]. Multiple & variable sources 5]. Allo & synthetic grafts avoid autograft harvesting & donor site morbidity

Disadvantages: 1]. Disease transmission e.g. xeno & allografts 2]. Unavailable technology for the recombinant and genetically modified options 3]. Decreased osteogenic efficacy as compared é autografts 4]. Cell expansion & differentiation still under trials 5]. Osteoconductive matrices are still expensive

Properties of Bone Graft Materials

Material Osteoinductive Osteogenic Osteoconductive Integrity AAUUTTOOGGEENNOOUUSS CCAANNCCEELLLLOOUUSS BBOONNEE ++ +++ +++ - AAUUTTOOGGEENNOOUUSS CCOORRTTIICCAALL BBOONNEE + + + ++ VVAASSCCUULLAARRIIZZEEDD AAUUTTOOGGRRAAFFTT + + + +++ AALLLLOOGGRRAAFFTT "+/-" - + + BBOONNEE MMAARRRROOWW + + + - DDBBMM + - ++ - CCOOLLLLAAGGEENN - - + + CCEERRAAMMIICCSS - - + + BBMMPP "++" - - - "+ = Moderate" " ++ = Marked " " - = None" " +/- = Some "


CCaarrttiillaaggee SSuubbssttiittuutteess • No consistently reliable means to regenerate joint cartilage currently exists. • As with bone tissue engineering we have three basic elements for cartilage:

1- Growth Factors. 2- Chondorogenic cells. 3- Matrices ( Scaffolds).

1- GGRROOWWTTHH FFAACCTTOORRSS

a. IINNSSUULLIINN--LLIIKKEE GGRROOWWTTHH FFAACCTTOORR ((IIGGFF)) :: • IGF.s, beside their effect on osteoblasts, known to be differentiative and mitogenic for

cartilage tissue. Their importance lies with their role in osteoarthritis.

b. BBOONNEE MMOORRPPHHOOGGEENNEETTIICC PPRROOTTEEIINNSS ((BBMMPP..SS)):: • BMP.s based on their variable functional expression, are able to modulate

chondrogenesis • Theoretically, BMP.s are optimal growth factors to recruit undifferentiated stem cells

for the repair of full-thickness articular cartilage defects as they are unique in that they can initiate the formation of cartilage by a process similar to the endochondral ossification that occurs in the growth plate..

c. HHEEPPAATTOOCCYYTTEESS GGRROOWWTTHH FFAACCTTOORR ((HHGGFF)):: • HGF has been reported to have mitogenic effects on chondrocytes, meniscal cells, and

ligament cells.

d. BBAASSIICC FFIIBBRROOBBLLAASSTT GGRROOWWTTHH FFAACCTTOORR ((BBFFGGFF)):: • Several reports have shown that bFGF is capable of inducing repair of superficial or

partial thickness articular cartilage defects when injected intraarticularly..

e. TTRRAANNSSFFOORRMMIINNGG GGRROOWWTTHH FFAACCTTOORR--ββ ((TTGGFF--ββ)):: • TGF-β is produced by articular chondrocytes and remains in the cartilage in a latent

form. It has been reported to influence the proliferation of human articular chondrocytes.

2- CCHHOONNDDRROOCCYYTTEESS AANNDD UUNNDDIIFFFFEERREENNTTIIAATTEEDD MMEESSEENNCCHHYYMMAALL CCEELLLLSS

• Produce a new cartilage matrix. • Selective transfer of gene expression to chondrocytes or chondroprogenitor cells may

be preferable to synovial cell transfer. These studies are encouraging for the future use of ex vivo gene transfer to chondrocytes to treat cartilaginous defects.

3- AARRTTIIFFIICCIIAALL MMAATTRRIICCEESS

• ingrowth of new cells • matrix formation • Protective • Different methods for holding matrices and cells in articular cartilage lesions may

include GGLLUUEESS,, FFLLAAPPSS,, PPIINNSS….etc. • Intra-Articular Inj of Hyaluronan is an example of matrices in cartilage tissue engineering


