The lumbopelvic complex consists of the lumbar spine and sacroiliac joint (SIJ). This complex can be the source of many symptoms, both serious and benign. Any clinician examining and treating this region must have a sound understanding and knowledge of the anatomy and biomechanics as abnormalities of the lumbar spine and SIJ may lead to compensatory or secondary abnormalities in other portions of the spine, pelvis, or hip.
Because there is no gold standard for diagnosis of SIJ dysfunction by physical exam, the diagnosis often becomes one of exclusion.
The lumbar spine consists of five lumbar vertebrae, which, in general, increase in size from L1 to L5 in order to accommodate progressively increasing loads. Between each of the lumbar vertebrae is the intervertebral disc (IVD). Anatomically, the SIJ is a large diarthrodial joint that connects the spine with the pelvis. Three bones comprise the SIJ: two innominates and the sacrum. The motions at the lumbar spine predominantly occur around the sagittal plane and comprise flexion and extension, whereas the motions occurring at the hip occur in three planes and include the one motion that the lumbar spine does not tolerate well, i.e., rotation. Thus, the pelvic area must function to absorb the majority of the lower extremity rotation. In addition, when the body is in an upright position, the SIJ is subjected to considerable shear force as the mass of the upper body must be transferred to the lower limbs via the ilia.1–3
Low back and leg pain can arise from both local and distal structures. The Agency for Health Care Policy and Research (AHCPR) has grouped back pain into three categories: nonspecific back symptoms, sciatica, and potentially serious spinal conditions (Table 13-1).4,5