Abstract
Background:
Vertebral body tethering (VBT) offers an alternative treatment for patients with idiopathic scoliosis. We present our finalized Food and Drug Administration Investigational Device Exemption (IDE) study results on VBT.
Methods:
We retrospectively reviewed patients with Lenke Type IA/B curves who underwent VBT between 2011 and 2015. Clinical, radiographic, perioperative, and complications data were prospectively collected.
Results:
Fifty-seven patients (mean age 12.4 ± 1.3 years) were enrolled and followed for 6.6 ± 1.6 years (range: 3.0-10.2 years). Thoracic Cobb angle measured 40.4 ± 6.8° preoperatively with correction to 14.5 ± 9.0° at 2 years and slight regression to 22.1 ± 12.4° at last follow-up. Median Sanders and Risser were 3 and 0, respectively. At last follow-up, 71% of patients (39/56) had curves ≤30° and 98% (55/56) had achieved skeletal maturity. There were no major neurologic or pulmonary complications. However, 10 patients required 13 revisions; 2 required conversion to fusion. Percent predicted forced expiratory volume and forced vital capacity dropped from 85% and 87%, respectively, to 80% and 82% at the final follow-up.
Conclusion:
These finalized IDE results highlight VBT as a safe, effective treatment for skeletally immature patients with idiopathic scoliosis. However, complication and revision rates remain concerning, and patients should be counseled appropriately.
Level of Evidence:
Level III. See Instructions for Authors for a complete description of levels of evidence.
Introduction
Anterior vertebral body tethering (VBT) has gained traction as a growth-sparing technique for the treatment of adolescent idiopathic scoliosis (AIS). Although posterior spinal fusion (PSF) remains the gold standard, VBT has been used under the auspices of Food and Drug Administration (FDA) Humanitarian Device Exemption approval since 2019, specifically among skeletally immature patients. The primary benefits of VBT are motion preservation and reducing the risk of adjacent segment disease1. However, given the associated higher complication rates compared with PSF2, patient selection for VBT requires careful decision making.
Previously, we reported our interim FDA Investigational Device Exemption (IDE) study on VBT3, in which 80% of patients achieved main thoracic curves <30° at last follow-up. Although subsequent studies have further explored the role of VBT in AIS, concerns still remain regarding its complications, revision rates, and longevity. Much of the existing literature averages 2 to 3 years of follow-up and has shown approximately 50% reduction in curve magnitude with VBT4-6. Hoernschemeyer et al.7 recently published their mid-term VBT outcomes and showed diminishing efficacy with longer follow-up: 74% of patients initially achieved a tethered Cobb ≤30° but decreased to 64% with the mean follow-up of 5.7 years. Among patients who were followed >5 years, revision rates were 24%7,8. As an increasing number of mid-term to long-term studies examine outcomes data, VBT's viability as a treatment strategy will gain clarity. In this study, we present our finalized report on the results from our original interim FDA IDE study on VBT3.
Materials and Methods
We identified all patients who underwent VBT at our institution between 03/2011-08/2015 with minimum 2-year follow-up. VBT was performed as part of a clinical trial (NCT02897453), and the full list of inclusion/exclusion criteria is presented here: https://clinicaltrials.gov/study/NCT02897453. The FDA IDE was conditionally approved in 02/2015 and fully approved in 03/2015.
Informed consent was obtained, and the study conducted with IRB approval. The first patient was enrolled in July 2016. Study enrollment completed in 2019, and patients were followed through study closure in June 2024. Final enrollment included 57 patients, of which 56 (98%) formed the “per protocol” treatment cohort. Primary end point was the thoracic Cobb angle at 2-year follow-up. Secondary end points were longitudinal changes in patient-reported outcome measures (PROM), coronal Cobb angle at last follow-up, and radiographic parameters beyond the coronal Cobb angle. One patient enrolled but failed screening because they had additional surgical intervention on their curve within the 24-month period specified in the protocol. At the time of submission, 55 of 56 patients completed the study, defined as Sanders 8 and age ≥18 years. The remaining patient was lost to follow-up 59.6 months postoperatively. The mean follow-up was 6.6 ± 1.6 years (range: 3.0-10.2 years), and the mean time between surgery to enrollment was 3.6 years. See Fig. 1.
Fig. 1.
Patient enrollment, screening, and completion per protocol. Study enrollment was completed in 2019. One enrolled patient ultimately failed screening because their curve required surgical intervention before the 2-year time point specified within the protocol. For accurate reporting, this patient's baseline demographic, operative, and adverse events summaries were included in our cohort but were excluded from statistical analysis. At the time of study submission, 55 of 56 patients had reached Sanders 8 and age ≥18 years. The 1 remaining patient was lost to follow-up after last visit 59.6 months postoperatively but had completed per protocol treatment.
