Week 3a Joint and Tissue Mechanics Lecture

diarthroses joints characteristics

Synovial fluid filled cavity, permit moderate to extensive movement

Diarthroses joint examples

Shoulder, apophyseal (facet) joint of spine, knee, ankle

Synarthroses joint characteristics

Reinforced by combination of fibrous and cartilaginous CT, permit slight to no movement
Two types: fibrous and cartilaginous

Fibrous Joints

inflexible layers of dense connective tissue, holds the bones tightly together
EX: skull sutures

Cartilaginous joints

allow only slight movement and consist of bones connected entirely by cartilage
EX: symphysis pubis, IVD

Degrees of Freedom

Number of independent directions of movement at joint, can have up to 3

Hinge Joint

1 DOF, 1 axis of rotation
Angular motion primarily in plane located at right angle to the axis
EX: elbow

Pivot joint

1 DOF, 1 axis of rotation
ANgular motion is parallel to axis of rotation
EX: forearm

Ellipsoid joint

2 DOF, 2 axis of rotation
Elliptic surfaces restrict spin between the two
EX: wrist

Ball and Socket Joint

3 DOF, 3 axis of rotation
Surfaces allow spin without dislocation
Movement in all 3 cardinal planes
EX: hip, shoulder

Condyloid joint

2 DOF, 2 axis of rotation
Ligaments or bone structure (come in pairs) limits third degree of freedom
EX: Knee

Saddle joint

2 DOF, 2 axis of rotation
Concave surface on convex surface or vice verse
EX: 1st metacarpophalangeal joint

Planar joint

1 rotational DOF, 2 translational DOF, lack definitive axis of rotation
EX: 2nd-5th metacarpals

Diarthrodial joint elements

Always
- Synovial fluid, articular cartilage, joint capsule, synovial membrane, ligaments, blood vessels, sensory nerves
Sometimes
- Intra-articular discs/menisci, peripheral labrum, fat pads, bursa, synovial plates

CT Composition

ground substance, fibers, and cells

Periarticular CT

Dense Connective
- Regular = tendons and ligaments
-Irregular = joint capsules
Cartilage
- Articular
- Fibro
Bone

Stiffness

= change in Force/change in length

Refer to elastic properties graph

Stress

= Force / Cross Sectional Area

Strain

= elongation (change in length) / original length

Viscoelastic Materials

have both elastic and viscous response

Elastic materials

Strain when stretched and immediately return to original state when stress removed, they are time-independent

Viscous material

resist shear flow and strain linearly with time when stress is applied, they are time dependent

Young's Modulus

stress/strain

Elastic region of stress/strain curve

Added stress to the tissue results in greater deformation, though the tissue returns to its resting length if the stretch force is not maintained. Tissues with greater stiffness will have a steeper slope in this portion of the curve

Plastic region of stress/strain curve

The addition of more stress results in permanent deformation even after the stretch force is no longer applied due to the failure of bonds between the collagen fibers at the yield point

Yield point of stress/strain curve

Max strain without permanent changes

Ultimate failure point of stress/strain curve

complete tissue failure

Toe in region of stress/strain curve

Beginning part, many fibers are stretching causing a gradual increase in stiffness

Bone in stress/strain curve

Yield point reached quickly when strain is applied
Has a high Young's Modulus, more stiff

Tendon and ligaments in stress/strain curve

Yield point reached later when strain applied
Has a low Young's Modulus, more compliant

Viscoelastic Properties

Time dependent
- Creep
- Stress Relaxation

Creep

Load constant, displacement is changing

Continued deformation/displacement/length over time under constant load

Deformation occurs until state of equilibrium

EX: tree branch, with weight at distal end

Stress Relaxation

Load changes, displacement constant

Continued length over time while force decreases until equilibrium reached

LLPS

Adaptations in one session due to viscoelastic properties and neurological changes

Long term due to remodeling

Creep LLPS

Prone hangs using weights or gravity

Stress Relaxation LLPS

Splints, dynasplints

Loading rate

Speed at which a force is applied
A higher loading rate means viscoelastic materials are more stiff

Pre-conditioning

Multiple cycles to increase muscle length for the same force

Hysteresis

When some energy of load is dissipated (related to metabolic activity) so that not all energy is returned --> creates displacement

High Load Brief Stretch

Changes due to pre-conditioning and loading rate dependencies
EX: static and dynamic stretching