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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: The influence of design features in the biomechanical performance of a fixator for the lumbar spine

Author: ALKALAY, Ron

Year: 1997

Supervisor(s): Dan Bader

Spinal fixation systems using pedicular screws have gained popularity in managing the damaged spine. However, the loadings 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 separate loading regimens. The fixator was instrumented with strain gauges and 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 ineffectiveness of the transverse elements under 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 increase in screw insertion depth and a decrease in 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 has emphasised the importance of a full biomechanical evaluation of any future designs of spinal fixators.