Most scoliosis has no single, identifiable cause. The most common form, called idiopathic scoliosis, accounts for roughly 80% of cases and develops from a combination of genetic and environmental factors that researchers are still working to untangle. Scoliosis affects 2 to 3 percent of the population, with the primary age of onset between 10 and 15 years old. But the reasons someone develops a curved spine vary widely depending on their age, underlying health, and the type of scoliosis involved.
Idiopathic Scoliosis: The Most Common and Least Understood
A spine qualifies as scoliotic when it curves sideways by at least 10 degrees. In adolescent idiopathic scoliosis, that curve appears during the growth spurt years with no obvious structural or neurological explanation. “Idiopathic” literally means the cause is unknown, but that doesn’t mean scientists have no leads.
Current research points to several biological systems that may contribute: hormonal imbalances, abnormal bone or muscle growth, and differences in how the nervous system controls posture. One active area of investigation involves melatonin, the hormone most people associate with sleep. Melatonin also plays a role in bone growth and mineralization. Studies have found that people with idiopathic scoliosis tend to have lower bone mass and show signs of disrupted melatonin signaling in their bone cells. Researchers have identified a specific variation in the gene for one of melatonin’s receptors that appears more frequently in people with the condition. The working theory is that faulty melatonin signaling may weaken bone development during the critical years of spinal growth.
None of these factors act alone. The prevailing view is that idiopathic scoliosis results from multiple small biological pushes happening at the same time, during a period when the spine is growing rapidly and is most vulnerable to asymmetric forces.
The Role of Genetics
Scoliosis runs in families. If a parent or sibling has it, your risk is higher. But the inheritance pattern isn’t simple, like a single gene passed from parent to child. Researchers believe many genes are involved, some contributing to whether scoliosis develops at all and others influencing how severe the curve becomes or whether it worsens over time.
Large-scale genetic studies have identified a gene called LBX1 as a strong susceptibility factor in both Asian and Caucasian populations. LBX1 is involved in the development of neurons that help coordinate movement and posture. Specific variations near this gene can either raise or lower the risk: one common variant increases risk by about 56%, while a protective variant cuts risk by roughly 35%. Even so, LBX1 variations explain only about 1.4% of the overall variation in who gets scoliosis. That tiny number underscores how many other genetic and non-genetic factors are at play.
Congenital Scoliosis: Present From Birth
Some babies are born with spinal curves caused by vertebrae that didn’t form correctly during early pregnancy. This is congenital scoliosis, and it stems from problems during the first six weeks of embryonic development, when the vertebral column is taking shape.
The defects fall into a few categories. The most common is a hemivertebra, where part of a vertebra fails to form, leaving a small, triangular “half vertebra” that tilts the spine to one side. In other cases, adjacent vertebrae fail to separate properly, fusing together on one side (called an unsegmented bar) and creating a tether that forces the spine to curve as the child grows. Some children have a combination of both. These structural problems are unrelated to anything the parents did during pregnancy and are not the same condition as idiopathic scoliosis.
Neuromuscular Conditions
When a condition affects the nerves or muscles that support the spine, the resulting imbalance can pull the spine out of alignment. This is neuromuscular scoliosis, and it develops as a secondary effect of conditions like cerebral palsy, muscular dystrophy, spinal cord injuries, and spina bifida.
The mechanism is straightforward: the spine relies on balanced muscle tension on both sides to stay straight. When nerve damage or muscle weakness disrupts that balance, one side can overpower the other, gradually pulling the spine into a curve. The likelihood of developing this type of scoliosis is directly related to how much nerve and muscle function is affected. Children who can’t walk or sit independently are at higher risk than those with milder involvement. Neuromuscular curves also tend to be longer, stiffer, and more likely to progress than idiopathic curves.
Degenerative Scoliosis in Older Adults
Scoliosis doesn’t only start in childhood. Adults over 50 can develop new spinal curves as the structures that keep the spine aligned wear down with age. This is degenerative scoliosis, and it’s driven by the same arthritic processes that affect knees and hips.
The spine is a stack of bony vertebrae connected by discs and small joints called facet joints. Over decades, the discs lose moisture and flatten unevenly. The gel-like center dries out, and the tough outer ring can develop cracks. At the same time, cartilage in the facet joints wears thin, leading to bone-on-bone contact, bone spurs, and inflammation. When this degeneration happens unevenly (worse on one side than the other) it can tip the spine sideways. Osteoporosis compounds the problem by weakening the vertebrae themselves, making them more susceptible to compression fractures that shift alignment further.
Degenerative scoliosis typically causes back pain that worsens with standing or walking, along with stiffness that may develop so gradually it goes unnoticed for years.
Functional Scoliosis: Temporary and Reversible
Not every curved spine is a permanent structural problem. Functional scoliosis refers to a minor curve (typically under 10 degrees) caused by something outside the spine itself. The most common culprit is a difference in leg length. When one leg is even slightly shorter than the other, the pelvis tilts, and the spine compensates by curving to keep the head centered over the body.
This leg length difference can be real (a measurable difference in bone length) or functional, meaning the bones are the same length but tight hip muscles, a tilted pelvis, or soft tissue imbalances make one side sit higher than the other. Muscle spasms from an injury can also create a temporary curve. The key distinction is that functional scoliosis is correctable. Address the leg length difference with a shoe insert, release the tight muscles, or resolve the underlying spasm, and the curve straightens out.
Why Growth Spurts Matter So Much
For adolescents with idiopathic scoliosis, the pubertal growth spurt is the highest-risk period. During this phase, the spine is growing rapidly, and a curve can worsen dramatically in just a few weeks. Doctors track skeletal maturity using a scale called the Risser grade, which measures how much a specific part of the pelvis has calcified, rated from 0 (no calcification, still growing fast) to 5 (fully mature).
The greatest danger of progression happens during the transition from Risser 0 to Risser 1, which corresponds to peak growth velocity. Between Risser 2 and Risser 3, growth slows but the curve can still worsen, so treatment typically continues. Once a young person reaches Risser 5, the skeleton has finished growing and the curve is unlikely to change on its own. This is why age at diagnosis matters so much: a 10-degree curve in a 10-year-old with years of growth ahead carries far more risk of progression than the same curve in a 16-year-old who is nearly done growing.
Other factors that increase the odds of a curve worsening include the size of the curve at first detection (larger curves progress more often), the location of the curve (thoracic, or mid-back, curves tend to be more aggressive), and sex. Girls and boys develop scoliosis at equal rates, but girls are up to eight times more likely to have curves that progress to the point of needing treatment.