Critical Analysis of the Sequence and Extent of the Result of the Pathological Failure Of Self-Bracing of the Sacroiliac Joint

Abstract

When the sacrum is loaded with superincumbent weight, the sacroiliac joint functions as a self-compensating force couple. This couple creates and tends to rotate around an axis perpendicular to itself. The location of this axis may vary and depends upon the forces applied to the couple. As ligamentous tension increases in the couple, the sacroiliac joints are drawn tightly together through a mechanism of self-bracing. Self-bracing and its normal release or diminution provides for the storage and release of energy, which enhances the efficiency of normal ambulation and which modifies external forces. Failure of the force couple causes failure of the transverse axis of rotation of the sacroiliac joint and failure of the self-bracing mechanism. Because of the decrease in friction and stability, the innominate bones rotate anteriorly on the sacrum on an acetabular axis with the onset of pain and an alteration in apparent leg length. The resultant dysfunction may range from slight to severe, from minor ligamentous sprains to major sprains, muscle separations, and rents in the joint capsule. These rents may leak synovial fluid to the fifth lumbar nerve root, the lumbosacral plexus, and other tissues; and the resulting lesion may mimic disc dysfunction or create the impression of a multifactorial etiology. However it is seldom included in assessment of low back pain. A critical analysis of the biomechanics demonstrates the sequence and extent of involvement of adjacent tissues and structures, and it provides some direction for the restoration of normal function.

Key Words::

• Sacroiliac Joint
• Self-Bracing
• Low Back Pain