Dental implants: Mandibular neurovascular considerations

Introduction:

An ideal alveolar ridge with adequate bone height and width is essential for a successful dental rehabilitation. One of the most difficult surgical challenges to the implant surgery is severe resorption of the posterior mandible processes 1. The knowledge of intra-osseous course of inferior alveolar nerve and vessels is mandatory for placing an implant in body of the mandible. Damage to inferior alveolar nerve can lead to motor and sensory abnormalities. So, distribution of intra-osseous distribution of the inferior alveolar nerve is important in the accurate preoperative planning for the placement of mandibular implants 2. In an atrophic mandible, implant placement can be done by two ways,

  • One, by increasing the bone level above the inferior alveolar canal by use of autogenous bone grafting, allografts, xenogenic, or alloplastic materials with or without guided bone regeneration and distraction osteogenesis 3.  
  • Another method is by nerve lateralization 4. It carries a risk of epineurial damage or ischemic stretching.

Anatomy of mandibular nerve:

Mandibular nerve is branch of trigeminal nerve. Before we read in detail about the mandibular nerve and inferior alveolar nerve, let us discuss briefly the trigeminal nerve.

Diagrammatic representation of mandibular nerve and its branches

Mandibular nerve

The trigeminal nerve is the largest of the cranial nerves. It is a mixed nerve with sensory supply to the face, greater part of the scalp, the teeth, the nasal and oral cavity, the dura mater, the blood vessels of cerebrum and motor supply to muscles of mastication, mylohyoid and the anterior belly of digastric. It originates from the brainstem at the lateral surface of the pons, near its upper border, by a smaller motor and a larger sensory root. It exists the brain entering the trigeminal ganglion or Gasserian ganglion, where it gets divided into three major branches.

The upper branch of the trigeminal nerve is the ophthalmic nerve (V1). It passes forward in the lateral wall of the cavernous sinus and gains access to the orbit via the superior orbital fissure. The middle branch is the maxillary nerve (V2). Maxillary division exits the middle cranial fossa through the foramen rotundum and enters into the pterygopalatine fossa where it gives off several branches for the dura, the maxillary teeth and associated gingiva, the maxillary sinus, the upper lip, the lateral surface of the nose, the lower eyelid and conjunctiva, the skin of the cheek and of the side of the forehead, the nasal cavity and the mucosa of the hard and soft palate. The lower branch is the mandibular nerve (V3). V3 runs along the floor of the cranium then exits through the foramen ovale into the infratemporal fossa and innervates the dura, the temporomandibular joint, the skin over the side of the head above the ears, the auricle, the tongue and its adjacent gingiva, the muscle of the floor of the mouth, the mandibular teeth and associated gingiva, the mucosa and skin of the cheek, the lower lip and the chin and the muscles of mastication.

For detailed description please read “The Trigeminal Nerve”.

The mandibular nerve is the largest branch of the trigeminal nerve. As already stated it contains both afferent and efferent fibres. It leaves the skull through the foramen ovale and enters the infratemporal fossa where medial to the lateral pterygoid muscle it divides into a smaller anterior trunk and a larger posterior trunk.

Anterior trunk of mandibular nerve:

This division presents a larger amount of motor nerve fibers than sensory ones. The following branches were observed:

Masseteric nerve:

It is responsible for the motor innervation of the masseter and probably for sending tiny branches that supply the temporomandibular joint (TMJ).

Nerves to lateral and medial pterygoid:

These nerves arise from the anterior division of the mandibular nerve and they supply the motor innervation of these muscles.

Deep temporal nerves:

In number of two, one anterior and one posterior, they supply the anterior and posterior parts of the temporal muscle, respectively.

Buccal nerve:

Although pertaining to the anterior division, it bears sensory nerve fibers. It supplies the cheek skin and mucous membrane, except that of the region supplied by the posterior superior alveolar branches from the maxillary nerve.

Posterior trunk of mandibular nerve:

This division has a larger amount of sensory nerve fibers than the motor ones. It has following branches,

Auriculotemporal nerve:

It usually has 2 roots, arising from the posterior division of mandibular nerve. It encircles the middle meningeal artery (MMA) and runs posteriorly passing between the sphenomandibular ligament (SML) and the neck of the mandible. It carries somatosensory and secremotor fibres of the mandibular nerve and the glossopharyngeal nerve.

Lingual nerve:

The lingual nerve lies between ramus and medial pterygoid within the pterygomandibular raphe. It passes Lies inferior and medial to the mandibular 3rd molar alveolus. It provides sensation to anterior 2/3rds of tongue, lingual gingiva, floor of mouth mucosa, and gustation (chorda tympani).

Nerve to mylohyoid:

The mylohyoid nerve branches from the inferior alveolar nerve before it enters the mandibular canal. It provides motor nerve fibers to these muscles and sensitive nerve fibers to the neighbouring skin.

