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Lingual Nerve Injury: Surgical Anatomy and Management


June 4, 2018
by Yasser Alali, DDS MSc (candidate); Harshdeep Mangat, BSc, MD, DMD, MSc; Marco F. Caminiti, BSc, DDS, Med, FRCD(C)

Introduction
The Lingual nerve (LN) is a branch of the mandibular division of the trigeminal nerve (V3) that is responsible for general somatic afferent (sensory) innervation. It supplies the mucous membranes of the mandibular lingual gingiva, floor of the mouth and the ipsilateral two-thirds of the tongue. 1 It also carries specialized taste fibers and parasympathetic innervation to salivary glands. While it should be an infrequently encountered nerve during routine and basic oral and maxillofacial surgical procedures encountered in daily dental practices, its vulnernable position poses a risk of iatrogenic injury. The purpose of this paper is to enlighten readers with regard to the anatomy of this nerve in light of potential risks, which unfortunately are not uncommon. Unfortunately, current treatment options yield minimal success in the improvement or restoration of function of the lingual nerve following injury.

The aim for this paper is to:
I. Review and detail the anatomy of the lingual nerve;
II. Explain the methods of injury that have been reported and
III. Describe methods to mitigate against injury and help alleviate symptoms of nerve damage

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I. Anatomy (Table 1)

Table 1

After exiting the skull base via the foramen ovale, the mandibular division of the trigeminal nerve divides in the infratemporal fossa into the auriculotemporal, inferior alveolar and lingual nerves (LN). 3 The LN also carries nerve fibers that are not part of the trigeminal sensory system. The Chorda Tympani (CT), which is a branch of the cranial nerve VII (facial nerve), transmits taste sensation to the anterior two thirds of the tongue via preganglionic parasympathetic fibres to the submandibular and sublingual salivary glands. 1,2

The LN arises from the posterior division of the trigeminal nerve. 3 The CT joins the LN lower in the infratemporal fossa, approximately one centimeter below the bifurcation of the lingual and inferior alveolar nerves, after it exits the base of the skull via the pterygotympanic fissure. 3 Therefore, beyond this point, the LN contains: 1. autonomic fibers destined for the submandibular ganglion, 2. fibers for taste derived from the facial nerve; and, 3. general trigeminal sensory fibers to the anterior two third of the tongue 3 (Fig. 1). It then travels in anterior direction just medial to the inferior alveolar nerve and the lateral pterygoid muscle. 2 Once the LN enters the pterygomandibular space, its traverses between the lateral aspect of the medial pterygoid muscle and medial aspect of the mandibular ramus 1,2 (Fig. 2).

Fig. 1
The course of the third division of cranial nerve five and the associated peripheral branches. Trigeminal nerve – mandibular branch (V3); Chorda tympani (CT); Inferior alveolar nerve (IAn); Lingual nerve (Li); Mental nerve (Me).

Fig. 2
Innervation pattern of the lingual nerve with accessory peripheral branches and the demonstration of the rich neuronal supply in and around the area of the mandibular dentition. Chorda tympani (Ct); Inferior alveolar nerve (IAn); Lingual nerve (Li); Mental nerve (Me); Submandibular Ganglion (Gang); Nerve to Mylohyoid (My).

At the third molar area, the mylohyoid muscle is inferior to the LN. At the posterior attachment of the mylohyoid muscle to the mandible, the LN makes an anteromedial turn and travels superficial to the mylohyoid muscle. The location of the LN with respect to the height of the crestal alveolar bone varies, as reported in different studies examining the distance between the third molar and mandibular ramus in addition to the configuration of the alveolar ridge 4 (Fig. 3). These studies illustrate that the LN is potentially susceptible to injury during a variety of procedures including administration of local anesthesia, third molar extraction, preprosthetic surgery, to more extensive procedures like orthognathic surgery 4 (Fig. 4). The location of the LN is reported to be at the level of the alveolar crest or higher in 17.6% of cases. 5 In the retromolar region, the distance from the lingual alveolar crest to the LN is averaged at 4.45 mm and ranges between 3.01 mm to 2.28 mm in the third molar area. 5,6 As the LN courses along in the region of the mandibular second molar, the submandibular ganglion can be found suspended inferiorly by preganglionic parasympathetic fibres secretory fibres from the LN. 1 The LN then starts to move anteromedially, beneath the submandibular duct. The nerve then enters the ventral mucosa of the tongue anterior. 1

