A biomechanical assessment of thoracic spine stapling

An in vitro immature bovine study of thoracic spine fixation using shape memory alloy for minimally invasive adolescent scoliosis treatment.

The purpose of this study was to detect differences in thoracic spine range of motion due to the placement of spinal staples manufactured from a common shape memory alloy.

Scoliosis surgery attempts to both correct the existing deformity and prevent further curve progression. The gold standard in surgical treatment of scoliosis is posterior instrumentation and fusion using pedicle screws or hooks. Fusionless techniques generally use less invasive procedures than fusion methods. One such technique, anterior stapling of the vertebrae, theoretically enables preservation of growth, motion, and spinal function. However, the degree of stability afforded by this method has not been reported.

Eight immature bovine specimens (T4-T9) were used in this study. Nitinol staples were cooled to below the transition temperature and placed on the spine such that they spanned the disc space. The specimen was then heated above the critical temperature and the staple tines engaged the vertebral endplates. A common flexibility protocol was used to determine the ranges of motion (ROM) in flexion-extension, lateral bending, and axial rotation. The intact spine and 4 construct variants (combinations of staple type and placement) were evaluated using this testing protocol. ANOVA statistics with post hoc testing was used to discern statistical differences.

Not all staple variants were able to achieve significant reductions with respect to the intact condition. ROM was significantly restricted in axial rotation and lateral bending with the introduction of staple instrumentation. Further, there seemed to be a mechanical equivalence between a single double-prong staple and 2 single-prong staples. Our data indicate that staple fixation does not result in consistently elevated adjacent segment motion.

Our results imply that staples are able to significantly restrict motion while not achieving motion reductions that one would achieve with fusion-promoting instrumentation. The choice between double- and single-prong staples remains a matter of preference. Neither staple variant provided a mechanical advantage. The single-prong staple did allow more control in the placement of the staple over the disc space. The addition of an anterior staple significantly reduced the overall flexion-extension ROM.