Introduction

Tooth loss is a common occurrence despite the advances in technology and modern techniques used in dentistry. The modern solution to tooth loss is dental implants. Implants are widely used in dentistry to support missing teeth, but not without complication. The most serious complication to the patient following dental implant surgery is injury to the inferior alveolar nerve (IAN). Implants related IAN injuries range from 0% to 40% (Shavit & Juodzbalys, 2014). These injuries occur either during osteotomy preparation or implant placement. These injuries are directly related to the depth of preparation or the width of the implant. Local anaesthetic maybe a contributing factor.

To prevent neural complications the clinician must have a sound knowledge of the anatomy relevant to implant surgery. If a complication does occur, the clinician must also be able to classify and manage the complication. Appropriate care must be taken when damage to the IAN or lingual nerve occurs. The patient should be referred to a micro neurosurgeon for further management.

Anatomy

The Trigeminal Nerve

The trigeminal nerve (fifth cranial nerve) is the largest cranial nerve responsible for sensation in the face and certain motor functions such as biting and chewing. The name Trigeminal implies 3 births of origin. There are three branches on either side of the pons.

These three branches include: the ophthalmic nerve (V1), the maxillary nerve (V2) and the mandibular nerve (V3). The ophthalmic and maxillary nerves are purely sensory in their function whilst the mandibular has both sensory and motor functions (Agur & Dalley, 2004; Bagheri & Meyer, 2011).

The trigeminal nerve has different sensory, motor and parasympathetic supply:

Sensory supply:

There are three of the terminal branches of the trigeminal nerve that innervate the mucous membranes, maxillary sinuses, teeth and skin.

Motor Supply:

The mandibular nerve is the only branch of the trigeminal nerve that has a motor supply. It supplies the muscles of mastication: medial pterygoid, lateral pterygoid, masseter and temporalis

Parasympathetic supply: The post-ganglionic neurones of the parasympathetic ganglia travel with branches of the trigeminal nerve (Agur & Dalley, 2004).

The Maxillary Nerve(V2)

The maxillary nerve has 3 branches: zygomatic, pterygopalatine and posterior superior alveolar. They carry sensory information from the eyelids, cheeks, upper lip, teeth and gingiva, the nasal mucosa together with all the upper facial sinuses (Agur & Dalley, 2004).

The Zygomatic Branch

Divides into zygomatical temporal and zygomatical facial nerves. The lacrimal nerve, which is a branch from the zygomatical temporal nerve, running from the lateral wall of the orbit, has post ganglionic fibres, derived from the sphenopalatine ganglion that causes lacrimation. The skin on the cheek is supplied by the zygomatical facial nerve (Agur & Dalley, 2004).

The Sphenopalatine Ganglion

The pterygopalatine ganglion, the largest of the parasympathetic ganglia associated with the branches of the maxillary nerve, is deeply placed in the pterygopalatine fossa, close to the sphenopalatine foramen.

The pterygopalatine ganglion supplies the lacrimal gland, paranasal sinuses, glands of the mucosa of the nasal cavity and pharynx, the gingiva, and the mucous membrane and glands of the hard palate. It communicates anteriorly with the nasopalatine nerve (Bagheri & Meyer, 2011; Agur & Dalley, 2004).

The Posterior Superior Alveolar Nerve

Arises from the bulk of the nerve before it exits the to become the infraorbital nerve. Whilst it descends the tuberosity of the maxilla, several branches which innervate the gingiva and mucous membranes of the cheek are given off.

Whilst it traverses the infra temporal fossa of the maxilla, it communicates with the middle superior alveolar nerve that supplies the lining of the maxillary sinus and posterior maxillary teeth (Agur & Dalley, 2004).

The Mandibular nerve(V3)

The mandibular nerve is the largest branch of the trigeminal nerve. It has mixed sensory and motor fibres and passes through foramen ovale. It enters the mandible through the mandibular foramen. It travels forward within the mandibular canal and traverses the mandible from lingual to buccal. The IAN divides into the mental nerve and the incisive nerve. The incisive nerve continues in the mandibular canal and the mental nerve exits through foramen mentale (Bravitz, et al., 1993). The mental foramen has three emerging branches. One innervates the skin in the mental area, the other two innervate the skin of the lower lip, mucous membrane, and gingiva up to the second premolar. The incisive branch of the IAN supplies the canine and incisive teeth (Gintaras, et al., 2011).

