1. The lumbosacral junction is an extremely important region, and a study of its anatomy and mechanics explains many of the symptoms and sequelae of lumbosacral derangements.
2. Mobility at the lumbosacral junction is influenced by the shape and disposition of the constituent vertebrae and their articular processes, the resiliency of the connecting structures, the anatomical type of the individual, and, under certain circumstances, by the presence of anomalous developments or pathological processes.
3. The strength and stability of the junction are mainly dependent on the integrity of thé lumbosacral intervertebral disc, the longitudinal spinal ligaments, the ligamenta subflava, and the articular processes; these processes, being arranged almost in the coronal plane, impinge upon each other when the last lumbar vertebra tends to slide downward and forward and thus prevent anterior dislocation.
4. Muscular support is singularly lacking on the anterior aspect, but the psoas major laterally and the erector spinae and multifidus posteriorly, along with other smaller and less important muscles, have an important function in the maintenance of the stability of the junction and act as "ligament sparers".
5. Much of the potential weakness of the lumbosacral junction in man is due to the skeletal modifications necessary in changing from the ancestral quadrupedal to the modern bipedal condition.
6. There is considerable disagreement concerning the size of the "sacrovertebral angle". To obviate confusion, it is suggested that the "lumbosacral angle" should be used as a standard of comparison.
7. An interrelationship exists between the degree of sacral obliquity, the lumbosacral angle, and the amount of lumbar convexity. A method is described by which these interrelationships may be calculated.
8. Ullmann's test for lumbosacral olisthesis is too delicate and may lead occasionally to an erroneous diagnosis of early spondylolisthesis.
9. A brief résumé is provided of the chief causes of weakness at the lumbosacral junction.