Static Versus Dynamic Loading in the Mechanical Modulation of Vertebral Growth

Abstract

Measures of absolute and relative growth modulation were used to determine the effects of static and dynamic asymmetric loading of vertebrae in the rat tail. To quantify the differences between static and dynamic asymmetric loading in vertebral bone growth modulation. The creation and correction of vertebral wedge deformities have been previously described in a rat-tail model using static loading. The effects of dynamic loading on growth modulation in the spine have not been characterized. A total of 36 immature Sprague-Dawley rats were divided among four different groups: static loading (n = 12, 0.0 Hz), dynamic loading (n = 12, 1.0 Hz), sham operated (n = 6), and growth controls (n = 6). An external fixator was placed across the sixth and eighth caudal vertebrae as the unviolated seventh caudal vertebra was evaluated for growth modulation. Static or dynamic asymmetric loads were applied at a loading magnitude of 55% body weight. After 3 weeks of loading, growth modulation was assessed using radiographic measurements of vertebral wedge angles and vertebral body heights. The dynamically loaded rats had a final average wedge deformity of 15.2+/- 6.4 degrees, which was significantly greater than the statically loaded rats whose final deformity averaged 10.3 degrees +/- 3.7 degrees (P < 0.03). The deformity in both groups was statistically greater than the sham-operated (1.1+/- 2.0 degrees) and growth control rats (0.0+/- 1.0 degrees) (P < 0.001). The longitudinal growth was significantly lower on the concavity compared with the convexity in both the dynamically (0.34 +/- 0.23 mm vs. 0.86 +/- 0.23 mm) and statically (0.46 +/- 0.19 mm vs. 0.83 +/- 0.32 mm) loaded rats (P < 0.001). These growth rates were significantly less than the sham operated and growth control rats (P < 0.001). A variety of fusionless scoliosis implant strategies have been proposed that use both rigid and flexible implants to modulate vertebral bone growth. The results from this study demonstrate that dynamic loading of the vertebrae provides the greatest growth modulation potential.