Biomechanical modelling of the human sacroiliac joint.

From a mechanical point of view, the human pelvis can be considered as a stable, complex three link structure. This three-link closed-chain system explains why there is so little motion in the sacroiliac joint. Based on the minimum total potential energy principle, a quasi-static model of the human pelvis with its three joints is developed. In the model, the articular cartilage linings of the joint surfaces are considered as thin layers with a geometric non-linear behaviour. They lie between two rigid curved surfaces that are represented by small three-node elements. Accessory ligaments and capsules are represented by a number of non-linear springs. A primary model is developed based on a female cadaver. According to the primary model, the translation of the sacroiliac joint in the direction of force is about 0.5 mm in the lateral direction, about 1.8 mm in the antero-posterior direction, and about 1.5 mm in the superior or inferior direction, when a load of 1000 N is applied to the sacrum. When a load of 50 N m-1 is applied to the sacrum, the rotation in the load direction is about 1.6 degrees in axial rotation, about 1.0 degree in flexion or extension and about 1.1 degrees in lateral bending.