Healing of Stable and Unstable Osteotomies in Rats

In two groups of rats a partial, transverse osteotomy at the mid-shaft of the femur was made. The osteotomy was manually broken in one of the groups after the operation. The healing process of the osteotomies was evaluated at 15, 30, 60, and 90 days following operation. The unstable osteotomies healed by the formation of external callus, in contrast to the stable osteotomies, which healed by so-called primary bone union. At 90 days the bending moment and rigidity of the instable osteotomies were regained as compared to the intact right femurs. The stable osteotomies were only 67% as strong as the intact bones (P less than 0.01). The bending rigidity of the stable osteotomies was equal to that of the intact bones during the experimental period. It is concluded that instability at the fracture site is conducive to fast healing.