Bone strength and histomorphometry of the distal femur.

The ultimate bone strength of the distal femur was measured radially, by indentation testing, around the transepicondylar line in 3 mm depth steps up to 12 mm below the subchondral bone plate. Specimens from 10 cadavers were used. This orientation of specimens was chosen as a way to provide measurement in a more physiologic orientation for load bearing and to standardize the assessment. Bone hardness declined sharply over the first 6 mm below the surface, tending to plateau at deeper levels. Within the top 6 mm layer the lateral condyle was softer than both the medial condyle and the central patellofemoral area (P < .05), but at deeper levels it maintained greater hardness. Of the histomorphometric parameters, those showing the greatest consistent correlation with hardness were bone volume fraction and trabecular separation. When the tibiofemoral and patellofemoral compartments were compared it was found that for a given value of bone volume fraction, condylar bone is marginally harder than patellofemoral bone. The data are relevant to the design of implants that match their geometric and material properties to the shape and strength of the underlying bone.