Is the spinal motion segment a diarthrodial polyaxial joint: what a nice nucleus like you doing in a joint like this?

Abstract

This review challenges an earlier view that the intervertebral joint could not be classified as a diarthrodial joint and should remain as an amphiarthrosis. However, a careful analysis of the relevant literature and in light of more recent studies, it is clear that while some differences exist between the spinal articulation and the generic synovial joint, there are clear structural, functional and developmental similarities between the joints that in sum outweigh the differences. Further, since the intervertebral motion segment displays movement in three dimensions and the whole spine itself provides integrated rotatory movements, it is proposed that it should be classified not as an amphiarthrose, "a slightly moveable joint" but as a complex polyaxial joint. Hopefully, reclassification will encourage further analysis of the structure and function of the two types of overlapping joints and provide common new insights into diseases that afflict the many joints of the human skeleton.

Figure 1

Schematic showing similarities between an idealized diarthrodial join and an intervertebral disc. The diarthrodial joint comprises a thin layer of cartilage that covers both the articulating bone surfaces, and an innervated but relatively avascular fibrous joint capsule. The inner surface of the capsule is lined by a synovial membrane that secretes the synovial fluid. The intervertebral disc has two articulating cartilaginous surfaces, the endplate cartilages that are separated by a second proteoglycan rich tissue the nucleus pulposus. Nucleus pulposus cells secrete and organize a complex extracellular matrix. The gel-like nucleus pulposus is surrounded circumferentially by a fibrocartilagenous annulus fibrosus. Sharpey fibers of the annulus are inserted into the end plate cartilages and the vertebral bone.

Figure 2

Intervertebral disc (A, B) and knee joint (C, D) from newborn mice were analyzed by in situ hybridization with isotope-labeled RNA probes for lubricin (B, D). It is evident that lubricin is expressed by the annulus fibrosus (af) and superficial zone chondrocytes of the articular cartilage. af, annulus fibrosus; np, nucleus pulposus; ep, endplate cartilage; fe, femur; ti, tibia, me, meniscus. Scale bars: 85 mm for A, B; 150 mm for C, D.