Background: Although external fixation is widely used for fractures and limb lengthening, evaluation of the time for removing the external fixator is dependent upon radiographic examinations and clinical findings, and a useful method has yet to be established clinically. This study aimed to measure the bone electrical impedance (Z values) non-invasively by using external fixation pins as electrodes, and clarify the relationship with bone union.
Methods: Thirty rabbits received the external fixation at the right tibia and were assigned to a control group (group C; n = 5) and a fractured group (group F; n = 25). Z values were measured once a week following surgery. The animals of group F were assigned to 5 groups (weeks 2, 3, 4, 5, and 6 after osteotomy, each n = 5). The resistivity (rho) of the electrical property between electrodes was measured prior to euthanasia, and fracture cross-sectional area (FrA) of the conduction pathway and maximum bending stress (Bmax) were measured following excision of the tibia.
Results: Although Z values in group F increased through 5 weeks after surgery, Z values in group C remained constant at 3 weeks, and significant differences were observed between groups at 4, 5, and 6 weeks. The rho values and FrA in group F decreased through 5 weeks; while Bmax increased, reaching a plateau at 5 weeks.
Conclusions: Narrowing of conduction pathway due to the decrease in the contour of fracture area accompanying bone remodeling resulted in an increase of Z values. Both Z values and Bmax in group F reached a peak at 5 weeks, this was believed to be the optimal time for removal of external fixation. These results suggest that measurement of Z values makes it possible to evaluate bone union.