MMeeddiiccaallllyy aapppprroopprriiaattee aaddmmiinniissttrraattiioonn 1- Osteoarthritis of knee is documented with radiographic evidence: and 2- Osteoarthritic knee pain that interferes with functional activities such as; ambulation,

prolonged standing , etc.; and 3- Lack of functional improvement following a trial of at least three months of conservative

therapy; and/or Inability to tolerate NSAID therapy 4- Failure of at least one injection of a steroid product into the knee resulting in unsatisfactory

relief or relief that lasted less than three months. • Repetition of a cycle (3 to 5 injections) every 6mo, if symptomatic relief from the previous

course of ttt has been confirmed and documented, is considered MMEEDDIICCAALLLLYY AAPPPPRROOPPRRIIAATTEE.. MMeeddiiccaallllyy IInnaapppprroopprriiaattee aaddmmiinniissttrraattiioonn 1- Active inflammatory joint disease or synovitis affecting the knee (e.g., crystal synovitis,

rheumatoid arthritis) 2- Presence of infection of the target joint or skin surrounding the proposed site of injection 3- Allergy to birds, feather, eggs etc 4- Pregnancy. • Repetition of treatment cycles (3 to 5 injections), more frequently than every six months, is

considered NNOOTT MMEEDDIICCAALLLLYY AAPPPPRROOPPRRIIAATTEE.

TTeerrmmiinnoollooggyy:: 1- AAUUTTOOGGRRAAFFTT: is tissue transplanted from one area to another in the same individual. 2- AALLLLOOGGRRAAFFTT: is tissue transplanted from one individual to another. 3- XXEENNOOGGRRAAFFTT: is tissue transplanted between animals of different species. 4- OORRTTHHOOPPTTIICC: anatomically appropriate graft. 5- HHEETTEERROOTTOOPPIICC: anatomically inappropriate graft. 6- OOSSTTEEOOGGEENNEESSIISS: bone formation with no indication of cellular origin. This may be graft or host

origin (i.e. osteogenesis refers to augmentation of bone formation). 7- OOSSTTEEOOIINNDDUUCCTTIIOONN: refers to recruitment from the surrounding bed of mesenchymal-type cells,

which then differentiate into cartilage forming and bone forming cells. Osteoinduction is mediated by graft-derived factors.

8- OOSSTTEEOOCCOONNDDUUCCTTIIOONN: refers to the three-dimensional process of ingrowth of sprouting capillaries, perivascular tissue and osteoprogenitor cells from the recipient bed into the structure of the graft. Simply, the graft functions as a scaffold, for the ingrowth of new host bone.

9- BBIIOOMMAATTEERRIIAALL: A non-viable material used in a medical device, intended to interact with biological systems.

10- BBIIOOCCOOMMPPAATTIIBBIILLIITTYY: The ability of a material to perform with an appropriate host response in a specific application.

11- BBIIOOIINNEERRTT: No host response to the material. 12- IIMMPPLLAANNTT: An object made from non living material that is inserted into the human body where

it is intended to remain for a significant period of time in order to perform a specific function. 13- PPOOLLYYMMEERR BBOONNEE GGRRAAFFTT: nonviable engineered materials formed from polylactic acid and/or

polyglycolic acid, nylon, animal derived collagen and other materials. Depending on the material it degrades through inflammatory or metabolic processes. In some formulations its mechanical properties allow it to be used as a resorpable plate or screw.

14- CCEERRAAMMIICC BBOONNEE GGRRAAFFTT: nonviable brittle dense engineered material, solid or with formed or natural porosity, generally available in powders, granules or standard geometric shapes such as blocks and wedges. Most formulations degrade very slowly with some newer formulations being reported to degrade more rapidly.


Zobad Topic: Definition Notes

Load Is the force applied (newton) Stress(ð) (nominal, engineering)

F/A= N/m²= Pa= the force applied over a surface unit area. (measure of the force on an object)

True stress (ðt) uses true csa instead of original csa (as with nominal stress).; ðt= [F/original csa] x [1+ nominal strain].; ðt>>ðn because of lower csa ('Necking')

Strain (ε) L-Lo/Lo = a ratio between the change of length : original length = how far atoms are displaced apart

true strain= ln(1+ nominal strain).

