Among the dose–response relationships of the three SMs (PT, PI-LL, and SVA) with BP-specific QOL, PI-LL showed a distinctive nonlinear relationship. Additionally, only PI-LL-based SRS-Schwab classification category was associated with a worse BP-specific QOL. These findings suggest that PI-LL might be the most sensitive modifier determining BP-specific QOL among the three SMs.
Our findings partially agree with those of previous studies showing the correlations between the SMs of the SRS-Schwab classification with both BP-specific and general health-related QOL measurements [3, 4, 7]. However, although the relationships between the SMs and BP-specific QOL are not new concepts, we believe that our findings fill a gap in the literature. First, we were able to validate independent associations between the three SMs as continuous predictors and BP-specific QOL after adjustments for likely confounders. Second, this study revealed the clinical impact of the high values of three SMs on RDQ scores: participants with more than 95th percentile values of PT, PI-LL, or SVA have RDQ scores of ≥ 3 points on average, which corresponds to the minimally clinically important difference in the RDQ score . In other words, the three SMs can be used to screen 5% of community-dwelling elderly individuals to identify people with disabilities affected by BP. Third, potential superiority of the PI-LL for evaluating BP-specific QOL impairment reinforces the recent evidence that PI is the only constant parameter providing key information about adult spino-pelvic degeneration [6, 16]. For example, the global alignment and proportion score based on PI is a tool for predicting the likelihood of mechanical complications after corrective surgery for patients with spinal deformity .
Interestingly, the finding that RDQ scores increased when PI-LL exceeded 10° may suggest that BP-specific QOL worsens rapidly when the compensatory mechanism against malalignment exceeds a critical value. This finding is the first empiric demonstration of the concept of the ‘cone of economy’ : it is maintained within a particular range in sagittal alignment (e.g. within 10° in PI-LL); however, when it fails (e.g. > 10° in PI-LL), patient symptoms and health-related outcomes worsen rapidly. Collectively, our findings may suggest the need to revisit the traditional classifications and strategies for spinal surgeries such as instrumented fusion surgery based on a more reliable classification.
This study has several strengths. First, this is the first survey to focus on the shapes of the continuous dose–response relationships (i.e. not only correlation) between the three SMs and BP-specific QOL independent of the important confounders. Second, our data were thoroughly retrieved from a large population in the community. Recent studies have shown that sagittal alignment is widely involved not only in spinal deformity but also in many spinal disorders such as degenerative disc disease, osteoporosis, and facet joint arthritis [17,18,19]. An evaluation of sagittal alignment is not limited to corrective surgery for patients with spinal deformity; it should also be considered when treating localised spinal disorders . During our study, alignment and QOL data were evaluated at the individual level via whole-spine radiographs and a questionnaire. Therefore, our findings reflect the actual general community setting and are applicable to other populations with any type of lumbar disease.
Despite these strengths, our study has some limitations. First, unmeasured factors leading to sagittal malalignment might have affected the results. Inter-vertebral disc degeneration, osteoporotic vertebral fractures, and trunk muscle mass are clinically important and might affect the relationship between sagittal alignment and BP-specific QOL . Second, causality could not be assessed because of the cross-sectional nature of our study. However, the impact of altered SPA on BP-specific QOL is biologically plausible, and we believe that reverse causation is unlikely to occur given the traditional mechanisms of health-related QOL, in which biological changes in alignment and symptoms such as BP lead to a decline in function and perceived well-being . Decreased LL triggers spinal deformity and causes sagittal imbalance, as reflected by an increased PI-LL value and SVA. Sagittal plane imbalance shifts the body towards the periphery of the ‘cone of economy’. Consequently, several compensatory mechanisms are deployed to help re-centre the body over the pelvis and to maintain an upright posture. When these compensatory mechanisms fail, efficiency performing daily living activities and QOL decline, and disruption of sagittal alignment leads to further decrease in QOL. Third, because the setting of this study was rural, the results may not be applicable to people living in urban areas. However, sagittal alignment parameters in this study were comparable to those of other Japanese cohort studies (Supplementary file 3) [23,24,25]. Therefore, our participants may represent the general Japanese population, and the findings may be reasonably applicable to all. Fourth, cervical spine alignment and lower limb joint alignment were not evaluated. Patients with deformity experienced changes in their alignment from the cervical spine to the ankle and used compensatory mechanisms to correct the imbalance.