Fig. 3
A: Prior to dissection. B: After removal of the mucosa. C: Exposure of the lingual nerve (arrowhead). Note the lingual nerve passes lateral to the medial pterygoid muscle and below the superior pharyngeal constrictor muscle. SPCM; superior pharyngeal constrictor muscle, MPM; medial pterygoid muscle.The Clinical View for Dissection of the Lingual Nerve With Application to Minimizing Iatrogenic Injury J. Iwanga Clinical Anatomy 30:467–469 (2017).

Fig. 4
Course of the lingual nerve (arrowhead) in different tongue positions after removal of the superior pharyngeal constrictor muscle. A: Neutral position. B: Protruding, left-deviated and elevated position. C: Lower position. *tooth #47
The Clinical View for Dissection of the Lingual Nerve With Application to Minimizing Iatrogenic Injury J. Iwanga Clinical Anatomy 30:467–469 (2017).

Multiple studies have investigated the branching patterns of the LN from the inferior alveolar nerve (IAN). 4,7 The bifurcation of the LN and IAN bifurcation most often occurs between the otic ganglion and the sigmoid notch. Furthermore, less common branching patterns of the LN from IAN can occur in the upper half of the ramus, above the lingula or demonstrating a plexiform branching pattern – in order of incidence respectively. With respect to the relationship of the mandibular third molar to the LN – studies have failed to identify any statistical relationship between the presence or absence of the mandibular third molar and LN position. Furthermore, no significant relationship between distance from the LN to the lingual alveolar crest and the degree of mandibular crest resorption has been reported within the literature. 8

Clinicians must remember that the nerve has complex sensory and secretomotor function. Therefore, any damage to the LN can result in altered salivary secretion on the affected side and loss of taste to the anterior two-thirds of the tongue in addition to temporary or permanent general sensory changes to the anterior two-thirds of the tongue and floor of mouth. These can manifest as anesthesia, paresthesia, dysesthesia or hypoesthesia. 9-11 This loss of sensory function can cause speech changes, pain, burning sensation, drooling, and tongue biting 12,13 (Table 2).

Table 2

II. Injury: (Table 3)

Table 3

The potential for injury to the LN varies based on the type of procedure. In certain settings such as head and neck oncology, injury sustained by the LN is often unavoidable. However, more alarming is the fact that the majority of iatrogenic LN injuries are the result of elective non-oncologic procedures. 14 The cause of LN injuries include: administration of local anesthetic 15, removal of mandibular third molars 12, the surgical insertion of dental implants 16, and inadvertent instrumentation during mandibular osteotomies. 17 The incidence of LN injury during extraction of mandibular third molar varies, with the majority of studies reporting an incidence of 0.6% to 2.0%. 8,9,17 Fortunately, permanent injury to the LN from third molar surgery appears to be less common and has ranged from 0.04% to 0.6%. 12 The incidence of LN injury after performing a sagittal split ramus osteotomy ranges from 9% to 19.4%. 12

Several factors influence LN injury following extraction of mandibular third molar surgery. Although some studies have shown that the avoidance of the “lingual flap technique” in addition to preserving the lingual plate of the mandible can reduce the incidence of temporary lingual nerve injury. 14,18,19 A systematic review has shown no difference in permanent LN injury rates whether a lingual retractor was used or not. 13 There is no relation between the incidence of LN injury and the angulation of the mandibular third molar and LN injury. 20

The increased age of patients, degree of difficulty of the position of the third molars and surgeon experience have been shown to be the only significant predictors in determining risk of permanent LN injury as reported in a prospective study by Jerjes. 21

Fig. 5
1st week after third molar removal. It shows a unilateral lingual atrophy of the fungiform papillae of right hemitongue accompanied by signs of recent bites.
Martos-Fernández M, de-Pablo-Garcia-Cuenca A, Bescós-Atín MS. Lingual nerve injury after third molar removal: Unilateral atrophy of fungiform papillae. J Clin Exp Dent. 2014.

