Tn Bio Mech Color

1Basic Biomechanics & l fBiomaterials for

Orthopaedic Surgeons

Tariq Nayfeh, M.D./Ph.D.

Outline Introductionoduc o Basic Definitions Joint Mechanics Mechanics of Materials Bending Theory Bending Theory Biomaterials

2Why Study Biomechanics and Biomaterials To Pass Exams?

This area is actually a low yield area for time spent studying and the number of questions asked

So Why Study It? The basis of all implants and devices we use

The basis for most trauma we see The basis for most trauma we see The basis for most of our interventions

Basic Definitions Biomechanics is the science of the o ec a cs s e sc e ce o e

action of forces, internal or external on the living body.

Statics is the study of forces on bodies at rest

Dynamics is the study of the motion Dynamics is the study of the motion of bodies and the forces that produce the motion

3Basic Definitions Kinematics is the study of motion in y

terms of displacement, velocity, and acceleration with reference to the cause of the motion

Kinesiology is the the study of human movement and motion

Principle Quantities Basic Quantitiesas c Qua es

Length Time Mass

Derived QuantitiesVelocit (length/time) Velocity (length/time)

Acceleration (length/time2) Force (mass length/time2)

4Scalars and Vectors Scalar quantities have magnitude Sca a qua es a e ag ude

but no direction. Time, speed (not velocity), mass, volume

Vector quantities have magnitude and directionand direction.

Velocity, Force, Acceleration

Vectors A vector can be resolved F

Fy

into its individual components

Vectors can be added to form a new vector by adding their components

Fx

adding their components or graphically by the parallelogram method

5Moments A moment (torque) is the rotational o e ( o que) s e o a o a

effect of a force about a point.

=

FM

=d

M = F x d

Free Body DiagramsThe forces acting gon a body may be identified by isolating that body part as a free body diagram

Beer and Johnston, Mechanics of Materials

6Example Free Body Diagrams

Basic Laws of MechanicsNewtons Laws First Law:

An object at rest will remain at rest and an object in motion will continue in motion with a constant velocity unless it experiences a net external force

Inertia is the tendency of an object to either remain at rest or to maintain uniform motion in a straight line

The weight of a body is a vector quantity that is equal to the force of gravity acting on it

7 By combining the first and second

Basic Laws of MechanicsNewtons Laws

y co b g e s a d seco dlaws: For equilibrium to occur the sum of the forces and moments must be equal to zero

0F 0F

Basic Laws of MechanicsNewtons Laws Third Law:

For every action there is an equal and opposite reaction.

8Joint Mechanics How do joints j

maintain stability? What produces

joint movement?

Joint Mechanics Joints are stabilized by the

ti f th l action of the muscles, ligaments and bony structures.

The muscles are located at a distance from the joint

Muscle action produce Muscle action produce moments about the joint center

9Joint Mechanics Joint reaction o eac o

forces occur at the joint center

These reaction forces can be greater than the weight of the body segment or the entire body

Joint Mechanics When the muscle and joint reaction e e usc e a d jo eac o

forces are balanced equilibrium occurs and the body segments do not accelerate

When there is an imbalance of forces acceleration (or deceleration) forces acceleration (or deceleration) of the body segment occurs

10

Illustrative Problem0 xF

W=20 N

G= 15 N

x 0yF

352015

0

BRBR

WGRB

0M 0MN 275

315153020

030153

B

WGB

N 240R

Illustrative ProblemHip Reaction forces in single leg p eac o o ces s g e egstance

Buckwalter, et al. Orthopaedic Basic Science

11

Illustrative ProblemThis person is trying to lift a 20 kg s pe so s t y g to t a 0 gobject.

The force from the upper extremities is 450 N

The estimated moment arm of the upper extremities is Lw = 2cm

The estimated moment arm of the weight is Lp= 30 cmweight is Lp= 30 cm.

Mspine = 450x0.02 + 200 x0.30

Mspine = 69 Nm

Illustrative ProblemIf the person bends pforward

Lw = 25 cm Lp = 40 cm

Mspine = 450x0.25+200x0.4Mspine =192.5 Nm

Nordin and Frankel, Basic Biomechanics of the Musculoskeletal System

12

Forces across the hip and knee Hip joint contact forcesp j

Single leg stance 2 to 3 x BW

Walking - 3 x BW

Stairs, running - 5 to 7 x BW

Knee Tibiofemoral forces Rising from a chair 4 x BW

Walking 3 x BW

Stairs Ascent 6 to 7 x BW

Stair Descent 7 to 8 x BW

Mechanics of MaterialsIn order to understand how materials o de o u de s a d o a e a s

behave we need to define some basic quantities.

