Screw Pull Out under Cyclic Fatigue Loading in Synthetic and Cadaveric Bone

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The Ohio State University

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Fracture fixation devices, used to stabilize fractures internally to reduce the recovery time, are often held in place by screws. While the devices themselves are frequently tested for failure, the interface between the bone and the screw is usually not. Very few studies have been conducted on this interface failure which causes concern as to whether this mode of failure is accurately modeled by the composite bone models frequently used for testing. The integrity of the screw-bone junction is essential to the functional stability of the device as a whole. This project tested bone-screw interface fatigue in both synthetic and cadaveric bones. Cortical bone screws (3.5 mm diameter) were inserted bi-cortically in both human femurs and Sawbones® composite bones. Each screw was cyclically loaded between 200 and 1000 N at 7.0 Hz until failure or 200,000 cycles (chosen as the clinically appropriate load cycle for fracture healing applications). The distance the screw was pulled out over the course of the testing was recorded. A two sample t-test was performed to compare the pull out in cadaveric and composite bone. None of the screws pulled out of the bone. The screws inserted in cadaveric bone moved 0.094 ± 0.030 mm over the 200,000 cycles; the screw in the composite bone moved only 0.057 ± 0.026 mm. While statistically significant (p<0.001), this result is not clinically relevant as neither screw pulled out a clinically meaningful distance over the course of the testing.



cyclic loading, screw displacement, screw fatigue, composite bones, cadaveric bones