Relevant data were prospectively collected on demographics, radiographic parameters, operative details, PROMs, and outcomes/complications. All patients’ radiographs underwent rigorous analysis by a radiologist and measurements were reported in concordance with literature standards. Clinical measures included Visual Analog Scale (VAS) for pain, pulmonary function tests, scoliometer readings, and quality-of-life (QOL) measures (Scoliosis Research Society 22 [SRS-22], PEDSQL, and the Adolescent Pediatric Pain Tool [APPT]).
Statistical Analysis
Wilcoxon signed rank test was used to compare preoperative vs. postoperative outcomes due to the non-normality of the data set. P values (<0.05 for statistical significance) were tested for multiple comparisons and adjusted accordingly. All statistical analyses were performed with SAS version 9.4 (SAS Institute).
Results
Demographics
Between 2011 and 2015, 142 patients underwent VBT. Fifty-seven patients met inclusion criteria (mean age 12.4 ± 1.3 years [range 10.1-15.0 years]). The majority of patients were Risser 0 (68.4%), and the most common Sanders stage was 3 (35%) (Supplementary Table 1). At the time of final follow-up, 100% of patients were Risser 4/5 and 98% were Sanders 8, confirming that all patients had attained skeletal maturity at the time of study completion.
Perioperative Variables
All patients underwent a purely thoracoscopic approach (56%) or thoracoscopic plus mini-open approach (44%) (Supplementary Table 2). The mean number of tethered levels was 7.5 ± 0.6, and average surgical time was 223 ± 79 minutes. Estimated blood loss averaged 106 ± 86 mL, with 8 patients (14%) requiring intraoperative blood transfusion. Intraoperative neuromonitoring was used for all patients, and there were no neuromonitoring changes. All patients were admitted to the ICU postoperatively (average length of stay 1.5 ± 0.7 days). Average length of hospitalization (LOH) was 4.8 ± 1.4 days. Of note, this hospitalization may be longer than expected because many of our families travel to undergo the procedure at our institution.
Radiographic Parameters
Preoperatively, the main thoracic Cobb angle averaged 40.4 ± 6.8° and corrected to 19.9 ± 8.6°, 14.5 ± 9.0°, and 22.1 ± 12.4° at first erect (FE), 2-year, and final follow-up, respectively. When comparing the preoperative Cobb angle to FE and 2-year time points, there were significant improvements noted in the thoracic Cobb angle (p < 0.01 for both). However, loss of correction was seen at 5 years and last follow-up, averaging 19.3 ± 12.0° and 22.1 ± 12.4°, respectively (Supplementary Table 3). This trend is likely attributable to tether breaks, which can often be clinically inconsequential. Figs. 2-A through 2-D shows a patient who had satisfactory correction outcomes following VBT. At the time of final follow-up, 1 patient had a residual curve >45° and 3 had curves >40°.
Fig. 2.
Figs. 2-A through 2-D An 11-year-old girl (Risser 0, Sanders 3, premenarchal) presented with a 50° right thoracic and 26° left lumbar curve (Fig. 2-A). She initially underwent T5-T11 VBT (Figs. 2-B and 2-C), and at the time of last follow-up, radiographs showed an 8° right thoracic and 11° left lumbar curve (Fig. 2-D).
Regarding sagittal parameters (Supplementary Table 4), T5-L2 kyphosis measured 15.5 ± 10.0°, 16.7 ± 9.9°, and 17.4 ± 13.3° at preoperative, FE, and last follow-up, respectively. While this difference was significant (p < 0.01), the magnitude of improvement was fairly modest. L1-L5 lumbar lordosis measured 51.9 ± 11.4°, 50.7 ± 10.6°, and 51.4 ± 13.2° at preoperative, FE, and last follow-up; these differences were not significant. There were also no significant differences in sagittal or coronal balance.
When defining tether success as a main thoracic Cobb angle <30° or <35°, the majority of patients achieved a successful radiographic outcome (Supplementary Table 5). All patients had a Cobb angle <35° at 2 years; however, this number decreased to 77% at last follow-up. If defining success more stringently (Cobb angle <30°), 98% of patients met that criterion at 2-year time point; this number subsequently decreased to 71% at last follow-up. Again, these findings suggest that some loss of correction occurs with longer follow-up.
