The Design and analysis of a HA/PLA pedicle screw for spinal fusion

Metallic pedicle screws have been widely used in the orthopaedic field to treat spinal diseases. Although spinal screws have improved over the last few decades, there is still sometimes a need for a second painful surgery to remove the instrumentation due to late-onset pain or discomfort due to breakage or infection. Polylactic acid has been heavily studied recently and is emerging as a viable absorbing material for bone fixation devices with good biological response; however there have been no optimization studies for its use and practicality as a lumbar pedicle screw. In addition to the lack of pedicle applications for the material, there are no FEA studies analyzing the objective values of simultaneous bending and pull-out of a pedicle screw or a pedicle screw augmented with calcium phosphate bone cement. Finite element analysis, Taguchi method, and an artificial neural network were used as the optimization methodology. Three-dimensional finite element method was used to create an arranged L18 orthogonal array of a model simultaneously applying a bend and pull-out test. These simulations were used to calculate two objective values for analysis. Artificial neural networks were used to estimate an optimum design which had the lowest surface stress in the screw. The obtained design was used for experimental testing to verify computational results and analyze practicality of clinical use. The optimal screw was shown to have a maximum surface stress of 81.83 MPa. The reaction in the CPC was shown to have a surface stress of 43.69 MPa. The screw and CPC stresses are below their yield strengths, which should result in non-failure. However, the biggest concern is rigidity of the device, which was tested experimentally. An unrigid device will prevent bone fusion and will make the product unfeasible