[Biomechanics of femoral interlocking nails at the bone-implant transition]

Today there is a variety of different interlocking intramedullary nail designs available for the femur-each designed with a different approach to achieve stability for fracture fixation. We compared different nail types in the bone-implant complex (BIC) of four unreamed solid nails and a slotted, reamed nail to see if there are major differences in stiffness for axial load, bending and torsion. We simulated comminuted mid-shaft fractures by a 2 cm defect osteotomy in paired human cadaver femora. Each bone was tested intact in a Universal testing machine. The results were recorded, osteotomy and osteosynthesis were performed, and the BIC was tested. Relative stiffness was calculated for each individual bone. For P-values less than 0.01 ('least significance difference test') the difference between groups was considered to be significant. In torque testing the unslotted solid nails showed significantly more stiffness (0.6-1.8 Nm/degree) compared to the slotted nail (0.2 Nm/degree). Compared to intact bone (6.9 Nm/degree), both groups of nails were significantly less stiff (relative stiffness 2-20%). In axial load and bending testing, the large-diameter unreamed nail showed greater higher stiffness (32-68%). This study shows that stiffness of the BIC in interlocking femoral nails is more dependent on nail profile than on the press-fit of nails in the medullary canal. For torque stiffness the absence of a slot is of special importance. According to our study, all of the unslotted nails tested give adequate stability for fracture fixation.