The mental foramen is one of two foramina (openings) located on the anterior surface of the mandible. It transmits the terminal branches of the inferior alveolar nerve and vessels. The mental foramen descends slightly in toothless individuals (Soikkonen, et al., 1995).The shape of the foramen is round to oval and is located more coronally to the mandibular canal. Its size ranges from 2.5 -5.5mm in diameter and width. It lies either at the apex of the second mandibular premolar or between the apices of the premolars. There may be one foramen, however additional foramen and accessory mental foramen may occur and have been noted in studies conducted (Greenstein, 2006).

Anterior Loop of the Mental Nerve

Can be defined as an extension of the inferior alveolar nerve, anterior to the mental foramen, before exiting the canal. It is also referred to as the anterior loop of the neurovascular bundle progressing firstly forward and then looping back anteriorly before emerging from the mental foramen. To establish the length and prevalence of the anterior loop various diagnostic methods including:

Panoramic x-ray, Cone beam computed tomography (CBCT) scans, and cadaver jaws have been utilised (Guo, et al., 2009).

Atrophied Posterior Mandibular Ridge

Many patients may present with teeth in the anterior segment but with a loss of teeth in the posterior region. Usually a partial denture will be constructed for the replacement of these teeth, but due to loss of denture stability some patients will consider implant surgery. In these cases, the ridge may be atrophied. This may complicate implant placement because the inferior alveolar nerve bundle is located much closer to the alveolar crest than normal. In these instances, several procedures can be performed to create space for implant placement (Steinberg & Kelly, 2015).

Nasopalatine nerve of the anterior maxilla

Nasopalatine nerve related injury in the anterior maxilla is not common but can occur. Injury usually occurs as the nasopalatine nerve exits the incisive canal posteriorly to the central incisors. Utilizing Computed tomography (CT) scans of the segment can help identify the anatomy, dimension and morphology of the canal (Mraiwa, et al., 2004). The presence of a wide canal can result in subsequent volume bone loss bucco-palataly inhibiting the placement of an implant in the central incisor region. Studies have shown that implant placement can be performed in the atrophied maxilla, but neurosensory implication should be considered before treatment is provided.

Neural Complications in implant dentistry and nerve injury classification

Surgical complication during implant placement are not uncommon. The Seddon and Sunderland classification of nerve injuries are commonly used for classification.

Misch et al. (2008) diagrammatically described the types of implant related complications in dentistry. The types of complications include: treatment plan related, procedure related, anatomy related and other causes. Nerve injury is listed as an anatomy related complication in implant dentistry. The Seddon and Sunderland classifications of nerve injuries are widely used. These classifications are based on the level of anatomical injury and severity of the nerve disruption (Fig 1.) Steinberg et al. (2015) explained that implant related nerve injuries are mostly permanent in nature when compared to injuries from oral surgical procedures. Oral surgery related nerve damage is more transient. Steinberg et al. (2015) further explained that 75% of nerve injuries, resulting from implant placement, cause permanent nerve damage.

The Seddon classification describes nerve injury as one of the following terms: neuropraxia, axonotmesis, and neurotmesis.

Classification of nerve injuries

Neuropraxia There is no loss of continuity of the nerve; it has been stretched or has undergone blunt trauma. The paraesthesia will subside, and feeling will return in days to weeks.

Axonotmesis Nerve is damaged but not severed; feeling returns within 2 to 6 months.

Neurotmesis Severed nerve; poor prognosis for resolution of paraesthesia

Clinicians should be aware of these nerve injuries and standardized neurosensory examinations should be performed to determine the degree of change in sensation, know possible outcomes and decide whether to refer to a micro neurosurgeon. Before a dental implant is placed, an evaluation of the patient’s neurosensory function should be done especially in patients with a history of alteration of sensory function of the IAN associated with previous implant placement or with impacted wisdom teeth extraction. There are many neurosensory tests available to assess the neural damage after the implant has been placed and these tests vary from easy procedures, requiring simple instrumentation to more advanced procedures, requiring high-technology equipment (Alhassani & Alghamdi, 2010).

