Intro to tissue mechanics slides

Forces

• “push” or “pull”

• Internal (muscles, tendons, skin, etc.) or external (DB, KB, gravity, etc.)

• Parallel (direction matters, forces summed), perpendicular (resultant vector), oblique (y-component rotational ; normal force, X-component compressive/distractive ; tangential force)

• Vector (magnitude and direction)

Types of forces

• Tensile: perpendicular, to stretch

• Compressive: Perpendicular to compress

• Shear: two forces in opposite directions parallel to tissue

• Bending: 3 points of contact, parallel, 1 side compressive, 1 tensile

• Torsion: Twist, bending around Z-axis, 1 side compressive

Stress

The result of force that is applied to a tissue (measured in sigma 𝜎)

• Normal stress: Applied perpendicular to fiber orientation

• Shear Stress: Applied parallel to fiber orientation

Strain

The measure of the degree of deformation due to stress applied to a tissue (measured in epsilon ε)

• Normal strain: results from normal stress (perpendicular)

• Shear strain: Results from shear stress (parallel)

Stress-Strain curve

• Strain is the % of stretch past the resting length

• Elastic region is the region under the curve up to the yield point, so the area under young’s modulus. Tissue resists change and returns to resting position once stress is removed

• Plastic region: Area under curve after yield point and ending in failure point. Stress in this region leads to permanent deformation.

• Failure point: Magnitude of stress where the tissue will break/tear.

• Ultimate stress: Highest stress magnitude on graph, generally close to or at failure point.

Structural properties of tissues

• Strength: magnitude of load/stress at ultimate stress point (usually at or just before failure point)

• Stiffness: Slope of the stress/strain curve ; load required to deform tissue a given amount.

• Compliance: Measurement of the ease of deforming a tissue (reciprocal of stiffness).

• Toughness: The area under the curve of the stress/strain curve. Defined as work/energy. (how difficult is it to create a change in elongation?)

Ideal materials

• Solids: Everything but fluids. Lots of variety.

  • Elasticity: a property of solids that provides perfectly linear relationship. Ability of materials to return to starting position when load is removed. TIME INDEPENDENT (will always return to resting position regardless of how long)

Not “ideal” materials

Viscoelastic materials are all biomaterials. They’re not ideal in the sense of material properties being predictive, but they serve our bodies well. Viscoelastic materials have differing degrees of viscosity and elasticity, but usually aren’t solely one or the other. They are TIME DEPENDENT and RATE DEPENDENT

• Viscous materials are more resistive to compressive loads (cartilage is more viscous than elastic). Viscous materials resist change.

• Elastic materials are more resistive to tensile forces due to having a higher collagen content (tendons/ligaments are more elastic than viscous. Elastic materials return to resting position.

Viscoelastic material mechanical properties

• Hysteresis: the “lag” in rebounding. Decreases with increased heat. The area between young’s modulus and hysteresis is dissipated as heat.

• Rebound resilience: Ratio of area under unloading curve to area under loading curve as %. Want it to be close to 1 which indicates a low hysteresis. Closer to 1 means tissues prep for next rep quicker.

• Creep: maintain stress over period of time, get elongation over that period of time.

  • Cyclic creep tests: mimic athletic activities better. Examine fatigue failure on viscoelastic materials (tendonitis or any other itis)

• Stress relaxation: Maintain elongation over time and stress reduces.

Tissue protective responses to stress

• Uncrimping: collagen fibers go from being “wavy” to straight with an applied stress

• Viscoelasticity: Elastic deformation, Creep, Stress relaxation discussed above

• Stress reaction: Tissue responds to repeated stresses to make itself longer (Wolff’s law, happens in bones). Very important in resistance training with tendons

Material properties descriptions

• Soft

• Hard

• Stiff: not much deformation, or requires a lot of stress to deform

• Weak: lots of deformation, requires less stress to deform

• Brittle: fractures easily, doesn’t deform, but fractures instead.

• Ductile: flexible like rubber, deforms a lot, and doesn’t really fracture

What happens to curve if “normal'“ length tissue becomes “tight”?

Slope of the curve gets steeper, so the elastic zone shifts to the left. Less stress can be applied before tissue enters the plastic zone, and permanent deformation occurs.

What would happen to tissue if repeated stresses are performed up to but not beyond the yield point?

There would be a temporary change in the start/zero position (shift to the right). This is due to creep, and hysteresis

What would happen to the stress/strain curve if a prolonged stretch was applied beyond the yield point?

