Relative versus Absolute Modulation of Growth in the Fusionless Treatment of Experimental Scoliosis

• a - Medtronic Sofamor Danek

Purpose: To differentiate relative and absolute changes in growth on the concavity and convexity of an experimental scoliosis treated with anterior thoracic stapling.

Methods: Progressive, structural, scoliotic curves convex to the right in the thoracic spine were created in 12 Spanish Cross-X female goats using a posterior asymmetric tether (ref ). After 7-13 weeks all tethers were removed and goats randomized into stapled (n=7) and untreated (n=5) groups. Stapled goats underwent anterior thoracic stapling with four shape memory alloy staples along the convexity of the maximal curvature. All goats were observed for an additional 7-13 weeks. Twelve additional goats matched for age, sex and weight were 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 (two adjacent vertebrae and the intervening disc). Relative growth modulation was assessed using Cobb measurements across T9,10. Absolute growth modulation was assessed using measurements of the concave and convex apical spinal segment height with growth controls for comparison. Standard statistical analysis was utilized.

Results: Scoliosis: All goats achieved progressive, structural, scoliotic curves of significant magnitude during the tethering period (average 61.1°, range 49° to 73°) (P= 0.001). During the treatment period stapled goats corrected 7° (P=0.03), whereas untreated goats changed little (-1°).

Relative Growth Modulation: Apical spinal segment wedging progressed in all goats from 11.1° to 22.4° during the tethering period (P=0.001). During the treatment period wedging corrected 2.2° (22.5° to 20.3°) in the stapled goats but progressed 3.5° (22.3° to 25.8°) in the untreated goats (P<0 .05).

Absolute Growth Modulation: Apical spinal segment growth in all goats was decreased on the concavity by 78% and increased on the convexity by 30% when compared to growth controls (P<0 .001). During the treatment period, growth on concavity of apical spinal segment stapled goats was decreased by 10% when compared to controls and 47% untreated goats. Growth convexity 18% The difference between total decrease in at versus significant (P="" 0.04).

Discussion: The data in this study demonstrate the ability to modulate growth, according to the Hueter- Volkmann Principle, at the apical spinal segment of a progressive experimental scoliosis. During the tethering period, all goats demonstrated increased apical spinal segment wedging consistent with a relative decrease in concave growth and increase in convex growth compared to growth controls. Absolute measures of growth during this period confirmed lagging concave growth and exuberant convex growth. During the treatment period, stapled goats demonstrated modest correction of wedging at the apical segment that was significant when compared to progression of wedging in untreated goats. Absolute measures of growth confirmed the ability of shape memory alloy staples to control exuberant convex growth. However, concave growth was also diminished suggesting the inability of anterior thoracic staples to fully reverse the Hueter-Volkmann effect.

Ref: Braun et al. Experimental Scoliosis in an Immature Goat Model. Spine 2003

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