Simple tests are classified into mechanoreceptive and nociceptive tests.

Clinical neurosensory tests

• Mechanoreceptive

1. Static light touch detection- Patient is asked to tell when he/she feels light touch on the face and to point to the exact location.

2. Brush directional discrimination- Patient is asked to tell when he/she feels the brush and to determine the direction of movement.

3. Two-point discrimination- Patient is asked to determine single and 2 points of touch. The examiner uses any 2 instruments by which the patient can change the distance between them.

• Nociceptive

1. Pin pressure nociception -Patient is asked to determine the feeling of a pin prick.

2. Thermal discrimination- Patient is asked if he/she feels cold or heat (Poort, et al., 2009)

When these tests are performed, the patient’s eyes should be closed, he or she should be in a comfortable position and away from any distractions. The contralateral side should be used as a control and the results should be recorded.

The Sunderland classification of nerve injury is classified as first degree to fifth degree. This classification focus on the fascicular construct of the nerve and the integrity of the nerve after injury has occurred.

The axon continuity is maintained in a first-degree injury. The injury is transient, and the effect is not of a permanent nature. The altered sensation, in the region innervated by the affected nerve, should only last for a few months. The sensation should return to normal.

The term Neuropraxia also defines a first-degree injury.

A second-degree nerve injury is also defined as Axonotmesis according to the Seddon classification. The injury is more severe than a first-degree injury. Regeneration is still possible, but the nerve may take several months or even longer to heal. Wallerian degeneration occurs distal to the area of a crush injury.

A third-degree injury involves disruption of the fascicular endoneurium. The degree of injury most likely results in permanent sensory disturbance as the regeneration process is affected.

A fourth-degree injury can be defined as a disruption of the perineurium. Regeneration is affected by factors including scarring, fibrosis and/or neuroma formation at the injury site.

A fifth-degree nerve injury results in permanent nerve damage.

There is a complete transection of the nerve in fifth degree damage, also called Neurotmesis. The patient will experience complete anaesthesia at the anatomical structures innervated by the nerve. Nerve regeneration in a fifth-degree injury is highly unlikely.

Aetiology of neural complications in implant dentistry

Steinberg et al. (2015) correlates with Misch et al. (2008) with respect to the aetiology of nerve complications in implant dentistry. Both authors state that inadequate treatment planning may lead to nerve complications. Steinberg et al. (2015) elaborated that practitioners may not always be aware of the exact location of important anatomical structures.

Greenstein et al. (2015) explained that injury to a branch of the trigeminal nerve can occur from trauma, inflammation and/or infection. He elaborated that injury can also occur from bone harvesting during a bone graft.

The bone quality of the mandible and/or maxilla may be a factor contributing to nerve injury if proper care is not taken. During the osteotomy procedure, a sudden change in bone density may cause deeper drilling into bone than initially intended. This is due to the pressure applied by the surgeon onto the bone. The drill will follow the path of the least resistance and may damage an adjacent nerve (Steinberg & Kelly, 2015).

As mentioned previously, the immediate placement of implants may be an aetiological factor for neural complications. Surgeons may often aim to place an immediate implant slightly apically to the extraction socket. The reason for this may be to gain primary stability. If a nerve or neurovascular bundle is near the apex of a tooth, the nerve may be damaged with a more apical osteotomy. Steinberg et al. (2015) explained that the mandibular premolar region is of importance here as the mental foramen can easily be injured because of anatomical variation. Compression of the implant body into a nerve canal can cause significant nerve damage (Misch & Wang, 2008).

