Biomech of Cartilage Mpt (2)

CARTILAGE

• Cartilage is a highly specialised connective tissue.It is not as hard and rigid as bone, but it is stiffer and less flexibile than muscle.

• It’s function is to provide smooth lubricated surface for articulation.

• Articular cartilage is devoid of blood vessels, lymphatics and nerves and is subject to harsh biomechanical enviornment.

• It has a limited capacity for intrinsic healing and repair.

• Cartilage is more flexible and compressible than bone and often serves as an early skeletal framework .

• Cartilage is produced by chondrocytes that come to lie in a small lacunae surrounded by the matrix they have secreted.

• Cartilage clearly performs a mechanical function.It provides a bearing surface with low friction and wear, and because of its compliance, it helps to distribute the loads between opposing bones in a synovial joint.If cartilage were a stiff material like bone, the contact stresses at at a joint would be much higher, since the area of contact would be much smaller.

• As cartilage is not innervated and therefore it relies on diffusion to obtain nutrients. This causes it to heal very slowly.

• Normal articular cartilage is white,and its surface is smooth and glistening. Cartilage averages 2.21 mm in humans.

• Injury to articular cartilage is recognised as a cause of significant musculoskeletal morbidity.

• The unique and complex structure of articular cartilage make treatment or repair or restoration of the defects challenging fpr the patient, surgeon and the physical therapist.

• The preservation of articular cartilage is highly dependant on maintaining its organised archiecture.

STRUCTURE AND FUNCTIONS OF CARTILAGE TISSUE

• There are 3 different types of cartilage that have slightly different structures and functions.

HYALINE CARTILAGE.FIBROCARTILAGEELASTIC CARTILAGE

• HYALINE CARTILAGE-Hyaline cartilage is the most abundunt of the 3

types of cartilage.It is found in many locations in the body including- bronchial tubes, larynx, trachea.

Covering the surface of bones at joints especially in areas where damage due to wear may lead to osteoarthritis including ends of long bones and the anterior ends of ribs.

• STRUCTURE OF HYALINE CARTILAGE-Hyaline cartilage consists of bluish white, shiny

ground elastic material with a matrix of chondroitin sulphate into which many fine collagen fibrils are embedded.It contains numerous chondrocytes.

• FUNCTIONS OF HYALINE CARTILAGE TISSUE-It provides smooth surfaces, enabling tissues to

move/slide over each other eg facilitating smooth movement at joints.It also provide flexibility and support.

• FIBROCARTILAGE-Examples include intervertebral disc,menisci, pubic

symphysis, also in the portions of the tendons that insert into the cartilage tissue, especially at the joints.

• STRUCTURE OF FIBROCARTILAGE TISSUE-Fibrocartilage is a tough form of cartilage that

consists of chondrocytes scattered among clearly visible dense bundles of collagen fibres within the matrix. Fibrocartilage lacks a perichondrium.

• FUNCTIONS OF FIBROCARTILAGE TISSUE-Fibrocartilage tissue provides support and

rigidity to attached/surrounding structures and is the strongest of the three types of cartilage.

• ELASTIC CARTILAGE- In the body it is present in Auditory ( Eustachian tubes)External ear ( Auricle)Epiglottis.

STRUCTURE OF ELASTIC CARTILAGE TISSUE-

It is yellowish in colour, the cartilage cells are located in a thread like network of elastic fibres within the matrix of cartilage.A perichondrium is present.

• FUNCTIONS OF ELASTIC CARTILAGE TISSUE-Elastic cartilage provides support to

surrounding structures and helps to define and maintain the shape of the area in which it is present.

COMPOSITION OF ARTICULAR CARTILAGE

• Articular cartilage is a living material composed of a relatively small number of cells known as chondrocytes surrounded by a multicomponent matrix.

• Mechanically, articular cartilage is composite of material of widely differing properties.

• Approx 70-80% of the weight of the whole tissue is water. The remainder of the tissue is composed primarily of proteoglycans,collagen and relatively small amount of lipids.

STRUCTURE OF PROTEOGLYCANS

• Approximately 30% of dry weight of articular cartilage is composed of proteoglycan.

• Proteoglycan consists of a protein core to which glycosaminoglycans ( chondroitin sulphate and keratin sulphate) are attached to form a bottle-brush like structure.

• These proteoglycans can bind or aggregate to a backbone of hyaluronic acid to form a macromolecule with a weight upto 200million daltons.

• Proteoglycan concentration and water content vary through the depth of the tissue.

• Near the articular surface,proteoglycan concentration is relatively low, and the water content is highest in the tissue.

