Current issues in the treatment of adolescent idiopathic scoliosis: a comprehensive narrative review

Current issues in brace treatment

Bracing is generally recommended for skeletally immature patients with progressing curves exhibiting a Cobb angle of 25°–40° [1]. The purpose of bracing in AIS is to prevent curve progression rather than correct the existing deformity. Historically, there has been some skepticism regarding the efficacy of bracing in AIS patients; however, following the publication of a randomized controlled trial (RCT) by Weinstein et al. [5], the effectiveness of bracing in preventing curve progression in skeletally immature patients is now widely accepted. A recent meta-analysis by Zhang and Li [6], which analyzed seven RCTs involving 791 patients, also highlighted that bracing is an efficient and safe method for AIS patients. Furthermore, a study by Ikwuezunma et al. [7] reported superior cost-effectiveness of bracing over observation in cost-utility analysis.

The literature reveals significant discrepancies in recommendations for brace-wearing time. While some authors advocate for full-time wear (more than 23 hours per day), others suggest part-time wear (more than 18 hours per day) or night-time bracing (8 hours) using overcorrection braces such as the Charleston brace [8]. The Bracing in Adolescent Idiopathic Scoliosis Trial recommended more than 13 hours of daily wear for high-risk patients. Recent meta-analyses indicate insufficient evidence to support part-time wear [9,10]. However, considering the potential stress of full-time wear on sensitive adolescents, there is a growing preference for night-time bracing, which has shown favorable clinical outcomes in recent studies [9,11,12].

Although brace treatment is maintained until skeletal maturity, there is no definitive consensus on the appropriate weaning time. Curve progression after brace weaning can be disappointing for those who have endured long and stressful brace treatment, prompting researchers to investigate the optimal timing for brace weaning. Traditional suggestions include Risser stage 4 for girls and stage 5 for boys, 2 years post-menarche, and a cessation of height growth for 6 months [13]. A recent suggestion from Cheung and Cheung [14] posits that brace weaning can be safely conducted after reaching Sanders stage 7b or ulnar grade 8 in the distal radio-ulnar classification.

Numerous types of braces are available for patients with AIS (Fig. 1). In contrast to the conventional rigid braces (e.g., Boston and Charleston brace), soft braces use elastic bands and pads for deformity correction. Although soft braces offer enhanced wearability and comfort, previous studies have shown lower success rates compared to rigid braces [9,15]. Recently, hybrid braces have been developed, combining the advantages of both rigid and soft braces (Fig. 1D).

Fig. 1

Different types of braces used in the treatment of adolescent idiopathic scoliosis. (A) Milwaukee brace. (B) Boston brace. (C) Charleston brace. (D) Hybrid brace.

Recent innovations in bracing technology have focused on enhancing compliance and efficacy. Notably, sensor-integrated braces utilizing temperature or pressure sensors have shown improved wear compliance among AIS patients, as demonstrated by a recent meta-analysis by Cordani et al. [16]. Additionally, researchers have explored the integration of 3D deformity analysis into brace fitting and efficacy assessments [17,18]. These advancements will contribute to the development of next-generation braces for AIS.

The effectiveness of bracing for AIS patients with a relatively larger curve close to a surgical indication (Cobb angle >40°) is another research hotspot. According to a meta-analysis of nine studies (n=563), 32% of patients improved, 26% remained stable, and 42% worsened when bracing was attempted for large curves [19]. Another meta-analysis of eight studies by Babaee et al. [20] reported favorable outcomes for bracing in this population. Despite significant heterogeneity among the included studies and potential biases, these findings indicate that bracing may be considered for AIS patients requiring surgical correction but who are hesitant to undergo surgery.

