Mechanical testing of small fracture implants for comparison of insertion and failure torques.
BACKGROUND
Small fracture screws are among the most commonly used implants in the field of orthopedic surgery. The goal of this study was to compare the insertion and failure torques of three screw types: cortical, partially threaded cancellous, and fully threaded cancellous from three manufacturers: Zimmer, Richards, and Synthes.


METHODS
Each type of screw was subjected to biomechanical tests to determine the insertion ( n=6/group) and failure ( n=10/group) torques.


RESULTS
Two-factor ANOVA tests were run to determine whether the insertion or failure torques were different for the different screw types and manufacturers. In the case of insertion torques, neither the screw nor the manufacturer had any significant effect. In the case of failure torque, significant differences were found based on both the screw type and the manufacturer, with the cortical screws manufactured by Zimmer being the strongest. Although there were strength differences, the most important comparison clinically is between the failure torque and the insertion torque of each screw. In all cases, the failure torques were approximately 20 times larger than the insertion torques, and therefore no failures should occur if only torsional loads are applied during insertion. This comparison shows that factors other than screw strength and manufacturing processes may be involved in cases of screw failure during insertion.


CONCLUSIONS
All three screw types from all three manufacturers appear to be mechanically reliable, with the proper insertion to failure torque ratio. The surgeon's choice of implant should be based on other considerations.
Department of Orthopaedic Surgery, University of Texas Medical School at Houston, 6431 Fannin Street MSB 6.144, Houston, TX 77030, USA. craig.r.glauser@uth.tmc.edu
Craig R
CR
Glauser
Z Maria
ZM
Oden
Catherine G
CG
Ambrose
Mark B
MB
Willits
Kevin J
KJ
Coupe
D000704:Analysis of Variance; D001696:Biomechanical Phenomena; D001863:Bone Screws / Q000009:adverse effects*; D019544:Equipment Failure Analysis; D006801:Humans; D008422:Materials Testing; D019415:Torque