• In the deeper regions of the cartilage,near the subchondral bone,the proteoglycan concentration is the greatest,and the water content is the lowest.

STRUCTURE OF COLLAGEN

• Collagen is a fibrous protein that makes upto 60%-70% of the dry weight of the tissue.

• Type II is the predominant collagen in articular cartilage,although other types are present in smaller amounts.

• Collagen architecture varies according to the depth of the tissue.

ZONES OF ARTICULAR CARTILAGE

• There are 4 zones between the articular surface and subchondral bone.

Superficial tangential zone.Intermediate or middle zone.Deep or radiate zone.Calcified zone.• The interface between the deep zone and

calcified cartilage is known as TIDE MARK.

SPLIT LINES

• Split lines are formed puncturing the cartilage surface at multiple sites with a circular awl

• The resulting holes are elliptical,not circular and the load axes of the elipses are aligned in what is called the split lines direction.

• In the plane parallel to split line,the collagen organised in broad layer or leaves, while in plane orthagonal to the split lines the structure has rigid pattern that interrupted as the edges of the leaves.

• In the calcified and deep zones collagen fibres collagen fibres are oriented radially and arranged in tightly pack bundles.The bundles are linked to numerous fibrils.

• From the upper deep zone into the middle zone,the radial orientation becomes less distinct, and collagen fibrils forms a network that surrounds the chondrocytes.

• In the superficial zone, the fibres are finer than in the deeper zones, and the collagen structure is organised into several layers.

• An amorphous layer that does not appear to contain any fibres is found on the articular surface.

• Scanning electron microscopy is also used to investigate the structure of osteoarthritic cartilage. These investigations demonstrate two primary structural changes associated with degeneration – rolling of delaminated sheets into fronds, and formation and propogation of large cracks.

MECHANICAL BEHAVIOUR AND MODELLING

• The mechanical behaviour of articular cartilage is determined by the interaction of its predominant components- collagen, proteoglycans and interstitial fluid.

• In an aqueous enviornment proteoglycans are polyanionic,that is the molecule has negatively charged sites that arise from its sulphate and carboxyl group.

• In solution, the mutual repulsion of these negative charges causes an aggregated proteoglycan molecule to spread out and occupy a large volume .

• In cartilage matrix, the volume occupied by proteoglycan aggregates is limited by the entangling collagen framework.

• The swelling of the aggregated molecule against the collagen framework is an essential element in the mechanical response of the cartilage.

• When cartilage is compressed, the negatively charged sites on the aggregan are pushed closer together,which increases their mutual repulsive force and adds to the compressive stiffness of the cartilage.

• Nonaggregated proteoglycans would not be as effective in resting compressive loads, since they are not as easily trappedin the collagen matrix.

• Damage to the collagen framework also reduces the compressive stiffness of the tissue,since the aggregated proteoglycans are contained less efficiently.

• The mechanical response of the cartilage is also strongly tied to the flow of fluid through the tissue. When deformed fluid flows through the cartilage and across articular surface.

• If a pressure difference is applied across a section of cartilage,fluid also flows through the tissue.These observations suggest that collagen behaves like a sponge albeit one that doe not allow fluid to flow through it easily.

BIPHASIC MODEL OF CARTILAGE

• Fluid flow and deformation are interdependant has lead to the modelling of cartilage as a mixture of fluid and solid components. This is referred to as the BIPHASIC MODEL OF CARTILAGE.

• In this modelling, all of the solid like components of the cartilage, proteoglycans collagen, cells and lipids are lumped together to constitute the solid phase of the mixture.

• The interstitial fluid that is free to move through the matrix constitutes the fluid phase.

• Typically, the solid phase is molded as an incompressible elastic material, and the fluid phase is molded as incompressible and inviscid, that it has no viscosity.

• Under impact load cartilage behaves as a single-phase, incompressible elastic solid there is simply no time for fluid to flow through the solid matrix.

CLINICAL RELEVANCE

• The Biphasic model shows that fluid pressure sheilds the solid matrix from the higher level of stress that it would experience if cartilage were a simple elastic material without significant interaction of its fluid and solid components.

• In osteoarthritic cartilage that is more permeable than normal, stress sheilding by fluid pressurisation is diminshed, and more stress is transferred to the solid matrix.

MATERIAL PROPERTIES

• A confined compression test is one of the commonly used methods to determine material properties of cartilage.

• A disc of cartilage is cut from the joint and placed in an impervious well. Confined compression is used in either creep mode or relaxation mode.

• In creep mode a constant load is applied to a cartilage through a porous plate, and the displacement of the tissue is measured as a function of time.

