Strength Pattern of Cancellous Bone at the Ankle Joint

The strength pattern of cancellous bone was examined in 17 ankles by a cylinder compression test and in eight distal tibial specimens by an osteopenetration test. With the compression test, 6 mm levels were examined, five levels at the tibia and three levels at the talus. Cylinders were obtained from all levels from corresponding locations. The cylinders were tested at a constant deformation rate of 5 mm/min until failure. From the load deformation curves ultimate strength, yield strength, yield strain, ultimate strain and elastic modulus were calculated. The penetration test involves resection of the joint surface just above the subchondral bone plate at the distal tibia. Measuring force and displacement, a 2.5 mm conical needle was pushed into the bone at 25 locations in a 5 × 5 matrix. The force-displacement curves were integrated at intervals of 2 mm. Three levels were evaluated, corresponding approximately to the first level in the compression test. The study indicates the presence of a strong antero-lateral part and a slightly weaker postero-medial part at all levels in the tibial and talar bone. The compression test at level 1 and the penetration test at levels 1, 2, and 3 in the distal tibia revealed a high strength area crossing the central part, in contrast to the proximal levels of the tibia where the central parts were very weak. At the talus, a high strength area ran along the lateral and posterior border. There were no significant differences in the profiles of ultimate strength, yield strength, and elastic modulus.