Fatigue properties of a twelve-hole versus a five-hole intramedullary supracondylar nail.

Abstract

To evaluate a design change intended to improve fatigue properties of Intramedullary Supracondylar nails (IMSC). Fatigue testing was performed on 12-hole and 5-hole IMSC nails in 18 plastic-fiber composite femurs. Fractures were simulated by a 1 cm wide osteotomy. Nails were cycled until failure with femoral loads ranging from 133 N to 1869 N (30 to 420 lb.) for the 12 mm nails and 133 N to 1335 N (30 to 300 lb.) for the 11 mm nails. A simple finite element computer model (FEM) was also created to determine the difference in stress distribution between the two designs. All mechanical testing was performed using a servohydraulic test fram (MTS Systems, Minneapolis, MN). The FEM was performed using I-DEAS software (SDRC, Milford, OH) running on an Apollo 9000/735 workstation (Hewlett-Packard, Palo Alto, CA). The nails were locked proximal and distally. The number of cycles to failure and the maximum tensile stress (FEM) was measured and compared between the two designs. The 11 mm and 12 mm 12-hole nails failed at an average of 53,514 cycles and 102,433 cycles respectively. No nail failures were seen in the 5-hole design. The FEM showed a 400% increase in maximum tensile stress due to the transverse screw hole in the cannulated nail. Elimination of screw holes at the level of a supracondylar metaphyseal defect significantly improves the fatigue life of the IMSC nail.