The concept of bracing as a holding strategy in large AIS curves for patients with substantial growth potential remaining has not been well-documented in the literature. This study addresses this gap by retrospectively analyzing a cohort of AIS patients with severe curves treated with night-time bracing. We found curve progression in both the main and secondary curves, and that curves became less flexible during bracing. However, in the vast majority of patients (79%), the Lenke curve type did not progress during bracing. Notably, in those who did progress, the majority progressed in the proximal thoracic which was expected, since bracing of the proximal thoracic curve is challenging [11]. Also, this region is of less concern since fusion of the proximal curve is not likely to significantly affect the patient’s quality of life. Looking at the lumbar curve, six patients (7%) progressed from a non-structural to a structural curve, which would typically mean an indication for fusion of the lumbar curve. Fusion of the lumbar curve can result in early lumbar degenerative changes and decreased patient satisfaction [12]. In our study, age at brace initiation was identified as a significant factor associated with progression, with younger patients at a slightly higher risk (p = 0.01). However, there were no significant differences in terms of curve size, curve type, or curve flexibility at the start of brace treatment between the progression and non-progression groups. Our study does not assess the optimal time for surgical intervention. Historical data have suggested that crankshaft phenomenon can be avoided by waiting for closure of the triradiate cartilage [13, 14], while distal adding-on seems to occur significantly more frequent in Risser grade 2 or less at the time of surgery [6].

This is the first study to report on changes in flexibility during bracing, and as such, there are no comparable data in the literature. Vertebral body tethering may provide an alternative to bracing for treating skeletally immature scoliosis patient, either as a stand-alone procedure or as a hybrid (lumbar tethering and thoracic fusion) [15,16,17]. The available data suggest that the lumbar unfused curve also corrects in most cases [16, 18]. While these techniques have shown good results in carefully selected patients, they require a substantial amount of growth potential and are not suitable for patients who have completed the growth spurt [19]. Bracing could be a better alternative in these patients. To our knowledge, no study has examined curve flexibility before and after tethering, and a complication rate of more than 20% and low efficacy in moderate skeletal immaturity should be taken into consideration [20]. Physiotherapeutic scoliosis-specific exercises in combination with bracing have gained popularity in some centers, but whether this can limit curve progression and maintain flexibility in severe curves is unknown [21].

In the group of patients who did not progress with regard to Lenke type, a subset (38%) exhibited changes in the LTV, which moved distally by one or two levels. The LTV has gained increased focus as a suitable selection for the lowest instrumented vertebra in selective thoracic fusion [22,23,24]. As such, the distalization of the LTV has the potential implication of an added final fusion by one or two levels. In the lower lumbar area, this can have substantial impact on the surgical outcome [25, 26], while the available data do not show a deleterious effect of an added fusion level in the lower thoracic/upper lumbar area [27]. However, the risk of adding-on is increased from 12% to 19% (Risser stages 0–5) and 13% to 43% in patients with open versus closed triradiate cartilage [28]. These considerations may favor bracing as a holding strategy until relative skeletal maturity. While this study provides insights into the potential benefits of night-time bracing in skeletally immature AIS patients with surgical-range curves, several limitations should be considered. The main limitation is the lack of a control group. We cannot address the fundamental question of whether the brace treatment changed the natural course of the severe deformity. Also, this study focused on flexibility changes during bracing, not on efficacy of bracing in terms of preventing curve progression. Patients were only included if they had undergone surgical treatment (including a second set of flexibility radiographs), while patients with large curves that decided not to undergo surgical treatment were not included. Also, the indications for bracing were not standardized, and maturity assessment was based on a variety of factors. This may influence the external validity, although we consider our approach to reflect real-life clinical practice.