Unit 4 (Biomechanics) for Biology for engineers conducted at the CHRIS UNIVERSITY

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BIOSCIENCE- Chapter-4
Biomechanics of Ligaments
Name:……………………………………
Electronics & Communication Dept.

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Syllabus
Key mechanical concepts - 9 fundamentals of biomechanics -
Muscle action, Range of motion principle, Force motion
principle - Biomechanics of passive muscle tendon unit-
Biomechanics of bone - Biomechanics of ligaments

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● Ligaments are soft collagenous tissues that connects bone to bone.
● Plays a important role in maintaining the stability of a joint and also hold joints
together. It is also capable to restrict joint motion.
● Shares similar structure as that of tendon.
● Ligament microstructure can be visualized using polarized light that reveals collagen
bundles aligned along the long axis of the ligament and displaying an underlying
"waviness" or crimp along the length.
● Crimp is thought to play a biomechanical role, possibly relating to the ligaments
loading state with increased loading likely resulting in some areas of the ligament
uncrimping, allowing the ligament to elongate without sustaining damage

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Structure of Ligaments
● Ligaments are dense connective tissues that contain collagen, elastin, proteoglycans,
water, and fibroblasts.
● Approximately 70 to 80 percent of the dry weight of ligament consists of Type I
collagen, which is a fibrous protein.
● The collagen fibril is the basic load bearing unit of ligament.
● The fibril consists of bundles of microfibrils held together by biochemical bonds (called
cross-links) between the collagen molecules.
● Because these cross-links bind the microfibrils together, the number and state of the
cross-links are thought to have a significant effect on the strength of the connective
tissue.

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Schematic diagram of ligament

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BASIS FOR COMPARISON TENDONS LIGAMENTS
Definition Tendon connects muscles to
bone, and are present at the end
of skeletal muscles. These are
the fibrous connective tissue
non-elastic.
Ligaments connect one bone to
another bone and so are found
in joints. These are also the kind
of connective tissue which is
stronger and flexible and helps
in movements of the bones.
Nature Tendons are inelastic and tough. Ligaments are elastic and
strong.
Fibres Fibres are present as compact
parallel bundles.
Fibres are compactly packed
and not arranged in parallel
bundles.
Fibroblasts In tendon, fibroblasts lie in
continuous rows.
In ligaments, fibroblasts are
scattered.
Formed of Tendons are made of white
fibrous connective tissue.
Ligaments are made of yellow
fibrous connective tissue.
It joins Tendons connect end of the
muscles to any place of the
bone.
It connects bones to bones at a
joints.
Classification No classification. They are divided into three
categories-peritoneal ligaments,
fetal remnant ligaments, and
Ligaments Vs Tendon

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Biomechanical aspect of Ligaments
● Ligaments are passive collagenous structures that act primarily as tensile restraints to
control the distance between their attachment points.
● Ligaments normally traverse joints, and so they act to control the relative separation of
the bones that they are attached to.
● The ligaments control the patterns of movement, or kinematics, of joints, as well as
ensuring the stability of joints.
● In addition to this simple mechanical description of the role of ligaments, they provide
more subtle control of joint motion and stability via proprioceptive feedback to the
muscles.

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Biomechanical aspect of Ligaments
● Ligaments are viscoelastic structures with unique mechanical properties.
● The ligaments are pliant and flexible.
● It allows natural movement of the bones to which they attach, but are strong and
inextensible so as to offer suitable resistance to applied forces.
● Sustain chiefly tensile loads during normal and excessive loading.
● When injury happens, the degree of damage is related to the rate of loading as well as
the amount of load.

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Biomechanical aspect of Ligaments
● Ligaments are often evaluated by using mounted specimens such as a bone-ligament-
bone specimen.
● However, recent advances have allowed for some instrumentation to be used in the
measurement of in situ forces in humans.
● These include the use of buckle transducers, instrumentation at insertion sites,
magnetic resonance imaging, kinematic linkage measurements, and implantable
transducers.

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Stress-Strain Property of Ligaments
● When a tensile force is applied to the ligament at its resting length, the tissue
stretches.
● Force-length curves can be normalized to subtract out the effects of geometry.
● Thus, force can be normalized by dividing by the cross-sectional area of a tissue,
while length can be normalized by dividing by the initial length of the tendon or
ligament.
● The resulting stress-strain curve displays three characteristic regions: the toe region,
the linear region, and the failure region.

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If one neglects viscoelastic behavior, a typical stress strain
curve for ligaments and tendons can be drawn as:

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Toe region:
● As the load increases so does the recruitment of collagen fibres causing them to
‘uncrimp’.
● This occurs when collagen is stretched to approximately 2% of its original length and
returns to normal length when the force is removed, thus it is within its physiological
range.
● It is characterized by relatively low stiffness. There is a non-linear relationship on the
stress-strain curve at this stage.

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Linear region (Elastic phase):
● As the collagen fibrils become gradually uncrimped, the fibril itself is being stretched.
● There now becomes a linear relationship between deformation and load, as the tissue
becomes relatively stiffer.
● This occurs when collagen is stretched to 2-4% of its original length and returns to its
original geometric shape. The tissue is said to be elastic.

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Yield and Failure region (Plastic phase):
● The continued increase in load past 4% causes micro failure to the fibrils and damage
to cross-links.
● It results in a plateau effect on the curve: this point represents the ultimate tensile
strength of the tendon and is termed the ‘yield point’.
● The yielding of fibres occurs when the deformation is approximately 4-10% of the
resting length.
● Stiffness is reduced and the fibrils do not return to normal length on release, the tissue
then becomes ‘viscous’. This is known as ‘plastic’ deformation.

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● Finally, complete failure occurs as the ligament/tendon ruptures.
● Obviously this is a non-physiological range and there would be an inability to support
load or function.
● Collagen demonstrates various mechanical and physical properties in response to
load and deformation to allow it to withstand high tensile stresses.
● The point between the elastic and plastic region is where gross integrity is disrupted.

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Factors That Affect the Biomechanical Properties of Ligaments
● Numerous factors affect the biomechanical properties of tendons and ligaments.
● The most common are :
● Aging,
● Pregnancy,
● Mobilization and immobilization,
● Diabetes mellitus, connective tissue disorders, renal disease etc.
● Pharmacologic agents (steroids, nonsteroidal Anti-inflammatory drugs or NSAIDs).

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Video tutorial
https://www.youtube.com/watch?v=U7PE736mRVk
https://www.youtube.com/watch?v=LaIVBd_wJ_M