12 Tendon and Ligament: Anatomy, Function and Mechanics
Jim Jastifer M.D.
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Tendon vs. Ligament
Tendon: muscle to bone Ligament: bone to bone
Provide joint stability
Compositional differences between tendons and ligaments:
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Tendon: Basic Structure
Transmits forces created in the muscle to the bone Composed of collagen and elastin embedded in a matrix of proteoglycan and water Synthesized by tenocytes and tenoblasts Each muscle has proximal and distal tendon
myotendinous junction: tendonmuscle osteotendinous junction: tendonbone origin: proximal tendon-bone insertion: distal tendon-bone
(From “Human Tendons” by Józsa and Kannus)
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Tendon: Macroscopic Structure
60% water Composition (% of dry mass):
65-75% collagen (mostly type I, some type III) 2-3% elastin 2% proteoglycan
White in color Varies in shape (wide and flat, cylindrical, ribbon shaped) Powerful muscles have short and broad tendons (e.g. quadriceps) Muscles carrying delicate and subtle movements have long and thin tendons (finger flexors)
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Tendon: Surrounding Structures
Tendons have to help a force (produced by muscle) act (on the bone).
Requires lots of structures to efficiently do so
Sheaths Pulleys Linings
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Tendon: Surrounding Structures
Synovial sheaths
closed duct around tendons gliding on bone surfaces frequently observed in tendons of hand and feet the sheath is formed of two membranes: inner (visceral) and outer (parietal) sheets
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Tendon: Surrounding Structures (From “Human Tendons” by Józsa and Kannus)
Epitenon fibers at 60 to the tendon axis. Reorient to 30 after stretching
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Tendon: Surrounding Structures
Endotenon:
Thin network of crisscross collagen fibrils Envelopes the primary, secondary and tertiary fiber bundles together Proteoglycans are present between endotenon and tendon fibers hydration Allows fiber bundles to glide with respect to each other Carry blood vessels, nerves and lymphatics to tendon (From “Human Tendons” by Józsa and Kannus)
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Tendon: Internal Architecture (From “Human Tendons” by Józsa and Kannus)
Blood Supply
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Tendon: Internal Architecture
Crimping of tendons:
Wavy formation within fascicles Believed to result from crosslinking of proteoglycans Disappears when stretched and reappears when unloaded Removal of crimp dominates low strain range (<4%)
(From “Human Tendons” by Józsa and Kannus)
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Tendon: Other components
Proteoglycans of tendon
Glucosaminoglycan’s (GAGs) of tendon
Function similar to cartilage Tensional zones of tendon have 0.2% GAG (mostly dermatan sulphate) Pressure zones have 5% GAG (mostly chondroitin sulphate)
Glycoproteins of tendon
“adhesive” function fibronectin (involved in repair processes) thrombospondin (cell-matrix adhesion) laminin (observed in myotendinous junction)
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Tendon: Other Components
(From “Human Tendons” by Józsa and Kannus)
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Tenocytes: Tendon-cells
Direct maintenance
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Tendon: Biomechanics
Biomechanical characteristics of tendons:
Tensile strength: due to molecular and supramolecular organization of collagen Adequate flexibility: elastin fibers Inextensibility: efficient transmission of force from muscles to bones Inferior resistance against shear and compressive forces
Adaptation
Tension in all directions: fibers interwoven Tension along one axis: parallel ordering
Tendon: Biomechanics
Tendon and ligament are similar
Tendon generally stronger in tension because of more focused function Nonlinear behavior “Toe region”
Longer for ligaments because less organized
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Properties not as sensitive to loading rates as other tissues (ALL) Strength highly age dependent (ACL)
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Tendon: Mechanical Properties Tangent Modulus
Stress
B
C III
FAILURE
II
LINEAR
I
TOE
A
2
4
6
8
Strain (%)
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Tendon: Biomechanics Force
Extension
Effect of increasing tissue cross-sectional area on loadextension: greater load, greater stiffness
This is how your body solves force-extension problems…or…
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Tendon: Biomechanics Force
Extension
Effect of increasing tendon length on load-extension: less stiff, similar strength
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Tendon: Biomechanics
Secondary biomechanical functions of tendons:
Eliminates unnecessary length of muscleallows body to optimize muscle fiber length Enables muscle belly to act at a distance from the joint Absorbs energy: limits damage to muscles and bone
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Tendon: Mechanical Properties
Parameters measurable from loadextension curve:
Slope in region II: tangent stiffness/modulus linear load: load at the end of region II maximum load strain at maximum load strain to failure energy to failure (area under the curve)
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Tendon: Mechanical Properties
In vitro tensile strength: 50-100 MPa
Tendon with 1cm2 area can carry 0.5-1 tons
Tensile strength of tendon is about twice the strength of the muscle it is attached to
Strength increase during maturation and then decreases
No gender-related differences in strength
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Tendon Biomechanics
Parallel arrangement of collagen fibers to the direction of tensile force give tendons one of the highest tensile strengths of any soft tissue in the body. Two ways to characterize the tensile properties of tendon
Mechanical properties of the tendon
Stress-strain relationship
Structural properties of the bone-tendon-muscle unit
