Biomechanics of Tendon, Cartilage, and Ligaments

Question 1

Articular Cartilage Biomechanics/Functions:

Answer 1

Lines the articulating ends of bones

Functions
Distribute joint loads over wide area
↓ stress concentration (pressure)
Reduce friction and joint wear
Shock absorption
↑ time of loading

Question 2

Articular Cartilage Structure

Answer 2

Solid components
Collagen (15-22%)
Strong in tension
Weak in compression
Proteoglycans (proteins) (4-7%)

Water (65-80%)

Question 3

Collagen Fiber Structure

Answer 3

Filled with water, proteoglycans & cartilage cells
Sponge-like:
 Water moves out 
  then returns
 Shock absorption
 Nutrition

Question 4

Articular Cartilage - Viscoelastic Nature

Answer 4

Deformation depends on rate of loading
Rapid loading → Elastic & Stiff
Constant magnitude load → Creep
Constant load application causes a slow, but constant increase in deformation

Question 5

What damages articular cartilage?

Answer 5

1. Increased Contact Pressure
   Misalignment, obesity, co-contraction, loss of menisci
2. Repetitive Impacts
   Hard and fast loading without shock absorption (abrupt landings which increase force)
3. Inadequate Contact Pressure
   AC has no blood supply, it needs to be loaded and used to obtain nutrition from synovial fluid
4. Frictional Abrasion
   Instability from insufficient ligament and joint support
   Shear forces from muscles
   Reduction in synovial fluid

Question 6

Tension and Ligament Biomechanics- Functions

Answer 6

Tendon
Execute motion by transmitting mechanical forces from muscles to bones

Ligaments
Provide joint stability
Resist tensile forces
Guide joint motion

Mainly undergo tensile loads

Question 7

Tendon and Ligaments Structure

Answer 7

Cellular (20%)

Extracellular matrix (80%)
Collagen fibers, Elastin fibers, & Ground substance (30%)
↑ collagen content in tendon
↑ elastin content in ligament
Elastin: similar to collagen, but more elastic in nature
Water (70%)

Question 8

Tendon & Ligament - Collagen Fiber Arrangment

Answer 8

Unloaded → Crimped (wavy)
Loaded → Straighten (take out slack)

Collagen content tendon > Collagen content ligament

Question 9

Tendon & Ligament Bone Insertion

Answer 9

Gradual alteration
Collagen fibers intermesh with fibrocartilage
Fibrocartilage becomes mineralized
Merges into cortical bone

↓ stress concentration → stronger attachment

Question 10

Tendon & Mechanical Properties: Stiffness

Answer 10

Stiffness
Tendon > Ligament
Tendons: support large loads with small deformation
Efficient transfer of force (muscle → bone)

Question 11

Tendon & Mechanical Properties: Strength

Answer 11

Tendon > Ligament
Why differences?
Material properties (collagen content)

Question 12

Tendon & Mechanical Properties: Viscoelastic (collagen fibers)

Answer 12

Viscoelastic (collagen fibers)
Load-deformation response is time dependent
Stress relaxation & Creep
Tendon
↑ Loading Rate → ↑ Stiffness & Strength

Question 13

Tendon & Mechanical Properties: Clinical Application

Answer 13

Creep: 
Constant low load application may lead to permanent changes
Treatment of deformities
Foot deformities, scoliosis
Serial casting & braces

Stress Relaxation:
Static stretching → acutely ↑ ROM

Question 14

Tendon & Ligament: Mechanical Failure

Answer 14

Load elongation curve
Means of analyzing mechanical properties of tendon & ligament
Application of constant elongation until failure
Tension

Question 15

Why the toe region?

Answer 15

Uncrimping of collagen fibers
Tissue stretches easily, without much force
Stiffness rapidly increases once fibers have uncrimped

High tensile strength
Physiologic loading only 1/3 of ultimate stress magnitude

Question 16

Tendon Injury

Answer 16

Mid-substance tendon tears are rare
Tendon tears common at bony insertion (avulsion #) or musculotendinous junction

Why?
Viscoelastic nature
High loading rate → ↑ Tendon stiffness (greater than bone)
Ultimate Tensile Strength
Tendon > Bone
Tendon > Muscle
Weakest link

Question 17

Ligament Injury

Answer 17

Mid-substance tears are more common
Not as much collagen, compared to tendon
Weaker
Ligament is more easily injured, compared to tendon
↓ strength & ↓ stiffness
Need to minimize mechanical loads to reduce risk of injury

Question 18

Factors Influencing Tendon & Ligament Mechanical Properties

Answer 18

Loading rate
Aging
Hormonal fluctuation
Immobilization & Exercise
Steroids

Question 19

Aging

Answer 19

↑ strength & stiffness during maturation
↓ Collagen fiber diameter in elderly
↓ strength & stiffness

Question 20

Hormonal Fluctuation

Answer 20

(only ligaments)

↑ estrogen & relaxin → ↓ collagen production → ↓ strength & stiffness

Question 21

Immobilization

Answer 21

“Wolff’s law”  “Davis’s Law”
↓ stiffness & ↑ elongation
↓ tensile strength
Up to 12-months to recover from 8-weeks of immobilization

Question 22

Steroids

Answer 22

Multiple steroid injections → ↓ strength, ↓ stiffness, ↓ failure point, ↓ energy absorption