The success of a novel scoliosis surgery hinges on when growth spurts hit. Can new tools better predict them?

Mia Schloegel was 11 when they found the curve in her spine.

She was at her yearly pediatrician’s visit when her uncle — also her doctor — had her lean over, in a standard childhood check for scoliosis. “He noticed I had kind of a hump on one side and not the other,” said Schloegel, a sign of the rib cage rotation that often occurs when the spine is curved side-to-side. At her uncle’s urging, she and her mother went to get an X-ray at a Kansas City hospital that same day.

The news was a shock to the sixth grader. “I remember crying in the doctor’s office, because I didn’t think there was anything wrong with me,” said Schloegel, now 21. But all of a sudden, a surgeon was recommending that she have her spine fused together. For decades, fusion has been the default surgical approach for severe curves: A permanent network of screws, hooks, and wires would attach her misaligned vertebrae to two metal rods, holding them in position as bone grafts took hold. The middle span of her spine would turn into a solid, ossified pole.


Schloegel’s mother, an occupational therapist, started hunting for more information. Her daughter swam, ran cross country, and played basketball; grafting her spine together and permanently robbing her of some flexibility couldn’t be the only option.

“My parents were researching and trying to learn everything they could about scoliosis,” said Schloegel. “And that’s when my mom found the new tethering procedure.”


In 2012, Schloegel’s parents came across two U.S. practices performing vertebral body tethering, an experimental surgery that aims to straighten children’s spines without limiting their mobility. The method runs a flexible cord down the outside of the curve, through screws drilled into the vertebrae. Surgeons speculated that by applying tension to the wedged, misshapen bones, they would reshape over time into the orderly, rectangular building blocks they were meant to be.

For Schloegel and her parents, it was a difficult choice to make: Fusion, which still accounts for the vast majority of the 30,000 scoliosis surgeries in the U.S. each year, limits certain movements, but it also has a 60-year safety record and only requires follow-up operations in 5% of cases. The new procedure might preserve her flexibility, but it also remained highly experimental.

“When we went in, they were still talking about their research on animals,” said Schloegel’s mother Bebe. “So that was pretty alarming.”

But Schloegel and her parents decided to try tethering, and at 12 years old, she became one of the first patients to undergo the procedure, flying from Kansas City to San Diego for the surgery. (The other option was in Philadelphia, but Schloegel was excited to go to California — it’d be a built-in vacation every time she needed to go for a follow-up visit.)

With so few tethers performed at the time, a crucial piece of evidence hadn’t yet emerged: The success of the surgery depends on how much growth a patient has left. Tether too early, and curves can overcorrect in the wrong direction. Tether too late, and kids won’t grow enough for the procedure to work.

“I started working on this concept of modulating growth in animals more than 20 years ago,” said Peter Newton, a pioneer of the procedure and the pediatric orthopedic surgeon at Rady Children’s Hospital who performed Schloegel’s surgery. “And 10 years into the clinical experience, I still don’t even know if it works well enough to do on patients.”

That is changing as tethering becomes more commonly available. In 2019, the Food and Drug Administration approved a tethering device through its humanitarian device pathway, which exempted it from proving efficacy before approval. Today, more than 100 surgical centers have been trained in the technique, and more than a thousand scoliosis patients have gotten tethers.

But as the number of tethering surgeries increases, the technique’s 10 years of data still leave many questions unanswered. And critically, orthopedists still don’t have a reliable and objective way to predict how much growing a kid has left.

“It’s very clear that patients have maybe even a more challenging decision to make today with the options that exist,” said Newton. It will take years of study — and investment in a new class of data-driven predictors for skeletal maturity — for tethering to reach the right patients.

The success of scoliosis treatment has always been a matter of timing.

Generations of kids lined up in gym class to have teachers check for the hump Schloegel’s uncle found. Such screening has fallen out of practice, but doctors still try to catch a curve early to allow noninvasive treatments like bracing their best shot.

In the case of adolescent idiopathic scoliosis, when a child’s spine is curved without a known cause, the concave side of each vertebral body is compressed. That creates wedge-shaped bones that worsen the curve over time, a pattern that orthopedist Ian Stokes coined “the vicious cycle of scoliosis.” A brace, molded to the curve and worn almost 24 hours a day, can stave off that progression when curves are mild.

Severe curves, more than 40 or 50 degrees, can worsen over time, eventually pushing into organs and making it difficult to breathe. Those cases, of which about 90% occur in girls, usually require surgery, and timing there is key as well: Fuse a spine too early, and a kid won’t be able to finish growing.

