CLBP in males was associated with reduced vertebral bone mineral measures acquired through lateral-projection DXA, independent of age, height and mass. There are a number of mechanisms which might explain this association, as reported recently , yet in this study potential CLNP-related predictor variables of psychological wellbeing, back muscle endurance and physical activity were not associated with bone health, suggesting that other factors may be implicated driving this relationship. These findings raise important questions regarding the mechanisms related to, and clinical impact of, this association and underline the potential clinical utility of lateral-projection DXA methods to assess vertebral bone parameters in some contexts. However, the results presented should be interpreted within the context of the small sample size in the study.
Although previous studies have examined the association between bone mineral measures and LBP, the relationships have been inconsistent, and are likely to be at least partly attributable to suboptimal densitometry methods and sites used. Other studies point to a negative association when bone mineral properties were measured in the lumbar spine [26–29]. For example, Bogdanffy et al , in a group of 15 patients (11 male) undergoing L4-S1 spinal fusion for CLBP and lumbar instability observed a significant mean decrease in lateral-projection aBMD at L3 of 10.1% and 11.9% at 3 and 6 months, respectively, after undergoing surgery compared to pre-surgical values. Notably, no significant decrease was observed for aBMD acquired using PA-projection DXA, consistent with our data.
In univariate models, vertebral DXA parameters were not associated with potential predictor variables including the DASS-21 subscales, back muscle endurance and vigorous physical activity (other than in females for some DXA variables). The absence of any associations is likely attributable to the distribution of the predictor variable scores being relatively equal between the CLBP and control groups or insufficient power to identify any differences. This finding suggests that other factors may be important in explaining the association between CLBP and bone health, such as nutrition, other aspects of physical activity (e.g. inactivity), posture, occupation, inflammatory markers and neuroendocrine factors. These data are somewhat contradictory to other reports where individuals with CLBP tend to have poorer psychological wellbeing , reduced back muscle endurance  and reduced vigorous physical activity  relative to those with no pain. The discrepancies may be accounted for by a range of reasons. First, there are differences in the measurement tools between this study and earlier research. Second, the cohort recruited for the JSHS may have predominantly adopted active coping strategies for pain and thus experienced less functional disability. Third, the levels of disability experienced among individuals in this cohort may have been insufficient to significantly influence psychological wellbeing and capacity to engage in vigorous physical activity. Fourth, the sample size was inadequate to detect small, but clinically relevant, associations between the potential predictor variables and the DXA parameters.
The multivariate models demonstrate that in males up to 55% of the variance in vertebral bone mineral parameters acquired using lateral-projection DXA could be accounted for by the combination of age, mass, height and the presence or absence of CLBP. The adjusted mean difference in aBMD and ap.vBMD between the male groups of 0.09 g/cm2 and 0.02 g/cm3, respectively represent 0.7 standard deviations below the raw male control group means, or a difference of 11.0% and 9.5%, respectively, comparable to earlier research . This difference appears to be clinically-relevant for this age group in light of the 1.2-2.0 fold increase in the odds of sustaining a vertebral fracture with each 1.0 standard deviation decrease in vertebral aBMD acquired through PA-projection DXA in older men [32, 33]. Further, the association between vertebral fracture risk and reduction in bone mineral apparent density (BMAD) acquired using a PA-scan - an apparent volumetric measure similar to ap.vBMD used in our study -is reported to be greater than PA-derived aBMD . Nonetheless, it is acknowledged that reliable vertebral fracture risk estimates have not yet been determined for lateral-projection DXA parameters and therefore the biological significance remains uncertain.
The association between vertebral bone mineral measures and CLBP were only observed for those variables measured using lateral-projection DXA. Evidence continues to emerge to substantiate the potential advantages of lateral-projection methods, such as enhanced diagnostic sensitivity for vertebral fracture  and superior predictive capacity for vertebral failure in ex situ models . The lateral approach may better identify reduced bone mineral parameters in the context of CLBP due to the selective inclusion of the metabolically-active trabecular bone. Moreover, age-related degenerative changes in the lumbar spine may obscure any association with CLBP when aBMD is measured using PA-projection methods.
No association between vertebral DXA parameters and CLBP was observed in females. There are a number of potential mechanisms which might explain this gender difference. Although females had lower disability scores, a shorter LBP history, fewer episodes of LBP in the last year, and CLBP causing less interference with recreational activities than males, these gender differences were not statistically-significant and, therefore, are unlikely to account entirely for the absence of an association among females. Hansson et al  reported that BMC measured at L3 was negatively associated with the lifetime duration of LBP (years since first onset), and not the severity, disability or duration associated with the current pain episode. Although a greater proportion of males had experienced LBP for ≥ 20 years (27.3% vs. 16.7% in females) in our study, this difference was not statistically significant. The gender difference in the association between CLBP and DXA variables could also be explained by gender-specific neuroendocrine factors, gender differences in the extent of intervertebral disc degeneration, a greater resilience to musculoskeletal pain among females and other osteoporosis risk factors which were not controlled for in this study, such as alcohol and calcium intake and inflammatory markers.
A particular strength of this study is use of both PA-and lateral-projection DXA modalities. Although Bogdanffy et al  also used a combination of PA-and lateral-projection DXA, their study was based only on repeated measures in a single group of 15 patients undergoing spinal fusion. To our knowledge, no other studies have used lateral-projection DXA in this context. While lateral DXA may have potential for useful clinical applications, the associations between lateral-projection DXA parameters and fracture risk remain largely unknown, and thus the clinical significance of our findings for vertebral fracture risk are uncertain. Further, inherent limitations exist with DXA, particularly the inability to measure true vertebral volumetric BMD - a parameter which may have demonstrated greater deficits in trabecular BMD between the groups - and indices of bone quality which are equally important as aBMD in mediating bone strength. The association between CLBP and bone quality is currently uncertain, yet this may be an important area for future research, particularly in the context of inflammation-driven back disorders. This study is also limited in the scope of measurement of potentially important correlates of bone health. Although vigorous physical activity was measured, the IPAQ question we used only related to activity undertaken within the previous seven days. More extensive assessments of volumes, frequencies and intensities of vigorous and sedentary activity may be warranted using accelerometry, given the association between CLBP and deficits in vigorous activity . Although several exclusion criteria were applied to this study, we were unable to account for the potentially confounding effects of other important correlates of aBMD including nutrition, inflammatory markers, neuroendocrine markers and other physical factors such as posture, intervertebral disc degeneration and occupation which may influence the association between the presence of CLBP and aBMD. Finally, this study is limited by the small sample size which may increase the risk of a type I error and therefore interpretation of the results presented should be considered within this context. Larger studies should now be undertaken to confirm the findings from this study.