An investigation into the response of a sectioned mental nerve in the rat

Abstract

The chances of mandibular injury and thus injury to the inferior dental nerve have increased with high speed transport, more contact sport and recent advances in major oral surgery. Clinically most of these nerve injuries resolve themselves with time. However, a review of the literature has not revealed any experimental study on the response of the mental nerve to injury. This investigation was to determine the mental nerve (branch of the inferior dental nerve distal to the mandible) response to a transverse section. The mental nerve in the Sprague Dawley rat was sectioned and the subsequent response was observed histologically. An attempt was also made to use an electrophysiclogical technique. This histological investigation demonstrated that degeneration and regeneration of the mental nerve occurred, in general, in a similar fashion to other nerves. However, because of the mental nerve’s composition, peripheral position and the fact that it is in a restricted compartment (i.e. the lip) the relative displacement of the sectioned trunks is reduced. Because of this reduced displacement, successful regeneration is enhanced. During the regenerative phase, most of the axon branches entered the distal trunk. Those that did not enter the distal trunk moved off peripherally via minifuniculi or degenerated. However a large proportion of axons did regenerate. Myelin maturation was observed in all the distal segments but not in all the minifuniculi. Unfortunately the electrophysiologica1 technique failed to demonstrate the physiological response of the sectioned mental nerve. The exact reason was not established. However, this failure could be due to afferent nerve fibres and/or leakage of current to the digastric neuromuscular apparatus. Thus there were no physiological responses recorded for the sectioned mental nerve. The histological results concurred with the clinical observation that surgical repair is not required for establishment of mental nerve function after sectioning. However, the quality of the return of sensation could not be determined in this experimental model.