Impact of exercise on curve improvement

Patient-specific scoliosis exercises (PSSE), such as the Schroth 3D exercise and Scientific Exercises Approach to Scoliosis, are commonly employed in AIS management. However, the efficacy of PSSE in improving radiological and clinical outcomes remains debated. A recent meta-analysis of 26 studies (10 RCTs and 16 observational studies) by Baumann et al. [21] found that patients who underwent PSSE experienced statistically significant, but clinically insignificant, curve improvement. Furthermore, there was no significant improvement in patient-reported outcome measures and angle of trunk rotation (ATR). Contrary to common belief that smaller curves would benefit from exercise, this study found no significant improvement in Cobb angle when stratified by curve magnitude (30°).

Conversely, another recent meta-analysis by Chen et al. [22] investigating the efficacy of Schroth 3D exercise alone across 14 studies reported statistically significant differences in the improvement of Cobb angle, ATR, and quality-of-life measures between the Schroth group and control. However, the mean differences in Cobb angle and ATR improvements were 3.3° and 2.2°, respectively, raising questions about the clinical significance of these statistically significant differences, given that the standard error of Cobb angle measurement is 3°–5° [23].

A recent Cochrane review also substantiated the uncertain efficacy of therapeutic exercise for AIS [24]. The review highlighted significant limitations in existing research, including heterogeneity among studies and small sample sizes. Moreover, the review revealed no superiority of PSSE over general exercises, such as Pilates and core strengthening, concerning curve improvement. Currently, the literature lacks conclusive evidence to justify the use of PSSE for curve improvement in patients with AIS.

Evidence on spinal manual therapy in adolescent idiopathic scoliosis

Spinal manual therapy is a commonly employed treatment for AIS, particularly in local hospital settings. However, these treatments are typically not covered by insurance, placing a financial burden on the patients and their families. Manual therapy involves skilled hand movements designed to mobilize and manipulate soft tissues and joints, aiming to alleviate symptoms and improve spinal alignment. The term manual therapy is often used interchangeably with other terms, such as chiropractic, massage, osteopath, and Tuina.

However, there is a scarcity of evidence regarding the efficacy of spinal manual therapy for patients with AIS. Two recent meta-analyses of RCTs investigating the effect of spinal manual therapy in AIS patients highlighted critical knowledge gaps due to the predominance of low-quality studies and insufficient data to inform definitive conclusions [25,26]. Therefore, the literature does not currently support the use of spinal manual therapy in AIS management.

Surgical indication and options

Surgical treatment is typically indicated for AIS patients with a Cobb angle larger than 45° and remaining growth potential. The goals of AIS surgery are to achieve a balanced spine in both coronal and sagittal planes by correcting the 3D deformity and to prevent functional deterioration, including pulmonary function impairment, resulting from the progression of spine and thorax deformities. Although fusion-less surgery, such as anterior vertebral body tethering (AVBT), is available for patients with AIS, the mainstay of AIS surgery is to fuse the segment causing 3D deformity. Therefore, preserving mobile segments as much as possible while achieving sufficient deformity correction is another goal in AIS surgery. Selecting the appropriate fusion level is a subject requiring extensive discussion, which is beyond the scope of this review [27].

Several surgical options are available for AIS patients: anterior fusion, posterior fusion, anteroposterior fusion, and fusion-less surgery (e.g., AVBT). This review will primarily discuss deformity correction using posterior fusion and instrumentation, the most commonly employed method. The subsequent discussion will delve into contentious issues and cutting-edge advancements rather than established surgical techniques, providing a nuanced exploration of the field’s evolving landscape.

Pedicle screw instrumentation using novel technologies

Pedicle screw instrumentation has revolutionized AIS surgery by providing excellent pull-out strength and correction power compared to preceding instrumentation techniques [28]. Pedicle screws enable novel correction maneuvers, such as direct vertebral rotation (DVR), allowing surgeons to shorten fusion levels in surgically treated AIS patients [29] (Fig. 2).

Fig. 2

Direct vertebral rotation (DVR) maneuver during adolescent idiopathic scoliosis correction. (A) DVR on the apical vertebra. (B) DVR on lower-instrumented vertebra.

