Biomechanischer Stabilitätsvergleich am distalen Femur: retrograde Nagelung versus polyaxiale, winkelstabile Plattenosteosynthese

The distal femur fracture is a complex fracture with an increasing incidence. There are intramedullar stabilization systems which are well known and practiced. With the newest Generation of extramedullar, angle stable stabilization systems there is an equivalent method for the treatment of these complex fracture type available. In this study the biomechanical stability and loading capacity of these osteosynthesis types will be compared. We used eight paired, formalin-fixed cadaveric femurs. Preexisting fractures and osteolyses of the bones were radiologically excluded. A standardized fracture model (AO 33-A3) with a defect zone of 2 cm was created. After randomization there had been eight retrograde M/DN™-femoral nail osteosynthesis implanted, four on the right side of a pair of femurs and four of the left side. On the other part of the femur pair there had been the plate-osteosynthesis implanted. In the diaphysis we used two cortical screws with the nail osteosynthesis and four cortical screws with the plate osteosynthesis. For the distal locking cortical and cancellous screws were used (nail-osteosynthesis: 3, plate-osteosynthesis: 5). A 2 cm high Technovit - base on the femur condyles was formed. Hereafter the femurs were tested in a material proof machine from the INSTRON firm (Modell 5566). There was a standardized protocol. The tests started with 1.000 N. After 500 cycles of the same load, the load was increased by 500 N up to a maximal load of 5.000 N. Abort criterions had been a load decrease of 20 % and a deformation of 30 mm. The statistical evaluation was done with the SPSS analyse software (IBM SPSS Statistics). Significance was shown as p < 0,05 in the student t-test. All osteosynthesis resist a load of 2.500 N. With this load there had been an average plastic deformation of 0,29 mm within the nail-osteosynthesis group and 0,76 mm within the plate-osteosynthesis group. There is no significant difference in plastic deformation at the loading level of 2.500 N (p=0,056). The average plastic deformation of the whole construction was 2,76 mm for the nail-osteosynthesis and 5,64 mm for the plate-osteosynthesis. The reversible deformation at the load level of 2.500 N was significant different (p=0,0036). The average deformation was 1,99 mm within the nail group and 3,99 mm within the plate group. Between the load-level of 4.000 N and 5.000 N 5 osteosynthesis failed in each group. There has been no significant difference between the two osteosynthesis methods (p=0,943). 3 of 8 nail osteosynthesis and 1 of 8 plate osteosynthesis didn’t fail. The failure occurred in most of the cases at the diaphysal screw connection. Only the sample 2 with the NCB-plate osteosynthesis failed through a condyle breakage. A deformation ≥ 30 mm was achieved by the samples 3 and 4 of the plate-osteosynthesis group. This study demonstrates that both systems generate a sufficient primary stability and a comparable biomechanical stability under axial, cyclic loading. The failure type is also comparable. There is only a significant difference in the reversible deformation which indicates a better flexibility. This can be an advantage in the fracture healing.