Fig. 6
6th month post-removal. A decrease in fungiform atrophied area was observed despite the persistence of bites signs.
Martos-Fernández M, de-Pablo-Garcia-Cuenca A, Bescós-Atín MS. Lingual nerve injury after third molar removal: Unilateral atrophy of fungiform papillae. J Clin Exp Dent. 2014.

Fig. 7
Exposure of distal and proximal LN stumps. Bagheri et al. Microsurgical Repair of Lingual Nerve Injuries. J Oral Maxillofac Surg 2010.

Lingual Nerve And Local Anesthesia
The incidence of LN and IAN injuries caused by local anesthetic block injections ranged between 1:26,762 to 1:800,000. 10 Epidemiologically, several reports have highlighted the association between LN injury and high concentration local anesthetics like prilocaine 4% and articaine 4%. The mechanism of injury often related to physical damage caused by the needle causing subsequent hemorrhage, inflammation and scarring resulting in demyelination. 10 Trauma to LN injury from needle injection would result in damage the nerve bundle and consequently neurotmesis because the average diameter of the most commonly used needle is 25 gauge is approximately 0.45 mm and the average diameter of the lingual nerve is 1.86 mm. 22 Chemical injury will occur if local anesthetics is deposited in the intrafascicular space or inside the nerve which will results in demyelination, axonal degeneration, and inflammation of the endoneural fibers. 22

Lingual Flap And Nerve Protection
Its strongly recommended to avoid lingual flap detachment as much as possible to decrease the incidence of lingual nerve damage. Using lingual flap to protect the LN should be restricted to cases with high risk of nerve injury. 22

Lingual Nerve In Lower Third Molar Region
One of the important prevention strategies to avoid iatrogenic LN injury is obtaining a thorough knowledge of lingual nerve anatomy and topography. The inclination of the alveolar lingual plate in addition to the prominence of the alveolar process influence the lingual nerve position at the lower third molar region. 22

Edentulousness and mandibular atrophy with a loss of muscle mass causes the LN to rest in a superficial position compared to dentated individuals. LN will be more cranial in position if there is a short distance between mandibular ascending ramus and the lower third molar. 22,23 Therefore, marked mandibular atrophy could be an important risk predictor for lingual nerve injury during surgery in the area with any lingual flap manipulation.

The lingual split technique in addition to lingual flap retraction is associated with an increased risk of temporary nerve damage compared with the buccal approach plus and the simple buccal approach. 24

Although tooth sectioning does not increase the incidence of LN damage, it could be a possible risk factor for LN injury during extraction of lower third molar. The nerve may be directly cut by the rotary instrument during tooth sectioning. Therefore, sectioning two thirds of the lower third molar followed by using straight elevator to complete the separation would prevent the risk of LN injury. Furthermore, LN damage is also associated with removal of periradicular bone at the distolingual or lingual sites. 10,22

Suture
Although there is little data reporting on damage to the LN due to suturing, it is evident that the LN could be damaged by direct trauma from the needle if it is inserted too apically. 22 The nerve can also be compressed if included within the flap of the tissue adapted by the suturing. Over exuberant and excess tissue bites for suturing must be avoided and the use of compressive/occlusive maneuvers for surgical hemostasis must take into account the position of the nerve. Interestingly if the nerve and associated blood supply has been damaged during surgery, the first sign is significant bleeding which is difficult to control. Methods to stop the bleeding (suturing, hemostat, cautery) can worsen the status of an already damaged nerve.

