Effects of rotational instability on healing of femoral osteotomies in the rat.

Three different degrees of rotational instability were induced in transversally osteotomized rat femora by means of intramedullary steel nails with various degrees of interlocking. Union was delayed in osteotomies with the greatest rotational instability. The strength, stiffness and energy absorption of the osteotomies at 4, 8 and 16 weeks were also lower in this group. At 25 weeks, the end-point of the experiment, there were no differences in incidence of union or in the mechanical properties between the experimental groups. We conclude that a high degree of rotational instability caused delayed fracture healing.