2008

Validating Requirements for Surgical Spinal Implants With Finite Element Analysis

Vertebral body replacement devices require thorough analysis and engineering testing to ensure their safety.

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.

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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.

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