Alhassani et al. (2010) explained that injury to the inferior alveolar nerve is a serious and common occurrence in implant dentistry. He elaborated that the lingual nerve can also be damaged during implant placement in the mandible. The causes of nerve injury from a clinical perspective include local anaesthetic injections, the osteotomy procedure and placement of the implant itself (Alhassani & Alghamdi, 2010). These authors agree on their views regarding the aetiology of mandibular nerve injuries. Misch et al. (2008) however mentioned that implant dentistry can cause neural complications of the incisive nerve and mental nerve as well. The cause of injury to the mental nerve can be due to a pulling/stretch action on the nerve when a flap is elevated. Flaps carry the risk of nerve damage when the flap is poorly designed or reflected traumatically (Misch & Wang, 2008).

Injection of Local Anaesthetic for Implant Procedure

There are 3 possible causes

1. Direct intraneural injection with mechanical injury to the nerve (The axons maybe severed, there may be partial or total scar tissue or neuroma formation, or Wallerian degeneration)

2. Interruption of Mesoneurium with peri and intraneural haemorrhage and secondary scar formation

3. Chemical toxicity of the anaesthetic solution from a contaminant, usually a sterilizing solution in a leaky carpule.

It has been recommended that all local anaesthetics be aspirated during the injection technique (Bagheri & Meyer, 2011).

Pathogenesis of neural complications in implant dentistry

The physiological disturbance, after a nerve injury, happens at the axons of the nerve. Anatomically, the neurons are divided into proximal and distal parts with regards to the injury site. Physiological reactions are different at the proximal and distal parts (Madura, 2012).

Proximal to the injury site: The proximal stumps retract and at the same instant the axons seal their axolemma. Regenerative sprouts are produced after retraction resulting in swelling at the cut tip of the axon. Here the reorganisation of microtubular cytoskeleton occurs. Regenerative growth cones or incompetent endbulbs form at the ends of the injured axons. Successful formation of the growth cone will be the starting point for regeneration at the proximal nerve stump (Madura, 2012).

Distal to the injury site: Wallerian degeneration, which starts almost immediately after the axons are cut, lasts between 3 to 6 weeks. 24 hours after the injury, the axons begin to disintegrate. Microtubular structures of the axons are cut, resulting in a pathological axonal transport. This process is followed by the disintegration of neurofilaments. Schwann cells play an important role at the injury site as they secrete immunologically active substances. The distal stumps can retain their excitability for up to 10 days. This must be kept in mind before making final diagnosis with regards to the degree of the nerve damage (Madura, 2012).

A fifth-degree nerve injury resulting from implant dentistry will cause associated muscle de-innervation. The biochemical and electrophysiological properties of the muscle changes. Subsequently atrophy of muscle fibres occur. This may result in physical changes like a drooping lip.

Implant related nerve injuries are usually severe, causing permanent damage, due to the complete laceration of the nerve with the use of a drill. Steinberg et al. (2015) elaborated that the mechanism of injury will determine the outcome. An implant, placed near the nerve canal, may cause compression of the bone onto the nerve. Subsequently this may result in ischaemia, resulting in pathological nerve conduction. Demyelization may occur which may cause neuropathic symptoms.

Incidence of implant related nerve injuries

The occurrence of nerve damage following implant placement is ranges from 0% to 36% (Greenstein, et al., 2015). These results were obtained from studies evaluating altered lip sensations following implant placement. In the past vestibular incisions were used to allow for implant placement, however currently we use midcrestal incisions as they reduce the prevalence of neural complications. The incidence of transient nerve damage after implant placement was established at 5 out of every 169 patients. 1.7% of patients will experience permanent nerve damage following implant procedures. Compared to implant surgery, the incidence of nerve damage was 0.26% less after oral surgery. An altered sensation of the lower lip after an implant procedure can last up to 2 weeks and sensation should return to normal. This must not be confused with nerve injury, but rather caused by oedema (Greenstein, et al., 2015).

Strategies to avoidance of Nerve Injury

Several factors need to be considered to avoid neural complications. The anatomy and anatomical variations of implant sites need to be understood by the clinician. In addition, the treatment site, the surgical technique and implant system utilised must be carefully understood and considered.

Knowledge of Anatomy

The inferior alveolar nerve is most frequently damaged according to the literature because of its anatomical relation. Very rarely can damage occur to the nasopalatine nerve. According to Chang et al. (2010) the lingual nerve is unlikely to be damaged in modern implant placement, because of its location in the lingual soft tissue. Implant placement in the molar area does not require a vertical releasing incision on the lingual aspect (Greenstein, et al., 2015).

