Canine leg lengthening by the Ilizarov technique. A biomechanical, radiologic, and morphologic study.

The mechanical properties of canine tibias lengthened by the Ilizarov method as well as the degree of healing--as determined by the osseous appearance of the limbs using three imaging modalities--were examined as a function of fixator application time. Sixteen mongrel dogs had right tibial lengthening using a two-ring Ilizarov external fixator. The average lengthening was 27 mm. Four groups of four animals were killed at four, six, eight, and ten weeks after the completion of lengthening. The operated and contralateral tibias were examined with biplanar roentgenography, computerized tomography, and technetium scintigraphy. The torsional strengths of the tibias were then determined and histologic analysis was performed. There was considerable variation in the healing rate of the lengthening sites, both within and among groups. Histologic analysis and results of the imaging modalities revealed different degrees of development of the lengthening sites among members of the same group. Two types of mechanical behavior were seen, corresponding to completely and incompletely ossified lengthening zones. The strongest samples were those that had ossified across their lengthening sites by four or six weeks. The degree of recorticalization of the new bone and the extent of formation of a marrow space correlated well with increased torsional strength. The average strength of the fully ossified bones decreased progressively with time, paralleling a decrease in cortical thickness outside the lengthening zone. This seems to be a consequence of stress shielding by the fixator and disuse by the animal.