This study assessed the effects of brace treatment on BMC in adult women who were diagnosed with AIS and braced in early adolescence. Our main finding was that after taking into account nutrition and physical activity, BMC was lower in the lower extremities in women with AIS who had been braced compared with healthy controls. However, no correlation was observed between BMC and reported brace wear duration, suggesting that other factors may be responsible for this lower BMC. It is suggested that one of these factors may involve weight-bearing physical activity. This study was novel insofar as potential confounding effects of physical activity and nutritional intake were taken into account, and BMC and BMD were evaluated at the upper and lower extremities, as well as the commonly assessed lumbar spine and femoral neck sites.

Femoral neck BMC was lower in the AIS-B compared to both, the AIS and the control group. This is in agreement with Courtois et al. [29] who examined the effects of bracing in a sample similar in age to our participants (age 30.5 ± 6 years), and reported that AIS-braced women had lower spinal BMD and consistently lower BMD at femoral sites than healthy women. No comparison was made to women with AIS who had not undergone brace treatment in the study reported by Courtois et al. [29]. The current study supports and extends Courtois et al’s [29]. findings by accounting for the plausible roles of dietary intake [18,46-49] and physical activity levels [11-14,37-45], which are known to effect bone accrual during adolescence.

Snyder et al. investigated the effects of bracing on BMD in adolescent girls, and concluded that brace treatment does not affect BMD at the spine and hip [25,26]. In the current study, group differences in BMD did not reach statistical significance except for the femur neck axis, which was lower in the AIS-braced groups. It should be noted that Snyder et al. [25] used a follow-up period of only one year and participants were still within their critical growth period. Additionally, while the overall rate of bone mineral accrual for the braced girls was not different than that reported for healthy adolescent girls of another study, the annual BMD increase at the femoral neck was lower in the braced girls [25]. Snyder et al. did not report rate of bone mineral accrual but their report of low BMD annual increase is in line with our findings of lower BMC and BMD in the femoral neck in the AIS-braced group. A similar pattern was observed in the femur shaft, Ward’s triangle and legs, although the differences between groups did not reach statistical significance.

In contrast to our hypothesis, we did not find any difference in the lumbar spine BMC or BMD between the three groups. While these findings are in agreement with previous studies [25,26], Courtois et al., observed lower BMD at the lumbar spine (L2-L4) in their AIS-braced group. The authors suggested that the low BMD was associated with the severity of the curvature of the spine [29]. In the presence of scoliosis, spinal BMD values, as determined by DXA in the traditional posterior-anterior direction, should be interpreted with caution because when the lateral curvature is accompanied by a rotation in the spine, as is the case in AIS, the curvature can affect the DXA results. DXA projects the three-dimensional bone structure into a two-dimensional image. Therefore, the measured BMD in the spine is likely to be negatively affected by any deformity or axial rotation of the vertebrae [51]. Girardi et al. demonstrated that the error can be as high as 20% [52]. Snyder et al. scanned six human vertebras in the sagittal plane and concluded that at axial rotations beyond 25 degrees, the pedicles came into view of the scan, influencing the bone parameters, and resulting in large errors in BMD and BMC values [25]. The differences between the frontal and sagittal plane spinal BMD ranged from 10 to 60% [25]. Cheng et al. [51] examined the effect of axial rotation of lumbar vertebrae on BMD and using DXA in the anteroposterior plane, with vertebral axial rotation in increments of 7.5 degrees, up to a maximum of 45 degrees [51]. Degree of rotation was negatively correlated with BMD, but not BMC. BMD decreased approximately 19% when the vertebrae were rotated by 45 degrees [51]. Those findings suggest that measurements of lumbar spine BMC are not affected by axial rotation, while BMD values may be underestimated. This suggestion is supported by findings that bone mineral measurements obtained from traditional posterior-anterior DXA were poorer predictors of vertebral ultimate load, compared with measurements obtained with lateral-projection DXA or micro-CT [53] Therefore, the lower spine BMD in the braced women, reported by Courtois et al., should be interpreted with caution.