III. Management When Injury Occurs (Fig. 8)

Fig. 8
Protocol for LN Injuries (Adapted from Robinson et al. (2004))

Witnessed or open injuries mandate documentation and urgent immediate referral to an OMFS with experience in microneurosurgical repair for early intervention. The patients should be also given anti-inflammatory medications following acute nerve injury including steroids, NSAIDs, or both .

This would include ibuprofen 800 mg TID for 14 days or Solumedrol Dose Pack (usually a six-day course). An unwitnessed LN injury will be noted at a postoperative follow-up or by patient complaints or calls to the office. This would mandate a baseline history and neurosensory examination to establish the appropriate management and is essential. The injury should be classified as being either dysesthesia, paresthesia, hypoesthesia, or anesthesia. The pain and decreased sensation could be quantified on a visual analog scale of 1 to 10 and mapped quickly on a representation of the tongue and floor of mouth in any clinical chart. In cases where patients complain of intermittent pain, the clinician should be able to determine whether the pain is stimulated or spontaneous. Patients with long standing injury usually present with constant pain which may be the result of a lack of afferent input from the periphery and also due to the formation of a neuroma (traumatic neuroma). The decreased level of sensation could be quantified on a level of 1 to 10 and compared with the contralateral side. 25 Interference with daily living activates and alteration of taste sensation (paraguesia) should be documented. 12,25

Clinical Neurosensory Testing is a standardized maneuver utilized to objectively assess the degree of sensory impairment, monitor recovery, and determine if microneurosurgery is required. 26,27 Clinical Neurosensory Testing should be performed at three levels A, B and C to assess mechanoceptive and nociceptive awareness of the affected area. The non-affected side should be tested first to determine the patient’s normal responses. 26,27 The areas that are reported abnormal by the patient should be mapped and the final outline will represent the area of alternated sensation. 26,27 At level A, large myelinated A-alpha and A-beta assessed by using fibers brushstroke directional discrimination with constant rate and pressure by using hair brush or fine sable could be used and the patient should be asked to identify the direction of movement (i.e., to the right or to the left). In level B A-beta fiber assessed by using Boley gauge with blunt tips to evaluate two-point discrimination. The reference distance should be determined in the normal area based on the closest distance in which the patient could recognize the two points then the affected area is tested. Level C assessed could be evaluated by using or 27-gauge needle or dental explorer tip to assess C fibers and A-delta. Finally, there is insignificant correlation between altered taste sensation has little with the degree of LN injury. 26,27

Conclusion
The lingual nerve and other divisions of the trigeminal nerve are variably positioned within the hard and soft tissue of the mandibular molars. Clinician’s must be cognizant of this vast neural network that exists and that improper soft tissue manipulation can lead to devastating nerve injury. Surgery, unintentional laceration, crush injury, penetrating trauma, stretch injury, chemical insults can all contribute to damage to the LN. The management of LN damage depends upon the mechanism of injury, the duration of the nerve injury and the patient’s symptoms.
Unfortunately, trigeminal nerve injury for dentoalveolar surgery has major medicolegal implications due to the problematic consequence.11 Resultant lawsuits are usually based on inadequate informed consent, poor planning and assessment, poor surgical technique and improper management of LN injury postoperatively.10 Clinicians need to be aware that the Lingual Nerve specifically is difficult to repair and although the field of microneurosurgical intervention has shown advancements, the best method to avoid these injuries is prevention, which begins as always with a solid knowledge of anatomy. OH

Oral Health welcomes this original article.

References

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About the Authors
Yasser Alali, Resident, Division of Oral & Maxillofacial Surgery, University of Toronto, Toronto, Canada

Marco F. Caminiti, Program Director, Division of Oral & Maxillofacial Surgery, University of Toronto, Toronto, Canada
Corresponding author: Marco.Caminiti@dentistry.utoronto.ca

Harshdeep Mangat, Resident, Division of Oral & Maxillofacial Surgery, University of Toronto, Toronto, Canada