That problem doesn’t exist with tethering, a procedure that involves deflating one lung so surgeons can approach the spine through the rib cage. Through small incisions along the torso, surgeons drill holes laterally through the vertebrae, place screws in each, and string the tether through, adjusting its tension bone by bone.

The mechanics are still unproven, but the most prominent theory suggests that balancing out the load on curved vertebrae helps them to grow into a straight stack. “When you do a tether, all you’re doing is trying to hold the outside of the curve nice and taut,” said Firoz Miyanji, a pediatric orthopedic surgeon at BC Children’s Hospital in Vancouver who has done hundreds of tetherings. “As long as you can get those inside discs parallel so you offload the force that’s on them, they start to grow normally, and then away you go.”

But that kind of growth modulation takes time.

“If you do this operation with too much growth remaining on too small of a curve, you will growth modulate more than required,” said Newton. His first tethering patient needed two follow-up surgeries; one to remove the tether from an overcorrecting curve and extend it to a worsening one, and another to fuse his spine.

“The challenge is defining ‘too small’ and ‘too young,’” Newton said. But a crop of researcher-clinicians are working to build predictive tools to easier to know when — and whether — to tether.

To measure how much growing a kid has left and how much their curve might progress, orthopedists have long used X-ray-based measures of “bone age.” The Risser grade looks at the pelvic bone to document its growth and ossification; the Sanders score does the same by looking at the left hand. When Schloegel had her surgery, she was at a Risser zero.

But the two techniques often disagree with each other. And neither can capture the potential for growth with enough specificity to predict surgical outcomes on their own.

Michelle Welborn, an orthopedic surgeon at Shriners in Portland, Ore., describes three of her tethering patients, all of whom were at Sanders stage five. “One of them somehow lost a centimeter, and she never corrected more than what I got her on the table,” she said. “One of them grew a centimeter and a half, and it is perfectly straight; like, it is glorious. And one of them grew 6 centimeters in six months and she’s the only patient I’ve ever had that overcorrected.”

Skeletal maturity, as measured by those X-ray-based scales, is merely a surrogate for the more important measure: growth potential. So at Shriners, Welborn has been researching a blood-based test to determine where patients are in their growth. As we age, the soft bone in our bodies is slowly converted into hard bone. “It’s that conversion process that we’re actually measuring,” said Welborn, by tracking levels of a protein called collagen X that facilitates ossification. The lower the collagen X level, the closer a kid is to being done growing. The test isn’t validated as a predictor, but Welborn is midway through a five-center study that tracks scoliosis patients’ collagen X levels through their entire growth curve.

At the University of Montreal, orthopedist Stefan Parent is leading predictive research that takes advantage of the 3D X-rays that have become standard care for scoliosis patients. Looking at basic information like curve size, age, menstruation status, and a combination of Risser and Sanders scores in 172 patients, they were able to accurately predict the curve’s final state about 50% of the time, said Parent; adding 3-D measures of wedging pushed it up to 65%. He suspects the rest might come down to how rigorously patients stick to wearing their brace; his next step is to follow patients’ growth while also tracking their brace adherence with built-in sensors.

Genetic tests could also help predict growth and curve progression. Researchers at Washington University School of Medicine in St. Louis, home to a DNA databank for children with musculoskeletal disorders, received a $3.2 million grant to study the genetic basis of scoliosis and whether predictive mutations identified in the past are generalizable across diverse populations.

“That’s a lot of what I think the future is,” said Welborn. “Where they can combine Dr. Parent’s 3D growth, our biomarker for how much growth you’ve got left, and then some of these genetic tests can really help to refine that process.”

Miyanji, the Vancouver orthopedist, said that while that kind of modeling would be ideal, “we’re nowhere near that.”

Schloegel’s surgery seems to have happened at the sweet spot for tethering. Looking retrospectively, Newton said, it’s clear that “those who have changed in height the most, got the most correction from their tether.” Schloegel gained an inch overnight when she had her surgery, and grew to a tall 5-foot-9.

scoliosis -Mia Schloegel
Before vertebral body tethering, the major curve in Mia Schloegel’s spine measured 47 degrees. Nine years later, she has two 35-degree curves. Courtesy Mia Schloegel

Immediately after the surgery, Schloegel’s 47-degree curve looked better, but her spine wasn’t straight — most aren’t, even after a fusion. “I used to ask my parents, ‘Why couldn’t they just straighten my spine in surgery fully?’” But her vertebrae still needed time to remodel. It took two years for Schloegel’s spine to hit its smallest measured curve, in the low 20s.

