Biomechanics of Ender rods, compression screw, and Zickel nail in the fixation of stable subtrochanteric femur osteotomies.

Compression screws, Ender rods, and Zickel nails were evaluated mechanically to determine their resulting stability in the fixation of experimental subtrochanteric femur osteotomies. Twenty-five pairs of human cadaver femurs were used. Experimental osteotomies were created and a random member of each pair was fixed with a compression screw (AMBI system, Richards Mfg. Co). The other member was fixed with either Ender rods or a Zickel nail. The specimens were oriented in a direction to approximate the in vivo position of maximum resultant force on the femur and then subjected to a single progressive load to failure on a materials testing machine. The ultimate strength of fixation with the compression screw was approximately 4.6 times greater than with the Ender rods and 1.8 times greater than that with the Zickel nail. Ender rods and Zickel nails failed primarily in external rotation, whereas the compression screw failed by bone fracturing and varus bending.