The functional morphology of the superior articular processes of the lumbar vertebrae.

The bony trabeculae in the superior articular processes of the lumbar vertebrae form a series of overlapping pointed arches that correspond to the lines of stress produced during rotation. By using a photoelastic model, it has been possible to demonstrate that this structural arrangement is a functional adaptation to alternate lateral and medial bending movements. The functional significance of the articular processes is obviously to limit rotation, which can take place in the lumbar segment of the vertebral column during any asymmetrical movement of the body. The alternate bending movements of the superior articular processes arise partly from the laterally directed pressure of the corresponding inferior process, and partly from the medially directed pull of the firm 'transverse strengthening ligaments' of the joint capsule.