Effect of bone diameter and eccentric loading on fatigue life of cortical screws used with interlocking nails

Objective-To test the effects of bone diameter and eccentric loading on fatigue life of 2.7-mm-diameter cortical bone screws used for locking a 6-mm-diameter interlocking nail. Sample Population-Eighteen 2.7-mm-diameter cortical bone screws. Procedure-A simulated bone model with aluminum tubing and a 6-mm-diameter interlocking nail was used to load screws in cyclic 3-point bending. Group I included 6 screws that were centrally loaded within 19-mm-diameter aluminum tubing. Group 2 included 6 screws that were centrally loaded within 31.8-mm-diameter aluminum tubing. Group 3 included 6 screws that were eccentrically loaded (5.5 mm from center) within 31.8-mm-diameter aluminum tubing. The number of cycles until screw failure and the mode of failure were recorded. Results-An increase in the diameter of the aluminum tubing from 19 to 31.8 mm resulted in a significant decrease in the number of cycles to failure (mean +/- SD, 761,215 239,853 to 16,941 2,829 cycles, respectively). Within 31.8-mm tubing, the number of cycles of failure of eccentrically loaded screws (43,068 14,073 cycles) was significantly greater than that of centrally loaded screws (16,941 2,829 cycles). Conclusions and Clinical Relevance-Within a bone, locking screws are subjected to different loading conditions depending on location (diaphyseal vs metaphyseal). The fatigue life of a locking screw centrally loaded in the metaphyseal region of bone may be shorter than in the diaphysis. Eccentric loading of the locking screw in the metaphysis may help to improve its fatigue life.