Strain types 1]. Bending: o 3 points bending o 4 points bending o Cantilever bending

2]. Compressive buckling 3]. Shearing 4]. Torsion

Stress Shielding Is the stress by pass from the less stiff material to the more stiff one when they are fixed together

HHOOOOKKEE’’SS Law Stress is directly proportionate to strain till the yield point (Robert Hooke, 1678) Yield stress Is the stress beyond which the material will express plastic deformation (yield point= elastic

limit) Tensile Strength Max stress the material can resist without breaking when exposed to a single load, beyond ώ

continuous deformation occur even with decrease of the stress Failure Strength Max stress beyond which the material eventually fail Fatigue strength Max cyclic loads the material can resist without breakage when exposed to 107 cyclic loads Endurance (Fatigue Limit))

Max cyclic loads below fatigue will not occur (theoretical for ortho implants) Polished steel endurance = ½ the tensile strength

YYOOUUNNGGSS Modulus of elasticity (E)

measure of SSTTIIFFFFNNEESSSS of a material. =stress/strain. Usually the same in tension & compression

Strain Energy(U) (Joules)

The increase in energy associated with the deformation of a structure, as a result of the application of a slowly increasing load. = Area under load-extension curve.

Strain Energy Density (u) (J/m³)

Energy associated é deformation of a structure, eliminating the effects of the structures size. =area under stress-strain curve. u= stress²/2x Modulus.

Strain energy density obtained by loading to rupture MMOODDUULLUUSS OOFF RREESSIILLIIEENNCCEE= energy per unit volume that the material can absorb without yielding (= area under elastic portion of stress-strain curve).

Strain Hardening

yield strength, ductility & toughness, unchanged modulus(stiffness).

Toughness Ability of a material to resist breaking (i.e. absorb energy & deform plastically) = energy/unit volume a material can absorb before failure. Tough material has ductility & yield stress & withstands stresses &

strains.

Toughness measurements:; a. MMOODDUULLUUSS OOFF TTOOUUGGHHNNEESSSS =The area under the

curve up to the breaking point b. Impact Tests- Charpy c. Fracture toughness: ability to resist crack propogation

Stiffness Ability of a material to resist deformation. Measured as Elastic Modulus.

1]. Axial Stiffness(A) = [pi/4]x [Do-Di]; 2]. Bending Stiffness= Area Moment of Inertia(I) 3]. Torsional Stiffnes= Polar Moment of Inertia(J)

Hardness Measure of a materials resistance to abrasion or indentation. Hardness is proportional to Tensile Strength.

Hardness Tests: ; 1. Brinell- 10mm steel ball, HB=F/½piD[D-sq.rt(D²-d²)]; 2. Vickers- pyramid-shaped diamond, HD=1.854F/d²; 3. Rockwell

Ductility/ Brittleness

Is the ability of a materials to deform before they break (elastic & plastic) / Is the resistance to plastic deformation before breakage

Measures of Ductility 1. Percentage Elongation 2. Percentage Reduction in cross-sectional area 3. Bend tests; 4. Cupping tests(Erichson); 5. Impact test(Charpy) - ductile absorb energy till #

SSOOUURRCCIILL Subchondral bone condensation at superomedial acetabulum (R is maximum at this point)

GGOOTTHHIICC AARRCCHH Remodeled bone at the acetabular roof above the sourcil Euler's Column Law

Determines critical load for scoliosis. Pcrit = C.(E.I/L²); Pcrit = critical load, C=end conditions, E = modulus, I = moment of inertia, L =column length.


Topic: Definition Notes Definitions Kinematics= Analysis of motion w/out reference to forces.;

Kinetics= Analysis of motion under the action of given forces or moments. (= static / dynamic); Statics= study of forces & moments acting on a body in equilibrium (at rest or constant speed) Dynamics= study of forces & moments acting on a body (accelerating/ decelerating)

Failure When a material lost its ability to satisfy the original design function.

Types(7): 1].FFAATTIIGGUUEE: failure 2ry to cyclic loading 2].FFRRAACCTTUURREE: failure 2ry to bending stresses into > 2 parts 3].BBUUCCKKLLIINNGG: 2ry to compression of a thin walled tube 4].CCOORRRROOSSIIOONN: 2ry to electrochemical action 5].WWEEAARR: mechanical deterioration of solid surface 6].CCRREEEEPP & deformation 7].LLOOOOSSEENNIINNGG: septic & aseptic

Failure Ductile metals may fail in a brittle manner at; low temps, thick sections, at high strain rates or where there are flaws.

Fatigue The of strength by the application of cyclic loads below the tensile strength of the material.

