Studies on mechanical strength of bone. II. Torsional strength of cortial bone after rigid plate fixation with and without compression.

Biomechanical properties of intact rabbit tibio-fibulae were investigated with torional loading 1 day to 36 weeks after fixation using 4-hole stainless steel ASIF/DCP plates with and without compression. During the first 12 weeks energy absorption and torque moment increased slightly as a result of subperiosteal new bone formation following application of the plates. There was a concomitant increase in the yielding properties of the bone, reflecting progressive porotic transformation. Thereafter, up to 36 weeks postoperatively, the values for energy absorption, torque moment and angular deformation gradually declined in both groups of bones. The differences between normal control bones and plated bones at 36 weeks were 70.1, 53.0 and 26.3 per cent, respectively. The decay of torsional strength was the same whether or not compression was used in the plate fixation. The results suggest that even in normal bone rigid plates induce a considerable loss of strength due to the cancellous transformation they are known to cause in cortical bone.