Biomechanical Effects of the Nail Entry Zone and Anterior Cortical Bone Loss on the Proximal Tibia

OBJECTIVES: The aims of this study were as follows: to determine if the presence of the nail entry zone alters the biomechanics of the proximal tibia after intramedullary (IM) nailing using a computational model; to determine if nail removal restores normal biomechanics to the proximal tibia; and to determine if these effects are magnified with anterior cortical bone loss. METHODS: Three-dimensional finite element (FE) tibial models were developed and used for this study: an intact tibia, a nailed tibia, and a tibia with the nail removed. One matched pair of fresh-frozen cadaver tibias was obtained to construct and validate the FE model. The tibias underwent computed tomography scanning, and geometric models were obtained from computed tomography data through volumetric reconstruction. The left tibia was implanted with an unlocked IM nail. The experimental validation of the models was performed by comparing experimental and FE data. Anterior cortical bone of the proximal tibia was removed down to the tibial tubercle on the models to simulate a worst-case scenario of its removal during an IM nailing procedure. Three load cases were considered for each FE tibial model: standing, walking, and single-limb kneeling. RESULTS: The principal strain values of the proximal tibia with the nail entry zone were increased in comparison to the intact tibia in all 3 loading scenarios (+350%-550%). These effects were greatly magnified with anterior cortical bone loss near the nail entry zone (6-fold increase). The presence of the nail increased the principal strain values in the proximal tibia both with an intact and a disrupted anterior cortex in all loading scenarios, and these values remain elevated even with removal of the nail. The values predicted by the FE model were in good agreement with the experimentally measured strains (R = 0.92). CONCLUSION: The nail entry zone in the proximal tibia greatly increases the principle strain values when standing, walking, and kneeling. The presence of the nail results in the highest strain values, but they do not return to normal when the nail is removed. These effects are significantly amplified with removal of the anterior tibial cortex near the nail entry zone.