Low cycle fatigue = max. stress in a cycle > yield stress. High cycle fatigue = max. stress in a cycle < yield stress. This by surface scratches.; PPEEEENNIINNGG= light hammering of the surface with a round-nosed hammer Fatigue Life by inducing residual compressive stresses in material

Fatigue Fracture Occurs in 3 steps:; 1]. Nucleation of a crack- occurs at locations of highest stress & lowest local strength. These are

usually at or near the surface & include surface defects, such as scratches or pits, sharp corners, inclusions, grain boundaries or dislocation concentrations.;

2]. Propagation of a crack- towards lower stress regions. The crack propagates a little bit further each cycle, until the load-carrying capacity of the metal is approached

3]. Catastrophic failure- in a brittle manner; implant buckles into 2 or more parts when the load is changed during service.

Endurance (Fatigue Limit))

Is the cyclic load limit below fatigue will not occur (theoretical for ortho implants) Polished steel endurance = ½ the tensile strength

Corrosion Destruction of metal by electrochemical action Corrosion Mechanism

1) GGAALLVVAANNIICC: between metals é different electrochemical potentials 2) FFRREETTTTIINNGG: surface breakdown 2ry to motion & loads between metal surfaces 3) CCRREEVVIICCEE: motion bet metals depassivate their surfaces 4) PPIITTTTIINNGG: surface abrasion galvanic corrosion 5) SSTTRREESSSS: load generated crack galvanic corrosion 6) MMIICCRROO--BBIIOOLLOOGGIICC: micro-org secrete corrosive metabolite 7) IINNTTEERRGGRRAANNUULLAARR: corrosion at welding points &not the metal structure failure (weld decay)

Corrosion by oo PPAASSSSIIVVAATTIIOONN (surface oxidation) ○○ Implants é one metal type o PPEEEENNIINNGG (light surface hammering) ○○ Implants é non modular components o PPOOLLIISSHHIINNGG ○○ Implants é inert metal oo Heat treatment oo Good surgical treatment technique to abrasions

Types of Material 1]. IISSOOTTRROOPPIICC= has same properties in all directions.; 2]. AANNIISSOOTTRROOPPIICC= different properties in diff. directions.; 3]. OORRTTHHOOTTRROOPPIICC= has same properties in a particular direction throughout the material 4]. BBRRIITTTTLLEE = has linear stress strain curve till failure; i.e. affinity for plastic deformation 5]. DDUUCCTTIILLEE = has great affinity for plastic deformation before failure 6]. VVIISSCCOOEELLAASSTTIICC = has time-rate dependant stress strain curve (e.g. bone and ligament)

Creep Continuous deformation under constant stress. It is stress, time & temp. dependent (fatigue is stress & time dependent only).

Stress Relaxation Decrease stress under constant strain over time. Hysteresis Viscoelastic phenomenon that exhibits a different loading & unloading patterns of stress-strain

curve when subjected to cyclic loading, EENNEERRGGYY ((JJOOUULLEESS)) The ability to do work. Potential E=MGH (static energy);

Kinetic E=½mw² (motion energy)a, w=angular velocity Energy cannot be created or destroyed.


Topic: Definition Notes Tribology concept 1]. Wear

2]. Friction 3]. Lubrication

1]. Wear Material shedding from solid surfaces as a consequent of the mechanical action. 1. AABBRRAASSIIVVEE: the harder grooves the softer material 2. AADDHHEESSIIVVEE: the softer material adheres on the harder surface 3. FFAATTIIGGUUEE, in which repetitive loading subsurface delaminate lost from the surface 4. TTHHIIRRDD--BBOODDYY WWEEAARR implies the retention of debris bet. sliding surfaces abrasive wear. 5. BBAACCKK SSIIDDEE WWEEAARR: bet PE & the metal backing. 6. RRUUNN IINN WWEEAARR: is the accelerated wear that occur in the 1st few millions of cycling

2]. Friction The undesirable effect when two surfaces move in contact with each other.

F=μR (μ= coefficient of friction) (frictional force, F, is proportional to the normal component of the reaction force, R); μ=tanø (ø is the critical angle on an incline when motion starts to occur= 'angle of friction')

Coefficient of Friction

the resistance encountered in moving one object over another.

