Conventional treatment strategies for adolescent idiopathic scoliosis (AIS) have typically emphasized the correction of coronal deformity despite the higher prevalence of sagittal imbalance in these patients compared with those without spinal deformity [1]. The omission of sagittal malalignment in the criteria for King’s classification of AIS corroborates this lack of emphasis on the three-dimensional nature of the deformity in AIS, characterized by lateral deviation, rotation, and relative lordosis. With a progressive understanding of the multiplanar deformity in AIS, Lenke’s classification incorporated thoracic sagittal alignment as well for grading AIS [2]. Lately, global sagittal alignment has been recognized as a major determinant of quality of life, more so than coronal deformity [3,4]; consequently, greater attention is being given to correcting sagittal malalignment. Thus, both coronal and sagittal plane deformities should be addressed in the surgical correction of spinal deformity in AIS [5].

The exact prevalence of thoracic hypokyphosis in patients with AIS with thoracic curves is unknown, presumably because of the various angle cut-offs adopted; however, anecdotal evidence suggests that about 14% of patients with AIS have thoracic hypokyphosis [6]. Schlösser et al. [7] analyzed AIS sagittal alignment patterns in 192 patients and found that 49% of the patients with mild thoracic scoliosis (Cobb angle: 10°–20°) exhibited sagittal malalignment, most commonly presenting as thoracic hypokyphosis. The anatomical basis of this thoracic hypokyphosis in AIS has been attributed to rotation of the apical vertebra and a relatively longer anterior column, particularly at the apical and para-apical levels [8]. The anteroposterior length discrepancy probably resulted from anterior column lengthening, mainly attributable to increased anterior intervertebral disc height, with partial contribution from the vertebral bodies, possibly an adaptive mechanism to the altered biomechanical loading on the spine, as well as posterior column shortening caused by interlaminar space compression rather than laminar growth disturbance [8].

Loss of thoracic kyphosis (TK) has significant clinical implications as it is associated with reduced thoracic volume and decreased pulmonary function [9–13]. Newton et al. [11] studied 631 patients with AIS and found that 29% of those with hypokyphosis had moderate or severe pulmonary impairment. Specifically, thoracic hypokyphosis may lead to reductions in forced vital capacity and forced expiratory volume in 1 second; the reduced thoracic sagittal diameter restricts lung expansion, potentially resulting in significant respiratory compromise and cardiopulmonary failure [11,14,15].

Thoracic hypokyphosis has also been implicated in compensatory increase of cervical kyphosis [16,17] and loss of lumbar lordosis (LL) [3,18–21], lower back pain due to spinopelvic incongruence, and poorer quality of life [3]. These findings suggest that the restoration of normal TK is an important goal in surgery for AIS. However, the correction of axial rotational deformity in AIS can have a paradoxical effect of further reducing TK, exaggerating the hypokyphosis [22]. The present study aimed to identify the predictive factors for a postoperative increase in TK by at least 15° [23] and evaluate the effectiveness of the surgical techniques employed at Okayama Rosai Hospital for correcting thoracic hypokyphosis. Additionally, we aimed to assess the associated reciprocal changes in LL and analyze the correlations between coronal and sagittal plane deformities in AIS.

In contrast to the lumbar spine, correction of hypokyphosis of the thoracic spine is more resistant to surgical correction [26–29]. Increasing the TK angle is challenging because of the underlying anatomy of the thoracic spine in AIS; the elongated anterior column at the apical and para-apical levels makes kyphosis correction difficult. It may worsen with derotation of the vertebra, compounded by the fact that the ribs and adjoining sternum anteriorly form a mechanically stable structure around the thoracic spine [8]. The existing literature has identified several patient-specific factors that may affect the ability to achieve good TK correction, which include curve type, TK angle, deformity flexibility, skeletal maturity, and the apex level of the main thoracic curve [24,30,31]. In terms of surgical factors, high instrumentation density, osteotomies, extensive soft tissue release, rod contouring angle, rod diameter, material selection, deformity correction maneuvers, fusion levels, apex relocation, and the application of sublaminar banding have been identified as factors affecting the amount of TK correction [24,25,27,32–34]. Interestingly, a multicenter, prospective study by Monazzam et al. [29] including 280 patients with Lenke types 1–4 AIS showed that the operating surgeon was the only significant predictor of restoration of normal kyphosis, highlighting the importance of experience and intraoperative techniques; in contrast, factors such as preoperative kyphosis, rod material, implant density, and Ponte osteotomies were not significant. Solla et al. [24] reported that the concave rod contouring angle and preoperative TK angle were essential factors in achieving good TK correction, whereas Cidambi et al. [26] reported that overcontouring the concave rod by 20° resulted in high degrees of TK correction.

At our institution, meticulous soft tissue release and multiple osteotomies were performed to achieve adequate thoracic spine flexibility. Simultaneously, we opted for high screw density, overbent, large-diameter cobalt-chrome rods, sublaminar tapings, and a translational reduction maneuver to achieve satisfactory deformity correction. Using the aforementioned surgical techniques helped achieve excellent coronal correction, with a mean improvement of 74.2% and a significant increase in the mean TK angle (13.8°; from 20.8° preoperatively to 34.6° postoperatively). Previous studies have reported that attaining a postoperative TK of at least 23°–26° may reduce the risk of sagittal plane decompensation and cervical malalignment following thoracic fusion for AIS [17,35], verifying the clinical significance of our results. The amount of kyphosis restoration reported in the literature ranged from only −1.4° to 5° [27,30,36]; however, with the use of patient-specific prebent rods, Solla et al. [24] were able to achieve a greater increase in TK (approximately 14°). Similarly, Presenti et al. [25] obtained kyphotic correction of approximately 13° with sublaminar taping. Sagittal profile correction is more difficult to achieve than coronal plane deformity due to the loss of rod curvature after implantation, which reportedly varies from 16° to 22° regardless of the rod material due to the substantial resistive forces of the spine [26–29].