CONFINED COMPRESSION TEST OF CARTILAGE

• In relaxation mode, a constant displacement is applied to the tissue, and the force needed to maintain the displacement is measured.

• In creep mode the displacement of cartilage is a function of time, since the fluid cannot escape from the matrix instantaneously. Initially, the displacement is rapid.

• This corresponds to a relatively large flow of fluid out of the cartilage.

• As the rate of displacement slows and the displacement approaches a constant value, the flow of fluid likewise slows.

• At equilibrium the displacement is constant and fluid flow has stopped.In general it takes several thousand seconds to reach the equilibrium displacement.

• By fitting the Biphasic model to the measured displacement, two material properties of the cartilage are determine.

Aggregate modulusPermeability.

AGGREGATE MODULUS- is a measure of stiffness of the tissue at equilibrium when all fluid flow has ceased.The higher the aggregate modulus , the less the tissue deforms under a given load.

• PERMEABILITY- The permeability of the cartilage is also determined from a confined compression test.The permeability indicates the resistance to fluid flow through the cartilage matrix.

• Permeability was first introduced in the study of flow through flow through soils.The average fluid velocity through a porous sample ( Vave) is proportional to the pressure gradient ( Vp).The constant of proportionality( k) is called the PERMEABILITY.

DEVICE USED TO MEASURE PERMEABILITY OF CARTILAG

• Permeability is not constant through the tissue . The permeability of articular cartilage is highest near the joint surface ( making fluid flow relatively easy) and lowest in the deep zone ( making fluid flow relatively difficult ).

• Permeability also varies with the deformation of the tissue .As cartilage is compressed, its permeability decreases.

• Therefore, as a joint is loaded, most of the fluid that crosses the articular surface comes from the cartilage closest to the joint surface.Under increasing load, fluid flow will decrease because of the derease in permeability that accompaines compression.

• An indentation test provides an alternative to confined compression. Using an indentation test , cartilage is tested in situ. Since disc of cartilage are not removed from underlying bone,as must be done when using confined compression indentation may be use to test cartilage from small joints.

• In addition, three independent properties are obtained from one indentation test but only two are obtained from confined compression.

• Typically, an indentation test is performed under a constant load.The diameter of indenter varies depending in the curvature of joint surface, but generally no smaller than 0.8mm.

• Under, a constant load the displacement of the indenter resembles that for a confined compression and require several thousand seconds to reach equilibrium.

INDENTATION TEST ON ARTICULAR CARTILAGE

• By fitting the biphasic model of the test to the measured indentation, following are determined.

Aggregate modulusPoisson’s RatioPermeability

Poisson’s ratio is typically less than 0.4 and often approaches zero.

• This finding is significant departure from earlier studies, which assumed that cartilage is incompressible and therefore had a poisson’s ratio of 0.5.This assumption was based on cartilage being mostly water, and ater may often be molded as an incompressible material.

• However, when cartilage is loaded, fluid flows out of the solid matrix, which reduces the volume of the whole cartilage. Recognizing cartilage as a mixture of solid and fluid leads to the whole tissue behaving as compressible material,although its components are incompressible.

• The PERMEABILITY influences the rate of deformation.If the permeability is high, fluid can flow out of the matrix easily and the equilibrium is reached relatively quickly.

• A lower permeability causes a more gradual transition from the rapid early displacement to the equilibrium.

CLINICAL RELEVANCE

• The lower modulus and increased permeability of osteoarthritic cartilage result in greater and more rapid tissue deformation than normal.

RELATIONSHIP BETWEEN MECHANICAL PROPERTIES AND COMPOSITION

• Correlations between mechanical properties of the cartilage and glycosaminoglycan content, collagen content and water content has been established.

• The compressive stiffness of the cartilage increases as a function of the total glycosaminoglycan content. As the total glycosaminoglycan decreases compressive stiffness also decreases.

• In contrast, there is no relation of compressive stiffness with collagen content.

• Permeability and compressive stiffness , as measured by the aggregate modulus are both highly correlated with water content.

• As the water content increases cartilage becomes less stiff and more permeable.

CLINICAL RELEVANCE

• Decrease in proteoglycan content allows more space in the tissue for fluid.

• An increase in water content with an increase in permeability, increasing permeability allows fluid to flow out of the tissue more easily, resulting in more rapid rate of deformation.

JOINT LUBRICATION

• Normal synovial joints operate with relatively low coefficient of friction , about 0.001.

• There are 2 mechanism that are responsible for the low friction in synovial joints.

Fluid film Lubrication.Boundary Lubrication.

