Forces acting on intrinsic joints of pelvis

Question 1

Compression

Answer 1

• The weight of the upper body passes down through the spine into the SIJs.
• The forces travelling up through the lower limbs pass through the pelvis into the SIJs
• Thicker cartilage on the sacral side of SIJs
• Denser cartilage on the iliac side of SIJs
• Forces at pubic symphysis absorbed by the disc

Question 2

Tension / Distraction

Answer 2

Normally resisted by:
- ligaments (e.g. iliolumbar)
- muscles (e.g. pelvic floor muscles, piriformis)
- Occurs in late pregnancy until childbirth when SIJ ligaments + the pubic symphysis are affected by hormones (relaxin)

Pubic symphysis traction with pelvic floor pressure
- Excessive traction may sometimes occur, whereby the symphysis can over-widen leading to pain + instability

Question 3

Shear forces

Answer 3

• Can be vertical or anteroposterior
• Range of displacement normally from 0.5 – 2mm
• Resisted by:
  
  • interlocking ridges and grooves in SIJ
  • Ligaments
  • Muscles (dynamic/active stabilisation)
• Occasionally traumatic + more severe displacement – often associated with multiple pathologies (e.g. L5 transverse process fracture)

Shear forces – dynamic stability
- Muscles that attach to the pelvis from both the lower limb + the trunk can work together to stabilise pelvic joints

Question 4

Torsion / twist

Answer 4

• Nutation + counter-nutation at the SIJ can occur bilaterally or unilaterally = creating torsional forces at the pelvis (e.g. during gait)
• Resisted by ridges + grooves of non-synovial part of SIJ
• Sacral ligaments resist excess nutation (most ligaments)
• Muscles also can dynamically control amount of nutation/counter-nutation