13

StressStress is the intensity yof internal force.

Normal stress are perpendicular to the surface

Shear stress are parallel

AF


AO/ASIF

14

Depending on how you slice the material you can get combinations of stress and sheer


In pure tension or compression

The plane of maximum shear is at 45 degrees to the axis of loading!!

15

StrainStrain (Engineering):

Relative measure of the deformation (six components) of a body as a result of loading.

LL

Can be normal or shear

**A relative quantity with no units. Often expressed as a percent


AO/ASIF

16

Shear strainUsually expressed as an angle radiansUsua y e p essed as a a g e ad a s


17

Hoop stress

Hoop stress is the stress in a direction perpendicular to the

prtpr

1

direction perpendicular to the axis of an item

***As the thickness of the item decreases the hoop stress

tpr22

the hoop stress increases***Why is this important?


Hoop StressAs humans age, the g ,diameter of their bones increase, but the thickness decreases

We will see later that this change is not bad for ordinary human activity. It matters most when we as surgeons intervene.

18

Material TestingIn order to characterize how materials o de o c a ac e e o a e a sbehave we have to create standardize methods to test them and document the behavior.

In the US the ASTM standards are the most widely used

In Europe the most widely used is the ISO standards

Materials TestingMaterials of standardized sizes and shapes are pplaced in testing machines and loaded following standardized protocols

19

Stress-Strain CurvesStandardized curves used to help quantify how a material will respond to a given loadhow a material will respond to a given load.

AO/ASIF

Quantities Derived from Stress-Strain Curves Yield Strength: The stress level at which g

a material begins to deform plastically Ultimate Strength: The stress level at

which a material fails Modulus of Elasticity: The linear slope

of the materials elastic stress-strain behavior.behavior.

Ductility: The deformation to failure Toughness: Energy to failure (the area

under the stress strain curve)

20

Elastic vs. Plastic Behavior

AO/ASIF

AO/ASIF

21

Types of failure

Ductile

B ittlBrittle

Elasticity vs. ductility and strength

All of these materials have the same modulus of elasticity

But they have different toughness ductilitytoughness, ductility and strength.


22

Force-deformation curves for materials having various combinations of structuralproperties


23

Stiffness

F

L

Unloaded: A=cross section areaE=Youngs modulus of elasticity

u

Longitudinal stiffness Sax = EAL

F = Sax u = SaxuEAL

Force-Displacement Curves Similar to stress-strain

curves

Not a material property, instead a measure of how the entire structure behaves

Depends onp Material

Geometry

24

Force-Displacement Curves


QuestionThe linear relationship between an applied stress and the resultant deformation defines a material's1- modulus of elasticity.2- brittleness.3- yield strength.4- ultimate strength.5- toughness.

If the question was changed to applied force, instead of applied stress. The answer would change to stiffness.

25

Bending of Beams Most bones and orthopaedic os bo es a d o opaed c

implants are subjected to axial, bending, and torsion loading

Most failures occur secondary to bending and torsion

Linear bending theory

M

26

Bending Theory Definitions Neutral Axis: The location where a beam

experiences zero stress (this is a theoretical axis and can actually be located outside of the structure)

Moment of Inertia: The geometric property of a beam/s cross section that determines the beams stiffnessdetermines the beams stiffness

There is a bending and a torsion moment of inertia (we will limit our discussion to bending)

27

on on on

centricload

eccentricload

eccentricload

ompr

essi

onte

nsio

ompr

essi

onte

nsi

ompr

essi

onte

nsio

co co co

L o w s t r e s sL o w s t r e s s

HighstressHighstress

28

The resistance of a beam to bending

Bending Resistancee es s a ce o a bea o be d g

is directly proportional to its moment of inertia

The moment of inertia depends on its cross sectional area and shapeits cross sectional area and shape

Bending resistance solid cylinder = / 64 diam4

Bending resistance of a hollow cylinder= / 64 (outer diam4 inner diam4)( )or for thin shells= / 8 diam3 shell thickness

29

The bending stiffness of a half pin is e be d g s ess o a a p sproportional to one half the radius of the pin to what power?

