The trigeminal nerve and its peripheral branches are susceptible to injury from maxillofacial trauma and iatrogenic causes in the practice of dentistry and medicine. These injuries can be significant for patients due to their effects on speech, mastication, food and liquid incompetence, and social interactions. Many of these sensory disturbances often undergo spontaneous recovery; however, some may be permanent with varying outcomes ranging from mild hypoesthesia to complete paresthesia. Some patients can also develop untoward outcomes such as neuropathic responses, leading to chronic pain syndromes in addition to their sensory disturbances.
The face and perioral region have one of the highest densities of peripheral nerve innervation in the body, which is why it is difficult for patients to tolerate neurologic disturbances in this region as compared with other areas. Pain, temperature, and proprioception are transmitted centrally via the lingual, mental, inferior alveolar, infraorbital, and supraorbital nerves. Each sensation is transmitted by different types of sensory receptors and nerve fibers with differing susceptibilities to injury and recovery. Each of these sensory modalities must be tested and monitored through serial examinations for spontaneous recovery in patients with peripheral trigeminal nerve injuries. The goal of trigeminal microsurgery is to create an environment in which those nerves not demonstrating spontaneous recovery are given the opportunity for regeneration and prevention of the development of neuropathies. This article reviews the indications and microsurgical techniques for repair of lingual and inferior alveolar nerve branch injuries.
Related surgical anatomy
Inferior Alveolar Nerve
The inferior alveolar nerve is the largest of the 3 branches of the mandibular division of the trigeminal nerve, which passes downward along with the inferior alveolar artery of the internal maxillary artery. The inferior alveolar nerve descends medial to the lateral pterygoid muscle and between the medial pterygoid muscle and the ramus of the mandible to enter the mandibular foramen of the mandible. The mandibular foramen is identified by an elevation called the lingula on the medial aspect of the ramus and the antelingula on the lateral surface of the ramus. In the posterior mandible, the inferior alveolar nerve is generally closer to the lingual cortical plate. The path of the inferior alveolar nerve is hyperbolic when viewed in both the sagittal and axial views from the mandibular foramen to the mental foramen. In the sagittal plane, the inferior alveolar nerve begins approximately 10 mm below the sigmoid notch and reaches its lowest point at the second premolar/molar region and then ascends superiorly to exit at the mental foramina. The inferior alveolar nerve becomes the mental nerve after it exits the mental foramen. As the inferior alveolar nerve approaches the mental foramen, it will often loop forward and then back before exiting the foramen. For this reason, the mental branch may be injured when surgical procedures are performed anterior to the foramen, even when the mental nerve is visualized intact outside of the mandible proper. The close approximation of the mental foramen to the dental apices accounts for the potential injuries associated with chemical, mechanical, or thermal endodontic treatment and periapical infections.
Lingual Nerve
As the lingual nerve enters the oral cavity, it travels medial to the mandibular ramus for about 3 cm. In the third molar region, the lingual nerve may be intimately associated with the third molar and/or the alveolar bone, protected by periosteum or within the soft tissues of the retromolar region. While traversing the retromandibular region, the lingual nerve can potentially cross the internal oblique ridge with only a layer of oral mucosa covering and protecting the nerve. The ultimate position of the lingual nerve at the third molar region depends on the flare of the ramus, superior position of the nerve in relation to the alveolar bone, and the third molar horizontal and vertical orientation. After looping around the submandibular duct, the lingual nerve then passes upward onto the genioglossus muscle as it enters the substance of the tongue. The lingual nerve provides several small branches that course into the mucosal lining of the medial mandible supplying the attached and unattached lingual gingival tissue up to the mandibular incisors. Injuries of the anterior tongue that are deep may also induce injury, with resulting neurosensory alterations of the tongue distal to the site of injury. In contrast to the inferior alveolar nerve, which is contained within the mandibular bone, the lingual nerve is more vulnerable to injury because of the variation of its anatomic position.
Indications for trigeminal nerve microsurgery
Indications for trigeminal nerve microsurgery include: (1) observed nerve transection, (2) no improvement in sensation for greater than 3 months, (3) development of pain due to nerve entrapment or neuroma formation, (4) presence of foreign body, (5) progressively worsening hypoesthesia or dysesthesia, and (6) hypoesthesia that is intolerable to the patient. Contraindications for trigeminal microsurgery may include: (1) development of central neuropathic pain, (2) clinical evidence of improving sensory function, (3) level of hypoesthesia that is acceptable to the patient, (4) severely medically compromised patient unable to tolerate general anesthesia for microsurgery, and (5) excessive time elapsed since the initial injury.
Nerve repairs are categorized as primary, delayed primary, and secondary depending on their timing. Primary nerve repairs are performed at the time of injury during an observed injury where the repair is immediately undertaken. If the primary surgeon is not skilled in trigeminal microsurgery, these patients may be sent to a microsurgeon who could perform the repair within a few weeks as a delayed primary repair. Unobserved injuries are the most common type of trigeminal nerve branch injury, which present to the surgeon after surgery has been completed during the postoperative period. Patients with these injuries should undergo serial neurosensory examinations to determine if trigeminal nerve microsurgery is indicated.
Indications for trigeminal nerve microsurgery
Indications for trigeminal nerve microsurgery include: (1) observed nerve transection, (2) no improvement in sensation for greater than 3 months, (3) development of pain due to nerve entrapment or neuroma formation, (4) presence of foreign body, (5) progressively worsening hypoesthesia or dysesthesia, and (6) hypoesthesia that is intolerable to the patient. Contraindications for trigeminal microsurgery may include: (1) development of central neuropathic pain, (2) clinical evidence of improving sensory function, (3) level of hypoesthesia that is acceptable to the patient, (4) severely medically compromised patient unable to tolerate general anesthesia for microsurgery, and (5) excessive time elapsed since the initial injury.
Nerve repairs are categorized as primary, delayed primary, and secondary depending on their timing. Primary nerve repairs are performed at the time of injury during an observed injury where the repair is immediately undertaken. If the primary surgeon is not skilled in trigeminal microsurgery, these patients may be sent to a microsurgeon who could perform the repair within a few weeks as a delayed primary repair. Unobserved injuries are the most common type of trigeminal nerve branch injury, which present to the surgeon after surgery has been completed during the postoperative period. Patients with these injuries should undergo serial neurosensory examinations to determine if trigeminal nerve microsurgery is indicated.
General principles of trigeminal nerve microsurgery
Microsurgery is performed in the operating room under general anesthesia with complete muscle relaxation. The operating room table can be turned 90° relative to the anesthesiologist to allow for placement of the surgical microscope if used; however, some surgeons prefer to use surgical loupes. An operating microscope with multiple heads is preferred by the author to allow the surgeon and assistant simultaneous views of the surgical field. Instrumentation minimally consists of micro forceps, scissors, needle holders, and nerve hooks ( Figs. 1 and 2 ). A beaver blade is useful for the preparation of nerve ends for neurorrhaphy. Basic surgical principles for trigeminal nerve microsurgery include exposure, hemostasis, visualization, removal of scar tissue or foreign bodies, nerve preparation, and anastomosis if indicated, without tension. Residual clotted blood in proximity to a nerve repair may increase the amount of connective tissue proliferation, leading to further scarring and compression-induced ischemia potentiating demyelination, hence the importance of maintaining a hemostatic surgical field. Hemostasis may be assisted through elevated head position, hypotensive general anesthesia, local anesthetic with vasoconstrictor, hemostatic agents, and bipolar electrocoagulation to minimize potential secondary injury.
