The "Z-effect" Phenomenon Defined: a Laboratory Study

The Z-effect phenomenon is a potential complication of two lag screw intramedullary nail designs used for fixation of intertrochanteric hip fractures, in which the inferior lag screw migrates laterally and the superior lag screw migrates medially during physiologic loading. The current investigation was undertaken in an attempt to reproduce the Z-effect phenomenon in a laboratory setting. Sixteen different simulated femoral head and neck constructs having varying compressive strengths were created using four densities of solid polyurethane foam and instrumented with a two-screw cephalomedullary intramedullary nail. Each specimen was then cyclically loaded with 250 N vertical loads applied for 10, 100, 1000, and 10,000 cycles. Measurement of screw displacement with respect to the lateral aspect of the intramedullary nail was made after each cyclic increment. The inferior lag screw migration component of the Z-effect phenomenon was reproduced in specimens with head compressive strengths that were higher than the compressive strengths of the neck. Specimens with the greatest difference in head-neck compressive strength demonstrated the most significant displacement of the inferior lag screw without any displacement of the superior lag screw. Specimens with a femoral neck compressive strength of 0.91 MPa of and a head compressive strength of 8.8 MPa resulted in more than one centimeter of inferior lag screw lateral migration after 10,000 cycles of vertical loading. Models where the femoral head had a higher compressive strength than that of the femoral neck may simulate fracture patterns with significant medial cortex comminution that are prone to varus collapse.