Normal joints= 0.008-0.02; metal-on-metal= 0.8; metal-UHMWPE= 0.02; metal-bone= 0.1-0.2; ceramic-ceramic= v. low; ceramic-UHMWPE= v. low; metal-ceramic= v. high

Coulombs Law of Friction

The shear stress is always parallel to the relative velocity & equal to the product of the contact pressure & the dynamic friction coefficient as determined from measurements on particular combinations of materials. [Shear Stress= Compressive stress × Coefficient of Friction]

Torque Rotational Force [Newton X Meters(Nm)]

T= I x α; [I = Mass Moment of Inertia (Nm.sec²); α = angular acceleration (radian/sec²)]

Frictional Torque Is the force transmitted from head-PE interface to bone interface through out the motion arc

3]. Lubrication 1]. EELLAASSTTOOHHYYDDRROODDYYNNAAMMIICC = the bearing materials deform elastically; friction is determined by the complete lubricant film that separate the bearing surfaces

2]. BBOOUUNNDDAARRYY LLUUBBRRIICCAATTIIOONN = the bearing surfaces come much closer together & friction is determined by the coefficient of friction of the non-deformable material surface (lubricant partially separate the surfaces)

3]. BBOOOOSSTTEEDD LLUUBBRRIICCAATTIIOONN = the bearing surfaces are partially separated by pools of lubricant ώ is trapped by areas of bearing surfaces

4]. HHYYDDRROODDYYNNAAMMIICC LLUUBBRRIICCAATTIIOONN = the load & motion influence the lubricant film between the bearing surfaces.;

5]. WWEEEEPPIINNGG LLUUBBRRIICCAATTIIOONN = in which fluid shifts from cartilage to loaded areas Wetability Is the affinity of a material to a

lubricant material Depends on the surface tension of the material = the angle of contact bet the material and a lubricant drop

Velocity the rate of change of the position of the body.

= a Vector (has magnitude, direction & sense). Speed is scalar (only has direction).

0.1% Proof Stress the stress which results in a 0.1% plastic strain. For materials where the yield stress is not easily identified (aluminium). (proof stress not usually quoted for polymers)

Line drawn on force-elongation/ stress-strain graph parallel to the linear part of graph & passing through the 0.1% strain value(=0.1% gauge length).

Annealing Process involving heating to & holding at a temp. high enough for recrystallization to occur and then cooling slowly.

Results in a softened state (more ductile), to facilitate cold-working, improved machine-ability and mechanical properties.; eg. Orthop. wires (stainless steel 316L, annealed)

BBOOYYLLEE''SS Law Pressure= Force/Area BBAARRBBAA''SS Law Takes into consideration the effect

of csa on % elongation in Tensile testing.

% Elong.= {[a x sq.rt.(csa)/gauge length] + b} x100

Bone Fracture Bone fails in TENSION. Shear failure is a tension failure, but crack propagates in spiral because of the ANISOTROPY of bone.; *Haversion canals help to prevent crack propagation.

Brittle Fracture Break a material, & the broken ends fit together perfectly (i.e. no reduction in csa).


Topic: Definition Notes Solid materials

1]. MMEETTAALLSS: High tensile strength & modulus of elasticity, medium hardness, can be ductile, poor resistance to corrosion, high electrical & thermal conductivity.

AALLLLOOYYSS- • Mild steel= iron & carbon; • Stainless steel- Fe, chromium, carbon & manganese (C strength, Cr R to corrosion) • Vitallium= chromium, cobalt & molybdenum alloy (historical).

2]. PPOOLLYYMMEERRSS: 1]. Thermosets= decompose when heated. Bakelite. 2]. Thermoplastics= soften when heated.

PPOOLLYY--EETTHHYYLLEENNEE. Low modulus of elasticity; low hardness; medium tensile strengths; ductile; low densities; high corrosion resistance; low electrical & thermal conductivities; tend to creep; properties depend on temp. Can withstand high strains, not high stresses.;

3]. CCEERRAAMMIICCSS: 1]. Brittle (Can withstand high stresses, not high strains) 2]. Hard 3]. High modulus of elasticity 4]. Stronger in compression than in tension 5]. Low electrical conductivity.