Regarding pedicle screw insertion techniques, the freehand technique has been shown to be safe and effective for patients with AIS [30]. However, recent advancements in stereotactic navigation and robotics have sparked interest in the potential for these technologies to further enhance accuracy and safety in pedicle screw instrumentation. Many studies have reported improved accuracy and fewer pedicle breaches by using computed tomography (CT) navigation assistance. A meta-analysis conducted by Chan et al. [31], encompassing 18 studies with moderate or low risk of bias, found moderate evidence that CT navigation reduces pedicle breaches compared to the freehand technique.

In contrast, several studies have highlighted the drawbacks of navigation-assisted surgery. In a study by Katz et al. [32], utilizing the National Surgical Quality Improvement Program database, the navigated surgery group had a longer operation time and transfusion rate than the conventional surgery group. Another large database study by Kaur et al. [33] reported no significant differences in neurological complications between the navigated and conventional groups. These comparable outcomes may reflect the previous findings that the freehand technique is already sufficiently safe, and pedicle screw malposition is not the sole cause of neurological complications in AIS surgery.

Increased radiation exposure is another significant concern associated with the routine use of navigation assistance in AIS surgery. Several studies have documented heightened radiation exposure to patients without discernible benefits when intraoperative navigation is employed [34–36]. Moreover, a recent study by Striano et al. [37] indicated that intraoperative navigation may increase the projected lifetime cancer risk in patients undergoing AIS surgery. Considering these findings and the increased costs of using navigation, it is recommended that intraoperative CT-based navigation be reserved for select patients, such as those with dysplastic pedicles, rather than being used routinely for all AIS patients (Fig. 3).

Fig. 3

(A–D) An illustrative case of adolescent idiopathic scoliosis with dysplastic pedicles surgically corrected using stereotactic navigation-assisted pedicle screw instrumentation.

Robotic systems in spine surgery employ shared-control platforms, where surgeons maintain primary control of surgical instruments, while the robot ensures an optimal screw trajectory and steady manipulation [38]. Previous studies indicate that robotic assistance enhances the accuracy of pedicle screw instrumentation, surpassing both freehand and navigation-assisted techniques [39,40]. However, studies have also highlighted that robotic-assisted pedicle screw instrumentation tends to be less accurate in the upper thoracic region [41]. Furthermore, because of the absence of tactile feedback during screw insertion via robotics, surgeons should always be aware of situations where navigation screens are misleading.

Efficacy of minimally invasive surgery

AIS deformity correction often entails significant drawbacks, including lengthy skin incisions, conspicuous scarring, intraoperative bleeding, and postoperative pain due to the stripping of paraspinal muscles. Consequently, many authors have proposed minimally invasive surgery (MIS) for AIS, highlighting its efficacy. Sarwahi et al. [42] first introduced MIS for AIS in 2011. Despite a steep learning curve, they reported favorable outcomes of MIS in their subsequent studies [43]. Many authors have presented their own MIS techniques for AIS, yielding promising results [44–46]. Besides the cosmetic advantages, MIS for AIS aims to reduce intraoperative bleeding and postoperative pain and promote enhanced recovery after surgery (Fig. 4).

Fig. 4

(A–C) An illustrative case of adolescent idiopathic scoliosis surgically corrected using minimally invasive surgical techniques. Written informed consent for the publication of this image was obtained from the patient. (Courtesy of Prof. Jae Hyuk Yang and Prof. Seung Woo Suh from Korea University).

Although there is a lack of high-quality RCTs comparing conventional and MIS for AIS, recent meta-analyses have sought to identify the merits and challenges of MIS in AIS patients. A meta-analysis by Neradi et al. [47] revealed that MIS conferred significant benefits in terms of reduced duration of surgery, decreased blood loss, and lower analgesic requirements. Conversely, the conventional approach yielded superior correction rates and functional outcomes in their analysis. According to a more recent meta-analysis of 15 non-randomized studies (n=1,369) by Kim et al. [48], patients who underwent MIS for AIS had a significantly lower estimated blood loss and shorter hospital days, but longer operation time compared to the conventional group. This study found no significant between-group differences with respect to radiological outcomes, such as coronal Cobb angle and thoracic kyphosis.

