When Spinal USA, a manufacturer and distributor of advanced surgical spinal products, designed a new series of spinal implants called vertebral body replacement (VBR) devices, they needed to meet United States Food and Drug Administration (FDA) requirements for physical laboratory testing in order to obtain approval for use. Designed to be inserted by a surgeon into a patient’s spine during a spinal fusion procedure, the VBR devices required thorough engineering testing to ensure that they were safe and effective. In order to meet the FDA requirements, Spinal USA contracted Saba Metallurgical and Plant Engineering Services (SMPES) to perform finite element analysis (FEA) of the various VBR designs using ALGOR FEA software to virtually predict the behavior of the VBR designs under the required test conditions. Through a combination of computer simulation and physical laboratory tests of prototypes, the VBR devices obtained FDA approval.

Figure 1. Comparative Stress Analyses of VBR Devices were performed using FEA software. Shownhere (bottom left) are MES results for the axial compression load rating test case, and (bottom right)linear static stress analysis results for the torsion (bending) fatigue test case.

The titanium VBR devices were designed to treat patients with leg or back pain caused by spinal trauma, tumors, or degenerative disc disease. During a spinal fusion procedure, the surgeon removes the damaged disc and replaces it with the VBR device and bone graft material. This realigns the vertebral bones, lifting pressure from pinched nerve roots. Over time, the bone graft will grow through and around the implants, fusing the vertebra above and below and thus stabilizing the spine.

Each VBR device had to undergo three types of FDA-mandated physical laboratory testing: axial compression load rating, axial fatigue, and torsion (bending) fatigue. The method used was to perform comparative ALGOR analyses for all designs under all test conditions. From the FEA results, the weakest designs were determined. If the weakest designs passed the physical laboratory tests, then FEA could be used to demonstrate that the other designs were stronger and thus did not need physical laboratory testing.

Solid models of the VBR devices were created using computer-aided design (CAD) software. Then, the CAD models were opened in the FEA software and Mechanical Event Simulation (MES) and linear static stress analysis (LSSA) were performed to simulate the FDA-required tests.

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.

Figure 2. A plate-screws-VBR device Assembly Mounted in the Spinal Column was modeled and analyzed. MES results show a plot of displacement magnitude and von Mises stress contours for the plastic collapse test case.

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 .

NASA Tech Briefs Magazine

This article first appeared in the January, 2008 issue of NASA Tech Briefs Magazine.

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