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.
