Asymmetrical flexible tethering of spine growth in an immature bovine model

A 12-week bovine survival study of tethering anterior spine growth that included untethered control subjects.

To determine the effects that a flexible cable attached to the anterolateral aspect of the thoracic spine has on spine growth in rapidly growing calves.

The search for a way to correct scoliosis without the use of an arthrodesis continues in an attempt to maintain normal spine mobility. Experience in the hemiepiphyseal stapling of long bones has provided a background rationale for attempting growth modulation in the spine. It is postulated that a mechanical tether to the anterior and lateral growth of the spine in a growing child with scoliosis may allow spontaneous correction of sagittal and coronal plane deformity obviating the need for an arthrodesis.

Eight calves (age, 3-4 weeks; weight, 47 +/- 4.6 kg) underwent right-side thoracotomies exposing the thoracic spine. Laterally directed anterior vertebral body screws were placed into each body and two vertebrae, either T6 and T7 or T8 and T9, were tethered with a stainless steel cable. After 12 weeks, radiographs were obtained to evaluate the degree of deformity that had developed. In addition, biomechanical testing to determine the range of motion in the tethered and untethered segments was performed.

The calves increased their weight 153% during the 12-week postoperative period. The radiographic analysis demonstrated scoliosis of 11.6 degrees +/- 4.8 degrees in the tethered levels, as compared with 0.3 degrees +/- 1.7 degrees in the control segments (P < 0.0001). Similarly, kyphosis developed in the tethered segments (5.1 degrees +/- 5.8 degrees ), as compared with -1.8 degrees +/- 3.1 degrees at the control levels (P = 0.01). There was a significant wedging of the disc in the tethered (6.8 degrees +/- 1.6 degrees ) as compared with the untethered (0.7 degrees +/- 2 degrees ) segments (P < 0.0001). There was a trend toward lower height of the vertebrae on the tethered right side, as compared with the left side in the tethered segments (P = 0.075), whereas no side-to-side difference was noted in the control subjects (P = 0.48). Biomechanical analysis showed that the tether did not affect the range of motion in axial rotation or flexion-extension. However, the tether did restrict lateral bending as compared with that of the control subjects. When the tether was cut, the range of motion returned to levels matching that of the untethered control subjects.

Anterolateral tethering of the spine creates kyphosis and scoliosis in this rapidly growing bovine model. The spinal tether limited motion primarily in lateral flexion. However, total lateral bending motion returned to levels comparable with control motion segments after removal of the tether. This method of spine growth modulation may provide a possible treatment for the correction of spine deformities without arthrodesis in patients who are skeletally immature. The exact mechanisms of growth modulation and the effects of tethering on disc function and integrity are unknown and deserve further study.