Background: Despite the use of intramedullary fixation devices for the stabilisation of intertrochanteric fractures, the rate of complications is still high. One of the main reasons for burdensome reinterventions in 9-15% of cases is the cutting out of the fixation device through both the spongious bone and the cortical bone at the apex of the femoral head. This phenomenon is strongly connected to the reduction of the fractures achieved, the technical performance of the operation with optimal implant positioning and the resistance of the trabecular bone in the femoral head against deformation by the fixation device. The latter is very low in cases of severe osteoporosis. To prevent the complication of cutting out, it seems sensible to find the limits of load-bearing capacity of individual osteoporosis-associated features (i.e. bone mineral density) at which special additional measures or other techniques for the treatment of these patients are desired.
Methods: In a first step a new biomechanical standard test for implants stabilizing unstable trochanteric fractures was developed, which would provide predictable results depending on bone mineral density. In a second step a cut-off limit was sought for the bone density in the proximal femur that would afford stable fixation as measured by QCT (quantitative computed tomography) and DEXA (dual-energy X-ray absorptiometry).
Results: The developed test is realistic; it can be used to study typical cutting out phenomena on cadaver femora. In an unstable fracture model (type A 2.3 of the AO classification), the implants DHS with TSP, PFN and TGN showed a stable long-term load-bearing capacity at a bone mineral density of >0.6 g/cm3. In 5 of 32 specimens a cutting out phenomenon could be demonstrated, in 4 cases if the bone mineral density of the proximal femur was below 0.6 g/cm3 as measured by DEXA, and in one case poor performance of the implant (short screw in the femoral head) was evident.
Conclusions: In cases of bone density of >0.6 g/cm3 in the proximal femur (DEXA), the standard implants for the fixation of unstable trochanteric fractures could guarantee fixation without cutting out. The critical value of sufficient bone density in our few cases seems to be around 0.6 g/cm3 as measured by DEXA. Further investigation is needed to define the limits of bone mineral density for a successful osteosynthesis. An appropriate augmentation of the trabecular bone of the femoral head or a new design of the central loading device could increase the load-bearing capacity and thus help to reduce the cutting out phenomenon. Another alternative could be the primary implantation of an endoprosthesis in the treatment of these patients.