Lecture 4: Ligament Structure and Biomechanics

Ligament attaches _____ to _____.

Ligament attaches bone to bone

When are ligaments taut and or loose?

Ligaments have different functional sub-units that either tighten or loosen with joint movement

Ligaments often attach intimately to…

Joint Capsules

How are most ligaments arranged?

Many ligaments are arranged to support each other

What are the functions of ligament?

• Stabilize joint

• Guide motion

• Prevent excessive motion

• Restrain specific joint motion (along with capsule)

• Usually have one primary collagen fiber direction with one or more directions

• Joint position sense (proprioception)

What causes collagen fibers to line up a specific way?

Collagen fibers form along the way they are loaded (this then tells us the force that it will prevent)

Ligaments are made up of what type of tissue?

Dense Connective Tissue

What is the exact (%) composition of ligament?

Dense Connective Tissue

• Cellular Material (20%)

• Extracellular Matrix (80%)

  • Water (70%)

  • Solids (30%) (collagen, elastin, ground substance)

Ligament has a greater amount of Type ___ collagen compared tendon. Why?

Ligament has greater amount of Type III Collagen compared to Tendon.

(Ligament has 10% Type III where Tendon has 1-5% Type III)

Ligaments have more “give” than tendon

What are the cells of ligaments?

Fibroblasts

What are the cells of tendon?

Tenocytes / Tenoblasts

Where are tenocytes and tenoblasts located?

Between the collagen fibers along the long axis of the tendon

Where in the ligament has the greatest blood supply (or where is it the most vascular)?

Ligaments are most vascular at the attachment sites near bone

Where does this blood supply to the ligament come from?

Typically from the outer covering

Nerves that innervate ligaments are the same of what innervates the ______.

Joints (and also typically the nearby muscle)

(pain and mechanoreceptors)

How do ligaments load?

Ligament loads almost exclusively in tension.

What factors of the load are important to consider?

Rate (speed) of impact and magnitude

What does a higher rate and/or magnitude indicate about the ligament?

The more uniform and denser the appearance of collagen fibers in ligament

If we quickly load a ligament, how does the ligament respond?

• Ligament handles quick loads well

• Slope/Stiffness increases (so strain/elongation decreases)

• Ultimate strength point increases (can handle greater amounts of force)

• Will reach its failure point with less strain / “quicker”

During normal activities, ligaments function at ___% strain

Ligaments function at 1-3% strain (occurs primarily in “toe” region)

• Maybe due to dampening of loads

What is occurring during the “Toe” Region for ligament?

3% elongation for ligament

• Increase in length is from collagen uncrimping

With increased load rates, does dampening occurs better in ligament or tendon?

Ligament

What region (of a stress-strain curve) are ligaments normally stressed?

Toe Region

In the elastic region for a ligament, what is the curve like?

Linearly proportional (Young’s Modulus)

What is the percentage (%) of ligament elongation in the elastic region?

~5% of ligament length

For ligament, what is occurring at the yield point of a stress-strain curve?

Limit of collagen has been reached and microfractures begin

(some bonds may break before this point, but the fibers remain intact)

At what percentage (%) of ligament strain is the yield point at?

Ligaments yield point is at ~5% strain

When we play sports, we are at our upper limit of strain/tension, however, ligaments typically do not tear. Why?

Don’t tear because we have active tissue (muscle) and joint capsules that can help to protect ligaments

Why do/can we get injured during regular movement?

Problem in the feedback loop

• CNS distracted

• Bad mechanoreceptors

• Bad muscles or nerves

In the plastic region for a ligament stress-strian curve, what is occuring?

The stress distribution is not uniform (also, permanent deformation occurs here)

Starts at one end and builds up progressively as more shear stress is exerted

Strength is directly proportional to…

The diameter of the collagen fiber

After the yield point ______ begin and will continue until the _______ point.

After the yield point tears begin and will continue until the fracture point

Where does maximum strength occur on a ligament stress-strain curve?

Very often, this is the same point as the fracture point

For ligament, how can/do we move the fracture point and maximum strength point closer together?

Increased speed/rate of load (this also overall increases the maximum strength point)

What type of “failure” is occurring at the maximum strength point?

Progressive failure (there are “dips'“ in load values)

Discuss the failure point of a typical ligament stress-strain curve.

As the load acceptance begins to decrease even as load increases —> failure occurs

Failure occurs by….

Tear (larger fibers) or by creep

Smaller diameter fibers prevent _________. Why?

Smaller diameter fibers prevent shredding. This is because of the high surface to volume ratio

What allows for better functional properties?

Multimodal diameter distributions

Why does collagen fail?