Load-elongation relationship
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Tendon: Biomechanics
Challenges in conversion of load-extension curve to stress strain curve:
How to measure deformation
Specimen length for strain
Tendons are slippery
Definition of original length: ideal initial length is in the body Pre-strains are removed following dissection inducing slackness
Specimen area for stress
area is seldom uniform along the length caliper measurements prone to error due to compressibility of tissue indirect measurement of area: volume/length laser based techniques
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Tendon: Deformation Mechanisms
Toe region
Changes are at light microscopic level Waviness of fiber bundles straightened out Continued elongation results in increased stiffness End of toe region ranges from 1.5-4% strain
Methods to quantify toe-region
Wertheim (1947) 2=c12+c2 (c’s constant) Morgan (1960) =c30.812 Elden (1968) =c42 Fung (1967) quasi-linear viscoelastic model L=L ec5(-)
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Tendon: Deformation Mechanisms
Linear region
Ranges from 2-5% Tendon will recover to its original length if not strained beyond the linear region Common parameter is elastic or linear stiffness
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Tendon: Deformation Mechanisms
Failure region
Collagen fibers slide past each other Possible rupture of crosslinks Reduction in stiffness Waviness reappears at an increasing rate indicating gradual rupturing of bundles Ruptured fibers/bundles recoil
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Tendon: Viscoelasticity Rate dependency (rat tail tendon):
Force
high-rate
low-rate
Extension
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Tendon: Viscoelasticity Preconditioning:
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Tendon: Biomechanics of the tendon-muscle unit
Tendons function as a unit of a muscletendon-bone system Healthy tendon is seldom the weakest link of the system
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Tendon: In vivo considerations
Patellar tendon forces:
5.2 kN during kicking 8.0 kN during jump 9.0 kN during fast-running 14.5 kN during competitive weight-lifting
3,300lbs
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Tendon: Aging •Increase in collagen content •Decline in water content •Decrease in crimp
Elastic Modulus
•Increase in collagen cross-links
Age
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Digression: Tendon vs. Ligament
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What is elastin? Take a guess Both get mechanical properties, for the most part, from collagen
Digression: Tendon vs. Ligament
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Ligament, Aging Important because age is probably more important than strain rate for failure
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BIOLOGICAL TISSUES RESPOND TO THEIR LOADING ENVIRONMENT!!!!!!!!!!
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Ligament Biomechanics
Like tendons, ligaments demonstrate time- and history-dependent behavior Clinically-relevant examples
ACL reconstruction: initial force applied to tension the graft decreases w/ time b/o stress relaxation
Intraoperative spinal distraction
Preconditioning can decrease amt of stress-relaxation by ~50% Can decrease peak forces on instruments and their insertions on vertebra b/o soft-tissue creep
Shoulder dislocations
Creep in capsular ligaments & soft tissues
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Example 4.1
Modulus values in literature must be cautiously applied based on strain values
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Clinical correlate ACL reconstruction
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50% chance of osteoarthritis at seven year follow up
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10-mm wide BPTB graft has stiffness and ultimate load values of 210 ± 65 N/mm and 1784 ± 580 N QSTG autograft, evolved from a single-strand semitendinosus tendon graft, has very high stiffness and ultimate load values of 776 ± 204 N/mm, 4090 ± 295 N, respectively). Which is better?
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Question During
a one handed grip, which can you squeeze with more force, a pencil, a beer can, or a coffee can? Why?
Muscle Mechanics Jim Jastifer MD
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Introduction
Muscle types:
Cardiac muscle: composes the heart Smooth muscle: lines hollow internal organs Skeletal muscle:
Skeletal muscle accounts for 40-45% of body weight
- 700 muscles
- 80 pairs produce vigorous movement
Dynamic & static work
Dynamic: locomotion & positioning of segments Static: maintains body posture
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Introduction
Cells as opposed to extracellular matrix Several Properties
Responsiveness (excitability)-touch a hot stove
Conductivity
shortens when stimulated
Extensibility
local electrical change triggers a wave of excitation that travels along the muscle fiber
Contractility
capable of response to chemical signals, stretch or other signals & responding with electrical changes across the plasma membrane
capable of being stretched
Elasticity
returns to its original resting length after being stretched
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Skeletal Muscle Voluntary striated muscle attached to one or more bones Muscle fibers (myofibers) as long as 30 cm Exhibits alternating light and dark transverse bands or striations
reflects
overlapping arrangement of internal contractile proteins
Under conscious control
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Motor unit
One nerve cell activates several muscle cells
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Sarcoplasmic reticulum
Network of tubules & sacs; Parallel to myofibrils Enlarged & fused at junction between A & I bands: transverse sacs (terminal cisternae) Triad {terminal cisternae, transverse tubule} T system: duct for fluids & propogation of electrical stimulus for contraction (action potential) Sarcoplasmic reticulum store calcium
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