The mental nerve is more complex as its anatomy is not simple. When there is advanced alveolar resorption the mental foramen maybe at the alveolar crest. This phenomenon allows the clinician to place a surgical incision on the lingual portion of the ridge (Kraut & Chahal, 2002).

In some individuals the mental nerve may have an anterior loop. Thus, surgery in the anterior mandible may injure the loop. According to Greenstein et al. (2015), “…if an implant is to be placed anterior to the mental foramen and its length is longer than the distance from the alveolar ridge to the infundibulum of the mental foramen, one has to verify radiographically that here is no anterior loop in the mental nerve. …” To avoid mental nerve injury the clinician must be aware of its variations.

The value of treatment planning in avoiding nerve injury

The use of periapical and panoramic radiographs to locate the position of the IAN and mental foramen are beneficial for implant placement. But these are two dimensional radiographs. A clinician should carefully consider relying solely on this type of radiograph for implant placement. Studies were conducted to evaluate the diagnostic accuracy of panoramic radiography, computed tomography, and cone beam computed tomography compared to actual anatomical situation (Pertl, et al., 2013). The results indicated that many of the measurements were 2.5mm too long when comparing to anatomical situation to the panoramic x-ray, thereby increasing likelihood of IAN damage. The use of steel ball as references placed above the planned implant position increased accuracy of measurements.

The introduction of cone-beam computed tomography scanning to implant dentistry as a three-dimensional (3D) imaging tool has led to a break-through, because these scanning devices result in lower radiation dosages than conventional CT scanners. CT scans or cone beam xrays can provide a more accurate determination of nerve position and variations and should be used more often in treatment planning to avoid possible nerve damage. It is widely known that CT images can be reconstructed into a three-dimensional model that can be used as an accurate surgical guide. However, CT is expensive compared to CBCT. Which is readily available and can be used to assess bone quantity but not bone quality (De Vos, et al., 2009).

CBCT and CT are far superior to panoramic radiograph. Its provides more information and represents the actual anatomical situation accurately. This assists the clinician in selecting the appropriate implant size for the implant site to avoid nerve or other complications.

In combination with implant planning software, the use of CBCT images has made it possible to virtually plan the ideal implant position regarding surrounding vital anatomical structures and future prosthetic needs. The resulting planning information is then used to fabricate so-called drill guides or surgical guides, and this process ultimately results in the transfer of the planned implant position from the computer to the patient, with the drill guide or surgical guide directing the implant osteotomy and implant insertion. Importantly, this entire process can be performed in such a way that the predicted ideal implant position can be achieved without damaging the surrounding anatomical structures. (Tahmaseb, et al., 2014)

Local Anaesthesia Administration

The placing of mandibular implants does not necessarily require an IAN block. He advantage is that the patient has awareness and perception around the IAN. If the osteotomy is close to the neurovascular bundle, the patent will present feedback to the clinician. Another advantage of not administering an IAN block is to prevent any needle-stick injury to the nerve trunk. If the patient has numbness following implant placement with an IAN block, there is confusion about the nerve injury. Did the needle damage the nerve trunk or is there evidence of nerve damage? (Steinberg & Kelly, 2015).

Short Dental Implants

The use of short implants in areas where the vertical height is relatively minimal superior to the IAN, is another technique to avoid nerve injury. However, studies have shown mixed outcomes. There are reports of increased failure rates of short implants compared to standard dental implants. The use of short implants is acceptable, if the bone quality is suitable for primary stability. If short implants are used, then restoration splinting is advised (Steinberg & Kelly, 2015).

Bone Augmentation Techniques

In atrophic mandibles, bone augmentation procedures provide for enough space for implant placement. These procedures include distraction osteogenesis, onlay bone grafting, interpositional sandwich grafting, grafting using a barrier such as a titanium mesh. Several bone sources may also be used such as autologous bone, allogenic bone, xenografts, alloplasts and bone morphogenic protein. These bone augmentation procedures may only be used in instances where there is sufficient inter arch space. If there is insufficient space, then a nerve lateralization procedure maybe used (Steinberg & Kelly, 2015).

