Distribution patterns of infraorbital nerve branches and risk for injury.

BACKGROUND: During oral and head and neck surgery, oral vestibular incisions may require a transverse incision on the upper lip mucosa, resulting in possible sensory disturbances in the area innervated by infraorbital nerve (ION) branches. Although sensory disturbances are attributed to nerve injuries, anatomy textbooks have not showed the precise distribution patterns of the ION branches in the upper lip. Furthermore, no detailed study has been available on this issue. This study aimed to reveal the precise distribution patterns of ION branches in the upper lip by dissecting the detached upper lip and cheek area using a stereomicroscope. METHODS: During a gross anatomy course at Niigata University (2021-2022), nine human cadavers were examined with special focus on the relationship between ION branches in the upper lip and the layered structure of facial muscles. RESULTS: The ION branched to the inferior palpebral (IP), external and internal nasal, and superior labial (lateral and medial) nerves. The ION branches in the upper lip did not run in a horizontal pattern from outside to inside but showed a predominantly vertical pattern. Considering their course, incising the upper lip mucosa transversely may cause paresthesia of the ION branches. The internal nasal (IN) and medial superior labial (SLm) branches tended to penetrate the orbicularis oris and descend between this muscle and labial glands, whereas the lateral superior labial (SLl) branches tended to innervate the skin. CONCLUSIONS: These findings suggest that a lateral mucosal incision is recommended for oral vestibular incisions of the upper lip and that deeper incisions to the labial glands should be avoided when incising the medial side to preserve the ION during surgery from an anatomical point of view.

Macroscopic Anatomy of the Layered Structures of Facial Muscles and Fasciae in the Temporal-Malar-Mandible-Neck Region.

The layered structures of facial muscles and their topographical relationship with facial fasciae are still not fully understood. This study aimed to clarify the layered structures of facial muscles and fasciae in the temporal-malar-mandible-neck region. Thirty-four human cadavers were examined during gross anatomy courses at Niigata University (2017-2020). The face was composed of 3-layered (deep, middle, and superficial) fasciae and 4-layered facial muscles (first superficial, second superficial, third, and fourth muscle layers) according to the attachment of muscles and their topographical relationship with the fasciae. The deep fascia covered the temporal and masseter muscles. The parotid gland and facial nerves were enveloped in the middle fascia. The superficial fascia was continuous with the second superficial muscle layer. The connection between fourth and superficial muscles was at the malar and buccal areas, where the platysma blended with the masseter and the plural muscles blended with the buccinator. Our findings suggest that cooperation between the 4-layered structure of the facial muscles surrounding the apertures of the eyes and mouth and the superficial fascia enables humans to produce complex facial expressions. Furthermore, the spread of inflammation in the face may be owing to the layered facial muscles and fasciae, as these layered structures separate tissues into multiple compartments.