Early-onset scoliosis (EOS) is a spinal deformity that develops before age 10 [1,2]. Regardless of etiology, progressive spinal curvature leads to chest wall rotation, reduced thoracic volume, and impaired pulmonary development. The primary goals of congenital EOS treatment are to halt curve progression, correct the deformity, preserve spinal growth, and maximize thoracic volume to support cardiopulmonary development and prevent long-term respiratory and cardiac complications [1–3]. Initial management typically involves bracing and/or corrective casting to delay or avoid surgery. However, in rapidly progressive curves, these methods are often insufficient, necessitating the use of surgical, growth-friendly corrective techniques [1,4–6].

Apical curvature deformity is a complex three-dimensional (3D) phenomenon characterized by spinal rotation, coronal deviation, and passive anterior column lengthening [7,8]. The optimal magnitude and approach for achieving the required convex–concave and anterior–posterior length adjustments to restore physiological spinal morphology remain unclear. In principle, 3D correction should involve concave lengthening, convex shortening, axial plane derotation, and medial translation of the apex toward the midline in the coronal plane [9–11].

Several surgical techniques have been developed to preserve spinal growth and thoracic development while minimizing complications. A central principle in EOS management is achieving control of the apex of the spinal curve, which is essential for deformity correction without compromising growth potential.

The SHILLA technique, an early posterior approach, aimed to control the apex by fusing the apical region while allowing the rods to glide proximally and distally. Although SHILLA preserved some spinal growth, it was associated with a high incidence of crankshafting. This occurred due to continued anterior apical growth despite posterior fusion, resulting in progressive rotational deformity.

To overcome these limitations, the active apex correction (APC) technique was developed as a modification that incorporates principles of guided growth and posterior tethering. Unlike SHILLA, APC uses a non-fusion strategy for apical correction, focusing on growth guidance through targeted modulation. APC entails the application of controlled compression or distraction, medial translation, and axial derotation to the convex side of the apical vertebrae. This method addresses the primary drivers of curve progression, including vertebral wedging, lateral shift, and axial rotation, and helps reverse unfavorable growth patterns without fusion. By tethering only the convex side, APC allows dynamic modulation of the apical vertebrae, increases the concave-to-convex vertebral height ratio, and gradually reduces vertebral wedging [12,13].

Anterior vertebral body tethering (VBT) is another well-established technique for apical control. VBT is a non-fusion procedure that corrects the spinal curve by tethering the vertebrae with screws connected by a flexible cord. This construct maintains curve correction and facilitates modulation of the tethered vertebrae. However, VBT is primarily used in growing patients with adolescent idiopathic scoliosis and has not been widely adopted for EOS or for nonidiopathic deformities [14].

The objective of this study was to assess whether APC enables modulation of the three apical vertebrae within the tethered segment during correction of congenital EOS.

This retrospective, single-center study evaluated 10 skeletally immature patients diagnosed with scoliosis or kyphoscoliosis. All patients demonstrated radiographic evidence of vertebral wedging at the apex of the curve. Each patient underwent APC as the index surgical procedure intended for deformity correction. Inclusion criteria were as follows: (1) onset before 10 years of age, consistent with EOS; (2) a major Cobb angle greater than 40°; (3) skeletal immaturity at the time of surgery; and (4) congenital scoliosis as the underlying diagnosis. Patients were excluded if the scoliosis was noncongenital, if APC was performed as revision surgery, if follow-up was less than 24 months, or if apex modulation data were unavailable. Demographic and perioperative characteristics of the study cohort are summarized in Table 1.

A total of 10 patients with congenital EOS underwent APC as their index surgical intervention. The mean age at surgery was 8.2±2.39 years. Demographic data and curve characteristics are summarized in Tables 1–3. Among this cohort, seven patients had a wedge vertebra, and three had a fully segmented hemivertebra. None of the patients demonstrated an intervertebral bar.

The spine plays a fundamental role in maintaining posture, enabling movement, and supporting cardiopulmonary development. In EOS, progressive deformity alters spinal morphology and function, warranting surgical intervention when conservative methods fail [17]. Scoliotic deformity is a complex 3D anomaly characterized by coronal deviation, vertebral rotation, and apical lordosis, influenced by both vertebral and intervertebral disc mechanics [7,8,10,18,19].

