A Method to Test Anterior-Posterior Construct Shear Fatigue Based on the Vertebrectomy Model

Dynamic stabilization systems may be subject to anterior-posterior shear loading. However, there is no standard method established for testing a vertebrectomy model in anterior-posterior translation (as may be seen with dynamic spondylolisthesis). A new anterior-posterior shear fatigue test was devised to test one such dynamic stabilization system in anterior-posterior translation. Similar to ASTMF1717 assemblies, constructs were assembled so that screws were placed in ultra high molecular weight polyethylene blocks to attach to the test fixture. The test setup ensures that the entire shear load is transmitted by the implant system. Unlike ASTMF1717 assemblies, the blocks were rotated 180 deg from each other to facilitate loading in the test frame. This configuration is modified from an expected in vivo usage. However, the resultant load vectors are appropriately similar to the shear loads being simulated. The constructs include polymeric components and thus were tested at body temperature in a fluid bath. The blocks were moved ±5 mm with respect to each other. The constructs were cycled at 2 Hz for a minimum of 5 × 106 cycles. The constructs successfully survived 10 × 106 cycles of anterior-posterior shear displacement. Examination of the constructs demonstrated similarities between components tested via this new method and components retrieved from patients. Comparisons indicate that the shear fatigue test may stress the components as much or perhaps more than what was seen clinically.