13 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

C III

FAILURE

LINEAR

TOE

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=c12+c2 (c’s constant) Morgan (1960) =c30.812 Elden (1968) =c42 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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13.1

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Regulatory & Contractile Proteins

Myosin & actin are contractile proteins

they do work of shortening the muscle

Tropomyosin & troponin are regulatory proteins

act like a switch that starts & stops shortening

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Overlap of Thick & Thin Filaments

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Beer is always the answer Length-Tension Curve

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Types of contraction

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Muscle Twitch

Threshold is minimum voltage necessary to produce contraction

a single brief stimulus at that voltage produces a quick cycle of contraction & relaxation called a twitch

Phases of a twitch contraction

latent period (2 ms) is delay between stimulus & onset of twitch contraction phase is period during which tension develops and shortens relaxation phase shows a loss of tension & return to resting length refractory period is period when muscle will not respond to new stimulus

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Production of Variable Contraction Strengths

Stimulating the nerve with higher voltage get stronger contractions because recruit more motor units Stimulate the muscle at higher frequencies (stimuli/sec) up to 10Hz, produces twitch contractions with full recovery between twitches 10 - 20, each twitch develops more tension than the one before due to failure to remove all Ca+2 20 - 40, each stimulus arrives before the previous twitch is over temporal or wave summation produces incomplete tetanus 40 - 50, no time to relax between stimuli so twitches fuse into smooth prolonged contraction called complete tetanus (normal smooth movements)

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Recruitment & Stimulus Intensity

Maximal recruitment

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Histology of muscle

Type IIA Type IIB Type I

Eye muscle (Rectus lateralis); Myofibrillar ATPase stain, PH 4.3

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Muscle Differentiation (types of fibers) I (slow-twitch oxidative)

IIA (fast-twitch oxidative glycolytic)

IIB fast-twitch glycolytic

Contraction speed

Slow

fast

Myosin-ATPase activity

Low

High

Primary source of ATP Oxidative production phosphorylation

Oxidative phosphorylation

Anaerobic glycolysis

Glycolytic enzyme activity

Low

Intermediate

High

No. of mitochondria

Many

Few

Capillaries

Many

Few

Myoglobin contents

High

Low

Muscle Color

Red

White

Glycogen content

Low

Intermediate

High

Fiber diameter

small

Intermediate

Large

Rate of fatigue

slow

Intermediate

Fast

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Functional arrangement of muscle

 pinnated angle of muscle

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The Musculotendinous Unit

PEC: parallel elastic component CC: contractile component SEC: series elastic component

Tendon- spring-like elastic component in series with contractile component (proteins)

Parallel elastic component (epimysium, perimysium, endomysium, sarcolemma)

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Muscle mechanics Static contractions Twitch, summation, tetanus: 60hz

Brooks et al.

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Motor unit recruitment All-or-nothing event 2 ways to increase tension: - Stimulation rate - Recruitment of more motor unit

Size principle Smallest m.u. recruited first Largest m.u. last

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Influence of parallel elastic component

Fp l0

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Force and Velocity •Force diminishes with increase in velocity •Maximum force can actually be generated with eccentric contractions (1.8 times isometric max) •Power= Force * Velocity

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Force-Velocity Characteristics

Concentric contraction

Muscle contracts and shortening, positive work was done on external load by muscle Tension in a muscle decreases as it shortens

Eccentric contraction

Muscle contracts and lengthening, external load does work on muscle or negative work done by muscle. Tension in a muscle increases as it lengthens by external load

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Length and velocity versus Force

Note: maximum contraction condition; normal contractions are fraction of the maximum force

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Muscle fatigue

Drop in tension followed prolonged stimulation. Fatigue occurs when the stimulation frequency outstrips rate of replacement of ATP, the twitch force decreases with time

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Muscle mechanics To review, determinants of force/power production by a muscle 1. # of motor units recruited (i.e., the cross-sectional area of the active muscle) 2. frequency of stimulation 3. length of the fibers relative to Lo 4. velocity (shortening and lengthening) a. myosin ATPase activity b. SR concentration 5. muscle architecture a. orientation of fibers to the long axis b. the # of sarcomeres in series

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Obesity

There is a dose-response relationship between obesity and Osteoarthritis.

Increase in body weight proportionatly increases forces across weight bearing joints.

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Changes in obesity

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