Effects of Different Distraction Frequencies on Axon Morphometry Following a Two-Month Consolidation Period in a Rabbit Model

Despite advances in limb lengthening, neurovascular complications still occur. Two key parameters for successful distraction osteogenesis are 1) an adequate period of consolidation and 2) distraction frequency. Several studies have described decreases in soft-tissue resistance and improved nerve conduction with high vs. low frequency distraction. Clinically, a distraction rate of 1 mm/day, to 30% of original length, followed by an appropriate consolidation period is often employed. Most limb lengthening peripheral nerve studies describe nerve morphometry either immediately following distraction termination or following distraction at a rate greater than 1 mm/day. Thus, the impact of clinically physiologic limb lengthening on the peripheral nerve remains unclear. The goals of this study were to determine the effects of clinically appropriate limb lengthening at high and low distraction frequencies on internodal length and nerve morphology (diameter and composition).

Ten rabbits were assigned to Group 1 (n = 5)—low frequency distraction (1 mm/day, 3 increments) or Group 2 (n = 5)—high frequency distraction (1 mm/day, 720 increments). External fixators were used to distract the left tibia until 30% diaphyseal lengthening was achieved. The contralateral limb served as control. Two months after distraction termination, the external fixators were removed and the tibial nerve was harvested. Axon teasing was performed to determine internodal length and nerve morphology. Student's paired t-test and chi-square were used for statistical analysis. The level of significance was 0.05.

Mean internodal lengths were not statistically significantly different between the high and low frequency distraction groups. However, the mean overall axonal diameter was decreased, compared to control, in both Group 1 (16.35 μm vs. 14.2 μm, p < 0.01) and Group 2 (16.73 μm vs. 14.3 μm, p < 0.01). This change in axonal diameter was significant in A alpha fibers with diameters between 15–20 μm (p < 0.0028, Group 1) and > 20 μm (p < 0.0001, Groups 1 and 2).

Thirty percent limb lengthening at a rate of 1 mm/day significantly decreased axon diameter, with the A alpha motor fibers exhibiting the greater change. Contrary to other studies, no increase in internodal length was observed. Thus, internodal length increases may be seen immediately after lengthening but return to normal lengths after a period of consolidation. However, studies are necessary to fully determine the effects of the consolidation period on axon morphometry and the physiology of axon repair. A greater understanding of peripheral nerve responses to elongation may not only increase limb lengthening capabilities, but improve function following acute or chronic nerve repairs.