Ricardo A. Boyce, DDS, FICD; Gary Klemons, DDS
- Nerve injuries in the maxillofacial region may happen as a result of trauma, neoplasms, infections, or secondary to a surgical procedure.
- Because of the limitations of such investigations there is a large range in the rates of injury reported from study to study.1
- To complicate the issue further, the data are usually stratified, separating transient impairment from permanent loss of function, and only occasionally is the presence or absence of painful neuralgias or dysesthesia included.
- Unlike nerve injuries as a result of most oral surgical procedures, which tend to be mostly transient, implant-related nerve injuries are typically permanent.
- Implant-related nerve injuries are typically permanent compared with other dentoalveolar procedural causes, which are more likely to be transient.
- Many implant-related nerve injuries are caused by inadequate planning and can be avoided with appropriate imaging and pre-procedural assessment.
- Techniques to avoid nerve injuries include using local anesthetic infiltration rather than regional blocks, implementing short implants to maintain appropriate safety zones from neurovascular structures, applying bone augmentation techniques to increase available bone, cantilevering hybrid restorations to avoid placing implants in atrophic posterior mandibles, and conducting a nerve lateralization procedure as a final option.
- A suspected nerve injury or a patient presenting with numbness following an implant procedure should be managed in a systematic manner to allow timely referral when appropriate.
Seddon’s nerve injury classification:
- Broadly classified by severity using descriptors that include neuropraxia, axonotmesis, and neurotmesis.13
Sunderland’s nerve injury classification
- Sunderland focuses on the fascicular construct of nerves and the remaining integrity following the injury.
- 5 categories with increasing severity: 14
- First-degree injury Neuropraxia: is an insult resulting in conduction blockade following nerve traction or mild compression. Axonal continuity is maintained. Sensory disturbances are expected to be transient, lasting a few months or more.14
- Second-degree injury Axonotmesis: is more severe, with wallerian degeneration occurring distal to the site of a crush injury. Axon regeneration can occur because the endoneurium remains undisrupted. Complete recovery occurs normally within several months, but may take 1 year or longer.14
- Third-degree injury:a more severe crush or traction injury that disrupts the fascicular endoneurium. Complete axonal regeneration is hampered, and a form of permanent sensory disturbance is expected.14
- Fourth-degree injury: additionally disrupts the perineurium with the likelihood of fibrosis, scarring, or neuroma formation further impeding regeneration efforts.14
- Fifth-degree injuries Neurotmesis: result from complete transection of the peripheral nerve structure, including epineurium, perineurium, and endoneurium. Complete anesthesia occurs in the distribution innervated by the specified nerve. Neuropathic pain syndromes can occur, with spontaneous nerve regeneration being unlikely.13,14
Mechanism of Injury
- The chances are higher that an implant-related injury will be permanent because the mechanism of injury usually involves the nerve being cut or severely damaged by the drill during preparation; partial or complete transections (axonotmesis or neurotmesis).
- Severe compression of the neurovascular bundle caused by implants compressing the bony housing of the nerve canal may cause an injury consistent with axonostenosis.
- Plain radiographs or computed tomography (CT) scans are useful in situations in which postoperative neurosensory disturbances are present following implant placement. If the imaging shows that the canal is intact or the superior border of the canal has been minimally violated, backing out or removing the implant is advised to decompress the neurovascular bundle.
Reasons for Nerve Injury
- Careful preoperative planning and the availability of appropriate radiographs are important not only for the proper placement of implants for restorative reasons but also to avoid injuries to the important structures in the surgical region. For many cases, plain radiographs, clinical measurements, and direct surgical visualization are sufficient for judicious implant placement.16
- The bone quality at the site of the implant is also a factor contributing to nerve injuries. If a surgeon puts pressure on the implant drill while preparing the osteotomy and encounters an unexpected change in bone density, the drill will be inserted deeper than intended.8
- Another cause of nerve injuries is the overzealous use of immediate implants in areas that are too close to the neurovascular structures. Many immediate implants require drill preparation and implant placement apical to the extraction socket in order to gain primary stability.
Strategies to Avoid Injuries
Local Anesthesia Administration
- When placing mandibular implants, it is not necessary to administer an inferior alveolar block injection for adequate local anesthesia. The primary advantage of not giving a block injection is that the patient has some awareness and perception around the IAN. Not giving a regional block injection eliminates the risk of a needle-stick injury to the IAN trunk.
- Local infiltration provides sufficient anesthesia to place an implant with either flap or flapless techniques. Studies have shown there to be no statistically significant difference in patients’ comfort having an implant placed under local infiltration versus regional block injection. 25, 26
- To use shorter implants in areas where the available vertical height is minimal superior to the IAN canal.
- The use of short implants is a valid approach to place implants in the posterior mandible providing that the bone quality is suitable for primary stability. If multiple short implants are placed, splinting of the restorations should be considered.27
Bone Augmentation Techniques
- In atrophic situations in which implant placement would encroach on the nerve canal, bone augmentation procedures may provide enough space for safe fixture placement.
- Procedures include distraction osteogenesis techniques, onlay bone grafting, interpositional sandwich grafting, and grafting using a barrier such as a titanium mesh.
- One important caveat about bone augmentation procedures is that they may only be used in situations in which there is sufficient interarch space for both the bone graft and a crown on the implant.
Nerve Lateralization Procedures
- Nerve lateralization is a useful procedure to facilitate the placement of implants in atrophic posterior mandibles, especially in situations in which bone augmentation would encroach on the interarch space.
- This procedure makes available dense cortical bone inferior to the IAN canal for stable implant placement.
Avoiding Implants in the Atrophic Mandibular Posterior by Cantilevering from the Anterior
- Many patients have early loss of posterior mandibular teeth and wear a bilateral distal extension saddle partial denture. This situation is classified as a Kennedy class I. Over time, the posterior ridges become atrophic and denture wearing may become more difficult and uncomfortable. At this point the patient may be referred for implant consultation to replace the posterior teeth to remove the need for a partial denture.
- Depending on the overall health of the remaining anterior teeth, and the long-term prognosis of these teeth, another approach may be used: if the remaining anterior teeth have a poor long-term prognosis because of caries and or periodontal disease, the patient may benefit from removal of these teeth and placement of multiple implants anterior to the mental foramina with the intention a fabricating a fixed-hybrid appliance that cantilevers teeth to the posterior regions.
Management of Implant-Related Injuries
- Concern for nerve damage intraoperatively warrants prompt evaluation of the patient for numbness.
- If an IAN block was administered, a local anesthetic reversal agent (eg, phentolamine [OraVerse]), may be administered to expedite examination.
- A positive finding of numbness requires radiographic imaging to evaluate implant proximity to the inferior alveolar canal or mental foramen.
- If the implant is noted to be well away from vital structures and the patient reports numbness, a regimen of anti-inflammatory medication (steroids or nonsteroidal medications) 17 is begun with continued observation for improvement.
- Radiographic evidence showing interruption of the canal demarcation or direct impingement on the nerve from either the implant or drill path necessitates further evaluation via CT scan.
- CT review showing an implant impinging on, but not through, the canal should either be removed completely or backed out until the impingement has been alleviated.
- Patients showing progressive improvement can assume a normal postoperative course; however, no improvement within 3 months should be referred for nerve repair evaluation.17
- The ominous finding of drill path or implant placement through the canal warrants prompt referral to a specialist capable of nerve repair.