Complications and Revisions
In sum, 10 patients (17.5%) required 13 total revisions. There were 2 serious non–instrumentation-related adverse events: 1 patient required readmission for pneumonia, and another developed progression of a preexisting lumbar spondylolisthesis. Six patients developed pneumothorax, 3 had pleural effusions, and 1 had a wound complication, but all patients were successfully managed without chest tubes or further intervention. No major neurologic injuries occurred.
Of the 10 patients requiring revision or reoperation (Supplementary Tables 6 and 7), 5 patients had overcorrection, 2 had tether breaks, 1 had screw migration, 1 had progressive spondylolisthesis, and 1 developed a new curve. All patients revised for their cord breaks had demonstrated progression of their curves. Two patients required conversion to fusion, 1 for overcorrection and the other for adding-on. Supplementary Figs. 1-A through 1-H shows a patient with overcorrection 6 months postoperative who required tether release. The curve continued to overcorrect, necessitating a second revision surgery 59 months after index VBT.
Visual and Clinical Assessment
Preoperative thoracic scoliometer readings measured 13.6 ± 3.9° and subsequently improved to 6.1 ± 3.0° at 2 years and 8.2 ± 5.2° at last follow-up; this appears consistent with the loss of radiographic correction shown previously with longer follow-up. Preoperatively, 75.5% of patients had uneven shoulders; this decreased to 23% at last follow-up. Similarly, trunk shift was noted in 78% of patients preoperatively, improving to 36% at last follow-up, and scapular asymmetry was seen in all patients preoperatively, improving to 57% at last follow-up.
Pulmonary Function Tests
Overall, 42 patients had pulmonary function tests (PFTs) at both preoperative and 2-year time points. The mean preoperative forced expiratory volume (FEV1) and forced vital capacity (FVC) were 2.29 and 2.67 liters, respectively. At last follow-up, these same parameters were 2.83 and 3.38 liters, respectively. However, both % predicted FEV1 and FVC were lower than expected values for normal patients. All changes in PFTs were statistically significant (p < 0.01)
PROMs
No preoperative SRS-22, PedsQL, and Adolescent Pediatric Pain Tool (APTT) scores were obtained. At the time of last follow-up, SRS-22 outcome scores were 4.5 ± 0.4, and average function, pain, self-image, and mental health scores were 4.8 ± 0.3, 4.4 ± 0.5, 4.4 ± 0.6, and 4.4 ± 0.6, respectively. At the time of last follow-up, PedsQL total score averaged 2,150 and APTT word graphic rating scale averaged 1.6.
Discussion
This finalized IDE report of our interim VBT study3 confirms the efficacy of VBT, with 71% of patients achieving a thoracic Cobb angle <30° at last follow-up. With 55 of 56 patients reaching Sanders 8 by the end of the study, this rate of tether success likely represents the true rates of expected correction at full skeletal maturity. Notably, we had previously reported approximately 80% of patients having a Cobb angle <30°, which suggests that some loss of correction can be expected with longer follow-up. This hypothesis is supported by the significant improvements seen between preoperative vs. 2-year follow-up (40.4 ± 6.8° vs. 14.5 ± 9.0°, p < 0.01), followed by a loss of correction to 22.1 ± 12.4° at the final follow-up (p < 0.01). Hoernschemeyer et al.7 initially reported that 74% of patients achieved a thoracic Cobb angle <30° with the mean follow-up of 3.2 years. When these same patients were revisited with the mean follow-up of 5.7 years, this success percentage reduced to 64% and there was a concurrent increase in revision rates (21% vs. 24%)8. Our own experience showed an increase in revision rates from 12.3% to 17.5%. Thus, patients should be counseled regarding possible changes in their correction over time, as well as the overall revision and complication rates.
We did not encounter significant intraoperative or perioperative complications, but 23% of patients experienced non–instrumentation-related complications. Of the 6 patients with pneumothorax, 3 with pleural effusion, and 1 with respiratory infection, only the latter patient required readmission for medical treatment. Ten patients underwent 13 total revisions, most commonly for overcorrection. However, not all patients who experienced a complication required revision surgery (Supplementary Table 6). For example, of the 8 cases of suspected breaks and 3 cases of confirmed breaks, only 2 patients needed revision surgery. Thus, the presence of a radiographic complication does not necessarily translate to additional intervention.
