Effects of instability on fracture healing in the rat.

The effect of bending and rotational instability on the healing of a femoral osteotomy was studied in rats using intramedullary fixation with nails made of either steel or flexible polyacetal. All osteotomies were made rotationally unstable by reaming the medullary cavity to a diameter wider than the nails. At 16 weeks, four osteotomies had healed, and 17 had not healed. However, bending tests gave higher values for strength and energy absorption in non-unions with flexible as compared to Stiff nails. Rotational instability thus caused a high rate of non-union in the present model, which has given safe healing when the osteotomy has been stabilized for rotation. Flexible nailing increased strength and energy absorption in bones with non-union, but did not affect the incidence of healing.