Although the risk of injuring the lingual nerve in the mandibular molar area during dental treatment is high, it can be repaired by nerve grafting. However, from the perspective of clinical dentistry, the pathway and histomorphometric characteristics of this nerve remain to be documented in detail. The purpose of the present study was to morphologically elucidate the pathway of the lingual nerve to clarify its significance in a clinical setting. A histomorphometric analysis was also performed in consideration of nerve grafting. The vertical distance between the occlusal plane and the superior margin of the lingual nerve showed a gradual decrease from the premolar toward the distal molar area. This suggests that the risk of injuring the lingual nerve increases gradually toward the distal area. The average total fascicular area of the lingual nerve was 1.90 mm2, which was larger than that of the sural nerve. It is the first-choice donor nerve for grafting. Therefore, even though the total fascicular area of the donor nerve is a little smaller than that of the recipient nerve, nerve grafting should be successful.
We compared the temporomandibular joint structure between species of the order Carnivora and investigated its variation among family lineages. We also investigated the effect of the masticatory muscle physiological cross-sectional area (PCSA) on temporomandibular joint structure. The masticatory muscle is composed of multiple muscles, which contract in different directions and exert pressure on the temporomandibular joint. We investigated the effect of the ratio of each muscle’s PCSA—an indicator of muscle force—and muscle size relative to body size on temporomandibular joint structure. The temporalis PCSA relative to body size showed the highest correlation with temporomandibular joint structure. When the temporalis PCSA is large relative to body size, the preglenoid projects caudally, the postglenoid projects rostrally and the pre-postglenoid angle interval is small, indicating that the condyle is locked in the fossa to reinforce the temporomandibular joint. Most Carnivora use blade-like carnassial teeth when slicing food. However, dislocation occurs when the carnassial teeth are used by the temporalis muscle. Our results suggest that the temporomandibular joint is reinforced to prevent dislocation caused by the temporalis muscle. In Mustelidae, the temporomandibular joint with a rostrally projecting postglenoid is suitable for carnassial biting using the temporalis muscle. In Felidae, the force of the masseter onto the carnassial teeth is diverted to the canine by tightening the temporomandibular joint. In Canidae, the masticatory muscle arrangement is well-balanced, enabling combined action. Hence, reinforcement of the temporomandibular joint by bone structure is unnecessary.