4]. CCOOMMPPOOSSIITTEESS: two different materials bonded together. More expensive to produce. Alloy A substance containing two or more metals mixed in ! liquid phase. Ceramics A substance chemically comprised

of metallic and non-metal elements/molecules (eg. ZnO, SiO, TiO2))

Properties determined by ionic bonds, stronger than covalent bonds of polymers & metallic bonds of metal.;

1]. High chemical resistance. 2]. High Elastic Modulus. 3]. Highly Crystalline -> Brittle. 4]. Hard -> High wear resistance. 5]. Inert (eg. calcium hydroxyapatite). 6]. Can éstand high stresses, but cannot produce

high strain NB- because of high melting point large ceramics are prepared by compressing small powder particles this always has small defects stress risers + Brittle WEAK.

Composites = a multiphase material. The constituents must be chemically dissimilar & seperated by a distinct interface. (matrix & dispersed phases). It should provide distinctive properties that cannot be obtained by the individual components alone. High strength to weight ratio.

Types:; 1]. PPAARRTTIICCLLEE RREEIINNFFOORRCCEEDD:

a. Large particle (concrete) b. Dispersion strenthened (atomic);

2]. FFIIBBEERR RREEIINNFFOORRCCEEDD: whisker, fiber, wire; continuous, discontinuous; aligned(anisotropic),random(isotropic);

3]. SSTTRRUUCCTTUURRAALL: a. Laminar(wood) b. Sandwich panels.

Finite Element Modelling/Analysis (FEM)

= The ability to model structures of complex geometry as an assemblage of simple elements.

The main requirement is to have, for a range of elements of varying shapes, solutions of the governing differential equations for arbitrary boundary conditions.

Instant centre of rotation

It is the point about which the joint rotates

Free Body Diagram

The segment of the body of interest. The segment is assumed to be in equilibrium.

IM Nails *Tubes é a wall thickness:radius ratio of < 1/8 tend to behave as curved sheets rather than tubes. These thin- tubes are subject to buckling. (Bone is thick-walled).; *A wider diameter hollow tube is stiffer than a solid smaller diameter tube with the same amount of material. A slot/slit torsional stiffness by 98% -> quicker healing with callus.


Topic: Def: Notes: PE - Glossary • BBAASSEE RREESSIINN - The PE granules or powder; the raw source polymer.

• MMEEDDIICCAALL--GGRRAADDEE PPEE is a very small percentage of the worldwide production of PE. Only ultra-high molecular weight material is used in the manufacturing of components for total joint replacement.

• CCAALLCCIIUUMM SSTTEEAARRAATTEE - A compound mixed with the PE powder (in some grades of the material) before it is formed into a solid. The calcium stearate serves as a scavenger of residual acid. In ram extrusion, it also acts as a lubricant, and has been shown to help prolong the life of the manufacturing equipment. It also results in the poly ethylene having a whiter color. However, some reports have indicated that fusion defects are more common in PE that contains calcium stearate. Fusion defects may make the component more susceptible to crack initiation and propagation. Consequently, many manufacturers now use grades of PE that do not contain calcium stearate. However, the quantitative effects of calcium stearate on the wear properties of PE components are a subject of ongoing debate. ; In several retrieval studies, components manufactured from 1900 PE resin have shown significantly lower levels of oxidation following sterilization by gamma irradiation in air. The reason(s) for this have not been clearly identified.

• CCHHAAIINN SSCCIISSSSIIOONN - Breakage of the long chains of PE into shorter molecules. Extensive chain scission can substantially increase the crystallinity, density, stiffness and brittleness of the PE, weakening the material. Oxidation is a primary cause of chain scission in PE.

• CCOOMMPPRREESSSSIIOONN MMOOLLDDIINNGG - A consolidation method that subjects the PE powder to high temperature and pressure, fusing it into a solid form, either into bulk stock for subsequent machining, or into net-shape components.

• CCOONNSSOOLLIIDDAATTIIOONN - The fusing of PE powder into a solid form by application of heat and pressure. The two principal methods of consolidation are compression molding and ram extrusion.

• CCRROOSSSS SSHHEEAARR - The particular type of stress applied to the surface of the PE component due to the crossing-path motion of the femoral ball. Crossing-path motion is also present, albeit to a lesser extent, in some designs of knee prostheses. Studies have shown that PE wear is 10 to 100 times greater with crossing path motion than with simple linear reciprocating motion.

• CCRROOSSSSLLIINNKKIINNGG - The process by which chemical bonds link carbon atoms in adjacent PE molecules by combining two free radicals. Cross linking has been shown in laboratory wear simulators (both hip and knee) to markedly the wear resistance of PE.

