The Influence of Design Features in the Biomechanical Performance of a Fixator for the Lumbar Spine

Abstract

Spinal fixation systems using pedicular screws have gained popularity in manging the damaged spine. However, the loading to which individual components of a fixator are exposed are largely unknown. This thesis describes the use of a Corpectomy injury model to investigate the mechanical response of a commercial internal spinal fixator and the resultant loads acting on its rods and screws, under four separatelo ading regimens. The fixator was instrumentedw ith strain gaugesa nd tested using specially designed jigs. The results were then compared to theoretical models and any differences highlighted. An evaluation was also performed on a range of transpedicular screw designs under tensile loads. An increase in the tightening torque of the fixator clamps, ranging from 5 to 15Nm, and the inclusion of transverse elements across its vertical rods produced a combined increase in overall torsional rigidity of 89%. However, no such changes were found under axial compression and both simulated flexion and extension tests. The relative ineffectivenesso f the transversee lementsu nder sagittal loads was probably due to their spatial relationship with the fixator. The results from the instrumented fixator indicated several load response pathways, as predicted by the theoretical analysis. These pathways were influenced by several factors including, the screw angulation, the boundary conditions of the test and the addition of the transverse elements. Clamp design was critical in minimising rotational slippage of both screws and transverse elements. The results from the instrumented fixator revealed that the transpedicular screws were exposed to complex loads under each of the tests. Under tensile loads, both the increasei n screw insertion depth and a decreasein screw pitch were found to be the important parameters which affect screw performance. Analysis showed the state of stress and strain along the thread was the overriding factor in the tensile performance of these screws. This work hase mphasisedth e importance of a full biornechanicale valuation of any future designs of spinal fixators.