Joints tutorial Wk 4

Question 1

Generally, the articular areas of bones are expanded relative to the diaphysis. Explain the benefits of having greater joint surface area.

Answer 1

Increase in contact area –> decrease stress (stress/strain force)

Medial (resist valgus) and lateral surfaces or jt surfaces are wider –> increase moment arm for collateral ligaments (further away from axis)- more efficient

Question 2

Explain ways in which the coefficient of friction in synovial joints is minimised?

Answer 2

1. Lubricates jt surface
   
   • Viscosity- low but not low enough to leave joint
   • Fluid film lubrication- pressure mechanism; physical separation of surcaes by pressurised fluid
   • Boundary layer lubrication- molecule that adheres to surface
2. Roughness of surface
   
   • Cartilage has smooth surface

Question 3

Describe the mechanisms which may lead to damage of hyaline articular cartilage and discuss the factors that affect the ability of hyaline cartilage to repair.

Answer 3

• Excessive loading
  
  • Put on weight, too much weight= obesity = fatigue failure
• Acute, impulse load (high rapid application)
  
  • Cartilage splits
  • Exceeds ultimate stress/strain capacity = acute trauma
  • Eg. femoral condyles
• Prologued loading in static position
  
  • Biphasic model (fluid and solid phase)
  • When compress, fluid is imcompressible, long term fluid redistributes and moves out
• Altered kinematics
  
  • Tissue is not adapted to force
  • Altered kinematics = injury
  • Point of contact has shifted
  • Immobilisation = lack of mechanical stimulus
• Local inflammation
  
  • Chemicals into synovial fluid affects chondrocytes

Factors that affect repair:

• Poor
• Avascular tissue (cant have inflammation process)
• Aneural
• Low cell density- hard to repair (each cells have large area to “look after, and can’t migrate to injury site)
• Cells have low metabolic rate
• Cant’t migrate to site of injury
• Mechanotransduction is less efficient due as they are isolated

Question 4

Explain how lower limb biomechanics can contribute to the development and progression of tibiofemoral OA.

Answer 4

Varus

Medial suffers from OA more forces go through –> mechanical line of axis (for load bearing) –> line is medial to middle (medial-lat direction)

Normal is varus –> but load bearing and ground reaction forces (external) will want to increase varus –> increase medial load (eg. tibia adducted compared to femur)

Does not like rapid and uncontrolled application loading

How to improve?

• Balance external varus but target internal muscles of valgus decrease valgus (eg. lateral head of gastron, biceps femoris, Sartorius (some) vastus lateralis, pelvic control- no contralateral pelvic drop)
• Heel strike gets adduction movement

Question 5

Explain how the arrangement of collagen fibres within a tibiofemoral meniscus relates to the types of stresses that occur during weight-bearing.

Answer 5

• Round condyle
• Shape of meniscus
• Hoop force (semilunar shape)- circumferential forces
• Collagen- irregular proteoglycans

Question 6

Radial: Inner vs Outer - AF has more type ___ collagen Circumferential: anterior to posterolateral part of discs

Answer 6

I

Question 7

Intervertebral discs – testing AF: Tensile modulus

Answer 7

• Degeneration does not affect tensile modulus (stiffness)
• Highest tensile modulus (stiffest)- anterior outer
• Most compliant- posterolateral inner

Question 8

Intervertebral discs – testing AF: Failure stress

Answer 8

• Most stress before failure- AO
• Least stress before failure- PI (common injury site) relative to anterior
• Degeneration will fail under less stress

Question 9

Intervertebral discs – testing AF: Failure strain

Answer 9

• AF degeneration doesn’t failure strain (not deforming abnormal)
• Inner parts strains more before failure
• AF- energy under cure

Question 10

Intervertebral discs – testing AF: Strain energy density to failure

Answer 10

• Cope with less energy before failure (degenerative)
• Same stiffness