The inferior alveolar nerve, also called the mandibular nerve, is a sensitive branch of the third division of the trigeminal nerve. Its lesion can give rise to changes in sensory function (dysaesthesia) that may be classified as an absolute lack of sensitivity (anaesthesia), reduced sensitivity (hypoaesthesia), or a pathological increase in sensitivity (hyperaesthesia). In total, 15-20% of patients may feel pain, a much worse situation than a loss of sensitivity. Such pain may be subdivided into painful anaesthesia (spontaneous pain with a feeling of “phantom limb” in the area of the injured nerve), hyperalgesia (a painful feeling in response to even a light-touch stimulus), hyperpathia (a pain sensation that is delayed in response to a pressure stimulus), and sympathetic-mediated pain (pain maintained by sympathetic tone, in response to cold or emotional contexts and alleviated by α adrenergic inhibitors).
The inferior alveolar nerve is most often at risk of a lesion because of wisdom tooth extraction (0.4 25% of cases) and the placement of dental implants in the posterior part of the mandible (0-2% of cases).
During the manoeuvres required in complex tooth extractions, dislocation of the roots can cause compression of the nerve. If the roots are removed completely during surgery, the recovery prognosis is generally positive. Sometimes, however, occupation of the nerve canal by spread roots can cause permanent damage; in such cases, their surgical removal is essential. Compressive damage can also occur when dissectors or retractors are used to hold the nerve away from the dental alveolus. Typically, functional recovery is good, varying in duration from a few weeks to as long as 3-4 months.
Most cases of nerve severing during implant placement are due to the contact with a rotating bur. Not surprisingly, this can severely damage the nerve!
Consequently, in cases of injury during the positioning of an implant, lifting or removal of the implant itself after a few days from the detrimental event does not constitute removal of the cause of the nerve damage. The lesion, in fact, is not typically due to compression by the implant. Its removal, the correct procedure, is performed, rather, with the intention of facilitating nerve healing within the mandibular canal. This should serve as a guide for the regeneration of the nerve in its interior. Although spontaneous nerve recovery is generally good, it is never complete.
This reparative mechanism does not occur in the presence of a foreign body in the channel, such as a dental root, a bony spicule, or an implant (which is why it is valuable to remove it in case its location is in the channel). Rarely, the lesioned channel can be occupied by newly formed bone as a result of some blood in its interior at the time of surgery and this obviously hinders spontaneous regeneration of the nerve.
Symptoms typically consist initially of lip and chin anaesthesia; the area reduces in size within the first 12 months, reaching a few centimetres. Thus, the initial damage is greater and may affect speech partially and lead to loss of fluids from the mouth, because the lip is not sensitive. After the self-healing process, discomfort is reduced but may remain primarily psychological. In fact, the patient is typically disturbed by the inconvenience of losing some of the fine sensitivity of the lip.
If the discomfort with the residual damage in sensitivity is not acceptable, it may be possible to improve the situation with microsurgery, by connecting the injured nerve to a tiny branch of the contralateral mental nerve.
The permanent loss of vitality of the dental elements from the side of the lesion is almost invariable. There are other potentially hazardous actions to the nerve, such as the removal of cystic lesions, the treatment of apical granulomas, and complex teeth extractions. In sum, any surgical procedure near the course of the inferior alveolar nerve can lead to its injury.
Extravasation of endodontic material into the mandibular canal can cause damage to the inferior alveolar nerve by ‘burning’ it. Common symptoms other than lip and chin anaesthesia are dysaesthesia and pain. The injury mechanism is irritation or burning due to chemical contact between the endodontic material and the nerve. That differs significantly from sectioning, tearing, or compressing the nerve because of a surgical lesion. Among those cases, as described previously, anaesthesia and hypoaesthesia are the symptoms in 80-85% of cases, with pain representing the rest.
In cases of a disturbance caused by an endodontic material in contact with the alveolar nerve, the prevailing symptom is pain. Endodontic material beyond the apex generally remains in the bone periapically, with asymptomatic bone irritation. However, if the material is pushed into the mandibular canal, because of the close relationship between the nerve and the tooth apex, this may lead to neural damage.
Injections to provide nerve block anaesthesia in the lower alveolar nerve can lead to injury of both the inferior alveolar nerve and the lingual nerve. Generally, such lesions are not painful and spontaneous recovery occurs within 1 year. If recovery does not occur, a microsurgical intervention may be indicated.
In cases where pain is the main symptom, the deterioration in the quality of life can be dramatic. The pain level is often high and constant, and the patient is typically motivated to seek surgical treatment to try to address the problem. It important, then, to take immediate action, because the results are usually very good once surgery is undertaken. Surgery may be effective even long-time after the nerve trauma but percentages of success decrease.
Microsurgical reconstruction of the inferior alveolar requires general anaesthesia; a nerve graft is also typically required. When surgery is not the treatment of choice, drug therapy plus psychological support should be considered.

The Seddon classification is still used widely today for its simplicity and practicality. It provides for three degrees of injury: neurapraxia, axonotmesis, and neurotmesis. Neurapraxia may occur with simple compression or traction of the nerve and corresponds microscopically to its oedema. Generally, complete spontaneous healing takes place within 2 months and no treatment is necessary.
Axonotmesis involves lesions of nerve fibres within the nerve despite its continuity. The nerve undergoes retrograde degeneration of the fibres involved and then their regeneration. Even in such cases, the spontaneous recovery of function, sometimes incomplete, is predictable but with a higher degree of variation, from 3 to 6 months.
Neurotmesis is characterised by a complete section of the nerve. The consequences are the complete loss of sensitivity, accompanied or not by spontaneous painful sensations. Limited partial recovery may occur because of partial innervation in the anaesthetic area by adjacent nerves.