Brace wear varied considerably among participants in terms of total duration, as well as the number of hours each day. For example, while most participants wore the brace for less than 3.5 years, one participant reported wearing the brace for 8 years. This may be regarded as a limitation to the study. However, it should be noted that despite the wide range, reported brace wear duration did not correlate with femur neck BMC. This lack of correlation was apparent even when excluding the participant with extended brace wear duration (Figure 2). Thus, it is suggested that other factors may be responsible for the lower femoral BMC observed in the AIS-B group. These factors may involve nutritional intake and type of physical activity.

Daily total energy intake, calcium intake and dietary vitamin D intake did not differ between groups nor did supplemental calcium and vitamin D intake. Moreover, no correlation was observed between calcium intake and BMC. This may be partly explained by the findings that most participants in this study reported sufficient calcium intakes and mean calcium intake in all groups was above the recommended daily intake. Plasma levels of 1,25(OH) vitamin D were not assessed. It is possible that plasma concentrations of vitamin D, rather than nutritional intake may have related to current bone mineral status. There were also no statistically significant group differences in levels of current and past physical activity although the AIS groups’ past and present physical activity tended to be lower, especially in the AIS-braced group. The questionnaires used in the present study, while reflecting general physical activity, unfortunately do not provide information specifically on weight-bearing high-impact activities (e.g., jumping, ball games). Therefore, we speculate that lower levels of weight-bearing high-impact activity specifically, rather than overall physical activity could partially explain the lower BMC in the lower-limbs of the AIS-B group, as recently suggested by others [54]. Their low BMC at the lower limbs and not at the upper limbs suggests that site-specific factors may be acting on the bone. The set-point for the effect of mechanical stress on bone, as defined in the Mechanostat theory [55], may not be constant and may vary from site to site [56,57], and between different activities [58]. Weight-bearing, high-impact physical activity is beneficial to bone accretion, especially at weight-bearing sites of the skeleton [37-45]. For example, Fehling et al. [59] demonstrated higher BMD in the legs and pelvis of gymnasts and volleyball collegiate female athletes compared with swimmers and non-athletes. Furthermore, following a 12 week jumping intervention program in children and adolescents, Johanssen et al. [60] reported a greater increase in leg BMC in jumpers vs. non-jumpers. We did not observe a significant difference in reported physical activity between groups. However, it is possible that brace wear does not adversely affect an individual’s ability to perform daily physical activity in general, but rather, it may hinder their ability to perform high-impact weight-bearing physical activity. Thus, we speculate that lower limbs’ bone accrual would be specifically affected. Indeed, Green et al. recently concluded that, although physicians encourage girls with AIS, with and without a brace, to participate in physical activity, the literature reporting such physical activity among girls treated with bracing is essentially anecdotal [61].

It should be noted that participants did not report the inclusion of specific scoliosis-related exercises in their treatment during brace wearing. Such physiotherapeutic scoliosis-specific exercises (PSSE) have recently been advocated for adolescents with idiopathic scoliosis [62]. The aim of these exercises is to reduce the lateral curvature of the spine and prevent curvature progression, but it does not appear that these exercises are aimed at preventing bone mineral loss or enhancement of bone mineral accrual. Thus, it is suggested that treatment for AIS, and especially brace treatment, should include PSSE, as well as weight-bearing physical activity that may prevent bone mineral loss and promote bone mineral accrual [54].

There are several limitations to this study. Since this is a cross sectional study we are unable to make causal inferences with confidence. Thus, we cannot rule out the possibility that the AIS-B group had low BMC in the lower limbs before diagnosis and brace treatment. However, the fact that no group differences were observed in BMC in the upper limbs suggests that the lower BMC in the lower-limbs may be related to the brace wearing. Additionally, there are important potentially confounding factors, such as age of menarche, endocrine disorders, extreme diets, history of osteoporosis, on which information was collected retrospectively. While these data were taken into account in participant inclusion or in the comparison between groups, a prospective study would be needed to examine the influence of these factors on the effects of bracing. The relatively small sample size and low statistical power may have affected the strength and associations of our findings and the probability of finding a statistically significant difference between groups. Nevertheless, despite the relatively small sample size, group differences in BMC were observed in the femoral neck axis, with a similar pattern in other skeletal sites. This overall pattern suggests that brace wear may affect bone accrual during adolescence indirectly, possibly through its effect on the pattern of physical activity. Finally, past dietary intake was not assessed in the current study. While such data could have been insightful, it should be noted that recall of food intake over a long time period is very difficult.