2 3 4 One third One fourth

Relative bendingresistance

Solid rod 1

Flat beam 3.5

I beam edge on 6Identical size

ofcross sectional

areaI beam flat 0.6

Hollow cylinder 5.3

Gozna et al. 1982

30

When the diameter of a spinal instrumentation rod is increased from 4 mm to 5 mm, the rod's ability to resist a bending moment is increased by approximately what percent?1- 10%2- 25%3- 50%4- 100%5- 300%

46464

4411 dR

%10044.1256256625

445

56464

4

44

1

12

4422

RRR

dR

850Kg. 800Kg. 60Kg. 20Kg.

Bone-implant composite AO/ASIF

31

Tension band principle

A properly done tension band shifts the neutral axis to the surface of the beam so that compression occurs across the entire cross section

32

Example of tension bands

Example of tension bands

33

torque

shear

Mechanical Properties of Materials Isotropy Anisotropypy

Material properties do not depend on direction

Steel Aluminum

py Material properties

depend on the direction of loading

Bone Tendons Ligaments Cement

34

Anisotropy Bone is an

anisotropic material

Hence failure depends on load direction and loading typeloading type


Bone MechanicsCortical bone is weakest in directions weakest in directions that cause tensile stresses.

In the transverse direction the bone isdirection the bone is acting as a brittle material

35

1. a compression crack begins at the fulcrum.

Three-point bending produces a predominantly transverse fracture because

p g2. bone is weaker in tension than in compression.3. bone is weaker in compression than in tension.4. the forces are equally resolved between tension and

compression.5. the forces are resolved into pure tension.

Bending forces in the long bones most commonly result in what type of fracture pattern?1- Short oblique2- Transverse with butterfly3- Linear shear of 454- Spiral5- Segmental

What type of loading is most likely to cause a pure spiral fracture?1- Crush2- Bending3- Tensile4- Compression5- Torsion

36

AO/ASIF

Bending forces in the long bones most commonly result in what type of fracture pattern?1- Short oblique2- Transverse with butterfly3- Linear shear of 454- Spiral5- Segmental

What type of loading is most likely to cause a pure spiral fracture?1- Crush2- Bending3- Tensile4- Compression5- Torsion

37

A 27-year-old patient sustains the A 27 year old patient sustains the closed femoral fracture shown. This fracture pattern is most likely the result of which of the following forces?

1. Pure torsion

2. Pure bending

3. Pure compression

4. Four-point bending

5. Torsion plus bending

Why are Long Bones Hollow?

For the same total cross sectional area a hollow tube has higher bending and torsional resistance than a solid tube

Most bones are loaded in bending and torsion Bone responds to Wolfes law and tries to maximize

the bone density where stress is highest and minimize it where stress is lowest

The thinner a bone is the easier it is for nutrients to reach the osteocytes

Less energy is required to maintain the bone

38

Clinical QuestionCase 1: A 75 year old female

with osteoporosis falls and with osteoporosis falls and sustains a supracondylar femur fracture. The patient undergoes ORIF with a locked supracondylar plate. She is allowed to increase her weight bearing to full weight bearing at 6 weeks. Two weeks later she Two weeks later she presents with increasing pain, swelling and can not bear weight.

Clinical ExampleCase 2: A 75 year old female with Case 5 yea o d e a e

osteoporosis falls and sustains an intertrochanteric hip fracture. She undergoes ORIF with an intramedulary device and is allowed to weight bear as tolerated the next day Her fracture goes on to heal day. Her fracture goes on to heal without complications.

39

Clinical ExampleCase 3: An 83 year old male y

with multiple medical problems presents with severe right hip pain and the inability to bear weight. He had undergone a revision of his right total hip 10 ea s ago to a hip 10 years ago to a cementless stem.

Clinical CaseWhy did the patient in Case 2 do well y d d e pa e Case do e

while the patients in Case 1 and Case 3 have their implants fail?

40

FatigueFatigue testing is done using the same type g g g ypof samples and machines that are used to create stress-strain curves. However, the samples are loaded cyclically to failure. The goal of testing is to determine how many loading cycles at a given load a material can withstand before failing. g

**The failure stress levels are not the same as the yield stress and ultimate stress.**

FatigueFatigue testing generated fatigue g g g glife curves.

Fatigue Endurance Limit The stress level below which a material does not fail (usually must last greater than 10 million cycles)

Fatigue life The number of cycles that a material can withstand at a given stress level

41

Fatigue Life

Endurance Limit

Fatigue LifeIn a fatigue test, the maximum stress under which the material will not fail, regardless of how many loading cycles are g y g yapplied, is defined as

1- endurance limit.2- failure stress.3- critical stress.4- yield stress.5- elastic limit.

