Assessment of distraction callus in rabbits by monitoring of the electrical impedance of bone

BACKGROUND AND PURPOSE Evaluation of distraction callus is important for determination of the optimal time for removal of external fixation. We attempted to determine whether there might be a relationship between the electrical impedance of bone and callus maturation, with a view to using impedance as a way of knowing when to remove a fixator. METHODS We applied an external lengthener to the right tibia of 24 rabbits and performed distraction at 1 mm/day for 10 days. Radiographs were taken and measurement of overall impedance between fixation pins was performed weekly after distraction. At weeks 2, 4, 6, and 8 after distraction (n = 6 each), resistivity of the bone to electrical conductivity was measured before killing. Cross-sectional area of the conduction pathway in callus and maximum bending stress were measured after excision of the tibia. RESULTS The overall impedance increased statistically significantly from 1 to 6 weeks after completion of distraction. The resistivity of bone decreased over 4 weeks and the cross-sectional area of callus decreased significantly over 6 weeks, while the maximum bending stress increased significantly over the same time. We observed a negative correlation between the cross-sectional area of callus and maximum bending stress. INTERPRETATION The impedance values, which are related to changes in electrical conductivity and the conduction pathway, increased due to the changes in the cross-sectional area of callus, despite the reduction in bone resistivity. Since the cross-sectional area of callus was correlated with maximum bending stress and the impedance values increased with the callus-remodeling process, we suggest that temporal increases in overall impedance reflect callus maturation.