Scoliosis is characterized by an abnormal, three-dimensional curvature of the spine. This condition involves a rotational component, meaning the vertebrae twist along their vertical axis as they curve. This twisting motion alters the surrounding structures, and the most visible external sign is often the presence of a rib hump.
Identifying the Convex Side Protrusion
The rib hump is consistently located on the side of the spinal curve that bulges outward, known as the convex side. The inner edge of the curve is called the concave side. The prominence of the ribs on this convex side creates the visible asymmetry of the back.
The spinal curve is named for the convex side; for example, a curve bowing out to the right is a right-convex curve. The rib hump will therefore be found on the right side of the back. This physical protrusion becomes more pronounced when the individual bends forward, highlighting the underlying rotational deformity.
The Role of Vertebral Rotation in Rib Hump Formation
The appearance of the rib hump is a direct consequence of the spine’s rotational component. As the spine bends to the side, the vertebral bodies rotate toward the convex side of the curve. This three-dimensional twisting mechanism differentiates structural scoliosis from a simple postural curve.
The ribs are attached to the thoracic vertebrae and are mechanically pulled along with the rotating spinal bones. This movement creates a differential effect on the rib cage. On the convex side, the rotation of the vertebrae pushes the attached ribs backward and outward, causing the characteristic posterior prominence.
Conversely, on the concave side of the curve, the ribs are pulled forward and inward, sometimes appearing flattened or closer together. This asymmetrical distortion of the rib cage is most pronounced in the thoracic section of the spine, where the ribs are directly connected to the vertebrae.
Clinical Assessment of the Spinal Deformity
Healthcare professionals primarily use the Adam’s Forward Bend Test to screen for and visually confirm the presence of a rib hump. During this procedure, the patient bends forward at the waist with their palms together, making the spinal asymmetry and trunk rotation most apparent. This position eliminates the influence of posture and muscle compensation, allowing for a clear view of the rib cage and back contour.
To objectively measure the degree of this trunk asymmetry, a scoliometer is used during the Adam’s test. This device is placed over the most prominent part of the curve to quantify the Angle of Trunk Rotation (ATR) in degrees. An ATR reading of 5 to 7 degrees or more suggests the presence of a significant underlying spinal curve, warranting further evaluation.
The ATR measurement provides a reliable, non-radiographic indicator of the rotational severity, but it is not a diagnostic tool for scoliosis itself. A definitive diagnosis and the measurement of the curve’s magnitude, known as the Cobb angle, require a standing X-ray. Studies have established a correlation between the ATR measured by the scoliometer and the Cobb angle, with a higher ATR generally corresponding to a greater curvature of the spine.