42

Bone Fatigue Bone has no in vitro endurance o e as o o e du a ce

limit! In vivo bone heals If bone fails to heal when subjected

to cyclic loads we get stress fracturesfractures

Clinical Examples In Case 1 above the patient was Case abo e e pa e as

allowed to weight bear before her fracture healed. In this case the stress from walking on the bone resulted in rapid failure with relatively few cycles.

43

Case 3 The applied stress e app ed s ess

to the small diameter implant again resulted in fatigue failure of the stem

Stress ConcentrationWhen a structural member contains a discontinuity such as a hole or a sudden discontinuity, such as a hole or a sudden change in cross section, high localized stresses may occur near the discontinuity.


44

Stress Concentration

The highest stress gconcentration occurs near a sharp point

a21

pny21max

At higher rates of loading, bone absorbs more energy prior to failure because

1. the modulus of elasticity decreases.

2. bone is anisotropic.

3 bone is viscoelastic3. bone is viscoelastic.

4. bone deforms plastically.

5. bone is stronger in compression than in tension.

45

ViscoelasticityViscoelasticty is a term used to describe ymaterials that demonstrate time-dependant behavior to loading.

Visco is derived from viscocity (fluid like) Elastic come from elasticity (solid like)

Most normal temperature metals are elastic Most biologic materials (bone tendon Most biologic materials (bone, tendon,

ligaments), glass, polymers, and metals at high temperature exhibit viscoelastic behavior

A simple model for an elastic

Viscoelasticitys p e ode o a e as c

material is a simple spring in which instantaneous displacement occurs to an applied load.

The ene g of The energy of displacement is stored as potential energy and recovered when the load is removed.


46

Viscous behavior can be modeled as

Viscoelasticityscous be a o ca be ode ed as

a dashpot (shock absorber). Deflection occurs in response to the rate of force application

I thi th In this case the energy produced from loading is dissipated as heat.


Viscoelastic Behavior is modeled as

Viscoelasticityscoe as c e a o s ode ed as

a combination of elastic and viscous materials.

The energy from loading is partially stored and partially dissipated


47

The biomechanical properties of ligaments p p gand bone demonstate

1. a time-dependent behavior.

2. a rate-independent behavior.

3. a straight-line load-deformation behavior.

4. modeling with linear elastic-spring g p gelements.

5. similar stress-stretch curves.

At higher rates of loading, bone absorbs more energy prior to failure because1- the modulus of elasticity decreases.2- bone is anisotropic.3- bone is viscoelastic.4- bone deforms plastically.5- bone is stronger in compression than in tension.

48

The change in strain of a material under a constant load that occurs with time is defined as1- creep.2- relaxation.3- energy dissipation.4- plastic deformation.

Time

5- elastic deformation.

Stress Relaxation Stress relaxation is the decrease of Stress relaxation is the decrease of

stress with time under constant strain.

Time

49

Stress Shielding Wolffs law Wolff s law

If you dont use it, you lose it!

Stress shielding occurs when an implant carries most of the stress and effectively unloads the bone

Examples are the proximal femur with an ingrown implant and loss of bone under a plate.

Stress Shielding

Assume the plate is stainless steel with E=190 GPa

Assume the bone is all cortical bone with E=17 GPa

Both the bone and the plate must deform the same

P P

bb

b

p

ssp

EE E

)10( bbpbbppbp

AAAAPFFP

AF

bbb

pp E

E 1017190

)10(10

)10(

bpp

bpb

AAPAA

P

50

In a 77-year-old woman who underwent total hip arthroplasty 10 years ago. What is the predominant cause of the proximal femoral bone loss?

1. Stress shielding

2. Polyethylene debris-induced osteolysisosteolysis

3. Senile osteoporosis

4. Modulus of elasticity of the femoral stem

5. Diffuse osteopenia

Examples of Materials Used for Implants

51

Which of the following properties is most commonly associated with titanium alloy implants when compared with cobalt-chromium alloys?