Nerve Lateralization Procedures

This process is useful in instances whereby there is limited interarch space, in the posterior mandible. This procedure requires skill and experience with manipulating nerves.

Once the nerves are repositioned, implants are placed in the centre of the alveolus (Steinberg & Kelly, 2015).

Avoiding Implants in the Atrophic Mandibular Posterior by Cantilevering from the Anterior

In the Kennedy class 1 situation, patients have lost the posterior mandibular teeth and wear bilateral distal extension partial dentures. As time progresses denture wearing becomes more difficult and uncomfortable. At this time the patient is then referred for dental implants.

If the remaining anterior teeth have a poor long-term prognosis because of caries and or periodontal disease, then these teeth maybe sacrificed in the greater dental treatment. These compromised teeth may be removed, and several implants may be placed interforamina with the intention of constructing a fixed -hybrid prosthesis that cantilevers teeth to the posterior areas.

Removing teeth to replace other missing teeth may seem extreme, however this is a more practical treatment approach than performing multiple surgical procedures to allow dental implant placement (Steinberg & Kelly, 2015).

Management of Nerve Injury Complications

Early identification and diagnosis of nerve injury is important.

According to Gintaras et al. (2011) who performed studies and literature reviews, found that similar protocols existed for the management of nerve damage post operatively.

Patients whom experience post-operative anaesthesia or pain, it is recommended that they be seen 24hours post operatively. The surgical site should be assessed for the healing process. Adequate pain management should be given. It is imperative that the exact nature of the patient’s complaint is ascertained. Neurosensory testing can also be performed to determine the level of sensory dysfunction or future follow- up as indicated. If there is improvement on subsequent visits, there is no need for removal or repositioning of the implant. Patient who fail to improve in 3-4 months should be referred to a micro neurosurgeon for nerve exploration and repair. Sensory function should improve 3 months after nerve injury (Greenstein, 2006).

Steinberg and Kelly (2015) recommend that if one suspects nerve injury during implant placement and an IAN block was given, a local anaesthetic reversal agent like phentolamine (PM) maybe used. The PM will increase the likelihood of full recovery as its use allows early detection of nerve damage.

Once the effects of the local anaesthetic have passed, sensory tests should be performed to assess the IAN. Use both mechanoreceptive and nociceptive methods.

Radiographic analysis should be used to evaluate the proximity of the implant to the inferior alveolar canal or mental foramen (Steinberg & Kelly, 2015). If the radiograph reveals no nerve injury, then a regimen of anti- inflammatory medications should be prescribed which include steroids or non-steroidal anti-inflammatory drugs (Al-Sabbagh, et al., 2015).

However, if the radiographic analysis reveals that either the drill or the dental implant has impinged on the IAN. The use of anti- inflammatory medication should continue, and further investigation is necessary by CT (Steinberg & Kelly, 2015).

If a CBCT scan reveals the implant has encroach on but not through the canal, the implant must be removed completely or slowly reversed so that it no longer impinges on the canal. Following the implant removal there is likely to be soft tissue oedema of the soft tissue surrounding the nerve, hence the anti-inflammatory regimen is mandatory.

A CT scan may reveal that the implant has penetrated the canal and damage to the IAN has occurred. If this occurs, implant retrieval will not improve the situation. The patient is referred for re-evaluation and possibly microsurgery is necessary. Micro surgery can help some but not all patients. Nerve damage differs within patients and treatment outcomes are unpredictable (Greenstein, et al., 2015).

Conclusion

Surgical implant complications are not uncommon and should be addressed immediately (Misch & Wang, 2008). Causality maybe iatrogenic, due to poor treatment techniques, or lack of communication between dental specialities. To prevent complications, much time must be spent planning the case, assessing radiographs, measuring models and using CBCT scan to make a proper diagnosis. In addition, a sound knowledge of anatomy should be reviewed in each case. As more inexperienced clinicians start placing dental implants, so will there be an increase in surgical complications. A competent clinician will be able to plan a predictable surgery and know what his or her limitations are.

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