Biomechanics in Physical Therapy

1. BIOMECHANICS OF
CARTILAGE
Dr Nishank Verma
MPT-Sports
Jamia Millia Islamia

2. CONTENTS
• CARTILAGE
• STRUCTURE AND FUNCTIONS OF CARTILAGE TISSUE.
• COMPOSITION OF ARTICULAR CARTILAGE.
• ZONES OF ARTICULAR CARTILAGE.
• MECHANICAL BEHAVIOUR AND MODELLING.
• BIPHASIC MODEL OF CARTILAGE.
• MATERIAL PROPERTIES.
• RELATIONSHIP BETWEEN MECHANICAL PROPERTIES AND
COMPOSITION.
• JOINT LUBRICATION.
• MECHANICAL FAILURE OF CARTILAGE.
• EXERCISE AND CARTILAGE HEALTH.

3. 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.

4. • 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 a joint would be much higher, since the
area of contact would be much smaller.

5. • 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.
• The unique and complex structure of articular
cartilage make treatment or repair or
restoration of the defects challenging for the
patient, surgeon and the physical therapist.

6. STRUCTURE AND FUNCTIONS OF
CARTILAGE TISSUE
• There are 3 different types of cartilage that
have slightly different structures and
functions.
HYALINE CARTILAGE.
FIBROCARTILAGE
ELASTIC CARTILAGE

7. • 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.

9. • 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.

11. • 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.

15. • FUNCTIONS OF FIBROCARTILAGE TISSUE-
Fibrocartilage tissue provides support and
rigidity to attached/surrounding structures
and is the strongest of the three types of
cartilage.

16. • 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.

19. • 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.

21. 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.

23. 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.

24. • 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.

26. 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.

27. 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.

30. 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.

31. 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.

32. • 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.

33. • 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.

34. • 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.

35. BIPHASIC MODEL OF CARTILAGE
• Fluid flow and deformation are interdependent
and 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.

36. • 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.

37. 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.

38. MATERIAL PROPERTIES
• CARTILAGE LOADING AND UNLOADING
• 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.

39. CONFINED COMPRESSION TEST OF CARTILAGE

40. • 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.

41. • 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.

42. • By fitting the Biphasic model to the measured
displacement, two material properties of the
cartilage are determined.
Aggregate modulus
Permeability.
 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.

43. • 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.

44. DEVICE USED TO MEASURE PERMEABILITY OF CARTILAGE

45. • 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.

46. • 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.

47. • 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.

48. • 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.

49. INDENTATION TEST ON ARTICULAR CARTILAGE

50. • By fitting the biphasic model of the test to the
measured indentation, following are
determined.
Aggregate modulus
Poisson’s Ratio
Permeability
Poisson’s ratio is typically less than 0.4 and often
approaches zero.

51. • 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 after 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.

52. • 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.

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

54. 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.

55. CORRELATION OF COMPRESSIVE STIFFNESSWITH THE
TOTAL GAG CONC.AS TOTAL GAG CONC DECREASES,
COMPRESSIVE STIFFNESS ALSO DECREASES.

56. • 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.

57. 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.

58. LUBRICATION PROCESS FOR
ARTICULAR CARTILAGE
Fluid film lubrication
Hydrodynamic Lubrication
Boundary lubrication
Squeeze-film Lubrication
Mixed Lubrication

59. • 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 velocity
As well as the stiffness of the surfaces.

60. • 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.

61. 1- HYDRODYNAMIC LUBRICATION
• Occurs when two non parallel rigid bearing surfaces
lubricated by a fluid film that moves tangentially with
respect to each other.
• Lifting pressure generated in wedge by fluid viscosity as
the bearing motion drags fluid into the gap.

62. 2- SQUEEZE-FILM LUBRICATION
• Occurs when weight bearing surfaces move
perpendicularly towards each other.
• Pressure in fluid film result of viscous resistance of fluid
that acts to impede its escape from the gap.
• Sufficient to carry high loads for shot duration.

63. • 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.

65. • 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.
• Primarily depends on chemical properties of
lubricants.
• Functions under high load and low relative
velocity preventing diect contact between
surfaces.

66. • MIXED LUBRICATION
1. COMBINATION OF FLUID FILM AMD BOUNDARY
LUBRICATION
• Boundary lubrication where occurs where the
thickness of flud film is on the same order as the
roughness of the bearing surfaces.
• Fluid film lubrication takes place in areas with more
widely separated surfaces.
• Most friction in boundary lubricated areas, most
load supported by fluid film.

68. 2- BOOSTED LUBRICATION
• Shift of fluid-film to
boundary lubrication with
time over the same
location.
• Articular surfaces
protected during loaded
by ultrafiltration of
synovial through the
collagen –PG matrix.