1- Lower elastic modulus

2- Lower corrosive resistance

3- Better wear characteristics

Elastic modulus and ultimate tensile strength of the most common orthopedic biomaterials, listed in order of increasing modulus or strength:3- Better wear characteristics

4- Lower notch sensitivity

5- Greater hardness

ELASTIC MODULUScancellous bone polyethylene PMMA (bone cement) cortical bone titanium alloy stainless steel cobalt-chromium alloy

ULTIMATE TENSILE STRENGTHULTIMATE TENSILE STRENGTHcancellous bone polyethylene PMMA (bone cement) cortical bone stainless steel titanium alloy cobalt-chromium alloy

Stainless Steel Used for fracture fixation and spinal p

implants Most common is 316L Contains chromium, nickel, molybdenum The chromium forms an oxide layer on

the out side of the implant that acts as a i i t t l d f th corrosion resistant layer and forms the

stainless quality to it Strong material but can get stress or

crevice corrosion with time Caused by cracking the Cr-oxide layer with loading

52

Cobalt-Chromium Alloys There are a number of different alloys used for

implants depending on what type of manufacturing is used

Consists mostly of cobalt with chromium added for corrosion resistance

Like stainless steel the chromium forms a surface oxide layer

Used for joint replacements, bearing surfaces and occasionally for fracture fixation devices

Not all Co-Cr is the same and the mechanical properties are a function of which alloy is used and how the alloy is processed

Titanium One of the most biocompatible metals Very good corrosion resistance

Resistance is generated by a rapidly formed oxidized layer on its surface and this layer makes the titanium implant more corrosion resistant that Stainless steel or CoCr implants

Most commonly used alloy is Ti-6Al-4V 6% aluminum and 4% vanadium Initially developed as a high strength to weight ratio material for

aircraft

Its modulus of elasticity is around half of that of stainless Its modulus of elasticity is around half of that of stainless steel or CoCr, hence using titanium implants my reduce the stress sheilding

Very notch sensitive leads to crack formation and decreased fatigue life

Not a good bearing surface in joint arthroplasty because it gets rough with time

53

Ceramics Ceramics are materials are inorganic materials formed

from metallic and nonmetallic materials held together by from metallic and nonmetallic materials held together by ionic and covalent bonds

Examples include silica, alumina, zirconia

Mechanical properties are very process dependant and can vary from manufacturer to manufacturer

Ceramtec a few years ago changed a single step in their process of making femoral heads (they did not change the material) which resulted in fracture of the heads in vivo

Ceramics are very stiff, very hard, demonstrate very little Ceramics are very stiff, very hard, demonstrate very little wear

Can be very brittle Very biocompatible if manufactured to a high purity level

Polymers Polymers are large molecules made from y g

combinations of smaller molecules Nylon, PMMA, Polyethylene

Their mechanical and biologic properties depends on their micro and macro-structure

A polymers molecular weight depends on th b f l l i it h ithe number of molecules in its chains

54

Polymers


Polyethylene Semi-crystalline polymery p y Basic momer is CH2 with a molecular

weight of 28 Its mechanical and wear properties

depend on its molecular weight, structure, oxidation, cross linking, processing method and sterilizationprocessing method, and sterilization

**Not all polyethylene is the same**

55

Highly cross-linked ultra-high molecular weight polyethylene has what effect on tensile and fatigue strength when compared with ultra-high molecular weigth polyethylene?

1. Increased tensile and fatigue strength

2. Increased tensile strength and decreased fatigue strength

3 Decreased tensile and fatigue strength3. Decreased tensile and fatigue strength

4. Decreased tensile strength and no change in fatigue strength

5. No change in tensile or fatigue strength

Crosslinking Crosslinking is done to create larger C oss g s do e o c ea e a ge

molecular polyethylene molecules that can theoretically be more wear resistant

There are two common methods for crosslinking g

Irradiation Free radical generating chemical

56

Crosslinking

Lewis, Biomaterials 22 (2001) 371-401

57

Crosslinking The major problem with crosslinking is that

ll hi h d f di ti hi h d usually higher doses of radiation which produce the greatest amount of crosslinking also may cause a degradation in the materials mechanical properties. Specifically a decrease in fracture toughness and fatigue strength and life.

Newer versions of highly crosslinked polyethylene are being released that are being treated by a combination of lower dose radiation treated by a combination of lower dose radiation and post irradiation melting and or annealing. These processes are showing promise for low wear rates and small changes to the mechanical properties of the polyethylene

Tribology

The study of Friction, Lubrication, and Wear.

58

The natural jointElements that influence the e e s a ue ce etribological function of a joint are: The articular cartilage The synovial fluid And to a lesser extent the subcondral

bone capsule soft tissues and bone, capsule, soft tissues and ligaments.

