Changes in canine peripheral nerves during experimental callus distraction

In 24 beagles, lengthening of the right tibia was performed by callus distraction after osteotomy and application of a ring fixator. Distraction was started the fifth day after surgery with a distraction rate of 0.5 mm twice per day and ended after 25 days. A control group of six additional dogs underwent tibial osteotomy and external fixation without distraction. Twelve animals with leg lengthening and three animals without leg lengthening were sacrificed immediately after the distraction period of 25 days (Group A), the remaining 15 dogs were sacrificed after an additional consolidation phase of another 25 days (Group B). From the distracted right leg and from the left control side of each animal, the tibial and peroneal nerves were removed and studied by means of morphometric analysis of semithin sections and electron microscopic study. The authors reported only minor alterations of the myelinated nerve fibers consisting in relatively thin myelinated remyelinating nerve fibers, single axon degeneration, or macrophages. There was no axonal sprouting. In contrast, electron microscopic study disclosed signs of late Wallerian degeneration and axonal regeneration of the nonmyelinated axons. Morphometric analysis of the peroneal nerves revealed a significant increase of fiber density and a reduced mean axon and fiber diameter in the consolidation period (Group B). Similar changes were found in the tibial nerves in Group A and disappeared in the consolidation phase (Group B). These features indicate that callus distraction leads to moderate degenerative changes, followed by repair mechanisms, almost complete recovery, and some nerve fiber growth.