Traditional methods for apical control include fusion-based techniques such as SHILLA and convex growth arrest (CGA). CGA involves epiphyseal plate fusion on the convex side with the aim of permitting continued growth on the concave side [20–22]. However, results have been inconsistent. Ginsburg et al. [22] reported that CGA halted curve progression in only seven of 10 cases and did not promote spinal growth, and Tauchi et al. [20] noted that failed hemiepiphysiodesis can compromise subsequent corrective efforts. Although posterior instrumented fusion provides structural correction, it restricts longitudinal growth and limits further modulation potential [23].

Hybrid approaches that combine convex fusion with concave distraction have attempted to balance deformity correction with continued spinal growth [24,25]. Alanay et al. [24] reported improvements in coronal alignment and concave-side growth, although these benefits were accompanied by frequent surgeries and an 80% complication rate. Similarly, Demirkiran et al. [25] demonstrated effective correction and growth preservation, but complications occurred in 72% of patients, and repeated distraction procedures were required.

More recently, techniques have shifted toward nonfusion strategies that aim to correct the deformity while preserving spinal growth and function [4]. APC, introduced 16 years ago, was developed to achieve dynamic apical modulation through posterior convex-side tethering and controlled compression. This facilitates medial translation and derotation of the apex [26]. The underlying concept is that maintaining anterior and posterior growth helps prevent the crankshaft phenomenon. Studies by Newton et al. [14] and Louer et al. [27] have demonstrated asymmetric growth modulation and vertebral remodeling through tethering, and EOS imaging has become a valuable tool for monitoring these morphological changes.

Our study is the first to evaluate 2-year outcomes of APC in congenital EOS with a specific focus on vertebral modulation assessed through EOS imaging. We observed significant improvements in Cobb angle, AVT, and spinal lengths (T1–T12 and T1–L5). Concave and convex vertebral heights increased significantly at all assessed levels, consistent with previous reports indicating that APC supports growth-friendly deformity correction [26,28].

Notably, the concave-to-convex height ratio at A1 and A2 increased significantly, indicating greater concave-side growth and demonstrating true modulation of the tethered vertebrae. In contrast, the untethered vertebrae showed no change in this ratio, highlighting the localized effect of APC-mediated modulation. Although A3 did not reach statistical significance, the observed trend suggests a clinically meaningful response that may have been constrained by the small sample size and limited follow-up duration.

Our findings also support recent observations that the apical vertebra is not always the most rotated and that adjacent vertebrae may contribute more significantly to trunk rotation [18,29]. This highlights the importance of including periapical vertebrae in the corrective strategy, which is reflected in the modulation detected at A1 and A3. In our cohort of patients with congenital scoliosis, postoperative correction was achieved in all cases using the APC technique. The posterior-only tether provides asymmetric growth by allowing distraction on the concave side, which contributes to deformity correction and helps prevent the crankshaft phenomenon.

The complication rate in our series was low (20%) with one rod breakage and one screw pull-out, notably lower than other growth-friendly techniques. None of the patients developed the crankshaft phenomenon, a major drawback of SHILLA [26]. In comparison, SHILLA and CGA-based techniques have reported complication rates of up to 80%, including implant failure and the need for revision surgery [22,24,25].

This study is limited by its small sample size and relatively short follow-up period. The degree of the applied compression force was not quantified, leaving its specific contribution to vertebral modulation uncertain. The lack of statistical significance at A3 also warrants further investigation in a larger cohort. Long-term, multicenter studies are needed to confirm the durability and growth-modulating potential of APC. Potential iatrogenic effects on the intervertebral disc and adjacent spinal segments require extended follow-up and are acknowledged as a limitation of this study.

Congenital EOS remains a challenging condition, yet advances in growth-preserving surgical techniques offer new therapeutic opportunities. APC is a novel method that achieves deformity correction through apical modulation without the need for fusion. This study is the first to demonstrate vertebral modulation following posterior tethering with APC, supported by short-term improvements in spinal alignment, growth parameters, and a low complication rate. These findings position APC as a promising alternative to fusion-based approaches. Long-term studies are needed to assess the durability and wider applicability of these outcomes.