Transcript Single Joint System

Advanced Biomechanics of
Physical Activity (KIN 831)
Lecture 2
Biomechanics of Tendons and Ligaments
* Material included in this presentation is derived primarily from two sources:
Enoka, R. M. (1994). Neuromechanical basis of kinesiology. (2nd ed.). Champaign, Il: Human Kinetics.
Nordin, M. & Frankel, V. H. (2001). Basic Biomechanics of the Musculoskeletal System. (3rd ed.). Philadelphia:
Lippincott Williams & Wilkins.
What do you know about the
macroscopic structure and
function of tendons and
ligaments?
What do you know about the
microscopic structure and
function of tendons and
ligaments?
Functions of Ligaments and
Joint Capsules
• connect bone to bone
• act as static restraint to:
– help with joint stability
– guide joint motion
– prevent excessive motion
Functions of Tendons
• connect muscle to bone
• transmit tensile loads from muscle to bone to:
– produce joint torque
– stabilize joint during isometric contractions and in opposition
to other torques
– cause joint motion during isotonic contractions
– act as a dynamic joint restraint
– interact with ligaments and joint capsule to mitigate loads that
they receive
----------------------------------------------------Interesting points:
• tendon extends the reach of muscle
• tendon may conserve muscle tissue mass (i.e., muscle
tissue not required to extend from origin to insertion)
Tendons and Ligaments
• Dense connective tissues (parallel-fibered collagenous
tissues)
• Sparsely vascularized
• Composed primarily of collagen (fibrous protein which
gives tendons and ligaments strength and flexibility)
• Consist of relatively few cells or fibroblasts (≈ 20% of total
tissue volume)
• Contain abundant extracellular matrix
– ≈80% of total tissue volume
– ≈70% of extracellular matrix is water and ≈30% solids (collagen
(≈75% of extracellular matrix), ground substance, and small
amount of elastin)
• Structure and chemical composition identical to other
animal species (extrapolate behavior from animals)
Tendons and Ligaments
• Tendons
– Join muscle to bone
– Organization of collagen
fibers to accommodate
specialized function
• Fibers longitudinal and
parallel
• Transmit tensile muscle
forces
• Ligaments
– Join bone to bone
– Organization of
collagen fibers to
accommodate
specialized function
• Fibers generally
longitudinal and
parallel, some oblique
and spiral
• Primarily transmit
forces in functional
direction, but also
multidirectional
How can you make string able to
support a large load?
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How do manufacturers of string
make it able to support a large load?
1.
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Collagen Molecule
• Synthesized by within fibroblast as procollagen
(precursor to collagen)
• Consists of 3 polypeptide chains ( chains) each
coiled in left hand helix
• 3  chains combined in a right handed triple helix
• Bonding (cross-linking) between  chains
enhances strength of collagen molecules
• Develops extracellularly into collagen molecules
Collagen
• Groups of 5 collagen molecules form
microfibrils
• Cross links formed between collagen
molecules that aggregate at the fibril level
• Cross links between collagen molecules
give strength to tissues (e.g., tendons and
ligaments) they compose
• Fibrils aggregate further to form collagen
fibers
• Fibers aggregate to form bundles
Collagen Fiber Arrangement in
Tendons and Ligaments
Macroscopic and Microscopic
Structure of Tendon and Ligaments
Macroscopic
and
Microscopic
Structure of
Tendon and
Ligaments
Macroscopic and Microscopic
Structure of Tendon and Ligaments
• Epitendidium -outer covering
• Fascicle - bundle of fibrils
• Fibril - basic load bearing unit of tendon
and ligaments
• Microfibril - 5 rows of triple helixes in
parallel (see figure)
Schematic illustration depicting the hierarchical
structure of collagen in ligament midsubstance
Macroscopic
and
Microscopic
Structure of
Tendon
Schematic representation of the microarchitecture of a tendon
Structural hierarchy of a tendon. Connective tissue layers or sheaths
envelop the collagen fascicles (endotenon), bundles of fascicles
(epitenon), and the entire tendon (paratenon)
Macroscopic and Microscopic
Structure of Tendon and Ligaments
• Collagen molecule - triple helix in series; 5
