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Fatigue properties of a twelve-hole versus a five-hole intramedullary supracondylar nail
M J Voor1, D A Verst, S W Mladsi
1Orthopedic Bioengineering Laboratory, University of Louisville, KY, USA.
Objectives:
To evaluate a design change intended to improve fatigue properties of Intramedullary Supracondylar nails (IMSC).
Design:
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.
Setting:
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).
Intervention:
The nails were locked proximal and distally.
Main Outcome Measurement:
The number of cycles to failure and the maximum tensile stress (FEM) was measured and compared between the two designs.
Results:
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.
Conclusions:
Elimination of screw holes at the level of a supracondylar metaphyseal defect significantly improves the fatigue life of the IMSC nail.