Mechanical Modulation of Growth During the Creation and Correction of Vertebral Wedge Deformities in an Immature Goat Scoliosis Model

· (a, b - Medtronic Sofamor Danek) San Antonio, Texas, USA

PURPOSE:
To create and correct apical vertebral wedge deformities in an experimental structural scoliosis by purely mechanical means.

METHODS:
Progressive, structural, lordoscoliotic curves convex to the right in the thoracic spine were created in 14 goats using a posterior asymmetric tether.¹ After a 7-13 week period all tethers were removed and goats randomized to treatment (n=8) and control (n=6) groups. Treatment goats underwent anterior thoracic stapling with four shape memory alloy staples along the convexity of the maximal curvature. All goats were followed for an additional 7-13 weeks. Serial radiographs were used to document progression or correction of the maximal scoliotic deformity as well as to measure wedging of the apical spinal segment (two adjacent vertebrae and the intervening disc). Statistical analysis was performed using a student t-test.

RESULTS:
During the tethering period all goats achieved a progressive, structural, lordoscoliotic curve of significant magnitude (mean 61°, range 49-73°). Wedging of the apical spinal segment measured 10.3° at the beginning and 20.6° at the end of the tethering period. The increase in apical spinal segment wedging of +10.3° (9.8° vertebral/0.5°disc) was significant (P=0.001). During the treatment period wedging of the apical spinal segment in the stapled goats measured 21.0° at the beginning and 19.1° at the end for an average correction of -1.9° (-0.6° vertebral/-1.3° disc); wedging of the apical vertebral segment in the control goats measured 18.8° at the beginning and 24.7° at the end of the treatment period for an average progression of +5.9° (5.9° vertebral/0.0° disc). The difference in apical spinal segment correction versus progression in the stapled (-1.9°) versus control (+5.9°) goats was significant (P=0.015).

DISCUSSION:
The creation and correction of vertebral wedge deformities has been described previously in a rat tail model using external fixation. ² This study demonstrates the ability to create and correct apical vertebral wedge deformities in the thoracic spine of a larger animal that approximates the size of a juvenile human. Additional study is required, however, to identify the optimal mechanical means of achieving vertebral wedge correction and subsequent scoliosis correction.

References:
1. Braun, JT et al: Experimental Scoliosis in an Immature Goat Model: A Method That Creates Idiopathic-type Deformity Without Violation of the Spinal Elements Along the Curve. Presented at the Thirty-fifth Annual Meeting of the Scoliosis Research Society, Cairns, Australia, October 21, 2000.

2. Mente, PL et al: Mechanical Modulation of Growth for the Correction of Vertebral Wedge Deformities. Journal of Orthopedic Research 17:518-24, 1999