Biomechanics of cantilever “plow” during anterior thoracic scoliosis correction

Abstract

Background context Anterior instrumentation is often used for correction of thoracic scoliosis. Loss of spinal correction may occur after failure at the bone–implant interface, and forces on the bone–implant interface during scoliosis correction remain unclear. Purpose Evaluate two different mechanisms of loading associated with anterior scoliosis correction. Study setting In vitro biomechanics lab. Methods Polyurethane foam and human cadaveric thoracic vertebral bodies were instrumented with transvertebral body screws. Bone–implant interface failure loads were measured during constrained, fixed-angle screw translation, as well as unconstrained translation allowing coronal plane screw rotation. Vertebral body staples were randomly assigned to both conditions. Results Data were consistent across foam and cadaveric specimens. Failures occurred at significantly lower loads during unconstrained translation (with rotation) compared with constrained translation. Staple usage significantly increased the load to failure in both testing modes. In cadaveric bone, the constrained plowing load to failure was 562N+110N versus 188N+20N in the unconstrained testing. With a staple, these values increased to 694N+53N and 530N+100N, respectively. Conclusions The 280% increase in cadaveric failure loads when a staple was added in the unconstrained testing method exceeds previous reports. The unconstrained method of plow simulated anterior scoliosis instrumentation when a rod was cantilevered and compressed into position. Supplemental vertebral body staples may be clinically indicated, particularly at the ends of the construct where residual deforming forces remain the greatest.