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.

Introduction

He who rejects change is the architect of decay. The only human institution which rejects progress is the cemetery. (Harold Wilson)

That the spine is supple is obvious to all that watch the movements of young children at play, the power service of professional tennis players and the sinuous movements of ballet dancers. Undoubtedly, flexibility decreases with age, while spinal movements at all stages of life can be severely limited by disease. Since vertebrae themselves are relatively inelastic, movement in the spine is mediated notably by the tissues of the intervertebral disk. Although the mobility of contiguous vertebrae (motion segments) can be viewed as limited, the integrated movements of the thirty three intervertebral disks permit all of the critical movements of the spine without compromising nerve or muscle function.

The importance of the intervertebral disk was first noted by the great European anatomist Versallius. In his treatise De Humani Corporis Fabrica published in 1555, he remarked that the fluid filled cushions of the disk permitted the movement of the head and presumably the remainder of the spine. The famous English anatomist Henry Gray (1827–1861) confirmed this view and classified articulations between vertebrae as “amphiarthroses in which the contiguous bony surfaces are either connected by broad flattened disks of fibrocartilage, of a more or less complex structure” [1]. Analysis of the articulations themselves indicated that vertebrae were interconnecting by three joints; the intervertebral joint, and the right and left zygapophyseal (facet) joints. Linking two contiguous vertebrae, the joint complex has six degrees of freedom since it can deform in three planes: axially, medial/lateral shear and anterior/posterior shear [2]. At the same time, less discursive coupled rotatory or translatory movements are evident in the other joints of the motion segment [3].

Just prior to the death of Gray, the debate was joined by the German anatomist Hubert von Luschka, (1820–1875) who had already described the clefting joints in the articulation of cervical vertebrae (uncovertebral joints also called von Luschka joints). He remarked that the spinal motion segment was equivalent to a synovial joint [4]. Now over 125 years later, it is tempting to think that the definition of the spinal joint being diarthrodial still holds. However, possibly because of the large differences in anatomic structure coupled with its unique developmental origin and overall limitation in movement, Grignon and Roland concluded “it would appear difficult to consider the IVD [intervertebral disk] as a diarthrodial joint” [5].

Since publication of the review by Grignon and Roland (2000) a considerable number of new findings related to both diarthrodial as well as spinal joints have emerged prompting a re-examination of the conclusion that the intervertebral disk should not be classified as a diarthodial joint. Certainly, on simple structural and functional criteria, the intervertebral joint deserves to be in a separate category; this view is reinforced by observations from the developmental literature where it has been shown that axial morphogenesis is governed by cues which may be separate from those of the appendicular skeleton [6], [7]. Nevertheless, more recent findings indicate a much closer functional and genetic relationship to a diarthrodial joint than has hitherto been recognized.

Finally, the question is raised, why should we care about joint classification? From a health perspective, if the joints share a number of phenotypic and functional characteristics, this information could be used to provide a deeper understanding of the pathogenesis of disease. For example, if degenerative disk disease is linked to changes in the nutritional status of the nucleus pulposus due to diffusion problems in the endplate cartilage, then could a similar defect modify the normal functioning of the articular cartilage of the synovial joint? Likewise, in osteoarthritis where there is accumulating evidence that this condition is associated with defects in subchondral bone, it begs the question, does a similar change in vertebral structure promote disk degeneration? In addition, the issue could be raised, why is the disk more likely to fail though herniation than the synovial joint; this latter structure maintains its stability even when its function is severely degraded. Hopefully, reclassification will encourage further analysis of the structure and function of the two types of overlapping joints and access disease-related information that has been previously associated with a specific type of joint. In this way, common new insights can be generated into the major diseases that afflict the many joints of the human skeleton.