[Bone formation and mechanical properties of the cancellous bone defect site filled with hydroxyapatite granules]

We investigated the co-relationships of the granule size of hydroxyapatite with the amount of new bone formation and with mechanical properties at the bone defect site filled with hydroxyapatite granules. In this experimental study, cylindrical bone defects 6.4 mm in diameter and about 1 cm in depth were prepared in the bilateral femoral condyles and in the tibial condyles of large albino rabbits (body weight 5.1-7.3 kg), filled with three types of hydroxyapatite granules 0.3-0.6 mm, 0.6-1.0 mm, and 1.0-2.0 mm in diameter, and examined after 2, 4, and 8 weeks after sacrificing the animals. Two animals (4 limbs) each were examined for the same granule size and the same period after filling. Undecalcified sections of the tibia at the defect site were examined histologically, and the volume and surface of hydroxyapatite, the volume of new bone, the surface of the new bone in contact with hydroxyapatite, and the percent of this surface relative to the entire surface of hydroxyapatite were determined. Columnar specimens 6.0 mm in diameter and 5.0 mm in length were prepared from the defect sites of the femur, compressed at a deformation rate of 2.4 mm/min, and the failure stress, stiffness, and energy absorption were calculated from the load-deformation curve. The normal cancellous bone from 2 unoperated animals (4 limbs) and bone defect sites not filled with hydroxyapatite granules of 2 animals (4 limbs) were examined as controls. A greater amount of new bone was formed as the granule size was smaller. After 8 weeks, the failure stress and energy absorption were higher as the granule size was smaller, and the mechanical properties of the bone defect sites filled with 0.3-0.6 mm granules were similar to those of normal cancellous bone.