Surgical Spinal Implants Verified with ALGOR FEA

When Spinal USA, a manufacturer and distributor of advanced surgical spinal products in Flowood, Mississippi, designed a new series of spinal implants called vertebral body replacement (VBR) devices, the company 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) in Baton Rouge, Louisiana to perform finite element analysis (FEA) of the various VBR designs. "When trying to bring multiple devices to market at once, prototyping and physical laboratory testing can be very time-consuming and expensive, costing in the tens of thousands of dollars for each shape and size," said Brent Saba, owner and principal engineer of SMPES. "Hence, it is very desirable to minimize the number of physical laboratory tests by using FEA."

Saba used FEA software from ALGOR, Inc. of Pittsburgh, Pennsylvania to virtually predict the behavior of the VBR designs under the required test conditions. Through a combination of computer simulation using FEA and physical laboratory tests of prototypes, Spinal USA's VBR devices obtained FDA approval. Since then, they have been used successfully by surgeons to help spinal-disorder patients.

Spinal USA, a manufacturer and distributor of advanced surgical spinal products in Flowood, Mississippi, designed a new series of spinal implants called vertebral body replacement (VBR) devices to treat patients with spinal disc disorders. Brent Saba of Saba Metallurgical and Plant Engineering Services performed comparative stress analyses of VBR devices using ALGOR finite element analysis (FEA) software. Shown here, at bottom-left, are ALGOR Mechanical Event Simulation results for the axial compression load rating test case; and, at bottom-right, linear static stress analysis results for the torsion (bending) fatigue test case.

Advancing Spinal Implant Technology

Founded in March 2005, Spinal USA's mission is to meet the needs of patients, surgeons and healthcare providers by providing cutting-edge technology for treating disorders of the spine. The company's products offer surgeons enhanced intraoperative flexibility, simplified surgical technique and superior instrumentation with the goal of further improving patients' speed of recovery and overall surgical outcome at a competitive price.

Spinal USA's 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. "In general, the FDA requires that new VBR devices be equal to or greater in all three categories than comparable devices presently on the market," explained Saba. The method that Saba used was to perform comparative ALGOR analyses for all designs under all test conditions; from the ALGOR results, he determined the weakest designs; so long as 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.

Analyzing the VBR Devices

Saba created solid models of the VBR devices using Alibre Design and IronCAD computer-aided design (CAD) software. Then, he opened the CAD models in ALGOR FEA software and performed Mechanical Event Simulation (MES) and linear static stress analysis (LSSA) 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," explained Saba. "Then, a plastic collapse analysis was run for the axial fatigue test using a load of 3,000 Newtons for 5-million cycles." Saba used ALGOR's result probes to identify the maximum stresses for each VBR and then compared them. "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," said Saba. In order to compare the analysis results for various VBR designs, Saba emphasized, "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. "Here again, the lower the alternating stress range, the higher the fatigue life."

FEA in Place of Prototyping and Physical Laboratory Testing

Saba also modeled a VBR device with two types of plate systems. He performed comparative analyses of the VBR-device-and-plate assemblies, 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.

Saba also modeled and analyzed a plate-screws-VBR device assembly mounted in the spinal column. Mechanical Event Simulation results show a plot of displacement magnitude and von Mises stress contours for the plastic collapse test case.

According to Saba, the benefits of using FEA included, "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. Catching potential design flaws during the design stage instead of machining devices that would later fail during physical laboratory testing provided tremendous savings in cost and time."

Saba added, "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 sometimes 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."

Future Plans for FEA

Saba plans to continue using ALGOR FEA for testing spinal implants as well as his other engineering applications. "ALGOR is an integral part of my business. My primary line of business has been advanced mechanical designs following ASME VIII Division 2, as well as pressure vessel/tank/piping fitness-for-service applications according to API 579." Saba indicated that his MES workload has increased recently and, with MES, "My company can offer motion-based FEA that few, if any, of my competitors can provide."

Other typical ALGOR applications performed by Saba include: mechanical simulation of lifting a dressed (piping, pipe supports and platforms) tower from the ground to vertical position; a heat exchanger failure analysis investigation involving combined flow-thermal stress analyses and evaluation; and load ratings of damaged tank/pressure vessel shells from impact, corrosion and/or vacuum damages.

Brent Saba, owner and principal engineer of Saba Metallurgical and Plant Engineering Services, used ALGOR finite element analysis (FEA) software to analyze various designs of VBR devices made by Spinal USA. Saba's FEA results were accepted as evidence of compliance with United States Food and Drug Administration requirements, thereby reducing the need for expensive and time-consuming prototyping and physical laboratory testing.

Brent Saba, PE-ME/MT, earned B.S. and M.S. degrees in Mechanical Engineering (ME) at Louisiana State University (LSU) and is presently pursuing his doctorate in ME at LSU. He is the principal engineer/owner of Saba Metallurgical and Plant Engineering Services (SMPES) in Baton Rouge, Louisiana. For more information about SMPES, visit www.smpes.com.