Idiopathic scoliosis with a curve greater that 10 degrees has a reported prevalence of 0.5 to 3 per 100 children and adolescents.
The reported prevalence of curves greater than 30 degrees is much less, from 1.5 to 3 per cent per 1,000. Thus, small to moderate curves are common, and severe curves that require treatment are rare. Most Idiopathic scoliosis presents during adolescence.
AIS theoretically develops after age 10 years and is associated with rapid growth. Roughly 2% of adolescents have scoliosis with a curve progression greater than 30 degrees. The ratio of boys to girls is equal for minor curves, but the number of affected girls rises as the curve magnitude increases, reaching a ratio of 1:8 for those requiring treatment.
Identified risk factors for scoliosis progression include sex, remaining skeletal growth, curve location, and curve magnitude, with progression being the most rapid during peak skeletal growth. Peak growth velocity of adolescence averages 6 to 8 cm of overall height gain per year. Bone age and menarcheal status help determine the growth spurt in girls, with the onset of menses generally following the most rapid stage of skeletal growth by approx 12 months. When the Risser sign is 1 or less, the risk for progression approaches 60% to 70%. However, if the patient is Risser 3, the risk is reduced to less than 10%. Unfortunately, many of the readily identified markers of maturity are variable and appear just after the adolescent growth spurt. Therefore, it is impossible to tell if a patient who is premenarcheal and Risser 0 is approaching, in the midst of, past the most rapid growth and thus, at risk for scoliosis progression. Closure of the triradiate cartilage of the acetabulum has been identified as a radiographic sign, which more closely approximates the time of peak growth velocity.
Curve pattern has also identified as an important variable for predicting the probability of progression. Curves with an apex above T12 are more likely to progress than isolated lumbar curves. Curve magnitude at the initial diagnosis appears to be a factor predicting progression. Larger curves are more likely to continue to increase in magnitude with growth.
Pulmonary function becomes limited as thoracic scoliosis becomes more severe. Forced vial capacity and forced expiratory volume in 1 second decrease linearly, with an approximate 20% reduction in predicted values with 100% curves. The associated deformity of the chest cavity causes restrictive lung disease. Thoracic lordosis also decreases lung volume and and increases the deleterious effects of scoliosis on pulmonary function.
Estimates of the frequency of back pain in affected individuals in adults with scoliosis vary, but most studies report higher rates of back pain in affected individuals compared with control groups. Although the risk of curve progression is highest during rapid phases of growth, not all curves stabilise after growth. In long term studies, many patients have progression following skeletal maturity. Curves less than 30 degrees tend not to progress, with the most marked progression occurring in curves between 50 degrees and 70 degrees at the completion of growth (progression continuing at a rate of nearly 1 degrees per year).
Lumbar curves are most likely to progress if they are greater than 30 degrees at skeletal maturity. This risk of progression in adults after skeletal maturity has led to many of the treatment recommendations regarding surgical management of scoliosis.