Monitoring the healing of a tibial osteotomy in the rabbit treated with external fixation.

Fracture site axial rigidity was monitored non-destructively at weekly intervals during healing of tibial osteotomies in adult rabbits. Two groups of 20 rabbits each were treated with external fixators of two different rigidities. Four animals from each group were killed at 3, 5, 6, 7, and 8 weeks to determine the bending moments at failure of the healing fractures. Normal fracture healing was accompanied by characteristic phases in the development of fracture site axial rigidity. From 0 to 3 weeks there was a period of low and approximately constant rigidity, followed by a linear increase during 3 to 5 weeks to an approximately three to four times greater rigidity. The maximum average normalized axial rigidities were reached at 6 weeks and were 57% (high rigidity group) and 77% (low rigidity group) of the untreated contralateral controls. The maximum average normalized failure moments occurred at 8 weeks and were 48% (high rigidity) and 44% (low rigidity) of controls. The differences due to fixator rigidity were not statistically significant except for a large increase in failure moments at 3 weeks for the low rigidity group. Axial rigidities were correlated (r2 = 0.74 and 0.53, respectively) with failure moments, but only during the first 6 weeks. The monitoring technique provides a nondestructive means for following the biomechanical progress of fracture healing in an animal model. The occurrence of the characteristic increase in fracture site axial rigidity at 3 to 5 weeks can also be used to distinguish between normal and abnormal healing.