Validating Requirements for Surgical Spinal Implants With Finite Element Analysis

Tuesday, January 01 2008 The axial compression load rating and axial fatigue tests were simulated using MES, which provides nonlinear, multi-body dynamics with large-scale motion, large deformation, and large strain with body-to-body contact. A nonlinear FEA technique called limit load analysis was used to determine an axial compression load rating. Then, a plastic collapse analysis was run for the axial fatigue test using a load of 3,000 Newtons for 5 million cycles. Result probes were used to identify the maximum stresses for each VBR and then were compared. The VBR with the lower maximum stress value had the longer fatigue life.

The torsion fatigue test was simulated using linear static stress analysis with surface forces totaling 200 Newtons in the bending direction. Because anticipated stresses were within the elastic range of the titanium material under the applied bending load, the use of linear elastic FEA was deemed suitable. In order to compare the analysis results for various VBR designs, it was imperative to have nearly identical mesh intensity and quality, and exact loading and constraints. Analyses were run for the VBRs and the results were compared. Again, the lower the alternating stress range, the higher the fatigue life.

A VBR device with two types of plate systems also was modeled. Comparative analyses of the VBR-device-and-plate assemblies were performed, which showed that one was stronger than the other. The stronger version was not laboratory tested because FEA was accepted as evidence of its compliance with FDA requirements.

With FEA, multiple shapes and height-varying sizes could be compared against each other to determine the weakest shape/size combination. Given that the weakest shape/size VBR passed FEA simulation and laboratory testing, further laboratory testing was not required. However, if any of the VBRs failed to meet the FDA criteria during FEA simulation, then design changes could be made and retested using FEA. Future changes or optimization of VBR products can be compared to the original version using FEA. If the new design is weaker, then changes can be recommended to make the device stronger or more flexible. If the new design is better than the old design, then the results can be documented showing that the new device is better. This documentation can be used to show that physical laboratory testing of the new device is not necessary.

The VBR devices have been used successfully by surgeons to help spinal-disorder patients.

This work was done by Brent Saba, PE-ME/MT, principal engineer/ owner of Saba Metallurgical and Plant Engineering Services (SMPES), using ALGOR software. For more information, click here.