Analysis of biomechanical load cases for the investigation of pedicle screws ASTM F1717 vs. Toggling.

BACKGROUND: The anchorage of pedicle screws is essential for spinal stability during surgical procedures. Static pullout tests differ substantially from the dynamic loading conditions that occur in vivo. This study aimed to compare two dynamic load cases (ASTM F1717 and Toggling) to assess the implementation of the test results to clinical failure cases as well as their implications for standardization. METHODS: The two loading cases were investigated under standardized conditions using a 6DoF sensor and two types of polyurethane foams. Both test configurations were examined on an adaptable test setup that allowed the integration of the 6DoF sensor and the foam blocks (10 PCF Blocks from Sawbones of the following types: open-cell and cellular foam). Measurements were performed under compression (10-110 N) and under alternating compression and tensile load (±110 N). Cyclic loading was applied for 15 cycles when using the 6DoF sensor and for 500 cycles when the screws were anchored in the foam. RESULTS: In the ASTM F1717 setup, the boundary conditions resulted in a significantly higher force in the z-direction and a positive moment around the test bench's rotational axis. In the Toggling setup, an alternating moment around the rotational axis and a displacement in the z-direction were observed. Most foam (100% open-cell foam and 50% cellular foam) reached the failure criterion before completion of testing. The cellular foam withstood the ASTM F1717 loading in both directions, whereas under Toggling, it completed 40 ± 18 cycles in compression and 11 ± 5 cycles under alternating compression and tensile loading. CONCLUSION: The 6DoF measurements yielded consistent results, which were determined by the specific boundary conditions of the two load cases. However, the isolated load cases used in ASTM F1717 and in Toggling tests do not reflect the full spectrum of physiological loading. For the development of a universal testing method for pedicle screws in human specimens, the implementation of a multiaxial cyclic loading should be considered. In the future, universal spine simulators could be alternative to the isolated loading protocols currently in use.