Scoliosis is a spinal condition defined by an unnatural, sideways curvature of the spine. It is a complex, three-dimensional deformation involving both a sideways curve and a rotation of the vertebrae. The most visually distinct sign of this structural change is the rib hump, a localized prominence in the back resulting from the underlying skeletal misalignment.
Understanding the Rib Hump
The rib hump, sometimes called a rib arch or thoracic prominence, is a structural deformation of the chest wall. It is a physical manifestation of the spine’s abnormal geometry, most commonly observed in the thoracic (middle and upper back) region. Since the ribs are attached to the thoracic vertebrae, spinal misalignment directly affects the rib cage position. The hump itself is not a muscle spasm or swelling, but a protrusion of the ribs that occurs when the spine pushes the rib cage out of its normal, symmetrical position. The presence and size of the rib hump are related to both the magnitude of the sideways curve and the degree of vertebral rotation.
The Mechanics of Spinal Rotation
The formation of the rib hump is linked to the rotational component of scoliosis. When the spine develops a lateral curve, the vertebrae within that curve twist around their vertical axis. This coupled motion means that a sideways bend is always accompanied by rotation. As the vertebrae rotate, they drag the attached ribs into an altered position. On one side of the back, the ribs are pushed backward and slightly upward, creating the noticeable prominence. Simultaneously, on the opposite side, the ribs are pushed forward and inward, disrupting the symmetry of the rib cage.
Locating the Hump Relative to the Curve
The rib hump is consistently located on the convex side of the scoliotic curve. The convex side is the outer, bulging side of the C- or S-shaped curve, while the concave side is the inner, compressed side. This relationship is a defining feature of the structural deformity. For example, if the spine bends to the right (a right thoracic curve), the rib hump will be present on the right side of the back. The vertebral rotation pushes the posterior ribs on that convex side backward. Conversely, the ribs on the concave side are brought closer together and pushed anteriorly (toward the front of the body).
Clinical Assessment of the Rib Hump
Clinicians use the Adam’s Forward Bend Test to identify and quantify the rib hump. During this non-invasive screening, the patient bends forward at the waist with their feet together and arms hanging freely. This flexed position makes the rotational deformity and the resulting rib hump most prominent and visible. The test involves observing the patient’s back from behind, looking for asymmetry in the rib cage or lower back. To objectively measure the degree of rotation, a specialized instrument called a scoliometer is used. This device measures the Angle of Trunk Rotation (ATR), providing a numerical value that quantifies the severity of the hump.