The menisci of the lumbar zygapophysial joints.

In a study of 82 lumbar zygapophysial joints three types of intra-articular structures were identified. They were adipose tissue pads and fibro-adipose meniscoids, both located at the superior and inferior poles of the joint, and connective tissue rims, located along the dorsal and ventral margins. Every lumbar zygapophysial joint contained at least one of these structures and 47 contained more than one type. Connective tissue rims are short, central projections of the joint capsule and do not enter between the articular surfaces. Adipose tissue pads are covered by synovium and fill the subcapsular space at the superoventral and inferodorsal poles of the joint. Fibro-adipose meniscoids, also covered by synovium, project from the joint capsule at the superior and inferior poles and enter between the articular surfaces. Adipose tissue pads and fibro-adipose meniscoids are probably derived from a common primitive mesenchymal meniscus which primarily differentiates into a fatty structure. The fibrous component of fibro-adipose meniscoids then secondarily develops as a result of compression of the tip of the fatty structure between the articular surfaces. The function of these intra-articular structures is not evident but may be related to protection of the articular processes as they subluxate during flexion and extension. Meniscus entrapment in the lumbar zygapophysial joints has been proposed as a cause of acute locked back, but the present morphological data are inconsistent with this view.