• FLUID FILM LUBRICATION-For fluid film to lubricate moving surfaces

effectively, it must be thicker than the roughness of the opposing surfaces.

The thickness of the film depends on the .Viscosity of the fluids.Shape of the gap between the parts.Relative velocityAs well as the stiffness of the surfaces.

• If cartilage, is molded as a rigid material, it is not possible to generate a fluid film of sufficient thickness to separate the cartilage surface roughness.

• However, models that include deformation of the cartilage and its surface roughness have shown that a sufficient thick film can be developed. This is known as MICROELASTOHYDRODYNAMIC LUBRICATION.

• BOUNDARY LUBRICATION-• A low coefficient of friction can also be

achieved without a fluid film through a mechanism known as BOUNDARY LUBRICATION.

• Boundary lubrication of the articular surface appears to be linked to a glycoprotein fraction in synovial fluid known as LUBRICIN.

• Lubricin may be a carrier for lubricating molecules known as surface active phospholipids that provide boundary lubricating properties for synovial joints.

• Surface active phospholipids are believed to be boundary lubricants not just in synovial fluids, but in other parts of the body such as pleural space.

MECHANICAL FAILURE OF CARTILAGE

cartilage is an anisotropic material, we expect that it has greater resistance to some components of stress than to others.

For example,it could be relatively strong in tension parallel to collagen Fibers, but weaker in shear along planes between leaves of collagen.

Tensile failure of cartilage has been of particular interest,since it was generally believed that vertical cracks in cartilage were initiated by relatively high tensile stresses on the articular srface.

More-recent computational models of joint contact show that the tensile stress on the surface is lower than originally thought, although tensile stress still exists within the cartilage [13–15].

Studies of the tensile failure of cartilage are primarily concerned with variations in properties among joints, the effects of repeated load, and age.

Repeated tensile loading (fatigue) lowers the tensile strength of cartilage as it does in many other materials.

Repeated compressive loads applied to the cartilage surface in situ also cause a decrease in tensile strength.

Properties of most biological materials change with the applied strain; the collagen network becomes aligned with the direction of the tensile strain, and the material becomes strongly anisotropic.

Dropping three different-sized spherical indenters (2, 4, and 8 mm) onto the articular surface produces three different states of stress and,in some instances, a crack through the surface.

Shear stresses do exist in cartilage, although the orientation of these stresses is not always obvious.

under rapid loading, cartilage behaves as an incompressible elastic material.

Cracks are common in articlar cartilage

EXERCISE AND CARTILAGE HEALTH

• Participation in certain sports also appears to increase risk of developing Osteoarthritis.

• Saxon et al concluded that activity that involveTorsional loading.Fast acceleration and decceleration.Repetitive high impactHigh levels of participation.Increase risk of developing osteoarthritis.

Track and filed events.Racket sports.SoccerAmong the sports that are involved in higher

risk of developing osteoarthritis.Swimming and cycling are not linked with an

increase risk of developing osteoarthritis at the hip, although cycling may be related to osteoarthritis of patella.

• Injuries to Anterior cruciate ligament, collateral ligament or meniscus are implicated in the development of Osteoarthritis in knee.

• Loss of ACL may impair sensory function and protective mechanism at the knee.

• Distruption of internal joint structures may alter joint alignment and the ares of cartilage that are loaded.

• If ligament damage results in loss of joint stability, then joint loads may be increased by active muscle contraction trying to stabilize the joint.

• Partial or total meisectomy can also be expected to increase the stress on the joint since joint force is concentrated over a small area.

• Despite an increased risk of developing osteoarthritis from excessive or abnormal joint loading, some level of loading or exercise appears to be beneficial for joint health.

• In an in vivo study with 37 healthy human volunteers, Tiderius et al shows that glycosaminoglycan content in medial and lateral femoral condyle cartilage is lower in sedentary subjects than those who exercise regularly.

• After an exercise regime there is also an increase in glycosaminoglycan content in the knee of patient at risk of developing osteoarthritis.

• These latter two studies using an MRI imaging technology known as d GEMRIC to quantitatively measure glycosaminglycan content.

• They show a biochemical adaptation to exercise , although there appears to be no adaptation of cartilage morphology to exercise as determined by tissue mass.

• Since exercise can enhance production of matrix molecules,it mat seem reasonable to expect that it can have positive effect on joint health.

CLINICAL RELEVANCE

• Exercise in people with osteoarthritis is shown to have positive effects on several outcome measures such as-

Pain. Strength. Self-reported disability. Observed disability in walking. Self- selected walking. Stepping speed. Although mild to moderate exercise is often

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