Cannulated hip screws: a study of fixation integrity, cut-out resistance, and high-cycle bending fatigue performance

One major advancement in the treatment of femoral neck fractures has been the development of cannulated screws. This study investigates the integrity of five commercially available cannulated screw systems. Transcervical fractures were created in 25 adult femora and repaired by three cannulated hip screws of a randomly assigned design. The repaired specimens were subjected to a triangular loading pattern for 1,000 cycles, then loaded to ultimate failure. Secondly, screws of each type were inserted into femoral heads to a point 5-mm from the chondral surface, then ramp loaded to determine the push-through stiffness and maximum load. Finally, individual screws were tested in high-cycle four-point bending fatigue in a custom fixture. In push-through, the Orthomet screws could withstand a significantly greater maximum load than the Synthes screws. The fatigue life of the Ace screws was significantly longer than the Howmedica, Synthes, and Orthomet screws, whereas the Orthomet screws performed significantly worse than all other screw types. Although cannulated screw systems uniquely address the problems of push-through resistance and high-cycle fatigue failure, certain designs are more susceptible to failure.