rows stacked side-by side (parallel)
• Triple helix - cross links occur both between
and within rows of triple helixes  strength
(# and state of cross links influence
strength)  determined by age, gender, and
activity level
Elastin
• tendons and ligaments contain protein elastin
• influences elastic properties of tendons and
ligaments (↑ elastin  ↑ elasticity)
• proportion varies by function
– little in tendons and extremity ligaments
– much present in ligamentum flavum between laminae
of vertabrae
• protect spinal nerve roots
• pre-stress the motion segment
• provide intrinsic stability to spine
Ground Substance
• amorphous material in which structural elements occur
• in connective tissues, composed of proteoglycans,
plasma constituents, metabolites, water, and ions
between cells and fibers
Ground Substance in Tendons and Ligaments
• Proteoglycans act as cement-like substance between
collagen microfibrils contributing to overall strength of
tendons and ligaments
Water and Proteoglycans
• Forms a gel
• Viscosity decreases with activity
– Thixotrophy (property seen in catsup)
– Increased ability to accommodate higher
velocity stretches
– Advantage of a warm-up
Vascularization of Tendons and Ligaments
• Dual Pathway for Tendons
– Vascular (tendon surrounded
by paratenon)
• receives blood supply from
vessels in perimysium,
periosteal insertion, and
surrounding tissues
• Ligaments
– Vascularity
• Originates from ligament
insertion sites
• Small size and limited blood
flow
– Avascular (tendon surrounded
by tendon sheath)
• Synovial diffusion
• Healing and repair in the
absence of blood supply
---------------------------------------------------------------Take home message:
• Amount of tissue vascularization is directly related to rate of
tissue metabolism and healing
• Tendons and ligaments have limited vascularization
Macroscopic and Microscopic Structure of
Tendon and Ligaments
• Tendons surrounded by loose
connective tissue (paratenon)
– Paratenon forms sheath
• Protects tendon
• Enhances gliding
– Epitenon
• Synovial-like membrane
beneath paratenon in locations
of high friction
• Absent in low friction
locations
• Surrounds several fiber
bundles
– Endotendon
• Surrounds each fiber bundle
• Joins musculotendinous
junction into perimysium
• Ligaments surrounded by
very loosely structured
connective tissue (not named)
– Vascularity
• Originates from ligament
insertion sites
• Small size and limited blood
flow
Tendon
Insertion
in Bone
What comes to mind when you hear
the word “toe”?
Load Deformation Relationships
in Collagenous Tissues
• Toe - collagen fibrils stretched to line up, from
zigzag to straighten
• linear region - elastic capability of tissue; elastic
modulus
• failure region - fibers disrupted
• Hysteresis – failure to return to resting length
StressStrain
Relationship
in
Collagenous
Tissues
Collagen Fibers – Unloaded (Toe)
and Loaded (Elastic Region)
Typical Load-Elongation Curve
Load-Elongation Curve of Ligaments
with High Levels of Elastin
Elastin (protein) scarcely present
in tendons and extremity
ligaments
Ligamentum flavum:
• Substantial proportion of elastin
• Connect laminae of adjacent
vertebrae
• Function to protect spinal nerve
roots
• Provide intrinsic stability to
spine
LoadDeformation
Relationships
for
Connective
Tissues
* 1kN = 224.8 pounds
Note that text gives value
of failure of ACL between
76.4 and 87.67 lbs (340390 N)
Is there any movement in isometric
contractions?
Physiological Loading of
Tendons and Ligaments
• P (max) of ligaments and tendons not
achieved during normal activities
• normally 30% of P (max) achieved
• upper limit during running and jumping  2
- 5 % P (max)
Ligament and Tendon Injury
Mechanisms
• Injury mechanisms similar in tendons and
ligaments
• Microfailures take place before yield point
• After yield point, gross failure results and
joint begins to displace abnormally
• Joint displacement can also damage
surrounding structures (e.g., joint capsule,
other ligaments, blood vessels)
Anterior Drawer Loading the
ACL to Failure
Anterior Drawer Loading the
ACL to Failure
•
Microfailure
begins before
physiological
loading range is
exceded
What is the numerical
categorization system used by
athletic trainers to differentiate
between levels of ligamentous
injury?
