Biomechanical analysis of peri‑implant fractures in short versus long cephalomedullary implants following pertrochanteric fracture consolidation.

Introduction: Pertrochanteric femur fracture fixation with use of cephalomedullary nails (CMN) has become increasingly popular in recent past. Known complications after fracture consolidation include peri‑implant fractures following the use of both short and long nails, with fracture lines around the tip of the nail or through the interlocking screw holes, resulting in secondary midshaft or supracondylar femur fractures, respectively. Limited research exists to help the surgeon decide on the use of short versus long nails, while both have their benefits. The aim of this biomechanical study is to investigate in direct comparison one of the newest generations short and long CMNs in a human anatomical model, in terms of construct stability and generation of secondary fracture pattern following pertrochanteric fracture consolidation.

Methods: Eight intact human anatomical femur pairs were assigned to two groups of eight specimens each for nailing using short or long CMNs. Each specimen was first biomechanically preloaded at 1 Hz over 2000 cycles in superimposed synchronous axial compression to 1800 N and internal rotation to 11.5 Nm. Following, internal rotation to failure was applied over an arc of 90° within one second under 700 N axial load. Torsional stiffness as well as torque at failure, angle at failure, and energy to failure were evaluated. Fracture patterns were analyzed.

Results: Outcomes in the study groups with short and long nails were 9.7 ± 2.4 Nm/° and 10.2 ± 2.9 Nm/° for torsional stiffness, 119.8 ± 37.2 Nm and 128±46.7 Nm for torque at failure, 13.5 ± 3.5° and 13.4 ± 2.6° for angle at failure, and 887.5 ± 416.9 Nm° and 928.3 ± 461.0 Nm° for energy to failure, respectively, with no significant differences between them, p ≥ 0.17. Fractures through the distal locking screw holes occurred in 5 and 6 femora instrumented with short and long nails, respectively. Fractures through the lateral entry site of the head element were detected in 3 specimens within each group. For short nails, fractures through the distal shaft region, not interfacing with the implant, were detected in 3 specimens.

Conclusion: From a biomechanical perspective, the risk of secondary peri‑implant fracture after intramedullary fixation of pertrochanteric fractures is similar when using short or long CMN. Moreover, for both nail versions the fracture pattern does not unexceptionally involve the distal locking screw hole.