69. • Solid component of synovial flood passes into cartilage during
squeeze film action yielding a concentrated gel of HA protien
complex that coats and lubricates the surfaces.
• As articular surfaces approach each other,this becomes
difficult for HA macromolecules to escape from gap between
surfaces.

70. MECHANICAL FAILURE OF CARTILAGE
• A characteristic feature of osteoarthrosis is cracking,
fibrillation,and wear 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.
• It is generally believed that vertical cracks in cartilage
were initiated by relatively high tensile stresses on
the articular srface.

71. • 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.
• Studies of the tensile failure of cartilage are
primarily concerned with variations in properties
among joints, the effects of repeated load, and
age.
• Kempson and coworkers report a decrease in
failure stress with age for cartilage from hip and
knee. However, they find no appreciable age-
dependent decrease in tensile failure stress for
cartilage from the talus

73. • 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.
• For any value of peak stress,the number of
cycles to failure is lower for cartilage from
older than younger individuals.

74. THE EFFECT OF REPEATED TENSILE LOADING ON TENSILE
STRENGTH OF CARTILAGE.AS TENSILE LOADING STRESS
INCREASES,FEWER CYCLES OF LOADING ARE NEEDED TO
CAUSEFAILURE.

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

76. • Radin and coworkers also show that cartilage
failure could be induced by shear stress
• under rapid loading, cartilage behaves as an
incompressible elastic material,that is,its
Poisson’s ratio is 0.5.
• The relatively compliant, but
incompressiblecartilage experiences large lateral
displacement (due to itshigh Poisson’s ratio)
when loaded in compression, but this expansion
is constrained by the stiff underlying bone .
• Under these conditions, high shear stress
develops at the cartilage–bone boundary.

77. EXERCISE AND CARTILAGE HEALTH
• Participation in certain sports also appears to
increase risk of developing Osteoarthritis.
• Saxon et al concluded that activity that involve
Torsional loading.
Fast acceleration and decceleration.
Repetitive high impact
High levels of participation.
Increase risk of developing osteoarthritis.

78. Track and filed events.
Racket sports.
Soccer
Among 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.

79. • 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.

80. • 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.

81. • 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.

82. • 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.

83. 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
recommended

84. ROLE OF BIOMECHANICS IN CARTILAGE
REPAIR
• Degeneration and inflammation of articular cartilage
leads to Osteoarthritis
• Osteoarthritis is a painful and debilitating disease
characterized by progressive degenerative changes in
the articular cartilage and other joint tissues.
• Biomechanical factors play a critical role in the
initiation and progression of this disease.
• Abnormal loads can lead to alterations in the
composition, structure, metabolism, and mechanical
properties of articular cartilage and other joint
tissues.

85. • Abnormal loading may be caused by various factors
such as obesity, immobilization, joint
instability,overuse, or trauma.
• OA, including increased hydration, collagen
disruption, and matrix turnover accompanied by
decreased tissue stiffness in tension, compression,
and shear.
• Obesity alters gait and joint biomechanics, obese
individuals walk slower, with shorter and wider steps,
and have longer stance durations compared with
normal-weight individuals.

86. • These altered biomechanics can affect the load-
bearing regions of the articular Cartilage.
• So person with cartilage damage is advised to loose
weight if obese and proper gait pattern need to be
taught for proper loading .
• Proper biomechanical loading,gradual joint
mobility and stabilty help cartilage to recover
fast.

87. • BIOMECHANICS IN CARTILAGE
REHABILITATION
• Biomechanics play a crucial role in cartilage rehab after
surgical repair
• Goal of biomechanics in rehab of cartilage is to minimize
disability,immobilization,and overall morbidity occuring due
to small arthroscop.ic procedures or large open procedures.
• In the early postop. Period after cartilage repair, continous
passive movement should be done to enhance metabolic and
nutritional activities of cartilage, and to prevent joint stiffness.
• In early postoperative phase passive movements should be
performed instead of active movements because active
movements may have higher joint pressure than passive
movements leading to irritation of cartilage.

88. • The mechanics of weight bearing and walking may
increase the risk of articular cartilage breakdown
when the normal mechanical environment and
balance between loading and biological
maintainance of cartilage is compromised.
• Small mechanical loads promote cartilage metabolic
activity and nutrition.
• Critical level and manner of joint loading is required
to regulate the normal homeostatic balance of
cartilage anabolism and catabolism.

89. • Degenerative changes are associated with
hyperphysiologic magnitudes of loading or by
alterations in the normal loading pattern of the joint.
• Normal mechanical regulation of chondrocyte
activity in cartilage may be detrimentally influenced
by the presence of proinflammatory mediators and
cytokines in the joint.
• Biomechanics plays a crucial role in rehabilitation
and healing of cartilage after injury and repair by
surgery with proper joint loading, maintaining
homeostasis and biomechanical movements.