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Anatomy of mandibular canal:

The mandibular canal lies within the mandible beginning at mandibular foramen on the medial surface of the ascending mandibular ramus. It runs obliquely downward and forward in the ramus, and then horizontally forward in the body till mental foramen. It carries inferior alveolar neurovascular bundle 5. Anterior to the mental foramen the mandibular canal is referred to as the incisive canal. The diameter of mandibular canal has been investigated by various researchers. A study done on 45 Asian adults demonstrated that the mandibular canal near the third molar region has 2.0 to 2.4 mm diameter 6. Another study found the average diameter of the mandibular canal in its horizontal part around 2.6 mm 7. Another similar cadaver study reported that canal is approximately 3.4 mm wide 8.  

On a radiograph, the mandibular canal appears as a dark ribbon of radiolucency flanked by two radiopaque white lines. A study 9 was done using 3612 radiographs to evaluate the vertical plane of mandibular canal. The radiographs were divided into four categories:

  1. High mandibular canal (within 2 mm of the apices of the first and second molars)
  2. Intermediate mandibular canal
  3. Low mandibular canal
  4. Other variations

It was found that 48% of the canals were high, 49% were low, and only 3% could not be fitted into the high or low canal categories.

Inferior alveolar neurovascular bundle:

The mandibular nerve enters the mandible through the mandibular foramen on the medial surface of the ascending mandibular ramus. After passing through the mandibular foramen, the nerve is called the inferior alveolar nerve. Within the mandibular canal, the inferior alveolar nerve runs forwards in company with the inferior alveolar artery, vein, and lymphatic vessels and together they are called the inferior alveolar neurovascular bundle. The inferior alveolar nerve runs in the mandibular canal as a single trunk giving branches for molar and premolar teeth. In the premolar region, the nerve gives the incisive nerve for premolar, canine and incisor teeth and the mental nerve.

The buccolingual positioning of the inferior alveolar nerve has been studied well. In a recent study, Kim et al. classified the bucco-lingual location of the inferior alveolar nerve into 3 types,

In most of the cases (70%) the inferior alveolar nerve canal follows the lingual cortical plate of the mandibular ramus and body. In 15% of cases, the inferior alveolar nerve canal is located in the middle of the mandibular ramus posterior to the second molar. It then runs lingually to follow the lingual plate. In 15% of cases, the inferior alveolar nerve canal is located near the middle of the ramus and body 10.

There are also anatomical variations of the inferior alveolar nerve. The inferior alveolar nerve can give multiple (extraosseous) branches before it enters the mandibular canal. Within the bony canal it may give rise to multiple intraosseous branches also. There may be the presence of accessory foramina associated with the presence of additional branches of the inferior alveolar nerve. It may lead to ineffective anaesthesia.

Peculiarities of mental foramen clinical anatomy:

One of the most challenged regions for implantation in mandible is mental foramen region. This is because there are many variations with regards to the size, shape, location and direction of the opening of the mental foramen. The shape of mental foramen can be round or oval: diameter ranges from 2.5 to 5.5 mm 11-14. The outer end of the mental canal occupies a higher and more posterior plane than its inner end. One study found different shapes of the mental foramen in the mandibles of Zimbabwean subjects: round in 14 of 32 (43.8%) mandibles and oval in 18 of 32 (56.3%) jaws 15. The average size of the mental foramen has been estimated to be 3.47 mm (range: 2.5 to 5.5 mm) in height and 3.59 mm (range: 2 to 5.5 mm) in width 16.  

The mental foramen is usually located by the apex of the second mandibular premolar or between the apices of the premolars. However, anatomical variations occur concerning the mental foramen’s location 17, 18. After extraction of teeth the resorption of alveolar bone takes place because of which mental foramen comes closer to the alveolar crest 19, 20.

CT scan showing the direction of mental foramen

Mental nerve CT scan

In extreme situations, the mental foramen and mandibular canal can be adjacent to the crest of the alveolar ridge 21. Mental foramen is usually located more coronal than the mandibular canal. It has also been reported that more than one mental foramen may be present 18, 22. One study assessed the frequency of accessory mental foramina in skulls in four population groups which included American Whites (frequency of accessory mental foramen = 1.4%); Asian Indians (frequency of accessory mental foramen = 1.5%); African Americans frequency of accessory mental foramen = 5.7%); and pre-Columbian Nazca Indians (frequency of accessory mental foramen = 9.0%) 18. The most common emergence pattern of the mental foramen of the mental canal is usually in posterior direction. However, a study done on Zimbabwean subjects frequently detected a right-angled exit path 23.

