Vertebral body tethering for adolescent idiopathic scoliosis: a review

Abstract

Purpose

Adolescent Idiopathic Scoliosis (AIS) remains the most common type of pediatric scoliosis, mostly affecting children between ages 10 and 18. Vertebral body tethering (VBT) offers a non-fusion alternative to the gold standard spinal fusion that permits flexibility and some growth within instrumented segments. This article will serve as a comprehensive literature review of the current state-of-the-art of VBT in relation to radiographic and clinical outcomes, complications, and the learning curve associated with the procedure.

Methods

A systematic literature review was conducted on PubMed, Scopus, and Web of Science from April 2002 to December 2022. Studies were included if they discussed VBT and consisted of clinical studies in which a minimum 2-years follow-up was reported, and series that included anesthetic considerations, learning curve, and early operative morbidity.

Results

Forty-nine studies spanning the period from April 2002 to December 2022 were reviewed.

Conclusion

This article illustrates the potential benefits and challenges of the surgical treatment of AIS with VBT and can serve as a basis for the further study and refinement of this technique ideally as a living document that will be updated regularly.

Appendix

Basic science

VBT requires growth for gradual scoliosis correction via the Hueter-Volkmann Law as the basis for growth modulation in skeletally immature patients. The principle leverages the observation that skeletal growth is diminished by compression of the growth plate and increased by decreasing compression or by inducing distractive forces.⁹ Seminal work on this principle in the spine was conducted by Stokes, et al. using a rat tail model. ⁸ Their group demonstrated vertebral wedging of individual rat tail bones after asymmetrical loading which progressed over time and leads to scoliotic deformity. In a follow-up study, their group showed the scoliosis can be corrected by reversing the applied load.²²

A number of larger animal studies were subsequently endeavored and have shown the feasibility of using non-fusion tether-based applications in skeletally immature spines to both create scoliosis and correct it while permitting at least some longitudinal growth. Disk integrity has also been shown to be maintained. Newton et al. conducted two in vivo studies in bovine²³ and porcine models²⁴ to evaluate the effects of intraoperative tensioning of flexible spinal tethers on spinal growth and motion using stainless steel cables. In the bovine model, his team first performed right-sided tethers of four thoracic vertebrae and four screws without tether as a sham operation on the contralateral side. Tethering caused scoliosis (11.6˚ ± 4.8˚) and disk wedging (6.8˚ ± 1.6˚) with decreased vertebral height. In the porcine model, pre-tensioning induced scoliosis and apical disk (T9–T10) wedging, but after 12 months, no radiographic differences were observed between groups, suggesting growth modulation is possible in untensioned states. Furthermore, spinal motion and disk health were not negatively affected after tethering. Histology showed normal disks and intact growth cartilage along with no foreign bodies or microabscesses on lymph tissue analysis.

Braun et al. created an immature goat model to predict scoliosis progression by analyzing the percentage of vertebral body wedging, specifically in the area of maximal deformity.²⁵ They induced scoliosis in 15 goats and observed them for 12 weeks. Seven goats developed progressive curves (mean: + 10.1˚), while eight goats did not show progression (mean: − 1.6˚). Their results indicated that a higher percentage of vertebral body wedging was associated with progressive curves, suggesting its potential as an indicator for risk of curve progression in idiopathic scoliosis.

Patel et al. created another porcine scoliosis model with curvature of 50 degrees achieved to potentially evaluate VBT.²⁶ Moal et al. used this model to explore the non-fusion correction of tethering.²⁷ The pigs were divided into three groups: the Scoliosis Model (SM) group served as the control and was euthanized after the scoliotic curve reached a specific angle; the Tether Release (TR) group had the inducing spinal tether removed and was observed for ongoing growth modulation; and the Anterior Correction (AC) group had the tether removed and received an anterior corrective tethering technique. The AC group demonstrated favorable realignment in all three planes and correction of vertebral wedging, indicating potential advantages over fusion-based approaches in preserving spinal growth and mobility. They found tethering offers advantages over fusion-based methods, preserving spinal growth and mobility.

Biomechanics

Various studies highlight the importance of instrumentation parameters in VBT such as tether tensioning, the amount of stress applied by the tether on the instrumented spine, screw positioning, the number of tethers used, and the positioning of the patient.

Lalande et al. explored tether tensions and pressures transmitted onto the vertebral end plates of a tether applying cyclical loads in a porcine model.²⁸ In a previous experiment on rat and mouse tail vertebrae by Valteau et al. axial cyclical compressions allowed for similar growth modulation as for standard statically loaded tethers, while providing better preservation of the intervertebral disks and soft tissues of the instrumented intervertebral segments. Reductions in growth plate thickness and the number of proliferative chondrocytes per column in static conditions compared to cyclical loading, suggesting differential response of growth plates of between the two loading mechanisms.²⁹ Lalande et al. then developed a cyclical VBT prototype with a motor box that applied automated cyclical tensioning. Five different tensions were tested, revealing pressure exerted on the end plate was linearly correlated to the mean tether tension (r² = 0.86). The results of these studies suggest some benefit of a cyclically loading VBT concept though further study is needed.

Cobetto et al. investigated the use of an anterior vertebral body growth modulation (AVBGM) in correcting pediatric scoliotic deformities.³⁰ The device aims to modify the compression distribution on vertebral body growth plates by applying compressive forces on the convex side of the scoliotic curve. The authors used patient-specific finite element models to simulate the effects of different instrumentation parameters and patient positions on the 3D corrective effects of AVBGM. They found that AVBGM can provide significant correction in the coronal and sagittal planes, but not in the transverse plane. The results offer valuable insights to improve the biomechanical knowledge and design of AVBGM and could help personalize the surgery to improve treatment outcomes.

Nicolini et al. investigated the impact of VBT on the range of motion (ROM) within the thoracolumbar spine.³¹ They tested different surgical reconstruction groups, including the native spine, one tether, two tethers, and a hybrid construction. Compared to the native spine, VBT with one or two tethers led to a slight reduction (less than 9.7%) in global ROM during flexion and extension, and up to 13.5% reduction in right axial rotation. In lateral bending, VBT significantly reduced global ROM by around 25–45%. The hybrid technique showed the least impact on global ROM in flexion–extension and axial rotation, with reductions of less than 10.5% and 10–14%, respectively. The results suggested that the double tether or hybrid surgery may still preserve global motion in flexion–extension and axial rotation.