The inferior alveolar nerve is midway between the buccal and lingual cortical plates in the first molar region. In about 1% of patients, however, the mandibular canal bifurcates in the inferior superior or medial lateral planes. Thus, a bifurcated mandibular canal will manifest more than one mental foramen. This may or may not be seen on panoramic or periapical films. Therefore it is suggested that clinicians should consider obtaining a preoperative CBCT to avoid nerve injuries prior to implant placement above the inferior alveolar canal. A mean incidence of neurosensory disturbance incidence after implant surgery was 6.1% (Goodacre et al., 1999) to 7% (Goodacre et al., 2003), with a range between 0.6% and 39%. Nerve damage can have results ranging from mild paresthesia to complete anesthesia or even disabling dysesthesia.
1. Neurapraxia - There is no loss of continuity of the nerve; it has been stretched or has undergone blunt trauma. The parasthesia will subside, and feeling will return in days to weeks.
2. Axonotmesis - Nerve is damaged but not severed; feeling returns within 2 to 6 months.
3. Neurotmesis - Severed nerve; poor prognosis for resolution of parasthesia.
Possible causes of nerve injury include poor flap design, traumatic flap reflection, accidental intraneural injection, traction on the mental nerve in an elevated flap, penetration of the osteotomy preparation and compression of the implant body into the canal. Nerve injuries may be caused indirectly by postsurgical intra-alveolar edema or hematomas that produce a temporary pressure increase, especially inside the mandibular canal. Direct traumas are the most frequent causes of nerve injury, and they may occur through five mechanisms: compression, stretch, cut, overheating, and accidental puncture. Finally, prolonged pressure from neuritis may lead to the permanent degeneration of the affected nerve.
An inferior alveolar nerve injury after implant placement.
The mental nerve is at particular risk of iatrogenic injury because it arises from asymmetric foramina and forms a concave loop anteriorly. In edentulous patients, it may be very close to the bone surface or the top of the crest.
The nerve injury may cause one of the following conditions:
1. parasthesia (numb feeling),
2. hypoesthesia (reduced feeling),
3. hyperesthesia (increased sensitivity),
4. dysthesia (painful sensation),
or anesthesia (complete loss of feeling) of the teeth, the lower lip, or the surrounding skin and mucosa
Overpenetration occurs when the cortical portion of the alveolar crest places resistance on the drill. However, as it enters the marrow spaces, a drill may drop into the neurovascular bundle unless the surgeon has excellent control.
For implants placed in the atrophic posterior mandible, the routine use of intraoperative periapical radiographs during the drilling sequence can help avoid the risk of injury to the inferior alveolar nerve. Periapical radiographs used intraoperatively to obtain working length measurements are similar in concept to techniques used in root canal therapy. This method can reliably determine safe distances between the implant and the inferior alveolar canal, thus avoiding the risk of injury to the nerve altogether. A safety margin of 2 mm between the entire implant body and any nerve canal should be maintained. Additionally, surgical placements of implants should be at least 3 mm in front of the mental foramen . When placing implants in proximity to the mental foramen, the clinician must take into consideration the anterior loop of the nerve and the available bone above the mental foramen, because the inferior alveolar nerve often rises as it approaches the mental foramen. Finally, although the depths of the implant bur are variable, the drill bur may be longer than the implant according to the manufacturers.
If an implant is in danger of violating the canal, its depth should be decreased in the bone (i., by unscrewing it a few turns) and left short of the canal or removed. Because the altered sensation may be due to an inflammatory reaction, a course of steroid treatment or a high dose of nonsteroidal anti-inflammatory medication (e.g., ibuprofen [800 milligrams] three times per day) should be prescribed for three weeks. Adjunct drugs such as clonazepam, carbamazepine, or vitamin B-complex might alleviate neuritis via their known neuronal anti-inflammatory actions.
If improvement is noted at three weeks on the basis of a repeated neurosensory examination, the clinician can prescribe an additional three weeks of anti-inflammatory drug treatment. If the improvement is seen, however, the patient should be referred to a microneurosurgeon.
Recommendations to avoid nerve injuries during implant placement
1. Measure the radiograph with care. Apply the correct magnification factor.
2. Consider the bony crestal anatomy: If the ridge is thin buccolingually, is this useless bone or should an augmentation procedure be done? Is the buccolingual position of the crestal peak of bone influencing the measurement of available bone?
3. Consider the buccolingual position of the nerve canal (CBCT)
4. Allow a 1 to 2 mm safety zone.
5. Use a drill guard.
6. Take care with countersinking not to lose support of the crestal cortical bone.
7. Take a radiograph after initial osteotomy.