1Biomechanics Laboratory, Department of Orthopaedics, University of Gothenburg, Sweden, and the
2Biomechanics Unit of the Department of Mechanical Engineering, Imperial College of Science and Technology, London, England
Previous in uiuo and in uitro experiments, using dye and radioactive glucose and sulphate, have demonstrated that the calcified zone of articular cartilage in normal adults is for practical purposes impermeable. Such impermeability, should it occur in the intervertebral disc, might be revelant to premature disc degeneration and would throw doubt on the importance of the osseous route for nutrition of the disc.
The present investigation has demonstrated that there is a statistically significant difference in permeability between the lateral portion of the end-plate, on the one hand, and the central portion of the endplate and annulus fibrosus on the other, the latter two usually being permeable, the former impermeable.
Histological examination revealed that permeability of the end-plate was always associated with the presence of “vascular buds” (vascular projections making wide ampulla-like contacts between the marrow space of the vertebral body and the hyaline cartilage of the disc’s endplate). Thus this study suggests that two nutritional routes are open for the intervertebral disc: (1) diffusion through the central portion of the end-plate from these marrow space cartilage contacts and (2) diffusion through the annulus fibrosus from the surrounding vessels. Our observations do not allow us to comment on the relative importance of these two routes in life; all we can say is that they are both potentially available.
Of 9 impermeable central portions of 75 end-plates tested, 7 were from segments with degenerated disc (Grades 2-3). Impermeability of both the central and lateral portions of 71 end-plates studied was seen in 8 end-plates. Of these 6 were from segments with degenerated discs.
Thus a statistically significant association was observed between impermeability of the end-plate and the presence of macroscopic disc degeneration of Grades 2 and 3. The pathological significance of this association is discussed, and it is suggested that the absence of marrow space contacts with the cartilage in certain end-plates results in impermeability of the end-plate, which in turn may interfere with disc nutrition and be a cause of disc degeneration.