Ultrastructural Characteristics of Axons in Traumatic Neuromas of the Human Lingual Nerve

Aims: To determine the ultrastructural characteristics of axons in traumatic neuromas of the human lingual nerve during the surgical removal of lower third molar teeth and to establish whether any characteristics were different between patients with dysesthesia and patients without dysesthesia. Methods: Transmission electron microscopy was used to determine the ultrastructural morphological characteristics of human lingual nerve neuromas (n = 34) removed at the time of microsurgical nerve repair. From a sample population of myelinated and nonmyelinated fibers within the neuromas, fiber diameter, myelin thickness, g-ratio, and the number of mitochondria per axon were quantified. Comparisons were made with normal control lingual nerve specimens (n = 8) removed at the time of organ donor retrieval. Results: Significant differences in ultrastructural morphology were found between the neuromas and control nerves. The neuromas contained a higher proportion of small (2- to 8-µm diameter) myelinated nerve fibers than controls, and the mean myelinated fiber diameter was significantly lower in neuromas than in controls. Mean myelin sheath thickness was significantly thinner in neuromas (0.6 ± 0.1 µm) than in controls. However, the g-ratio, which is a measure of the myelination status of the nerve fibers in relation to their diameter, was found to be similar in each group, suggesting a normal process of myelination in the damaged axons. Nonmyelinated axon diameter was also significantly smaller in the neuromas than in the controls, and Schwann cells were found to sheathe more nonmyelinated axons in neuromas than in controls. The ratio of nonmyelinated to myelinated axons was significantly higher in neuromas than in controls. However, no significant differences were found between patients with dysesthesia and those without dysesthesia. Conclusion: Damage to the lingual nerve results in marked changes to axon diameter, myelin sheath thickness, and Schwann cell-axon relationships. These ultrastructural changes could contribute to the altered electrophysiological properties of axons trapped within neuromas. However, no significant differences in the ultrastructural characteristics studied were found between specimens from patients with or without symptoms of dysesthesia. Keywords: dysesthesia, human lingual nerve, traumatic neuromas