Based on current findings, conventional open surgery remains the mainstay for AIS surgery. More evidence from high-quality studies is required on the clinical and radiological outcomes of MIS in patients with AIS to verify the efficacy and cost-effectiveness of MIS. Future studies should also examine the feasibility and effectiveness of MIS in larger and stiffer AIS curves.

Effect of implant density and materials

The concept of implant density, defined as the number of screws per treated level in AIS surgery, has garnered significant attention in the literature. Some authors advocate for high-density (HD) constructs, potentially offering better correction power, while others prefer low-density (LD) constructs for their cost-effectiveness. A recent meta-analysis by Aoun et al. [49] involving 24 studies and 1,985 patients showed that the HD group experienced significantly better improvements in ATR and lower-instrumented vertebra (LIV) tilt angle. Conversely, the LD group showed shorter operation times, lesser blood loss, and lower costs. All other variables were comparable between the two groups in this analysis. Although the LD construct offers evident clinical benefits, the clinical significance of improved radiological outcomes observed in the HD group remains uncertain.

Previous studies have also investigated the clinical implications of implant density, including the potential for HD constructs to minimize the extent of spinal fusion. Chang et al. [50] compared two surgical strategies in selective thoracic fusions (STF): (1) LD constructs without LIV DVR and (2) HD constructs with DVR (Fig. 5). The authors found that the HD construct with LIV DVR resulted in significantly shorter fusion levels and a more proximal LIV compared to the LD group. They suggested that the HD construct affords better control during the LIV DVR maneuver, potentially shortening the fusion level in STF without increasing adverse outcomes like adding-on and coronal decompensation.

Fig. 5

Illustrative cases of the two surgical strategies. (A) Low-density pedicle screw construct without lower instrumented vertebra direct vertebral rotation (LIV DVR). (B) High-density pedicle screw construct with LIV DVR.

Rod material and size in AIS surgery remains a topic of debate, particularly with regard to their ability to restore thoracic kyphosis. Research has yielded conflicting results; some studies reported improved thoracic kyphosis restoration with cobalt chrome rods compared to titanium alloy rods, while others—including an RCT—found no significant difference between the two materials in this respect [51–55]. Most previous studies have found comparable radiological and clinical outcomes across different sizes of rods. However, a recent meta-analysis by Bowden et al. [56] reported that 6.0 mm rods were associated with a significantly higher reoperation rate than 5.5 mm rods through indirect comparison. High-quality studies with sufficiently large sample sizes are required to determine the most suitable rod size and material for AIS surgery.

Is thoracoplasty necessary?

Rib hump or rib prominence is a major cause of dissatisfaction in patients with AIS even after surgical correction [57]. Thoracoplasty is performed to reduce rib hump in conjunction with spinal deformity correction. To minimize the surgical morbidity associated with thoracoplasty, modified techniques have been proposed [58,59]. In previous studies, thoracoplasty was found to effectively reduce rib prominence, as measured by inclinometer, and achieve higher Scoliosis Research Society-22 self-image scores compared to those who did not receive the procedure [60,61].

In contrast, several studies have reported decreased pulmonary function in patients who underwent thoracoplasty. Two recent meta-analyses indicated that additional thoracoplasty during posterior spinal fusion and instrumentation caused a significant deterioration in pulmonary function, characterized by reduced vital capacity and forced expiratory volume in 1 second, while posterior correction without thoracoplasty resulted in a minor improvement in pulmonary function [62,63]. Ultimately, statistical analysis cannot definitively determine whether improved appearance justifies the decline in pulmonary function resulting from thoracoplasty.

How promising is anterior vertebral body tethering?

