Response to “Vertebral fracture and intervertebral discs”

Field: Medicine (Health Sciences)

We thank Drs. Adams and Dolan for their interest in our work and the opportunity to further discuss the association between vertebral bone mineral density (BMD) and disc degeneration (DD) in the lumbar spine. Their comments arise from our discussion of study limitations, in which we acknowledged that "the age of our subjects was relatively young and a mean age of 50 years is probably too young to determine the full extent of the association studied." We went on to speculate that there may be a stronger association between greater vertebral BMD and more severe DD in older spines,1 which they question. This was, of course, only speculation, as the association in the elderly could not be addressed with our data and was based on a number of previous studies of elderly women that revealed an association between greater vertebral BMD and more severe lumbar DD. In one such study of 630 women older than 60 years (73.3 ± 6.9 years), higher dual-energy X-ray absorptiometry (DXA) lumbar spine BMD was associated with greater disc space narrowing and Kellgren-Lawrence DD scores.2 In another study of older women (>60 years, 72.8 ± 6.6 years), greater radiographic disc space narrowing was found to associate with both anterior-posterior and lateral DXA vertebral BMD measurements.3 An association between greater disc bulging and higher DXA lumbar vertebral BMD was also observed in women aged up to 86 years.4 Such associations, however, have been less studied in men, although a similar association between greater disc space narrowing and lumbar BMD acquired with DXA was observed in a cohort of 250 men (65.4 ± 9.0 years).5 We were not suggesting that the relation between the structure and function of the vertebrae and discs is a simple one. Although our study was not designed to investigate underlying mechanisms of the BMD-DD association we identified, particularly in older spines, it is an important and interesting topic that Adams and Dolan highlight. Adams and Dolan discuss a possible mechanism that could lead to focal bone loss–related specifically to severe disc narrowing, seen in late-stage DD, which may be particularly relevant in the elderly. In pursuing such knowledge, distinguishing different definitions of lumbar vertebral BMD is of great importance. Whereas BMD measured using DXA is area-adjusted, BMD measured with QCT or micro–computed tomography (µCT) is volume-adjusted. The measurements acquired in the study by Adams and colleagues,6 are substantially different from those used in our study. They used measures of regional bone architecture parameters, such as bone volume/total volume (BV/TV) and trabecular thickness (Tb.Th), to examine focal bone variations, whereas we measured global BMD of the whole vertebra and its vertebral body.1 Thus, the two studies are not directly comparable. Causal inferences are uncertain and the chicken and egg dilemma remains in cross-sectional studies, including those of the BMD-DD relation. Adams' and Dolan's view that "in middle age, dense vertebral bone may promote disc degeneration" is one possible explanation for the association observed between vertebral body BMD and DD, but there are others. We did not suggest and do not agree with the view that vertebral BMD is likely to interfere with metabolite transport through the vertebral endplate, because we did not observe an association between endplate BMD and DD in a previous study.7 On the other hand, we agree with Adams and colleagues6 that severe disc narrowing may shift more mechanical loading posteriorly to the facet joints and thus, stress-shield the anterior vertebral body. As for the measurement of disc degeneration, disc space narrowing was not selected because it is a nonspecific DD indicator and cannot reflect early DD. Furthermore, disc height measurements are prone to bias by osteophytes and endplate sclerosis, which likely are confounders of the BMD-DD association.1 Therefore, we used discography, which can display "successive stages in disc degeneration,"8 as a superior measure to disc space narrowing. As we have discussed in detail,1 although it is possible that DD is influenced by altered mechanical loading due to BMD changes, it is also possible that elevated BMD is an adaption of the vertebral bone to the changing mechanical environment related to DD. Therefore, we agree with Adams' and Dolan's view that "interactions between vertebrae and their adjacent discs cannot be summarized simply."