The aim of this study is to examine the coupling of lateral flexion and axial rotation in adolescent idiopathic scoliosis (AIS). Seventy six patients with AIS each had left and right supine lateral bending films before surgery. The vertebral rotation (Perdriolle) was measured from T5 to L4 on each radiograph. The change in rotation was calculated for each vertebra from left to right lateral bending = ‘lateral flexion induced rotational change (LFIRC)’

In AIS, the coupling of lateral flexion and axial rotation is similar to that seen in the normal spine. Curves which are stiff in frontal and transverse planes show little or no change in frontal plane or transverse plane rotation. Conversely, curves which are mobile in frontal and transverse planes show considerable change in frontal and transverse plane rotation. The presence of soft tissue tethers is the most likely explanation for the limited rotations of the stiff curves.

Between T7 and T10, the magnitude of the vertebral rotation on the pre-operative AP radiograph affects the direction of the lateral flexion induced rotational change; this is probably due to a change in the line of action of the ligaments and muscles responsible for this rotational change.

Further work is needed to examine the relation of the variation in rotational change to the progression of the rib hump after surgical instrumentation and evaluate the predictive worth of these findings.