AVBT is a non-fusion surgical technique that acquired US Food and Drug Administration (FDA) approval in 2019 [64]. Initially developed and introduced by Zimmer Biomet, the majority of current evidence on AVBT stems from this pioneering work. Since then, several other manufacturers have either received or are awaiting FDA approval, expanding the availability of AVBT systems. As AVBT systems have developed and advanced, evidence regarding their efficacy and safety has grown in the literature.

AVBT is considered a minimally invasive non-fusion technique typically performed using thoracoscopic assistance. AVBT can help avoid long spinal fusions and extensive posterior midline scars. The surgical procedure utilizes polyethylene terephthalate tethers to connect anchor screws inserted laterally from each vertebral body on the convex side. The tethers restrict the growth of the convex side of the curve and correct the coronal deformity of AIS based on the Hueter-Volkmann principle. Previous studies have reported favorable radiological outcomes following AVBT; for instance, a meta-analysis by Vatkar et al. [64] indicated that the preoperative mean Cobb angle of 48.5° decreased to 20.1° at the final follow-up (mean follow-up=34 months) [64].

Despite its promise, AVBT is not without risks, and complications and reoperations are notable concerns. Common complications include mechanical complications (such as tether breakage), pulmonary complications, and undesired outcomes, including overcorrection and curve progression. A recent meta-analysis by Roser et al. [65] revealed an overall complication rate of 23%, with tether breakage being the most common at 21.9%. Additionally, pulmonary complications, including atelectasis and pulmonary effusion, were observed in 6.7% of cases. Notably, overcorrection and reoperation rates were both 11.4%, while the conversion rate to spinal fusion surgery was 7.2%. The relatively high complication and reoperation rates suggest that AVBT has yet to overcome significant hurdles before it can supplant posterior spinal fusion as the mainstay of AIS surgery.

Measures for outcome improvement

The enhanced recovery after surgery (ERAS) protocol has been extensively investigated and implemented in various surgical fields, including AIS. A meta-analysis by Koucheki et al. [66], incorporating 14 studies and 2,456 cases, evaluated the efficacy of ERAS in AIS patients. Their analysis showed that ERAS reduced the operation time by 35.6 minutes, lowered blood loss by 112.3 mL, and allowed earlier patient ambulation by 29.6 hours compared to the conventional approach. Additionally, ERAS resulted in improved pain scores in the immediate postoperative period, reduced analgesic consumption, and a shorter length of hospital stay. These findings strongly support the adoption of the ERAS protocol in AIS management.

Surgical site infection (SSI) is a devastating complication following AIS surgery. To mitigate the risk, surgeons frequently employ prophylactic measures, including the use of vancomycin powder and povidone-iodine irrigation. Shinohara et al. [67] retrospectively reviewed 4,145 AIS cases from a multi-center registry and reported a delayed deep SSI rate of 1.0% (43/4,145). The incidence of SSI was significantly lower in patients receiving vancomycin powder (0.2%) compared to those who did not (1.9%). Although advancements in surgical techniques may contribute to reduced SSI rates, vancomycin powder appears effective in minimizing SSIs. Povidone-iodine solutions are also often incorporated into irrigation regimens, demonstrating efficacy in reducing SSI rates following AIS surgery [68,69]. However, there is no clear consensus on the optimal dosages of vancomycin powder and amounts of povidone-iodine irrigation for effective SSI prevention.

Intravenous tranexamic acid (TXA) administration is commonly used in various orthopedic surgeries to reduce intraoperative blood loss. Many spine surgeons utilize TXA to minimize blood loss and avoid transfusion during the perioperative period of AIS surgery. A recent meta-analysis by Liu et al. [70], synthesizing data from 10 clinical trials with 741 participants, demonstrated that TXA significantly reduced perioperative blood loss and transfusions without increasing the risk of thromboembolic events. These measures—ERAS, vancomycin powder, povidone-iodine solution, and TXA—can significantly enhance the clinical outcomes of contemporary AIS surgery.