A biomechanical study of the cortex-anchorage vertebral screw

To obtain a comprehensive understanding on the effect of the improvement of fixation strength and on the optimal design in various geometrical parameters of a new screw system through biomechanical analyses.

A new screw with the cortex-anchorage was designed and manufactured to improve the fixation of the instrumentation for osteoporotic spine. There were four expandable wings distributed around the screw after insertion.

Screw loosening or loss of correction caused by insufficient mechanical stability on the bone-screw interface is frequently found in osteoporotic subjects. Similarly, the removal and replacement of a screw in a revision procedure substantially decreases its mechanical fixation. Since cortex is the most rigid part in the vertebral body, emphasis on the cortex-anchorage may offer an optimal fixation of screws.

The biomechanical evaluation that consists of the pullout test and the finite element analysis was applied to identify the stabilizing effect and the optimal design for the new screw system. In the pullout experiment, the porcine vertebral body with a hollow block of cancellous bone was proposed to simulate an osteoporotic spine. This osteoporotic model was specially simulated the degeneration and destruction of the cancellous bone in vertebrae. In the finite element analysis, the reduction of elastic modulus was used in various levels of vertebral degeneration.

Pulling screws out of vertebral bodies with a hollow block of cancellous bone, the mean pullout force was 729 (SD 159) N for the conventional screws, and 1072 (SD 179) N for the new screw system. The finite element analysis showed that the longer screw with bi-cortex fixation was the better option in reducing the bony stress and increased the stability. As the height of wings changed, the stress distributed on vertebral body indicated the lowest in fixation by a screw with the largest wings. Nevertheless, there existed a least displacement of vertebral body and moderately low stress on wings' lateral end when assembled with the middle size wings.

The stabilization function of expansive wings of the new screw system was enhanced in the osteoporotic vertebra and better than that of a conventional screw. The finite element analysis showed a middle size wing could help the screw to reduce the risk of failure and to improve the vertebral stability.

Screw loosening or loss of correction caused by insufficient mechanical stability on the bone-screw interface is frequently found in osteoporotic subjects. From the biomechanical point of view, this study had shown that a new design of screw could improve the fixation of the instrumentation for osteoporotic spine. With further investigations that includes the clinical proof and the development of a cortex-anchorage vertebral screw may provide a valuable alternative to the spinal instrumentation for the patients with osteoporosis.