Development of an in vitro test method to simulate intra-operative impaction loading on lumbar intervertebral body fusion devices

Intervertebral body fusion devices (IBFDs) are commonly used in the treatment of various spinal pathologies. Intra-operative fractures of polyether-ether-ketone (PEEK) implants have been reported in the literature and to the FDA as device-related adverse events. The device and/or implant inserter failures typically occur during device impaction into the disc space and require implant removal and replacement. These additional steps may cause further complications along with increased surgical time and cost. Currently, there are no standardized test methods that evaluate clinically relevant impaction loading conditions on IBFDs. This study aims to develop an in vitro test method that would evaluate implant resistance to failure during intra-operative impaction. To achieve this, (1) surgical implantations of IBFDs were simulated in nine lumbar cadaver specimens by three different orthopedic spine surgeons (n = 3/surgeon). Impact force and mallet speed data were acquired for each surgeon. (2) Based on the acquired surgeon data, a benchtop mechanical test setup was developed to differentiate between two TLIF IBFD designs and two inserter designs (for a total of four IBFD-inserter combinations) under impaction loading. During implant insertion, impact force measurements indicated that lumbar IBFDs are subjected to high energy forces that may exceed their mechanical strength. Our test method successfully replicated clinically-relevant loading conditions and was effective at differentiating failure parameters between different implant and inserter instrument designs. The mechanical test method developed shows promise in its ability to assess impaction resistance of IBFD/inserter designs and evaluate potential risks of device failure during intraoperative loading.