Graduate Mechanical Engineer, University of Greenwich

Abstract

An in-depth investigation was conducted into the current state of the art of orthopaedic bone screw design and manufacture. To do this a review of medical journals, textbooks, and queries of surgeons and medical device distributors took place. Lessons learned from the literature review were used to guide a case study focussing on a commercially available example pedicle screw. The dimensions of the pedicle screw were obtained through various methods of reverse engineering, including laser scanning, touch-probe scanning, microscopic analysis, and manual measurement with digital callipers. These dimensions were used to create a 3D CAD model of the example pedicle screw. Three key parameters were modified in increments and decrements of 15%; Major Diameter, Root Diameter, and Pitch. The resulting table of dimensions were used to 3D model six new screw designs. In addition to these designs, two new designs were proposed based on the example pedicle screw that incorporated two different conical core designs. These eight new screw designs were tested virtually with Finite Element Analysis in order to better understand the relationship between the change in parameters and the change in stresses and forces experienced by the screw. The eight new designs, as well as the design representing the example pedicle screw, were subjected to three loading conditions; Force, Moment, and Displacement. Loading conditions and materials properties closely approximated conditions found in real-world applications. The results were analysed to find patterns that show the effect of parameter change. To validate the FEA results, tests were conducted using bone-analogue polyurethane foam. Comparison of the results of FEA testing and physical testing showed consistent results, adding credibility and value to the FEA results used for the rest of the study. After analysis of the results, any shortcomings were noted and suggestions were made for further study. 

Meshed CAD model. Ten screws were modelled based on a reverse engineered screw. The major and root diameter, and pitch were varied ±15%, as well as a variable pitch design. 

Custom made jig to test pull-out strength in polyurethane bone-analogue foam.

MATLAB program written to collate the data gathered from the tensile test machine.

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Prototype screw created with 3D printer.