Function of the Lumbosacroiliac Complex as a Self-Compensating Force Couple with a Variable, Force-Dependent Transverse Axis: A Theoretical Analysis

Abstract

The dense ligamentous structures surrounding the sacroiliac joints preclude free movement in the joints, yet some movement persists into old age. The nature of movement and function of the sacroiliac joints is obscure. I hypothesize that these joints function as part of the lumbosacroiliac complex, and act as self-compensating force-couples to balance forces of gravity, weight-loading, inertia, rotation, acceleration and deceleration. These forces are accommodated in variations in tensile ligamentous stress that is translated into compressive forces in the sacroiliac joints, which increases friction and stability, and allows increased weight-loading without shearing. This also allows storage of energy in the form of increased tensile ligamentous stress without injury to the joint. The joints appear to have both a transverse and an oblique axis, the location of which is dependent upon the moment of the applied forces, which may vary and change the location and direction of the axis. Dysfunction may occur with an anterior displacement of the line of gravity if anterior pelvic support fails to maintain self-bracing and friction. The resulting lesion may mimic disc dysfunction or give the impression of a multifactorial etiology and prevents normal function of the force couple.

Keywords:

• Sacroiliac Joints
• Innominate Bones
• Self-Bracing
• Pathomechanics