• EELLEECCTTRROONN BBEEAAMM IIRRRRAADDIIAATTIIOONN - Also known as E-beam, the PE is bombarded with high-energy electrons which induce crosslinking. Because there is more attenuation of an electron beam than gamma rays, a high beam energy (e.g., 10 MeV) is used to produce crosslinking in the PE. Residual free radicals generated by the electron beam can be extinguished by an appropriate post-crosslinking thermal treatment to avoid long-term oxidative degradation; EtO sterilization - A sterilization method that utilizes ethylene oxide gas (EtO). EtO does not induce free radicals or oxidation; it also does not induce crosslinking. To eliminate the toxic gas, components must be outgassed for a sufficient period prior to being implanted.

• FFRREEEE RRAADDIICCAALL - An electron on an atom that is a potential reaction site for oxidation or cross-linking.

• GGAAMMMMAA IIRRRRAADDIIAATTIIOONN - Irradiation by exposure to a radioactive cobalt source, which emits gamma rays. Gamma radiation (in air) has been the predominant method used to sterilize prosthetic joint components for more than two decades, with free radical production, oxidation and crosslinking being unintentional by-products. Only recently has gamma radiation been used to intentionally crosslink PE to improve its wear resistance.

• GGAASS PPLLAASSMMAA SSTTEERRIILLIIZZAATTIIOONN - A non-irradiation sterilization method in which a device is exposed to energized O2, nitrogen and argon gas particles and a peracetic acid gas in alternating cycles. The plasma sterilizes the product by inactivating microorganisms. As with EtO sterilization, gas plasma does not generate free radicals, induce oxidation or crosslinking.

• IINNEERRTT GGAASS PPAACCKKAAGGIINNGG - Sealing the PE component in a package flushed with an inert gas, such as argon or nitrogen, to remove the O2 present during sterilization and subsequent shelf storage.

• IISSOOSSTTAATTIICC MMOOLLDDIINNGG - A multi-step process that begins é manufacture of a cylindrical compact of UHMWPE powder from ώ most of the air is expelled. Subsequently, the compacted rods are sintered in a hot isostatic pressure (HIP) in an argon-filled pouch to oxidative degradation of the UHMWPE. Finished implants are then made by either turning or milling operations.


Topic: Def: Notes: PE (cont.) • OO22 LLEESSSS PPAACCKKAAGGIINNGG - PE components are sealed in a package in an atmosphere é minimal O2

during sterilization and subsequent shelf storage. Current versions include combinations of inert gases partial vacuum, and enclosing a packet containing an O2 scavenger.

• OOXXIIDDAATTIIOONN - Reaction of an O2 molecule with a free radical on the PE molecule. This typically leads to chain scission, indirectly increasing the crystallinity, density and stiffness of the polymer and reducing its resistance to fracture and wear.

• RRAAMM EEXXTTRRUUSSIIOONN - A consolidation process in which a ram is used to force the PE powder through a heated nozzle, resulting in a fused bar as large as six inches in diameter. Careful control of the processing variables (principally, the extrusion rate and the nozzle temperature) is required to produce a fully and uniformly consolidated material (e.g., containing minimal fusion defects).

• TTHHEERRMMAALL SSTTAABBIILLIIZZAATTIIOONN - Heating the PE to neutralize residual free radicals and, thereby, stabilize it against long-term oxidation. Bulk PE (molded blocks or extruded bars) can be heated above the melting temperature (about 150° C), held there for a number of hours, and then cooled ("remelting") to extinguish the free radicals before being machined into a final component. ; In contrast, if radiation crosslinking is applied to the finished components, they cannot be remelted, but they can be heated to below the melt temperature and held at that temperature for a number of days ("annealing") to substantially reduce the residual free radicals. Because annealing occurs with the PE still in a semi-crystalline state (i.e., below the melting temperature), it is not as effective as remelting in eliminating the residual free radicals.

• VVAACCUUUUMM PPAACCKKAAGGIINNGG - Typically, the PE components are placed in a barrier package, flushed with an inert gas (i.e., nitrogen) and then evacuated to minimize the O2 present during radiation, sterilization and subsequent shelf storage.

Polymers (Plastics) made up of long-chain molecules based on carbon & hydrogen.

