gms | German Medical Science

Objectives: Angular stable plate fixation is a widely accepted treatment option for periprosthetic femoral fractures with stable implants. There is no conclusive evidence for the optimal plate position concerning distance or overlap between the tip of the implants.

Methods: 38 composite femur models (SawboneTM) were reamed up to a diameter of 23 mm to create an osteoporotic bone model. A weber hip prosthesis stem (Zimmer™) was cemented (palacos™) into each femur using 3rd generation cementing techniques. A distal femoral NCB plate (Zimmer™) was attached to each femur with a distance to the tip of the stem varying from 8 cm distance to 6 cm overlap in 2 cm steps. Each specimen was mounted to a servohydraulic polyaxial testing machine (Instron 8874). The specimens were tested in axial load of up to 1500 N and torsional load of up to 50 Nm. 3D-surface strain on the femur around the tip of the plate was measured with a 3-dimensional image correlation system (Limess™). Load to failure was tested at the end of each trial in axial loading.

Results and Conclusion: 3 Specimen failed early in the axial and 4 in the torsion testing. All early failures occurred between 2 cm overlap and 2 cm distance. Specimens were divided into a far group with a distance between the implants or an overlap of more than 4 cm and a close group within 4 cm distance or overlap. Specific failure patterns could be identified. Strain was significantly higher in the close group, and the load to failure significantly lower.

In Conclusion placing the plate within 4 cm distance or 4 cm overlap to the femoral stem jeopardizes for implant failure in falls and for fatigue fractures. These results apply only for osteoporotic bone. A minimal gap of 6 cm or an overlap of the plate and the femoral stem of at least 4 cm is recommended in periprosthetic femur fractures to avoid stress risers.