Healing of osteotomies under different degrees of stability in rats

In rats, a partial, transverse osteotomy at the midshaft of both femurs was made. In two thirds of the bones, the osteotomy was manually broken and stabilized, either by a tightly fitting or by a loose intramedullary nail. The healing process of the osteotomies was evaluated at 30, 60, and 90 days after the operation. The stable osteotomies healed by direct bone union, and the unstable osteotomies healed by the formation of external callus. At 30 days, the bending moment, rigidity, and energy absorption of the stable osteotomies were significantly greater than those of the unstable osteotomies. At 60 days, however, there were no significant differences in these parameters. During the observation period, no significant differences in mechanical properties were found between the osteotomies nailed with a tightly fitting or a loose intramedullary nail. It is concluded that healing of rigidly fixated fractures is a rather slow, steady process in relation to healing of unstably fixated fractures. Different degrees of instability of intramedullary nailed fractures do not seem to be of significant importance as to the eventual recovery of mechanical properties.