The posterior tether in scoliosis.

The hypothesis that a localized lordosis, or tethering of the posterior elements of the spine, is the primary cause of the vertebral rotation in idiopathic scoliosis was investigated in anatomic specimens of human and calf spinal columns. The specimens were axially loaded with and without a posterior tether created using Zielke instrumentation. Lateral deflection and axial rotation were monitored roentgenographically. The vertebrae of tethered spines showed increased rotation in the direction associated with idiopathic scoliosis. The spinous processes moved toward the concavity at the apex of the induced lateral curve. Conversely, untethered spines either exhibited little rotation or rotated in the opposite direction; the spinous processes moved toward the convexity of the curve. Rotations toward the convexity occur in rotational kyphosis. Thus the hypothesis that idiopathic scoliosis is a rotational lordosis is substantiated; the characteristic rotation can be explained with the aid of a geometric model.