By Schloegel’s standards, the surgery was a success. She hasn’t needed a back-up fusion, isn’t in pain, and has followed in her dad’s footsteps to become a triathlete.

Still, her spine isn’t perfect. About four years after the surgery, Newton noticed that two of Schloegel’s screws had angled apart slightly, suggesting the tether connecting them had broken. Another two years later, it looked like the tether had broken in another spot. Her tethered curve settled at about 35 degrees, and a second curve above it has grown to balance it out.

“That tethering device has a limited life span; it eventually frays and breaks,” said Newton. And in patients like Schloegel, whose vertebrae have already remodeled, “the rope’s not doing much anyway, so it doesn’t go flying around or go anywhere or do anything crazy,” he added.

But that tether break, and Schloegel’s second curve, indicate that there’s still a lot surgeons need to learn. They’re now presenting data on patients with as many as five years of follow-up. Broken tethers are common. Depending on the study, follow-up surgeries are necessary up to 39% of the time; up to 13% of tethering patients ultimately get a fusion. As more surgeons get practice, those figures could decline.

The standards used to determine a successful surgery also still vary significantly; some groups consider any surgery that avoids fusion a success, while others also require a curve to be reduced below 30 degrees. And tethering’s primary advantage over fusion — its ability to maintain flexibility — still hasn’t been proven in long-term followup studies.

Zimmer Biomet, the company that makes the FDA-approved tethering device (called, simply, “The Tether”), is conducting a post-approval study as required by the FDA.

As that data is collected, and surgeons share outcomes through registries like the Pediatric Spine Study Group and Setting Scoliosis Straight, orthopedists have come up with their own tentative guidelines for treatment. Most of the surgeons STAT spoke with have come to the same consensus: Tethering is most likely to succeed in patients who have several years of growth left, with curves between 40 and 60 degrees.

In reality, though, there’s no telling when a kid will go through a growth spurt. And until long-term data and predictive tools combine to guide surgical decisions with more precision, the emotional burden of deciding between tethering and fusion will fall to patients and their families, whose choice is only complicated by the young age of many patients.

“Talking to a 13-, 14-, or 15-year-old,” the target age for many fusions, “the patient often is driving the conversation,” said Noelle Larson, an orthopedic surgeon at Mayo Clinic. But because tethering relies on continued growth, surgical conversations often come earlier. At 11, Schloegel’s parent’s were largely responsible for the decision to tether, which she understands now that her 11-year-old sister is being monitored for scoliosis. “I truly feel like many 10-, 11-, 12-year-olds are not able intellectually to make that type of medical decision about their back,” said Larson.

Most of the decision-making, then, ends up in the hands of the parents, many of whom are enthralled by the prospect that they could correct their kids’ curves while preserving their flexibility. Online forums for scoliosis are filled with largely positive conversations about tethering, and many providers said they’ve seen an uptick in families coming to them with questions about the surgery.

“In my early years, I remember doing a pre-op on a patient and when I walked in, the father said, ‘Hey, Dr. Miyanji, is my son’s X-ray going to look as good as so and so?’ And it was one I had just done the day before.” The patient had already posted them to a Facebook group.

I feel the pull, too: 18 years ago, a spinal fusion locked up 13 of my vertebrae. The first time I read about tethering, I felt a twinge of buyer’s remorse. I’m healthy, but I worry about my back pain worsening over time. What free discs remain in my spine continue to absorb the lion’s share of life’s jolts and shudders, which could lead to arthritis down the line.

“Everybody wants to not have a fusion, right?” said Newton. “But I think it’s important to have a realistic message, and it’s helpful for people to realize this just isn’t a slam dunk.” His hope, though, is that within the next 10 or 20 years, tethering can become a more standardized treatment for scoliosis.

“It truly is the first new technology that we’ve had for pediatric spine surgery in 30 years, so it’s very exciting,” said Larson. “Everyone in the field just wants to do a good job with it and make sure that it’s successful for as many children as possible.”

Schloegel, now a junior at the University of Kansas, sees that potential. She’s premed, and is studying mechanical engineering with an eye for its applications to medical devices. “I think me being one of those first patients to have the tethering procedure has inspired me to want to learn more about innovative medical devices and surgeries,” said Schloegel. “I know how important it is. I just know things can get better and there’s better ways to do things.”

Source: STAT