Permanent deformation would result. There would be a new normal/resting/zero position. Some fibers would be damaged. This is the technique used in contracture cases.

How would an increase in temperature effect the stress/strain curve?

It makes it behave more elastic. This also lowers the hysteresis slope, or decreases the amount of hysteresis (rebound lag). The rebound resilience gets closer to 1, there is decreased stiffness. Overall, the tissue will prepare for the next rep better which is the primary functional purpose behind doing a dynamic warmup.

Poisson’s ratio v(nu)

Ranges from 0-0.5. Structures closer to 0 are more brittle, thus breaking before deforming. Structures closer to 0.5 are more viscous, so they will be more able to deform.

Bone = 0.13-0.3, Sandstone/concrete = 0.1, metals are generally 0.25-0.33.

Fracture mechanics (not just bone)

Methods of fracture

• Single insult (macro trauma): one large blow takes tissue to plastic range. Changing the fracture point requires an increase in material strength

• Repeat low-level insults (microtrauma): cyclic loading, especially present in “creep” loading to fatigue failure. Occurs in tendonitis, or any other “itis”

Types of fractures: both types exacerbated by tissue defects causing tissue to break with less force (single insult case) or less cycles (fatigue case)

• Shatter: comminuted fracture, poor prognosis, high impact load

• Crack propagation: more from fatigue failure. Shredding of tissue ; stress fracture of bone

Key concerns

• Number of defects (micro or macro tears)

• Size of defects: longer and wider cracks lead to more damage

• Location of defects: Defect in femoral head is more concerning than defect near gluteal tuberosity due to femoral head being a higher stress point. Osteoporosis there would be more likely to lead to fracture.

Stress concentrations

Defects can become an area that stress is being primarily applied to

Natural areas of stress concentration are tissue interfaces (ligament or tendon transition to bone)

Rolling an ankle at different ages

In children, bone is still cartilaginous, so weaker than the collagen of ligaments, so an avulsion fracture is more likely

In adults, collagen of ligament is weaker than the osseus bone, so torn ligament is more likely.

Tissue types

Connective (the main focus of this course)

Muscle

Neural

Epithelial

Types of connective tissue

Irregular connective tissue (loose and dense)

Regular connective tissue (dense)

Cartilage

Adipose tissue

Haemopoietic tissue (structures in blood like plasma

Blood

Bone

3 major contents in connective tissue

• Extracellular matrix (ECM): Proteoglycan, Glycosaminoglycans (GAGs), Water. Proteoglycan and GAGs comprise “ground substance”

• Collagen: 28 types, but we mostly talk about types 1,2,3

• Elastin: provides elastic recoil

Ground substance

Proteoglycans: protein polysaccaride w/ protein core and sulfate chains attached

• Made of hyaluronic acid (hydrophillic, helps tissue absorb water) - high conc. of viscoelastic materials

• several hundred GAGs attached to it

• Stabilize collagenous skeleton of tendons and ligaments

Glycosaminoglycans (GAGs): 3 main types

• Aggrecan: largest, hydrophillic

• Biglycan: larger concentration of type 2 collagen

• Decorin: collagen synthesis and development

Collagen

Synthesized by fibroblasts, helps withstand stretching. Doesn’t specify protein or amino acid sequence

3 procollagens → (L. handed helix) Tropocollagen microfibril in R. hand helix → (uses vitamin C) Fibril → (multiple fibrils) Fiber

• Tropocollagen is the basic building block of collagen. Procollagen can become tropocollagen, or several other structures, but once it becomes tropocollagen, all factors are present for it to become collagen.

Increased diameter → increased strength.

Increased thread # & lower diameter → increased flexibility

Orientation of fibers depends on the tissue and load placed on it. Most fibers are parallel, ligaments have more cross fibers than tendons.

Types of collagen

1: Most common, found in all tissue types, binds to other types of collagen. Thick, stiff, strong ; Mature scars, bone, tendons, ligaments

2: Thin support (hyaline cartilage), thinner & slightly less tensile strength. Provides general framework for hyaline cartilage

3: Thin filaments, make tissue strong but supple and elastic. Fresh scars, skin (more elastic), blood vessels, uterus, GI tract.

Skin converts from 3 → 1 as you age

Collagen strength

Collagen fibers are only good at resisting tension along the length of the fiber. Little to no resistance to compressive loads.

Elastin

• Protein

• provides elastic recoil, helps structures return to their original shape

• Highest concentration in Ligamentum flavum (post. to spinal cord, ant. to lamina