Prospective Vertebral Body Tethering Data

Commentary I congratulate Larson et al. on their well-presented Investigational Device Exemption study that was initiated by the investigators rather than the company. This was no small undertaking, and I commend the authors for taking this path to prospective investigation of a technique. At the initiation of the study in 2015, there was no device cleared by the U.S. Food and Drug Administration (FDA) for this indication (the FDA Humanitarian Device Exemption occurred in 2019). Thus, much of the literature on vertebral body tethering (VBT) is retrospective. The cohort studied was a fairly uniform population with adolescent idiopathic scoliosis (AIS): most had a single thoracic tether, all curves were between 40° and 70°, and patients were enrolled and followed under a strict protocol. The amount of skeletal growth remaining is thought to be an important variable in determining the extent of growth modulation based on additional curve reduction following VBT. The range of remaining growth in the cohort, as assessed by the Sanders Maturity Score, was quite narrow. Two-thirds of the cohort were at Sanders 4 and another quarter were at Sanders 3. Thus, the results are most helpful in assessing the outcomes in this specific patient age range—beyond peak height velocity, and with modest growth remaining. The average T1-S1 spine height increased 20 mm over the first 2 years postoperatively. Standing height gained during follow-up was not presented. One of the challenges in reporting the outcomes of VBT is achieving consistent follow-up. The authors were able to follow a consecutive series of patients for a minimum of 2 years. Although 2 years is the minimum standard that we apply for publication, the cohort was undergoing growth and had a device that is known to fatigue and break, so we must interpret these results (like many that have been reported before) with an eye on what the additional follow-up times points will yield. Even with the minimum 2-year and average 3.8-year results presented, Larson et al. demonstrate clinical “success” that was declining over time: 85% at 1 year, 75% at 2 years, and 62% at 3.8 years. The sustainability of the correction may be related to the degree of growth modulation achieved in the years following implantation of the tether. If the growth and thus the modulation of vertebral shape are limited, the correction may be lost as the tether fails. More growth after implantation (selecting patients younger than Sanders 4) may yield growth modulation that leads to greater correction (or even overcorrection), and thus more potential for permanent vertebral shape change in the process, potentially resulting in a more sustainable long-term correction. The authors did not find the rate of suspected tether breakage to be as high as in some prior reports. Measuring every radiograph made at 6-month intervals may yield a higher rate of detecting the >5° change in angulation between screws at adjacent segments that is required to declare a suspected failure, compared with the annual measurements made in this study. The authors, like others, conclude that cord breakage does not necessarily result in clinical failure. This is an important point, and I absolutely agree. However, I also would suggest that the best chance for a lasting correction, given the knowledge that cord failure is coming eventually, likely requires greater growth potential than the majority of the patients in the present study had. The fact that the clinical success rate declined with each year of follow-up remains a concern that we must confront. How do we achieve an 85% success rate that lasts well beyond skeletal maturity? This paper is an important addition to the growing body of literature regarding the outcomes of VBT in the treatment of AIS. Nevertheless, the precise indications with regard to curve location and magnitude and remaining skeletal growth that will yield a durable long-term scoliosis correction continue to be defined and clarified.