Our results demonstrate that the group with greater (≥15°) postoperative TK gain had a lower preoperative TK angle (17.7°), consistent with previous reports [30,37–39], which increased to a mean postoperative angle of 39.0°. In contrast, the subgroup with a kyphosis gain of <15° exhibited a higher preoperative mean TK angle of 23.6° and a final mean postoperative angle of 30.6°. Additionally, the greater corrective potential observed in patients with a flatter preoperative sagittal profile was positively correlated with LL, as evidenced by the corresponding reciprocal increase in LL following TK correction. Although patients who exhibited ≥15° TK gain showed a significantly greater increase in LL than those with ≤15° gain, logistic regression analysis indicated that ΔLL was not an independent predictor for achieving >15° TK gain. This apparent discrepancy may be due to the considerable interpatient variability in ΔLL, as reflected by the significant standard deviation value. Therefore, while there was a general trend toward concomitant thoracic and lumbar sagittal correction, LL gain alone does not reliably predict substantial TK improvement in patients with AIS.

Another notable finding of our study was that a smaller preoperative PT was significantly associated with greater postoperative TK gain. This relationship may be explained by the reciprocal compensatory nature of sagittal alignment, in which a lower PT reflects anterior pelvic rotation secondary to increased LL. This hyperlordotic lumbar configuration likely represents a compensatory adaptation to preoperative thoracic hypokyphosis, aimed at maintaining overall sagittal balance.

No significant correlation was observed between the flexibility of the coronal curve on bending or traction radiographs and the degree of correction of TK. This suggests that preoperative coronal flexibility does not necessarily predict postoperative sagittal plane correction. Similarly, there was no correlation between the magnitude of the preoperative Cobb angle and either preoperative or postoperative TK. As the Risser’s grade was comparable between the two groups, skeletal maturity is unlikely to have influenced the extent of TK correction. However, the influence of screw density on deformity correction in patients with AIS remains controversial. In our study, higher screw density was associated with improved sagittal profile correction. In contrast, several authors have reported that higher screw density does not ensure superior coronal or sagittal radiographic outcomes, and is rather associated with increased operative time, blood loss, and overall cost [40,41].

Previous studies evaluating the impact of rod material on corrective outcomes in AIS have consistently shown no significant influence on either the main thoracic Cobb angle or TK restoration. A multicenter randomized controlled trial by Sakai et al. [32] demonstrated that coronal and sagittal corrections were comparable between titanium and cobalt-chromium rods. Similarly, Sia et al. [27] reported no significant difference in the extent of intraoperative rod deformation between the two materials during corrective surgery. Boissière et al. [42] also found no significant differences in radiographic outcomes between titanium and cobalt-chromium rods, suggesting that rod material may not substantially affect surgical results. These findings indicate minimal effect of material-dependent differences and rod composition in comparison with other factors, which likely play a more decisive role in determining postoperative alignment.

All patients in our cohort were instrumented with 6.0-mm diameter rods to optimize sagittal plane correction. Previous studies have reported that larger-diameter rods demonstrate superior corrective potential in the sagittal profile, likely due to their greater rigidity and resistance to spinal deformity forces during TK restoration [26,28,29,43–45]. In addition, screw density was identified as a significant factor influencing thoracic sagittal correction in our series. This finding is in line with several reports in the literature, which have suggested that higher pedicle screw density may enhance chances of correction of sagittal alignment, particularly in patients with preoperative thoracic hypokyphosis [39,46,47].

This study has several limitations. First, all procedures were performed by a single surgeon at a single center, which may limit the generalizability of the findings, as surgical techniques vary considerably among surgeons and institutions. Second, sagittal parameters of the cervical spine and their relationship to sagittal changes in the thoracic spine were not included in the present analysis. Further studies incorporating cervical alignment are required for a more comprehensive understanding of global sagittal balance. Third, variations in the sagittal profile among patients, particularly the level of the kyphotic apex, may influence the degree of correction achieved. Additionally, the preinsertion rod contour relative to the preoperative TK shape, and the relationship between the degree of rod contouring before implantation and subsequent rod flattening after insertion, were not evaluated. Assessing these relationships could help determine the optimal amount of precontouring required to achieve the intended postoperative TK correction and ensure symmetrical rod curvature upon correction completion.

Our findings suggest that preoperative sagittal morphology influences the degree of postoperative TK correction achievable following deformity surgery for AIS. Patients with hypokyphotic thoracic curves and lower PT demonstrated greater potential for postoperative restoration of TK (≥15°), which is likely correlated with the enhanced corrective capacity in patients with a flatter sagittal profile. A higher screw density was also associated with improved curve correction, indicating the biomechanical advantage of increased fixation points for achieving sagittal realignment. In contrast, those with higher preoperative TK and greater pelvic retroversion showed limited additional correction, possibly due to preexisting optimized sagittal balance or decreased thoracic spine flexibility. Therefore, preoperative TK angle, PT, and implant density should be considered to optimize sagittal plane deformity correction in AIS.