*viscoelastic (deformation time & stress dependent); *Hysteresis

Power (watts) The rate of doing work. P=W/t Slip =plastic deformation in metals, one

layer or plane of atoms gliding over another. Slip occurs step by step with the movement of 'dislocations' within the crystal.

Dislocations are faults or distorted regions. (types of dislocations= edge & screw dislocations); Material deformation occurs by slip or 'twinning'.

Mechanical Twinning

= a form of Deformation where atoms in each succesive plane within a block will move different distances, with the effect of altering the direction of the lattice so that each half of the crystal becomes a mirror image of the other half. As compared to 'slip' where all atoms in one block move the same distance.

Moment of Inertia

Represents the resistance a structure has to acceleration

1]. MMAASSSS MMOOMMEENNTT OOFF IINNEERRTTIIAA= R to angular acceleration 2]. AARREEAA MMOOMMEENNTT OOFF IINNEERRTTIIAA= resistance to Bending. 3]. PPOOLLAARR MMOOMMEENNTT OOFF IINNEERRTTIIAA= resistance to Torsion.

Polar Moment of Inertia (J)

Measure of the Torsional Stiffness of a column/ shaft

J = [pi/2]x[Ro4-Ri4] = 2.I; T/ø = JG/L (T/ø= torsional stiffness, T= torque, ø= angle of twist, G= shear modulus, L= length of shaft)

Second Moment of Area/ Area Moment of Inertia (I).

A property which measures the distribution of the material around the cross section. (a measure of bending stiffness)

The further the material is from the neutral axis, the stiffer the construct under a given load.; Circle: I = [pi.r4] /4 (hollow: r= outer radius-inner rad.); Bending Stiffness = E.I (where E is Youngs Modulus) Rectangle: I= w×h³ ÷12; The region of a bone/nail with the smallest I is subjected to the largest deformation under load & will fail first.; Indirect bone healing (thick periosteum) -> incr. I -> incr. stiffness & strength.

Momentum Linear momentum= m.v Angular momentum= I.w (I=mass moment of inertia)

Newton's Laws of Motion

1]. A body will remain at rest or continue moving with a constant velocity along a straight line, unless a resultant external force acts on it.;

2]. The acceleration of a body is proportional to the resultant force acting on it and is in the direction of this force. (F=ma);

3]. To every action there is an equal but opposite reaction.


Topic: Definition Notes Passivity Conditions existing on a metal

surface because of the presence of a protective film that markedly lowers the rate of corrosion.

Methods:; 1]. Surface Treatment- with a highly oxidizing solution

(nitric acid).; 2]. Some alloys & metals spontaneously form a passive film

(Type 316 stainless steel, Titanium).; 3]. Altered environment by incr. passivating/ oxidizing

agents (chromate, nitric acid); 4]. Applying a current (anodic protection)

Poiseuille's Law Rate of flow of fluid through a pipe/vessel is proportional to the fourth power of the radius, & inversely proportional to the length.

Poisson's Ratio The lateral/transverse compressive strain is proportional to the longitudinal tensile strain, within the elastic range of a material.

v(mu)= lat. strain/longit. strain.; It may take on values betw. 0(a fully compressible material) and 0.5(a material which maintains a constant volume during deformation). Values >0.5 imply expansion of vol during deformation.

Titanium vs. Stainless Steel

Ultimate Strength(failure): Stainless Steel > Titanium; Yield Strength(permanent deformation): Titanium > Stainless Steel

Implant fixation 1]. Interference Fit: (Press fit): depends on formation of fibrous tissue interface 2]. Interlocking fit (PMMA): Grout that allow for gradual transfer of stresses to bone = Microinterlock 3]. Biological Fit (Porous coating): bone ingrowth into the implant

Work (Joules=N.m)

Work is done by a force when the point of application of the force moves in the line of action of that force.

W=Fs ; W=Frø=Mø (for angular motion)

Perfect Material =

1]. Stiff ............................................................. resist deformation 2]. Hard ........................................................... resist surface abrasion 3]. Inert ........................................................... resist corrosion 4]. Tough ....................................................... resist breakage 5]. Ductile ....................................................... able to deform before breakage 6]. Adapt to loading 7]. Regenerate (reduce failure) = a composite = Bone (a ceramic phase (calcium

hydroxyapatite), dispersed in a collagen-based matrix).

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