Functional anatomy and biomechanics of the pelvis

This commentary focuses on the anatomy and biomechanics of the pelvic girdle and, specifically, the sacroiliac joints. In bipeds, the pelvis serves as a basic platform with three large levers acting on it (the spine and two lower limbs). Movement of the pelvic platform upon the hip joints relative to the femur, such as flexion and extension (pelvic ante- and retroversion), and rotation and abduction/adduction, strongly influences lumbar spinal movement. As well as this substantial external pelvic motion, internal pelvic motion through the sacroiliac joint is essential for effectively transferring loads between the spine and lower limbs. It has been postulated that the sacroiliac joints act as important stress relievers in the ‘force–motion’ relationships between the trunk and lower limb ( ). These joints ensure that the pelvic girdle is a flexible ring of bone that will not easily fracture under the great forces to which it might be subject, either from trauma or from its many bipedal functions ( ). Analysis of gait mechanics demonstrates that the sacroiliac joints provide sufficient flexibility for the intrapelvic forces to be transferred effectively to and from the lumbar spine and lower extremities ( ).

The sacroiliac joint is a highly specialized joint that lends stable (yet flexible) support to the upper body. Both the tightness of the well-developed dorsal fibrous apparatus and the specific architecture of the sacroiliac joint result in limited mobility. Numerous researchers have tried to model sacroiliac joint function by studying its principal displacement characteristics. A common assumption of these studies is that increased loading on the sacrum leads to tilting of the sacrum ventrally (nutation), a process by which most dorsal sacroiliac joint ligaments are stretched and the dorsal aspects of the iliac bones are drawn together ( , , ). Counternutation normally takes place in unloaded situations, such as lying. Nutation implies a forward tilting of the sacrum relative to a posterior rotation of the ilia, and vice versa in counternutation.