Can the human intervertebral disc be compared to a diarthrodial joint?

The joint between human vertebral bodies is traditionally classified as a symphysis, a major cartilaginous synarthrosis. However, it has been suggested by some authors to compare the intervertebral disc (IVD) to a diarthrodial joint. The bases of this comparison are reviewed and discussed. In his description of 1895, Luschka saw the IVD as a diarthrodial joint containing articular cartilages and synovium in the annulus fibrosus. Subsequently, histologic and ultrastructural investigations into the cells and extracellular matrix of the nucleus pulposus (NP) supported the same hypothesis. More recently, the appearances of the IVD in magnetic resonance imaging (MRI) and discography, as well as clinical considerations were interpreted in the same way to simplify understanding of IVD pathology. However, a number of objections involving general, morphological, embryological, biochemical, histologic, radiological and clinical considerations demonstrate the limitations of this hypothesis. The general structural and mechanical principles of joint classification are mentioned to highlight the basic differences between the IVD and a synovial joint. An overview of the development of these joints, and a review of the literature dealing with ultrastructural aspects of the NP, with histopathological studies of the IVD, and with the normal appearance of the IVD on MRI and its age-related changes has led us to refute the hypothesis of this analogy. The IVD cannot be compared to a synovial joint, explaining why most of its pathologic features are quite different from those of a joint of the appendicular skeleton.