Form + Force Closure

Question 1

Form Closure

Answer 1

• refers to a theoretical stable joint due to close fitting articular surfaces.
• the triangular sacrum is essentially wedged b/w the ilia in the pelvic ring, giving it a degree of form closure (“keystone”)
• Complete form closure = not practical in the pelvis as the SIJ motions allow gait, absorb shock + shear forces, + allow special adaptation for pregnancy + birth.
• Form closure at the SIJ is provided by:
  
  • ‘Wedging’ of the sacrum between the ilium,
  • Ridges + grooves of the articular surfaces b/w the ilium + sacrum
  • the dense ligaments of the SIJ.

Question 2

Force Closure

Answer 2

• refers to the extra forces required to stabilise the joint.
• occurs with sacral nutation, tension increases in the SI ligaments resisting further motion
• With force closure lateral force generated by muscles as well as friction help compress the SIJs + resist vertical load.

Question 3

Force closure – muscle components

Answer 3

• Lower limb muscles associated with SIJ force closure include: rectus femoris, sartorius, iliacus, gluteus maximus, and hamstrings in the lower limb,
• Trunk muscles include: the Transverse Abdominus, internal oblique(IO), multifidus, diaphragm, and pelvic floor muscles perform an anticipatory stabilizing role