Because of the adjacent tropocollagen molecules pull apart rather than the molecules themselves rupturing

What percent (%) strain does ligament failure occur?

10-14% strain

In ligament, do all fibers experience the same load at the same time?

No

How does stress distribution occur in ligament?

Stress distribution occurs as the load travels through the structure

At what loading rate does ligament failure result in an avulsion injury?

Slow Loading Rate

At what loading rate does ligament failure result in mid-tissue failure?

Fast Loading Rate

What age group does ligament failure result in an avulsion injury?

Younger individuals

What age group does ligament failure result in mid-tissue failure?

Older individuals

Does aging or strain rate have a greater effect on ligament? What does this mean/cause?

Aging has greater effect

Higher deformation occurs at or near bone insertion

What are the viscoelastic properties of a ligament?

• Hysteresis

• Creep

• Stress-Relaxation

Hysteresis is temperature _________.

Hysteresis is temperature dependent

A temperature increases —> hysteresis decreases

How does hysteresis compare between elastin/collagen to muscle?

Normal hysteresis loops for elastin and collagen are small compared to muscle

Considering creep testing, when is equilibrium reached in ligament?

Equilibrium is reached quickly (<30 min); often within 2-5 minutes

Considering stress-relaxation, how does the response differ between ligament and tendon?

Ligament has a greater response than tendon (but clinical application not known)

Equilibrium reached within minutes

Collagen fibers are ____________ sensitive, except for ____________.

Collagen fibers are strain-rate sensitive, except for extreme loadings

How is the load/elongation curve impacted by increased loading rates?

Load/elongation curves are steeper

If a load/elongation curve has greater stiffness with high loads, what does this lead to?

• More energy stored

• More force required to rupture

• Undergoes greater elongation

• Quicker time to failure

As load rate increases, bone shows ______________ in strength than ________.

As load rate increases, bone shows greater increases in strength than ligament.

How is failure impacted by slower rates of loading?

• Load to failure decreases by 20% (lower ultimate strength?)

• Less energy stored to failure (30%)

Is ligament or tendon more sensitive to rate loading?

Tendon is more sensitive to rate-loading

Is ligament or tendon more likely to tear with higher rates?

Ligament

What other factors/points are rate sensitive?

• Ultimate stress point

• Energy absorption

What does ligament fatigue failure occur from?

Occurs from cyclic loading that does not achieve ultimate tensile strength

How many continuous cycles would it cause for ligament fatigue failure?

300,000+ continuous cycles

Basically, no endurance limit (but if stress is higher, number of cycle can reduce)

How do cells respond to mechanical loading?

• If fibers can’t support forces, then cells susceptible (fiber fails first —> then cell)

• Loads too high lead to cell death (meaning less capability to heal)

In ligament, was does a decrease in temperature cause?

• Increasing ligament stiffness

• Greater hysteresis (takes longer for ligament to return to resting)

• Decrease in viscoelastic properties

During infancy, Type ___ collagen is converted to Type ___.

Type III collagen is converted to Type I

How is water content and collagen diameter size affected as we age? What does this cause?

Water content and collagen diameter size decrease as we age

This leads to increased stiffness and reduction in compliance

When does failure mode change from avulsion injury to mid-substance tear?

Changes during adulthood

When do avulsion injuries become more common regardless of loading rate? Why?

When we get older (in the elderly)

This is because bone strength is decreasing

Do males or females have more laxitity?

Females

What does tissue remodeling depend on?

Mechanical Demands

If we increase loading, what happens to strength and stiffness?

Increases

If we reduce loading what happens to strength and stiffness?

Decreases

What does immobilization do to ligaments? Why does this happen?

Causes ligaments to become softer and less stiff

Large reduction in the function of GAGs

Immobilization can decrease material properties in as little as…

8 weeks

After immobilization, what happens to energy absorption capacity?

Energy absorbing capacity may never return to pre-existing condition

A sedentary lifestyle can lead to an increase in…

• Ligament degeneration

• Higher risk of rupture

If a woman is pregnant, are her ligaments more or less stiff?

Less stiff (especially in pubic symphysis)

In an individual with diabetes mellitus, they show higher rates of…

Why does this occur?

Higher rates of…

• Tendon contracture

• Tenosynovitis

• Joint stiffness

• Capsulitis

Lose the viscous component of mechanical properties

In an individual with diabetes mellitus has the overall strength of tendons and ligaments changed?

No

What is the typical type of failure mode in an individual with diabetes mellitus?

Tensile fractures of the bone

How do we assess a ligament injury?

Put the joint in the position that the ligament is supposed to resist and apply force