<p>Disease‑modifying therapies (DMTs) not only have significantly extended survival for patients with spinal muscular atrophy (SMA) but also have brought to light potential new long-term orthopedic concerns associated with the disease. At <strong>Cure SMA 2025</strong>, Brian D. Snyder, MD, PhD, discussed these emerging orthopedic considerations during a session on orthopedic management in SMA.</p> <p><br></p> <p><em>Following this presentation, featured expert Brian D. Snyder, MD, PhD, was interviewed by </em>Conference Reporter<em> Associate Editor-in-Chief Mona Shah, PharmD. Clinical perspectives from Dr Snyder on these findings are presented here.</em></p>

The musculoskeletal complications of SMA typically include contractures from a limited functional range of motion at the shoulder, elbow, wrist, fingers, hip, knee, and ankle, as well as skeletal pathoanatomy that involves the axial skeleton and manifests as spinal deformity (including scoliosis and/or kyphosis) and hip instability (including proximal femoral coxa valga, excessive femoral anteversion, and acetabular dysplasia). While we previously thought that much of the skeletal pathoanatomy and functional disabilities in patients with SMA were related exclusively to muscle weakness from a lack of SMN protein during motor neuron development, we are now starting to identify a skeletal phenotype that not only may be related to SMN protein levels but may also be dependent on the SMN2 copy number.

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In the pre-DMT era (ie, around 2016), children with less than 3 SMN2 copies often died early. And so, before the introduction of DMTs, we did not typically see spinal deformities until later, usually not until age 5 to 8 years, and these presented as long, sweeping, C-shaped thoracolumbar kyphoscoliosis, typical of many neuromuscular conditions. However, the development of severe high thoracic kyphoscoliosis, hip instability, pectus carinatum, and micrognathia in children with 5qSMA under age 3 who are treated with DMT at an early age suggests that these pathoanatomical features may be part of an SMA skeletal phenotype that we did not see previously. In other words, we are now seeing how DMTs have changed the spectrum of SMA disease manifestations.

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As I discussed during my presentation at Cure SMA 2025, scoliosis is more than just poor posture; the early-onset scoliosis that occurs in 60% to 90% of children with SMA types 1 and 2 is often associated with concomitant pelvic obliquity, thoracic insufficiency, and cardiopulmonary compromise. Once the spinal deformity progresses to a Cobb angle of greater than 50 degrees, it starts to have a profound restrictive effect on pulmonary function, leading to thoracic insufficiency and a need for pulmonary assistance (eg, bilevel positive airway pressure).

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In addition to spinal deformity, children with SMA can also develop a contorted rib cage (often referred to as “parasol” ribs), which compresses the lungs and restricts the space available for the lungs to grow. However, the size of the thorax (ie, rib cage and spine height) in a patient who is 8 years of age is only 40% of that of an adult, so early-onset scoliosis before age 8 can have a profound effect on pulmonary function.

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Spinal deformity correction by spinal instrumentation is only partially effective in preserving lung growth and function. Thus, early treatment to mitigate scoliosis and limit progression to less than or equal to 50^° using a customized, contemporary spine brace is indicated. If nonoperative treatment fails, growth-preserving spinal instrumentation using magnetically actuated posterior rods that allow the spine and chest to continue to grow is indicated.

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When we stabilize the spine with posterior rod implants that anchor to the pelvis, lumbosacral spine mobility is lost. As a result, a previously asymptomatic, unstable hip now becomes symptomatic because lumbopelvic motion can only occur through the hips. So, stabilizing the spine to address scoliosis and preserve pulmonary function may lead to symptomatic hip instability that mandates hip reconstruction.

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In the pre-DMT era, the management of hip instability was suboptimal. With DMT, many children with SMA can walk with assistance, stand, and do pivot shift transfers. So, now there is new emphasis on treating hip instability by comprehensive hip reconstruction, addressing the coxa valga and femoral anteversion with varus derotational intertrochanteric osteotomy and the acetabular dysplasia with a posterolateral acetabuloplasty. These changes in management are important to minimize pain and maximize function for the patient.

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Finally, concerns about not lengthening a contracted muscle because it may weaken it need to be reassessed within the context of the functional consequences of the associated joint contractures. For example, if you have a crouched posture because of hip and knee flexion contractures, fractional lengthening of the joint-deforming hip flexors and hamstrings is indicated with the goal of achieving an upright posture, which is important for standing and walking. However, these surgeries must be combined with aggressive physical therapy and muscle-strengthening programs. So, there needs to be education to transform long-held views and to change orthopedic practice based on evidence-based studies.