Vertebral body tethering (VBT), a surgical alternative to spinal fusion for patients with severe scoliosis, is not in widespread use, but research shows it could help certain populations, young athletes in particular. Still, there are drawbacks.
The FDA approved VBT for use in pediatric scoliosis patients in 2019 as an alternative to spinal fusion. The procedure, which had already been in use in Canada and Europe, is performed at only a handful of hospitals around the world. But new research suggests it could have a promising future for the subset of patients deemed appropriate candidates for the surgery.
“The patients who are potentially candidates for VBT ... have remaining growth and ... moderate and flexible curves up to 60 or 65 degrees,” says Baron S. Lonner, MD, Chief of Minimally Invasive Scoliosis Surgery at Mount Sinai Hospital and Professor of Orthopaedic Surgery at Icahn School of Medicine. “Finding[s] from my own patients … [show they] did indeed have similar curve correction as for fusion in a two-year follow-up.”
How VBT Works
Zimmer Biomet Spine’s The Tether, so far the only FDA-approved VBT system in the U.S., employs anchors, bone screws and set screws made from titanium alloys used in common spine implants. These are placed on the side of the curvature on the patient’s spine. A cord, made from a strong flexible polymer, is then secured and tightened to partially straighten the spine.
“The concept is to have progressive curve correction over time as the patient matures and then reaches skeletal maturity with a corrected curvature,” Dr. Lonner says.
Unlike spinal fusion, VBT allows patients to retain their full flexibility, making it an appealing choice for young athletes, such as those who swim, dance or practice gymnastics.
Dr. Lonner worked with Zimmer Biomet Spine to create The Tether.
“Twenty-five years ago, I was one of the handful of surgeons in the United States performing scoliosis fusion through the side, like we’re doing now for tether,” Dr. Lonner says. “So the approach was very similar, and that provided a lot of insights for me. ... It was less muscle dissection, and there was some benefit to that.”
Drawbacks to VBT
Dr. Lonner is careful to emphasize that his results showing a similar curve correction from VBT compared with spinal fusion are just from his own patients — all carefully selected as candidates — and the results are only two years out.
“We really need five-year, 10-year and 20-year follow-ups and comparisons to fusion to determine the best route,” Dr. Lonner says.
Research conducted in Canada in the years following surgery (using a different tether than the Zimmer Biomet version) showed a higher than expected rate — 15.8% — of post-surgery complications.
“What we found is that every time point — from the first being in hospital, to 30 days, to one year, to two years — the complication rates and the risk of re-operation increased, at every one of those time points,” says Ron El-Hawary, MD, MSc, FRCS(C), Chief of Orthopaedics at the IWK Health Centre and Professor in the Department of Surgery at Dalhousie University. “That’s pretty common for a lot of surgical procedures, but you hope that things plateau over the long term. That’s why a two-year followup is usually a good benchmark for orthopedic study, because if an implant survives for two years, that’s usually good for the much longer term. What’s different with vertebral body tethering is that as time goes on, we suspect that there might be more complications that arise.”
Dr. El-Hawary also has found, in research to be published in 2022, that a patient’s weight can impact the effectiveness of VBT.
“If patients have a BMI greater than the 85th percentile for their age ... the VBT doesn’t seem to work as well over time,” Dr. El-Hawary says. “You can get a good correction in the OR, but that growth modulation where you hope the scoliosis continues to get better over time — it doesn’t seem to work as well if you’re overweight.”
Between 2% and 3% of the U.S. population — 6 to 9 million people — have scoliosis. Most children affected experience the onset of the condition between ages 10 and 15. Although both males and females are equally affected, females are eight times more likely to progress to a severe curvature that requires bracing or surgery.
Can VBT Use Become Widespread?
The more familiar risks of spinal fusion, combined with its wide availability, are likely to ensure it remains the most common surgical treatment for scoliosis in the near future.
“It’s very predictable in terms of what happens over the short term, you get a great correction of the scoliosis, and it’s a pretty safe operation,” Dr. El-Hawary says. “But the downside is that you lose some mobility of the spine and can end up with vertebral degeneration 20, 30, 40 years later. So that’s why we are thinking about fusion type surgery, as it might decrease those longer term issues.”
Ongoing research to track patients five and 10 years out from VBT may provide more answers to questions including how many patients are breaking the cord, how many patients are experiencing overcorrection in their curvature and how many patients ultimately need spinal fusion.
“We’re following our patients and seeing where the sweet spot is for the proper indications and the proper technique for applying the tether,” Dr. Lonner says. “So getting more data is going to be important for the next generation of surgeons. And training more surgeons is part of the process of increasing availability for patients and their families.”
Advanced robotics may also be able to improve accuracy and efficiency of placement of the screws, lowering complications. However, finding the correct window of growth for VBT surgery may be the most important path forward.
“I do think this will become more widely utilized and available for patients,” Dr. Lonner says.