Frictionis the resistance to motion that is experienced whenever one solid body experienced whenever one solid body Slides over another

LOADLOAD

DIRECTION OF MOTION

FRICTION FORCE DIRECTION OF MOTION

FRICTION FORCE

59

Lubrication.materials applied to the interface to the interface reducing friction and wear.

Lubrication.

Lubrication reduces FrictionLubrication reduces Frictionreduces Wear

Ability of a bearing to support a fluid fil ill i it bl i fl th film will inevitably influence the friction and wear of the bearing surfaces during articulation

60

Lubrication modesBoundary LubricationHydrodynamic Hydrodynamic Lubrication

Hydrostatic Lubrication

STRIBECK CURVE

Coefficient of Friction

() BL

Sommerfeld Number(viscosity x sliding speed x radius / load)

ML FFL

61

Boundary Lubrication

High Friction and Wear

Boundary Lubrication No pressure build up in the o p essu e bu d up e

lubricant. Loading is 100% carried by the

asperities in the contact area. The contact area is protected by

absorbed molecules of the lubricant absorbed molecules of the lubricant and / or a thin oxide layer.

The characteristics for boundary lubrication is the absence of Hydrodynamic pressure.

62

Fluid Film Lubrication

No Friction or Wear

Hydrodynamic Lubrication

Bearings are supported by a thin ea gs a e suppo ed by alayer of fluid which is pulled into the bearing through viscous entrainment, compressed, creating a sufficient hydrodynamic pressure to support load.

HD h >0.25mEHD h ~0.025m

- 2.5 m

h HD h >0.25mEHD h ~0.025m

- 2.5 m

h

63

Generation of fluid filmAs the ball rotates, fluid is drawn into the converging wedge andinto the converging wedge and builds up a pressure which carries the load

Hydrodynamic Lubrication Pressure builds as speed increases.speed increases. The surface asperities are completely separated by a lubricant film. The load and H d d i Hydrodynamic pressures are in equilibrium.

64

Hydrostatic LubricationBearings are supported on a thick ea gs a e suppo ed o a cfilm of fluid supplied from an external pressure source.

hP PhP PP PP P

Artificial joint surfaces Metal / Ceramic bearing on UHMWPE do / g

not benefit from fluid film lubrication they operate in a mixed fluid film regime.

Unavoidable wear results at a rate of approximately 200m of linear penetration per year giving a life expectancy of a 4mm thick cup about 20 expectancy of a 4mm thick cup about 20 years.

M-on-M and Ceramic on Ceramic perform in a fluid film regime therefore the resultant wear rate is significantly reduced.

65

Which of the following features improved fluid film lubrication in a metal-on-metal total hip arthroplasty?

1. Smaller diameter femoral head, a completely congruent fit between the socket and the congruent fit between the socket and the head, and sufficient roughness to allow for some microseparation between the head and socket

2. Smaller diameter femoral head, a slight clearance between the socket and the head, and no surface roughnessL di t f l h d l t l 3. Larger diameter femoral head, a completely congruent fit between the socket and the head, and minimal surface roughness

4. Larger diameter femoral head, a slight clearance between the socket and the head, and minimal surface roughness

5. Larger diameter femoral head, a slight

Clearance

66

What is Radial Clearance?

Radius Cup Radius Cup Radius Cup Radius Cup R2 R2 ---- Radius Radius Head R1Head R1--= Radial = Radial ClearanceClearance

--Clearance Clearance allows a fluid allows a fluid

Is there an optimal clearance?

N l i No one clearance, it is a ratio to head diameter

The bigger the head gets, the bigger the clearance gets

We must consider manufacturing capabilities Nominal and Ranges etc

The lubricant fluid in Vivo

V ti ht

67

Effect of clearance with bovine serum all tests to date

Too tightToo tightToo tight Too tight and too high and too high clearances clearances may end up may end up in high wear in high wear

t d tt d t

0

rates due to rates due to an increase in an increase in frictionfriction

Does reduced clearance make a difference?

BOA Manchester 2004 McMinn presented early McMinn presented early results of 20 controlled clearance cases implanted in 2004.

Radiolucencies observed in superior acetabulum in 10% of acetabulum in 10% of the cases to date.

Reduced Clearance bearings need further assessment.

68

24 hour Cobalt output in Regular and low clearance BHR

Regular BHR vs Controlled Clearance BHRUrine Cobalt Output

40

50

60

70

80

90

ne O

utpu

t g/

24hr

Regular BHR

Low Clearance BHR

0

10

20

30Urin

Pre Op 5 day 2 month 6 month 1 year 4 year

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