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

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.

► We critique the assumption that the intervertebral disk is an amphiarthosis. ► We compare the development, structure and function of the disk with a synovial joint. ► The intervertebral disk has many similarities with synovial joints. ► The intervertebral disk should be re-classified as a polyaxial diarthrodial joint.