Relative versus absolute modulation of growth in the fusionless treatment of experimental scoliosis

Study Design. Absolute and relative growth modulation of apical spinal segments were measured during creation and correction of an experimental scoliosis in a goat model. Objective. To differentiate relative and absolute changes in growth on the concavity and convexity of an experimental scoliosis treated with anterior vertebral stapling. Summary of Background Data. The creation and correction of vertebral wedge deformities have been previously described in a rat tail model using external fixation as well as in a goat model using anterior vertebral body stapling. Methods. Progressive, structural, scoliotic curves convex to the right in the thoracic spine were created in 14 Spanish Cross-X female goats using a posterior asymmetric tether. After 7 - 13 weeks, all tethers were removed, and goats were randomized into stapled (n = 8) and untreated (n = 6) groups. Stapled goats underwent anterior vertebral stapling with 4 shape memory alloy staples ( Medtronic Sofamor Danek, Memphis, TN) along the convexity of the maximal curvature. All goats were observed for an additional 7 - 13 weeks. There were 12 additional goats matched for age, sex, and weight used as growth controls throughout the study. Serial radiographs were used to document progression or correction of the maximal scoliotic deformity, and changes in relative and absolute growth at the apical spinal segment T9 - 10 (2 adjacent vertebrae and the intervening disc). Results. All tethered goats had progressive, structural, scoliotic curves of significant magnitude during the tethering period (average 61.4 degrees, range 49 degrees to 73 degrees) ( P = 0.001). There was 1 goat from each group eliminated from the study because its apical spinal segment did not match the T9 - 10 level used to establish normal growth in controls. During the treatment period, stapled goats had a correction of -6.9 degrees ( P = 0.03), whereas untreated goats had little change (-1.4 degrees). Apical spinal segment wedging progressed in all tethered goats, from 11.1 degrees to 22.4 degrees, during the tethering period (P = 0.001). During the treatment period, wedging corrected -2.2 degrees (range 22.5 degrees to 20.3 degrees) in the stapled goats but progressed +3.5 (range 22.3 degrees to 25.8 degrees) in the untreated goats ( P < 0.05). Apical spinal segment growth in all tethered goats was decreased on the concavity by 78% and increased on the convexity by 33% when compared to growth controls (P < 0.001). During the treatment period, growth on the concavity of the apical spinal segment of the stapled goats was decreased by 10% but increased in the untreated goats by 37% when compared to growth controls. On the convexity, apical spinal segment growth at T9-10 was decreased in the stapled goats by 18% and increased in the untreated goats by 29% when compared to growth controls (P < 0.04). Conclusions. Data in this study show the ability to modulate relative and absolute growth, according to the Hueter-Volkmann law, at the apical spinal segment of a progressive experimental scoliosis. However, anterior vertebral stapling, although able to control progressive wedging and scoliosis at the apical spinal segment, was not able to reverse fully the Hueter-Volkmann effect.