Current surgical techniques for scoliosis correction are successful in achieving good long-term outcomes. However these surgical approaches involve fusion of the spine resulting in stresses above physiologic levels on the adjacent spine segments and extensive spine exposure. Experimental surgical approaches are being explored as potential alternatives, including mechanically altering spine growth through staple hemiepiphysiodesis. The goal of staple hemiepiphysiodesis is to generate asymmetric compressive stresses across the growth plate to modulate growth rate, without critically decreasing intervertebral disc mobility. The immediate post-operative effects of spine stapling on the biomechanical properties of the spine have not been fully reported. Further, while the effects of this and related methods are a function of both loading direction and distance from the implant, no previous biomechanical study has reported bilateral differences in vertebral joint stiffness or displacement gradients as a function of loading direction or proximity to the staple. The purpose of this study was to develop a method to test whether staple hemiepiphysiodesis caused an initial coronal plane gradient in disc stiffness and mobility.
In-vitro biomechanical testing, applying pure unconstrained moments with an independent axial compressive preload, is one method for evaluating treatment effects on the spine. A fixed frame cable driven test assembly capable of applying pure unconstrained moments to the specimen with an independent axial compressive preload was designed and developed. Pure moments were applied to functional spinal units before and after staple implantation to measure changes in stiffness in lateral bending, flexion/extension and axial rotation.
Biomechanical tests were conducted on six thoracic functional spinal units of skeletally immature pigs in lateral bending, flexion/extension and axial rotation before and after stapling, with and without a 250 N axial compressive preload. Treatment effect was determined by paired differences for changes in vertebral joint neutral zone, tangent stiffness and displacement on the ipsilateral side. Preliminary results with only moment-rotation for the fifth loading cycle suggest there are no significant differences in joint neutral zone or tangent stiffness parameters following spinal stapling. Analysis of the displacement on the ipsilateral side in lateral bending towards the staple showed that the vertebral joint was not immobilized after stapling.
Staple hemiepiphysiodesis affects the distribution of static and dynamic compressive stresses in the disc and vertebral growth plate, for the purpose of asymmetrically modulating vertebral growth. Defining initial post-operative changes is important to the timing of differential growth treatments and the identification of possible side effects. The efficacy and side effects of mechanical growth modulation likely depends on the initial biomechanical changes due to the specific treatment as well. Towards these ends, a test assembly capable of spine segment testing has been developed and preliminary biomechanical results are presented.