Medial migration of intramedullary hip fixation devices: a biomechanical analysis

Intramedullary nails for fixation of extracapsular hip fractures have gained popularity recently. Although clinically successful, they are not devoid of complications. An infrequently reported complication is the medial migration of the femoral neck element (FNE) of the implant into the pelvis. The purpose of this study was to create a biomechanical model simulating this effect based on a clinical case radiographic analysis.

Eight clinical cases of medial migration were available for radiographic analysis. Medial migration was quantified and the fractures were classified. A biomechanical model was built comprising two fixtures containing the nail and FNE respectively. A pivot between the two fixtures, representing a deficient femoral calcar, simulated an unstable fracture type. Two pivot points were used for each nail. The constructs were tested using sinusoidal loading (40-800 N at 2 Hz) and medial migration was assessed. Five different nail designs (TFN, PFN, PFN-a, Gamma-3 and IMHS) were tested (overall 75 tests).

All the five implants demonstrated medial migration to a similar distance. The TFN required the highest number of cycles (3127 +/- 2569) and the IMHS the lowest (58.8 +/- 3.6) although this difference did not reach statistical significance (P = 0.07). Changing the pivot point for the medial calcar did not alter the results significantly. All eight clinical cases demonstrated an unstable intertrochanteric fracture pattern (AO/OTA 32A2).

Discrete biomechanical conditions are required to reproduce medial migration of the FNE in cephalomedullary devices.