TRANSFER OF LUMBOSACRAL LOAD TO ILIAC BONES AND LEGS .1. BIOMECHANICSOF SELF-BRACING OF THE SACROILIAC JOINTS AND ITS SIGNIFICANCE FOR TREATMENT AND EXERCISE

This study deals primarily with the stability of the base of the spine . The sacroiliac joints are vulnerable to shear loading on account of their predominantly flat surfaces. This raises the question of what me chanisms are brought into action to prevent dislocation of the sacroil iac joints when they are loaded by the weight of the upper part of the body and by trunk muscle forces. First a model is introduced to compa re load transfer in joints with spherical and with flat joint surfaces . Next we consider a biomechanical model for the equilibrium of the sa crum under load, describing a self-bracing effect that protects the sa croiliac joints against shear according to 'the sacroiliac joint compr ession theory', which has been demonstrated in vitro. The model shows joint stability by the application of bending moments and the configur ation of the pelvic arch. The model includes a large number of muscles (e.g. the gluteus maximus and piriformis muscles), ligaments (e.g. th e sacrotuberous, sacrospinal, and dorsal and interosseous sacroiliac l igaments) as well as the coarse texture and the ridges and grooves of the joint surfaces.