In adults with spinal deformity, sagittal balance has been reported to be related closely to HRQOL, and sagittal spinopelvic parameters are known to be important factors for treatment decisions [6]. However, spinal deformity in AS is a pathological condition that is different from adult spinal deformity, and the characteristics of sagittal balance and its relationship with HRQOL are largely unknown in patients with AS. In recent years, the importance of corrective treatment for AS spinal deformity has been increasingly acknowledged, and many studies have reported associations between sagittal spinopelvic parameters and HRQOL for the objective evaluation of surgical correction of AS spinal deformity [13]. However, studies on sagittal spinopelvic parameters and HRQOL in nonsurgical treatment for AS spinal deformity are scarce [3].
In recent years, interracial differences have been reported for the association between sagittal alignment of the spinal column and HRQOL in adult spinal deformity [14]. These differences suggest that the pathological evaluation of sagittal balance of the spinal column should be performed depending on the ethnicities of patients, not only in adults with vertebral deformity but also in those with AS. In particular, AS prevalence rates vary greatly among ethnicities, with rates ranging from 0.52% in the United States [15] and 0.19 to 0.54% in Taiwan [16] to 0.0065% in Japan [17]. This disparity is another justification for separate analyses by ethnicity.
In the sagittal balance comparison of AS and DLKS in this study, each gender ratio was different. This is because AS often occurs in males [17], and DLKS often occurs in females [18], as the respective disease characteristics. Although male AS patients show radiological progression, including the development of syndesmophytes [19], the effects of gender difference on sagittal balance have not been clarified. Furthermore, in healthy adults, it has been reported that females in their 70s have larger PTs and PIs than males [20], and spinopelvic parameters has been reported to be similar for males and females in their 30s [21]. However, there is no large-scale study on the impact of gender differences on sagittal balance, including the adult spinal deformity classification by Schwab et al. [9], and this is unclear in DLKS as well as in AS.
Sagittal balance of the spinal column and pelvic morphology in patients with AS are reported to be different from those in healthy individuals [1, 22]. Previous studies have compared the characteristics of sagittal alignment in patients with AS and healthy individuals [3, 4, 23], but to our knowledge, no comparisons have been made with DLKS. Given a comparison was being made with patients with DLKS, we were able to analyze the compensatory function of the sagittal alignment in patients with AS.
In this study, an AS population with relatively large thoracic kyphosis was compared with a DLKS population. The results showed that, despite a similar SVA in the two populations, the former had significantly greater thoracic kyphosis and significantly smaller posterior PT with no significant difference in LL. In addition, although SVA was equivalent, TPA [12], which combines SVA and PT information, was significantly smaller in patients with AS. This finding demonstrated a sagittal alignment characteristic of AS, suggesting that the effect of compensation by posterior PT, which is usually found in patients with DLKS, is small. One study presumed that pelvic retroversion restrictions (i.e., small PT angles) in AS patients are sagittal malalignment corrections by flexion of the knees and/or plantar flexion of the ankles [22] . However, since this mechanism cannot be substantiated by this study, further investigations involving the lower limbs are necessary.
Of the HRQOL measures, ODI, SRS-22 total, SRS-22 appearance, JOABPEQ social life dysfunction, and JOABPEQ psychological disorder scores showed statistical correlations with sagittal spinopelvic parameters in patients with AS. This result appears to indicate that compromised sagittal balance of the spinal column decreases the HRQOL of these patients.
A study on Korean patients with AS and near-normal and early-stage kyphosis reported that SVA, SS, and LL showed an important correlation with HRQOL for patients with AS [3]. A study on associations between HRQOL and SVA, TPA, spinosacral angle, and spinopelvic angle (SPA) in patients with AS and with a relatively large kyphosis has shown that SPA was significantly correlated with ODI [24]. In our study, the questionnaires most relevant for AS evaluation were the SRS-22 and JOABPEQ. Furthermore, the HRQOL was significantly correlated with SVA, SS, and GK. AS progression has been reported to be characterized by thoracic kyphosis and ankylosis [6]. In our participants who were likely to have relatively advanced thoracic kyphosis and ankylosis, GK was newly identified as an important sagittal spinopelvic parameter for AS evaluation in addition to SVA and SS.
Our study revealed the presence of a sagittal balance mechanism unique to patients with AS. In addition, thoracic kyphosis associated with AS progression was shown to be closely related to lower back pain, and its degree was closely related to SRS-22 and JOABPEQ scores. Furthermore, we identified SVA, SS, and GK as important sagittal spinopelvic parameters related to HRQOL.
This study has some limitations. First, the effects of coronary imbalance were not considered although patients with kyphoscoliosis were included as the control group. Second, because patients with kyphosis without scoliosis are scarce, the AS and DLKS groups were not precisely matched in age. Third, the effects of inflammation itself on HRQOL cannot be ruled out. Fourth, it was difficult to discuss possible effects of medical treatment and deformity correction surgery intervention because this was a cross-sectional study.