A fatigue life analysis of small fragment screws

Objectives: To conduct a comparative fatigue analysis of several commonly u sed small fragment screws. Design: Biomechanical laboratory study. Setting: Research laboratory. Main Outcome Measurements: A fatigue life analysis of seven different types of small fragment screws was conducted using a Wohler fatigue-testing mach ine. Four different types of 35-millimeter cortical screws were subjected t o fatigue analysis. These included solid stainless steel screws from Synthe s Ltd. (core diameter 2.4 millimeters), Zimmer Inc, (core diameter 2.4 mill imeter), and Smith and Nephew Richards Inc. (core diameter 2A millimeters) and cannulated. stainless steel screws from Synthes Ltd. (core diameter 2.5 millimeters). In addition, three types of 4.0-millimeter cancellous screws were tested. These included stainless steel screws from Synthes Ltd. (core diameter 1.9 millimeters), titanium screws from Synthes Ltd. (core diamete r 2.0 millimeters), and titanium alloy screws from DePuy-Ace (core diameter 2.8 millimeters). Fatigue lives, as reflected by mean cycles to failure, w ere compared. Results: The four types of cortical screws had longer fatigue lives than th e Synthes cancellous screws did (p < 0.001) but shorter fatigue lives than the DePuy-Ace cancellous screws did (p < 0.0001). Among the cortical screws , the cannulated and solid Synthes screws and the solid Zimmer screws did n ot differ statistically. The Smith and Nephew Richards cortical screws fail ed at statistically fewer cycles than the Synthes solid and cannulated cort ical screws did (p < 0.003) but did not statistically differ from the Zimme r screws. The DePuy-Ace titanium alloy cancellous screw had the longest fat igue life of the tested implants by a large margin (p < 0.0001). The Synthe s pare titanium and stainless steel cancellous screws did not significantly differ. Conclusions: This analysis supports core diameter as the principal factor d etermining fatigue life as the results paralleled implant geometry. This de sign modification to improve bending and fatigue strength may come at a pri ce to pullout strength, however, because of a decreased major-to-minor diam eter and increased pitch. Cortical screws differed in fatigue performance d espite identical dimensions, presumably highlighting the importance of impl ant processing and machining. Cannulated cortical screws performed well rel ative to solid screws, thereby supporting their clinical use. Pure titanium and stainless steel cancellous screws performed similarly in fatigue despi te differing material properties, presumably because of geometric design di fferences. This report highlights some of the differences in the in vitro f atigue performance among several commonly used small fragment screws.