Scoliosis is a sideways curvature of the spine greater than ten degrees, as measured on an X-ray. It is a three-dimensional deformity that disrupts the spine’s normal alignment, typically appearing during adolescent growth spurts. The visible sign of this structural change is often a prominence on the patient’s back, known as a rib hump or gibbus. This serves as a key indicator during initial screenings.
Understanding Vertebral Rotation and Curve Direction
Scoliosis is not simply a bend to the side, but a rotational deformity where the vertebrae twist along the vertical axis of the spine. This rotational component transforms the lateral curve into a three-dimensional condition. The degree of this twisting motion is often more directly related to the visibility of the hump than the overall magnitude of the sideways curve itself.
When the spine curves laterally, the curve has two distinct sides. The outer side, which bows outward, is called the convex side, while the inner side, which dips inward, is the concave side. The twisting of the spinal column ensures that the vertebral bodies—the main cylindrical parts of the bones—rotate toward the convex side of the curve.
The Mechanics: Why the Rib Hump Forms and Which Side It Appears On
The rib hump is a direct consequence of vertebral rotation occurring in the thoracic spine, where the ribs attach. Since the ribs follow the twisting motion of the spinal bones, as the vertebral bodies rotate toward the convex side of the curve, the ribs attached to that side are pushed posteriorly, or backward.
This posterior displacement of the ribs on the convex side creates the noticeable protrusion on the patient’s back. Simultaneously, the ribs on the opposite, concave side of the curve are pulled forward, or anteriorly. Therefore, the rib hump is consistently located on the convex side of the spinal curve. The size of the hump is correlated with the severity of the vertebral rotation, making it a reliable physical measure of the rotational deformity.
Clinical Assessment of the Rib Hump
Clinicians use the Adams Forward Bend Test to identify and evaluate the rib hump. During this test, the patient bends forward at the waist with their palms together, allowing the examiner to view the back from behind. This action flexes the spine and makes the rotational component of the scoliotic curve significantly more prominent, revealing the rib hump.
The presence of any asymmetrical elevation along the back during this test suggests a structural scoliosis. To quantify the severity of the hump, a tool called a scoliometer is placed over the maximum point of the prominence. This instrument measures the Angle of Trunk Rotation (ATR), providing a numerical value in degrees for the rotational deformity; a higher ATR corresponds to a greater degree of vertebral rotation and a more noticeable rib hump.