Biomechanical characterisation of osteosyntheses for proximal femur fractures: helical blade versus screw.

Proximal femur fractures are of main concern for elderly and especially osteoporotic patients. Despite advanced implant modifications and surgical techniques, serious mechanical complication rates between 4-18% are found in conventional osteosyntheses of proximal femur fractures. Clinical complications such as the rotation of the femoral head and the cut-out phenomenon of the fracture fixation bolt are often diagnosed during post-operative treatments. Therefore, efforts in new intramedulary techniques focus on the load bearing characteristics of the implant by developing new geometries to improve the implant-tissue interface. The objective of this investigation was to analyse the osteosynthesis/femur head interaction of two commonly used osteosyntheses, one with a helical blade and the other one with a screw design under different loading conditions. For the comparative investigation the helical blade of the Proximal Femur Nail Antirotation was investigated versus the screw system of the Dynamic Hip Screw. After implantation in a femoral head the loads for rotational overwinding of the implants were analysed. Pull-out forces with suppressed rotation were investigated with analysis of the influence of the previous overwinding. All investigations were performed on human femoral heads taken of patients with average age of 70.3+/-11.8. The bone mineral densities of the human specimens were detected by QCT-scans (average BMD: 338.9+/- 61.3$\frac[\mathit[mg]][\mathit[cm];[3]]$) Prior to cadaveric testing the experimental set-up was validated and special influences were analysed by the use of synthetic foam blocks (Sawbone). The helical blade showed a significant higher torque for the rotation of the femoral head compared to the screw system. The pull-out forces of the blade were substantially lower than of the comparative screw. Taken together the helical blade showed a higher potential of rotational stability, but after a rotation the lower pull-out forces demonstrate a higher degree of damage to the femoral head.