| A young patient with scoliosis
The term “scoliosis” comes from the ancient Greek word “skoliosis,” which means obliquity or bending. Deviation from the midline and the vertebrae rotational component are not fully understood.
The natural history of a curvature refers to the course of events in spinal maturation without any intervention. Two factors are of paramount importance in the natural history of a curvature: the amount of remaining growth and curve magnitude. Both of these are determined by the radiographic parameters identified in a standing thoracic spine X-ray.
- Skeletal maturity is measured by examining the appearance of the apophysis along the iliac crest known as the Risser sign. On a scale from 1 to 5, it measures the progression of ossification. Growth centers on the vertebral body and triradiate cartilage can also be identified, indicating relative remaining spinal growth. Skeletal maturation is closely linked to the hormonal changes that occur during puberty, which produce the more visible changes of adolescence known as the Tanner stages. This scale also provides clues to the state of skeletal maturity.
- Curve magnitude is an angular measure reported as the Cobb angle. Curves of 11 degrees or more are defined as scoliosis. Lesser curves are defined as spinal asymmetry.
It is important to recognize that our information about the natural history of a curvature is incomplete. Studying large groups of patients, however, has provided additional information on the probability of curve progression. For example:
- Patients that are relatively immature skeletally and have a small curve (an apophysis at Risser 0 or 1 and a Cobb angle of less than 20 degrees) have about a 22 percent chance of curve progression.
- Patients with low skeletal maturity (Risser 0 or 1) and a Cobb angle of 20 to 29 degrees have a 68 percent chance of progression.
- Patients with a skeletal maturity level of Risser 2, 3 or 4 with a Cobb angle of 19 degrees or less have only a 1.6 percent risk of curve progression.
- Larger curves (Cobb angle of 20 to 29 degrees) at the same level of maturity (Risser 2, 3, or 4) have a 23 percent chance of progression.
These studies point to a trend: the less mature the skeleton and larger the curve, the more likely the curve is to progress.
The molecular basis for adolescent idiopathic scoliosis (AIS) is not fully understood. Genetics play a role as 30 percent of families indicate that they have a history of scoliosis. Males and females have small curves in almost equal numbers. Atypical curve patterns and higher curve rigidity are more prevalent in males. This may contribute to the lower curve correction rates with conservative treatment in males.
In addition, it is noted that scoliosis is a three-dimensional deformity.
- Axial rotation of the vertebrae can also be present. This phenomenon can sometimes be observed during the clinical examination, when the vertebrae’s spinous processes are no longer visualized along the posterior surface during the Adam’s Forward Bend Test. Rotation may or may not be present with deviation. If rotation is present without an X-ray-confirmed deviation, clinical reexamination on a yearly basis is recommended to observe for possible changes in deviation.
- Truncal imbalance or compensation may also develop. If the trunk of the body no longer appears centered with the shoulders over the pelvis, and an imaginary plumb line from the top of the rib cage to the gluteal cleft is not in alignment, further evaluation is appropriate. Compensatory curves may develop to bring the head and pelvis into alignment.
Curves progress during periods of rapid growth. Most curves progress at a little less than 1 degree per month, or about 10-12 degrees per year. While studies tell us what the probabilities are for large groups, they do not tell us what will happen for each patient. Individual medical treatment plans are based on several factors.
Even after skeletal maturity is reached, curves can progress. Curve magnitudes of 30 degrees at skeletal maturity require orthopaedic observation over an adult lifespan. Curve magnitudes of 50 degrees at skeletal maturity are at significant risk of progression during adult life.