S-shaped scoliosis develops when the spine forms two curves in opposite directions, creating a shape that resembles the letter S on an X-ray. This “double curve” pattern typically involves one curve in the upper (thoracic) spine and a second in the lower (lumbar) spine. The causes range from genetics and growth patterns in teenagers to wear-and-tear joint damage in older adults, and the specific cause depends heavily on when the curves first appear.
What Makes an S-Curve Different
Most scoliosis involves a single curve, called a C-curve. An S-shaped curve is a double curve where the spine compensates for one bend by curving in the opposite direction at a different level. This compensation can happen naturally as the body tries to keep the head centered over the pelvis, or both curves can develop more or less simultaneously. Because there are two curves involved, S-shaped scoliosis can be harder to detect visually since the curves partially cancel each other out, making the torso look more balanced than it actually is.
Doctors measure each curve separately using the Cobb angle on X-rays. A curve under 10 degrees isn’t considered true scoliosis. Curves between 10 and 20 degrees are mild, 20 to 40 degrees are moderate, and anything above 40 degrees is severe. In S-shaped scoliosis, each curve gets its own measurement, and one is usually larger (the “major” curve) while the other is smaller (the “compensatory” curve).
Idiopathic Scoliosis: The Most Common Cause
About 80% of scoliosis cases have no single identifiable cause, which is why they’re labeled “idiopathic.” This is by far the most common reason a teenager develops an S-shaped curve. It typically appears during the growth spurt before puberty, between ages 10 and 15, and affects girls more often than boys in terms of curves that progress enough to need treatment.
Although no single gene causes idiopathic scoliosis, the condition clearly runs in families. Genome-wide studies have identified several gene variants linked to higher risk, with the most significant being a variant near the LBX1 gene. Other associated genes include GPR126 and BNC2, along with a region near the PAX1 gene found specifically in female patients. Rarer genetic variants involve genes responsible for connective tissue components like fibrillin and collagen, suggesting that the structural scaffolding around vertebrae may play a role.
Beyond genetics, several biological theories attempt to explain why the spine starts curving. One involves disruptions in melatonin signaling that affect bone-building cells. Another proposes that the front of the spine grows faster than the back, creating a mismatch that forces the spine to twist and bend. Researchers have also found that patients with idiopathic scoliosis have elevated levels of a calcium-regulating protein called calmodulin in their blood platelets and an uneven distribution of it in the muscles alongside the spine. None of these theories alone fully explains the condition, which likely results from multiple factors converging during rapid growth.
Why Growth Spurts Matter
The biggest risk factor for a curve getting worse is how much growing a child still has left to do. Doctors assess this using the Risser scale, which grades skeletal maturity from 0 to 5 based on how much of the hip bone’s growth plate has hardened. A Risser stage of 0 to 2 signals that the skeleton is still immature and the risk of curve progression is high. Stages 3 to 5 indicate the skeleton is nearing or has reached maturity, and curves are much less likely to worsen.
This is why S-shaped scoliosis diagnosed in a 10-year-old with years of growth ahead carries a very different prognosis than the same curve found in a 16-year-old who’s nearly done growing. The curves themselves don’t cause scoliosis, but rapid skeletal growth acts as fuel for whatever underlying process started the curvature.
Congenital Vertebral Defects
Some babies are born with vertebrae that didn’t form properly during fetal development, and these malformations can produce S-shaped curves early in life. The defects fall into three categories.
Formation defects mean part of a vertebra simply didn’t develop. The most common example is a hemivertebra, where only half of the bone forms, creating a wedge shape that tilts the spine to one side. Hemivertebrae can be fully segmented (with normal growth plates above and below, making them more likely to worsen), partially segmented, or fused to a neighboring vertebra.
Segmentation defects happen when adjacent vertebrae fail to separate properly. A unilateral bar, where one side of two or more vertebrae fuses together while the other side continues growing, creates progressive asymmetry. A bilateral fusion creates a solid block of bone that doesn’t grow at all in that segment.
Mixed defects combine both problems. A child might have a hemivertebra on one side and a bony bar on the opposite side at a different spinal level, which is particularly prone to producing an S-shaped double curve because the growth disturbances pull the spine in different directions at different levels.
Neuromuscular Conditions
When the muscles or nerves that support the spine don’t work properly, the spine can gradually collapse into curved patterns, including S-shapes. The most common neuromuscular conditions responsible include cerebral palsy, Duchenne muscular dystrophy, spinal muscular atrophy, Friedreich ataxia, spinal cord injury, and myelomeningocele (a form of spina bifida).
Neuromuscular scoliosis behaves differently from idiopathic scoliosis. The curves tend to be longer, sweeping across more vertebrae, and they often continue to worsen even after a person stops growing because the underlying muscle weakness or imbalance persists. Children who use wheelchairs full-time are at especially high risk because the upright forces on the spine are distributed unevenly when trunk muscles can’t provide balanced support.
Degenerative Changes in Adults
S-shaped scoliosis can also develop for the first time in middle age or later in a spine that was previously straight. Called “de novo” degenerative scoliosis, this happens when the discs between vertebrae and the small facet joints at the back of the spine wear down unevenly. If the left side of a disc loses height faster than the right, the vertebra above tilts, and the process repeats at neighboring levels.
This asymmetric breakdown creates a self-reinforcing cycle. Once the spine starts tilting, the uneven loading accelerates degeneration on the compressed side, which increases the tilt further. Osteoporosis compounds the problem, particularly in postmenopausal women, because weakened bone is more susceptible to compression fractures that add to the asymmetry. Over time, the combination of disc collapse, joint arthritis, and vertebral fractures can produce both lateral curves and forward slippage of one vertebra over another.
Degenerative S-curves most commonly affect the lumbar spine and tend to be smaller in degree than adolescent curves, but they can cause significant pain and functional problems because the degenerated joints and compressed nerves are themselves painful, independent of the curvature.
How S-Shaped Scoliosis Is Detected
The visible signs of S-shaped scoliosis are often subtle because the two curves partially offset each other. Still, certain asymmetries are telling: one shoulder sitting higher than the other, one shoulder blade protruding more prominently, one hip appearing higher, or one side of the rib cage rising more than the other when bending forward. That forward bend test is often the first screening tool used in schools or doctor’s offices.
Diagnosis requires imaging. Standing X-rays of the full spine remain the standard, allowing doctors to measure each curve’s Cobb angle and assess skeletal maturity. When curves appear atypical or suggest an underlying cause, MRI is used to check for spinal cord abnormalities, which are more common in these cases. A newer imaging system called EOS stereoradiography provides full-spine 3D images with significantly less radiation than traditional X-rays, though it’s expensive and not widely available. Ultrasound-based measurement of spinal curves is in early development as a radiation-free alternative, but isn’t ready for routine clinical use yet.
CT scans, while detailed, deliver a high radiation dose and are increasingly being replaced by MRI for scoliosis evaluation. When CT is used, it’s typically reserved for surgical planning in complex cases where precise bony anatomy needs to be mapped.
Multiple Factors, Not One Answer
In most cases, S-shaped scoliosis doesn’t have a single clean explanation. A teenager with idiopathic scoliosis likely has a combination of genetic susceptibility, connective tissue characteristics, and growth timing that together produced the double curve. An older adult’s S-curve reflects years of accumulated joint wear, bone density changes, and the biomechanical domino effect of asymmetric loading. Even congenital cases often involve multiple vertebral anomalies at different levels interacting with each other as the child grows. The S-shape itself is less a diagnosis than a description of how the spine responded to whatever underlying process set the curvature in motion.