gms | German Medical Science

Objectives: To biomechanically evaluate the stability of a diaphyseal anchored, cementless stem in presence of a proximal periprosthetic femoral medial wall defect compared to the stability of the same stem in an intact femur.

Methods: 22 human cadaveric femora were assigned to a fracture group, featuring a proximal medial wall defect, and a native group. A diaphyseal anchoring stem was implanted in each group. The specimens were tested under an increasing axial loading protocol until catastrophic failure. Implant loosening was determined at 1mm axial displacement of the implant. Load, cycles, and multiples of the respective bodyweight at implant loosening were calculated.

Figure 1 [Fig. 1]

Results and conclusion: Mean initial stiffness was 2243.9 ± 467.9 N/mm for the intact group and 2190.127 ±474.8 N/mm for the fracture group. Mean load to loosening in the intact group was 3210.5 ± 1073.2N and 2543.6 ± 576.4N in the fracture group. Mean cycles to loosening in the intact group were 27104.9 ± 10731.7 and 20431.5 ± 5763.7 in the fracture group. Mean multiples of the resulting bodyweight at loosening in the intact group was 548.3 ± 158.5% and 441.4 ±104% in the fracture group.

A medial wall defect involving 40% of the medial anchorage distance significantly decreases the axial stability of a diaphyseal anchored stem. Nevertheless, mechanical failure occurred at superphysiological stress levels as average hip contact forces usually don't exceed 2-fold bodyweight. At loosening rates of about 4-fold body weight in the fracture group, a "safe zone" remains of 0.5-fold body weight for peak loads and 2-fold body weight for average loads.