Experimental study on the effect of facial nerve retention on nerve regeneration and reconstruction of orbicularis muscle function

Objective To investigate the effect of the regenerated facial nerve (FN) with varying degrees on the reconstruction of orbicularis function post facial-hypoglossal side-to-side neurorrhaphy (FN-HN) in the incomplete facial palsy model. Methods A total of 40 male adult Sprague Dawley rats were divided into 4 groups randomly. All rats were performed with crushed FN injury and FN-HN anastomosis. Three months later, the 4 groups were performed with FN cross section with different ratios at the proximal end to the injured site: no intervention in group A, 1/3 of cross section in group B, 2/3 of cross section in group C and transection in group D. The degree of closure of the affected-side eyelids in rats was assessed using the eye blink score, the muscle active potentials (MAPs) were used to detect the action potential and the area under the curve (AUC) of the rat′s orbicularis oculi muscle on the affected side, the number of positive neurons in the FN and HN nuclei were revealed by retrograde labeling, and the myelinated axons were counted on semi-thin sections of facial nerves and nerve grafts. Results For the eye blink scores, the 4 groups showed significant difference during the three months (F=12.47, 11.00, 10.61, 19.13, all P<0.05) and after the cross section (F=29.06, P<0.05). Moreover, the score in group D was lower than that in group A and B(both P<0.01). One week after the cross-section of FN, all 4 groups showed significant difference in amplitude and AUC of MAPs when we stimulated the proximal end of anastomosis site of facial nerve and the middle of the nerve graft (F=27.56, 11.86, 6.33, 4.65, all P<0.05). In addition, the amplitude and AUC of MAPs in group D were significantly lower than those in group A (both P<0.05). Among the 4 groups, retrograde labeling indicated significant difference in the number of positive neurons in the FN nucleus (F=6.52, P<0.05), and the neurons in group D were less than those in group A and B (both P<0.05). The counts of myelinated axons in both FN trunk and nerve graft in the 4 groups were significantly different (F=8.33, 6.35, both P<0.05). In particular, group A had less myelinated axons compared with group C and group D (both P<0.05). Conclusion Preserving structural integrity of facial nerve during FN-HN seems to have positive significance. Retention of at least 2/3 regenerated facial nerve is beneficial for the recovery of the orbicularis oculi muscle. Key words: Anastomosis, surgical; Facial nerve; Hypoglossal nerve; Orbicularis; Rats