Two patients (4%) required conversion to PSF. This rate is lower than the reported rate of 7% across the literature2, as well as other studies where the revision rates were even higher9-11. Newton et al.10 were among the first to highlight the high revision rates associated with VBT, showing a 42% revision rate among 17 patients who underwent VBT, most frequently for overcorrection. Subsequently, they9 also compared VBT directly with PSF. Approximately 30% of patients required revision surgeries in the VBT group, with 3 patients undergoing conversion to PSF. No revisions were required within the PSF cohort. While these results merit consideration, their patients were younger (mean age 11 years) and the majority were skeletally immature (100% Risser 0, 94% open triradiates). By comparison, our cohort was more skeletally mature (32% >Risser 0, 37% open triradiates). Patient age and skeletal immaturity likely influence revision and complication rates, given the inherent difficulty of predicting growth potential. Indeed, we found that patients with open triradiates were statistically more likely to overcorrect following VBT regardless of index curve and initial correction12.
Pehlivanoglu et al.13 showed superior SRS-22 and SF-36 scores when patients underwent VBT versus fusion. Although our patients did not complete PROMs preoperatively, postoperative SRS-22 scores were similar to published reports9. In our cohort, pain scores were similar (average 4.4 in both studies), but we found higher satisfaction in function (4.8 vs. 4.3), self-image (4.4 vs. 4.1), mental health (4.4 vs. 4.3), and satisfaction (4.6 vs. 4.3). We administered supplemental surveys (PedsQL, APPT) to further assess impact on QOL. The PedsQL offers a general assessment of perceptions on QOL among children with chronic health conditions14. Higher scores indicate higher function and, by corollary, higher QOL. Our cohort demonstrated high QOL, with physical, psychosocial, and total scores of 738/800, 1,411/1,500, and 2,150/2,300, respectively. The APPT is a pediatric pain assessment tool that dissects specific pain generators by location, intensity, and quality15. In our cohort, word graphic rating scales (0 = no pain; 10 = worst pain) averaged 1.6 and pain quality descriptors averaged 3.4/67, suggesting relative comfort following VBT.
Regarding PFTs, both FEV1 (2.29 vs. 2.84 liters) and FVC (2.67 vs. 3.33 liters) showed improvements. However, % predicted FEV1 declined from 85% to 80% while % predicted FVC declined from 87% to 82%. Hwang et al.16 recently demonstrated that among AIS patients treated with VBT, baseline % FEV1 and % FVC were 79% and 82%, respectively. They showed significant improvements in FEV1 and FVC but unchanged % predicted values in FEV1 (79-80%) and FVC (80%-82%). These results are consistent with our current findings. Thus, while AIS patients may present with mild restrictive lung disease, the thoracoscopic or combined mini-open approaches appear to be well tolerated.
The strength of this report lies in its prospective follow-up data collection. Limitations include a small sample size, lack of preoperative PROMs, and no comparative PSF cohort. In addition, these results represent our earliest experience with VBT, and we have since implemented modifications in patient selection as well as surgical technique. Perhaps as a reflection of this, the mean Cobb angle was 40.4° ± 6.8°; while these curves still complied within the FDA indications range of 30 to 65°, we no longer routinely perform VBT for curves <45°. Thus, these results may vary when investigating outcomes for our most recently treated patients.
Nevertheless, our finalized IDE report reinforces our interim findings that VBT is a safe and effective treatment for patients with AIS. While the reported benefits of motion preservation remain enticing, it is equally important to counsel patients and their families regarding the risks for complications, potential need for revision surgery, and less overall correction compared with PSF. Furthermore, while this study cohort had reached skeletal maturity, effects on curve progression, development of new curves, and rate of disk degeneration will remain worthy of study as these patients move through adulthood.
Appendix
Supporting material provided by the authors is posted with the online version of this article as a data supplement at jbjs.org (http://links.lww.com/JBJSOA/A906). This content was not copyedited or verified by JBJS.
Footnotes
Conceptualization: A.F.S., J.M.P., S.W.H.; methodology: A.F.S., J.M.P., T.O., M.P., S.W.H.; investigation: T.O., M.P., E.N., A.F.S.; writing-original draft: T.O., A.F.S.; writing-review and editing: A.F.S., J.M.P., T.O., M.P., S.W.H.; supervision: A.F.S.; funding acquisition: A.F.S.
Investigation performed at Shriners Children’s Philadelphia, Philadelphia, Pennsylvania
This study was approved by WCG IRB #1161547 (PHL1508).
This study was funded and sponsored by Highridge Medical (formerly Zimmer Biomet), Westminster, CO.
Disclosure: The Disclosure of Potential Conflicts of Interest forms are provided with the online version of the article (http://links.lww.com/JBJSOA/A905).
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