Categorization of Ligamentous
Injury
1. Negligible clinical symptoms, some pain,
microfailure of some collagen fibers
2. Severe pain, clinical detection of some
joint instability, progressive collagen fiber
failure resulting in partial ligament
rupture, strength and stiffness may
decrease 50% or more, muscle guarding,
perform clinical testing under anesthesia
Categorization of Ligamentous
Injury
3. Severe pain, joint completely unstable,
most collagen fibers ruptured, loading
joint produces abnormally high stress on
the articular cartilage  correlated with
osteoarthritis
Additional Factors in Injuries to
Tendons
• Amount of force of contraction produced by
muscle attached to tendon
– Tensile stress on tendon directly related to force
of muscle contraction
– High levels of tensile stress can be produced by
eccentric contraction, possibly reaching failure
Additional Factors in Injuries to
Tendons
•
Cross sectional area of tendon in relation to
cross sectional area of its muscle
–
–
–
–
Cross sectional area of muscle directly related to
force of contraction
Cross sectional area of tendon directly related to
tensile strength
Tensile strength of healthy tendon may be more than
twice that of force of muscle contraction (clinically,
muscle ruptures more common than tendon ruptures)
Large muscles usually have large tendons
Viscoelastic Behavior (Rate
Dependency) in Tendons and
Ligaments
• Increased strain  increased slope of
stress-strain curve (i.e., greater stiffness at
higher strain)
• Higher strain rate  more energy stored,
require more force to rupture, undergo
greater elongation
Typical loading (top and unloading curves (bottom) from tensile
testing of knee ligaments. The two nonlinear curves, called the area
of historesis, represents the energy losses within the tissue.
Two Standard Tests of Viscoelastic Behavior*
1. Stress-relaxation test
– Loading halted in
safe region of stressstrain curve
– Strain kept constant
over extended period
of time
– Stress decreases
rapidly at first, then
gradually
– Decrease in stress
less pronounced with
repeat tests
*Viscoelastic – variation in mechanical properties of
tissue with different rates of loading
If you were asked to develop a creep
test, what would you use to make
measurements?
Two Standard Tests of Viscoelastic Behavior
2.
Creep test
–
–
–
–
Loading halted in
safe region of
stress-strain curve
Stress kept
constant over
extended period
of time
Strain increases
rapidly at first,
then gradually
Clinically used in
casting club foot
and bracing in
scoliosis
Schematic creep curve for ligament
Influence of Loading Rates on
Bone-Ligament-Bone Complex
• At slow loading rates (60 sec.; much slower
than in vivo injury mechanism), avulsion
produced
• At fast loading rates (0.6 sec.; simulates in
vivo injury mechanism), ligamentous injury
typical
Factors Affecting Biomechanical
Properties of Tendons and Ligaments
• Maturation and aging
– Up to 20 years of age,
• number and quality of cross-links in collagen molecules
increases  increased tensile strength
• Collagen fibril diameter increased  increased tensile
strength
– After maturation,
• Collagen content of tendon and ligaments decreases 
decreased tensile strength
Factors Affecting Biomechanical
Properties of Tendons and Ligaments
• Pregnancy and postpartum period
– Clinical observation – increased laxity of tendons and
ligaments in pubic area during latter stages of
pregnancy and during early postpartum period 
hormonal influence
– Research studies of rats– increased laxity of tendons
and pubic symphasis during latter stages of pregnancy
and during postpartum period; stiffness of these
structures later returned
Factors Affecting Biomechanical
Properties of Tendons and Ligaments
• Pregnancy and postpartum period (continued)
– Hormones may have influence on ligament laxity in
women at various stages of menstrual cycle 
influence ligamentous injury rates in females (e.g.,
higher incidence of injury in women in basketball and
soccer in comparison to men)
Factors Affecting Biomechanical
Properties of Tendons and Ligaments
• Mobilization and immobilization
– Tendons and ligaments remodel in response to
mechanical demands
• Become stronger and stiffer when subjected to increased stress
• Become weaker and less stiff when stress removed
– Physical training found to increase tensile strength of
tendons and ligament-bone interface
Factors Affecting Biomechanical
Properties of Tendons and Ligaments
• Mobilization and immobilization
– Immobilization found to decrease tensile strength of
ligaments
– Immobilization decreased mechanical properties of
bone-ligament-bone complex in knee of primates (8
weeks of casting)
– Considerable reconditioning required in primate knees
to regain former complex strength (approx. 12 months)
(see figure)
Influence of Immobilization on
Primate ACL Ligament
Influence of Immobilization on
Primate ACL Ligament
Factors Affecting Biomechanical
Properties of Tendons and Ligaments
• Nonsteroidal Anti-Inflammatory Drugs (NSAID)
(e.g., aspirin, acetaminophen, indomethacin)
– In animal studies, short term administration of NSAIDs
(indomethacin) found to increase the rate of
biomechanical restoration of tissues (tendons)