Pullout properties of 3.5-mm AO/ASIF self-tapping and cortex screws in a uniform synthetic material and in canine bone

Objectives-To compare the pullout properties of 3.5-mm AO/ASIF self-tapping screws (STS) to corresponding standard cortex screws (CS) in a uniform syn thetic test material and in canine femoral bone. The influence of screw-ins ertion technique, test material, and test-material thickness were also asse ssed. Study Design-In vitro experimental study. Sample Population-Two independent studies. a uniform synthetic test materia l and paired femurs from mature dogs. Methods-Mechanical testing was performed in accordance with standards estab lished by the American Society for Testing and Materials for determination of axial pullout strength of medical bone screws. Completely inserted STS, completely inserted CS, and incompletely inserted STS were tested in 3 grou ps of 10 test specimens each in 4.96-mm and 6.8-mm thick sheets of syntheti c material. In the bone study, group 1 consisted of 24 completely inserted STS compared with 24 completely inserted CS, and group 2 consisted of 24 in completely inserted STS versus 24 completely inserted CS. Comparisons were made between paired femurs at corresponding insertion sites. Pullout data w ere normalized, thereby eliminating the effect of test-material thickness o n pullout properties. Mean values were compared using 2-way ANOVA. Statisti cal significance was set at P < .05. Results-In both the 4.96-mm and 6.8-mm synthetic material, pullout testing of the completely inserted STS demonstrated significantly greater yield str ength and ultimate strength than completely inserted CS. There was no signi ficant difference between incompletely inserted STS and completely inserted STS. The 6.8-mm test material significantly increased yield strength and u ltimate strength for all test groups compared with the 4.96-mm test materia l. In canine bone, there was no significant difference in yield strength of completely inserted STS and completely inserted CS. Yield strength of comp letely inserted STS and completely inserted CS were significantly greater t han incompletely inserted STS. Conclusions-Pullout properties of completely inserted STS were significantl y greater than corresponding CS in a uniform test material. In canine bone, the pullout strength of STS and CS were not different. Incomplete STS inse rtion resulted in an 18% reduction in holding power as compared with comple tely inserted CS and STS in canine bone. Clinical Relevance-The length of STS used in canine bone should be such tha t the cutting flutes extend beyond the trans cortex to maximize pullout str ength. (C) Copyright 2001 by The American College of Veterinary Surgeons.