mechanical properties of specific tissues: tendon and ligament

why is tendon function important for efficient joint movement?

• it connects muscle to bone

• transmits muscle force to bone

• store and return energy to the msk/movement system (acts like a spring)

• provide joint stability

• different tendons have different functions/healing constraints

what can a tendon be compared to?

a tension spring

what is an achilles tendon rupture?

• traumatic injury

• typically an athletic injury of a rec athlete in their mid 40s

• long, inefficient tendon alters lower extremity function

• need for improved early rehab protocols that reduce long term disability

is early loading and early activity helpful for tendon mechanical properties?

YES! improved tendon mechanical properties and increased cross sectional area in animals that returned to activity at 1 week

what did a study find about early tendon loading?

early tendon loading results in increase cross-sectional area

how to ligaments guide joint movement?

• connect bone to bone

• constrain movement

• provide joint stability

what is the ACL - clinical problem?

• traumatic injury, athletic population

• < 50% return to prior level of sport

• may or may not require surgical reconstruction to regain knee joint stability

• knee laxity is reduced after reconstruction, but returns in post op period

What are the ACL biomechanics of graft tissue

• knee laxity increases over time

• knee laxity may relate to changes in graft mechanics

What is the study of cadaveric knees ACL?

• ACL reconstruction performed with pretensioned patellar tendon graft

• subjected knees to 1500 cycles of flexion

• substantial drop (-50%) in graft tension and increase in knee laxity after 500 cycles

Pretensioning is important and specific pre-tensioning protocol may need to be reconsidered

what is the most common collagen in ligaments and tendons?

Collagen 1, most common ; forms into aligned fibers

What are all of the types of collagens that make up ligaments and tendons?

• Collagen I (most common)

• Collagen III, V, X, XI, XII

what is collagen V?

regulates fibril diameter

What is collagen III?

scar! replaced by collagen I eventually

What is collagen XII?

found in between type I fibers, lubricates for sliding

what are the other non-collagenous components that make up ligament and tendon?

• proteoglycans

• elastin

• cells

• water

what are proteoglycans?

• hydrophilic

• regulate collagen fiber development

• viscoelastic properties

• lubrication and gliding

what is elastin?

allows tissue to return to original length after loading (it acts like a spring)

how do tendons and ligaments differ in composition?

tendon has more collagen and less elastin

ligament has more elastin and less collagen

is collagen organized in ligament and tendons?

yes, it is densely packed and highly organized

it is well designed to resist tensile stretching

what to tendon fibrils look like at rest?

at rest, tendon fibrils have a wavy appearance , or crimp

when tendon fibrils are are stretched, they straighten!

is collagen more aligned in tendon or ligaments?

they are more aligned in tendons

what is collagen alignment in tendon?

• highly aligned collagen in the direction of tension

• tendon able to withstand very high tensile loads along the primary axis of the tendon (up and down)

what is collagen alignment in ligaments?

• densely packed, but less aligned than collagen

• ligament is better able to withstand multi-planar stresses

what happens to a structure with scarring?

diaemeter changes, fill with fluid, doesnt strighten back out

are ligament and tendon tensile forces linear or non linear?

non-linear

what is the area of hysteresis?

history dependent: material becomes less stiff as you pull it (balloon example in class)

It goes back to stiff after long periods of rest

what does it mean that ligaments and tendons are anisotropic?

they behave differently when pulled on different axes

what is the structural properties?

• extrinsic measure

• depends on size and shape

EX: thick tendon vs thin tendon

what are material properties?

• intrinsic measure

• QUALITY of the tissue substance (size and shape does NOT matter)

what does modulus mean?

quality

what is creep?

will elongate with consistent load

what is stress relaxtion?

dont have to pull as hard overtime, length stays constant

what are the three major steps of ligament and tendon healing?

1. inflammatory (0-72 hours)

2. repair and regeneration (72 hours - 6 weeks) creates low quality tissue

3. remodel (6 weeks -12 months +): turnover of collagen, organization gets better

why are progressive loads on the tendon required to stimulate healing?

• PHYSICAL STESS THEORY

• physical stress (tensile load) leads to collagen tensioning and sliding

• in the tendon, shear forces in the tendon stimulate tenocytes setting communcation and biological processes in motion

• cell signaling of stimulated tenocytes important part of restoring homeostasis

what maintains tenocyte activity, cell signaling and tissue homeostasis?

tendon loading and collagen sliding

what happens when there is impaired collagen sliding?

phenotypic changes of the tenocyte (sad yellow avocado), impaired cell signaling and tissue degeneration

what does progressive tendon loading do?

it aims to place safe loads on the tendon to normalize tendon structure and cellular function

what are the two responses to loading?

• good: stimulate tissue repair and remodeling

• Bad: avoid overloading (tissue damage) or underloading (not enough to stimulate the tissue)

what happens when homeostasis is not maintained?

tendinopathy, boney chunks in tendon, disorganized and hypercellular, lipid in the tendon, modulus/quality decreases

what is the repair stage simulated bu?

mechanical loading, and is characterized by a tendon callus

what are some other considerations for tendon and ligament healing?

• immobilization

  • decreased ultimate load and stiffness

  • irregularities in collagen fibers

  • vascular changes

  • alterations in cell shape and number

  • adhesions of tissue to surrounding structures

• comorbid conditions

  • diabetes, hyperlipidemia

• age