The section of the nerve in front of the mental foramen and just before its ramification to the incisive nerve can be defined as the anterior loop of the inferior alveolar nerve. Although the length of the anterior component of the mental canal is not given, it is implied that it is consistently present. During implant placement any violation of this loop may result in neurosensory disturbances in the area of the lower lip and chin. Radiographic examination provides the clinician information about the mandibular canal and metal foramen. But, the ability of conventional two dimensional radiological methods (panoramic tomography, periapical radiographs etc.) to reveal the anterior loop is limited and their reliability and accuracy has been questioned 24, 25. Newer technologies like cone beam computed tomography (CBCT) which provide three dimensional image are more useful in detection of mental foramen opening, its direction and probably anterior loop.  

Anterior loop of Mental nerve 

Mental nerve

To avoid nerve injury, it is desired to place an implant leaving a 2-mm safety zone above the nerve and if the implant is placed anterior to the mental foramen then distal surface of the implant should be ≥2 mm mesial to the mental foramen 26.

Nerve injuries:

Seddon 27 described three types of nerve injury—neurapraxia, axonotmesis, and neurotmesis based upon the severity of tissue injury, prognosis for recovery, and time for recovery.

Neurapraxia:

It is a conduction block resulting from a mild insult to the nerve trunk. There is no axonal degeneration, and sensory recovery is complete and occurs in a matter of hours to several days. The sensory deficit is usually mild and characterized by a paresthesia, with some stimulus detection but poor discrimination and disturbed stimulus interpretation.

 Axonotmesis:

It is a more severe nerve injury as compared to neurapraxia. Afferent fibers undergo degeneration, but the nerve trunk is grossly intact with variable degrees of tissue injury. Sensory recovery is good but incomplete. The time course for sensory recovery depends on the rate of axonal regeneration and usually takes several months. The sensory deficit is characterized by a severe paresthesia.

Neurotmesis:

This is the most severe kind of nerve injury where complete disruption of the nerve takes place. Sensory recovery is not expected except when the nerve courses through a canal like the mandibular canal. The sensory deficit is characterized by anaesthesia.

Healing following the nerve injury:

After injury the nerve tries to repair itself by sprouting regenerating nerve units. These regenerating nerve units then try to grow down the nerve to re-innervate muscle or skin. The nerves regenerate at the rate of 1 inch per month. During regeneration of the nerve if correct connection is made i.e. motor nerve to muscle or sensory nerve to skin, then recovery of muscle function and skin sensation will occur. However, if the regenerating nerve fibers do not make a correct connection then no recovery will occur.

Inferior alveolar nerve lateralisation:

In many cases there is insufficient bone for implant placement in posterior mandibular region. The inferior alveolar never is the limiting factor, which should not be injured during the surgical procedure. To overcome this problem the inferior alveolar nerve lateralization procedure has been described. This surgical procedure involves the surgical exposure of the inferior alveolar nerve, releasing it from the inferior alveolar canal and placing it on the lateral aspect of the implant after the implant has been inserted upto the desired level in the bone.

Surgical procedure:

In 1987, Jensen and Nock 28 were the first to publish this technique developed for the translocation of the mental foramen. A full thickness periosteal flap is raised to expose the lateral aspect of the mandible upto the inferior border of the mandible. The incision for the flap begins at the retromolar region and continues forward to the mesial portion of the cuspid tooth area, where a vertical relaxing incision is made. The corticotomy starts usually 3–4 mm distal to the mental foramen and should should be extended 4–5 mm distal to the most distal implant position. It is recommended that to remove the trabecular bone over the neurovascular bundle, only hand instruments such as small curettes are used. The inferior alveolar nerve is mobilized and released from the canal and protected by placing a barrier like aluminium foil between the nerve and the bone.

After this, drilling is completed to place the implant. The implant is inserted while the nerve bundle remains retracted. In this way the apical portion of the implant is placed inferior to the level of the nerve. After the implant placement the nerve is repositioned over the lateral aspect of the implants. Complete healing takes places usually in six months.

The complications of this procedure include risk of temporary or even permanent irritation of the nerve, which may lead to anesthesia, hypesthesia or paresthesia. Various studies have been done to investigate the resolution of sensory complications which demonstrate variable results 29, 30. Because of the variability in results reported thus far, further investigations of the long-term neurosensory outcome of this procedure are needed. Patient must be told about the complications before attempting the procedure. Six to eight weeks of lasting paresthesia of the mental nerve, and the possibility of a permanent paresthesia should be told to patient.

Conclusion:

The in depth knowledge of mandibular nerve is required during placement of implants in the mandibular posterior area. Although we have various radiographic methods to identify the course of the mandibular nerve, but care must always be taken during implant placement because all imaging techniques have their own limitations.

Inferior alveolar nerve transposition is a useful adjunct for managing the atrophic posterior mandible with dental implants. CT scans are necessary to accurately locate the mandibular canal before undertaking nerve repositioning for implant placement, and it is important that only well-experienced surgeons perform this surgery. Because of the benefits of the inferior alveolar nerve lateralization procedure it is becoming more and more popular procedure. Lastly, it is the experience and efficiency of the operator which plays an important role in overall success of this treatment.

References:

Please contact author for references

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