By Howard. Holmes, DDS, Dip OMFS, FICD; David Lam, DDS; Taylor MC Quire, DDS and Peter Giolious DDS
Inferior Alveolar Nerve injury is a serious neurological complication which can result from a number of reasons, the most common of which is by performing oral and maxillofacial surgical procedures. The purpose of this paper is to review the anatomy of the inferior alveolar nerve (IAN) in the mandibular region and to review those factors that appear to be most associated with the development of functional impairment of this nerve.
Injury to the inferior alveolar nerve can be one of the most serious complications that can arise from the performance of a number of oral and maxillofacial surgery procedures. Due to this nerve's anatomical location, it may be iatrogenically traumatized during various surgical procedures carried out for the management of trauma, oncologic problems, preprosthetic problems, orthognathic surgery and most commonly third molar removal. As well, local anesthetic injections themselves may lead to either transient or permanent dysfunction, as can the placement of dental implants, or performing endodontic treatment (Table 1).
Finally, chemical substances commonly employed by dentist during root canal therapy or placed into extraction sockets can adversely alter nerve function. The surgical removal of mandibular third molars is by far the most common culprit, usually representing over 75 percent of the patients who are afflicted with this problem. Additionally one must always remember that systemic diseases may also compromise the functionality of this nerve (Table 1).
The subsequent distorted somatosensory sensation, be it anesthesia or paraesthesia can result in significant impairment in speech, chewing, sleep, work or socializing and ones psychological well being (Fig. 1). If dysesthesia results, not uncommonly a chronic neuropathic pain syndrome further compromises the patient.
In 1980 Bruce2 distributed questionnaires to oral surgeons regarding the presence of inferior alveolar nerve dysfunction after removal of mandibular third molars. Of the 990 cases evaluated in the survey there was in incidence of 4.4 percent.
In a similar manner Alling3 (1986) based on the extraction of 367,170 impacted mandibular wisdom teeth reported an overall occurrence of 0.41 percent. Of those cases where the nerve function was altered 3.5 percent of them persisted for greater than six months. Although these reported figures could not indicate the true incidence of IAN damage, they do reveal the scope of the problem.
The incidence of inferior alveolar nerve disturbance after third molar removal has been reported to vary widely from 0.04 percent to 8.0 percent (Table 3) when using the typical buccal approach. Temporary disturbances, are by far more common, however; permanent problems have been reported in a frequency of 0.6 to 2.2 percent.
Different surgical techniques for mandibular third molar removal have been felt to potentially affect the frequency of lingual nerve damage and as well the IAN. The "Lingual Split-Bone Technique" is considered to result in a higher frequency of nerve disturbances than the "Buccal Approach."
The lingual Split-bone technique was first developed in the United Kingdom (UK), where practices were largely hospital based and the use of chisels and osteotomes predominated. A purely lingual approach had been described by Warwick James. Sir William Kelsey Fry, as later described by Ward in 1956,4 developed the buccal approach in conjunction with the lingual split. This technique involves the reflection of tissues buccally, distally and lingually in order to expose the bone and tooth from all aspects.
A periosteal elevator, usually a Howarth's, is placed between the lingual periosteum and the lingual plate of bone. Chisels are used for socket saucerization and the tooth delivered distolingually. Ward has stated the advantages of this technique when the tooth is in lingual version, are "speed, elimination of dead space and better healing." The primary goal of using a lingual flap retractor is to protect the lingual nerve during bone removal and tooth elevation.
In contrast, a buccal approach, which avoided disturbance of lingual tissues by not retracting a lingual flap was developed for office based practices in North America (USA) by such early pioneers as G.B. Winter. Bone and tooth are removed using a drill from a buccal direction. The goal in both techniques is the same, namely, to minimize the incidence of lingual nerve injury during mandibular third molar removal.
Not uncommonly in our present era, some surgeons employ the "buccal approach," however additionally elevate the lingual tissue in hopes of decreasing damage to the nerve. Why there may be an increase in damage to the IAN is not as clearly understood.
Hence, the purpose of this paper is to:
1. Review the anatomy of the Inferior alveolar nerve.
2. Ascertain the variables with respect to the techniques that may be responsible for the nerve injuries which occur such that appropriate recommendations for their avoidance might be suggested.
ANATOMY OF THE INFERIOR ALVEOLAR NERVE IN THE THIRD MOLAR REGION
Although in most textbooks the anatomy of the IAN is consistently described as having a standard pathway, such is not the case. It is a branch of the third division of the trigeminal nerve (Mandibular Branch). The mandibular branch exits the cranium into the infra temporal fossa, through the foramen ovale, where it combines with the motor root to form a mixed nerve.
The nerve then quickly starts to divide to form the buccal, masseteric, pterygoid, temporal, auriculotemporal, lingual, and inferior alveolar divisions. From the IAN also arises mylohyoid (Fig 4).5 The IAN traditionally enters the mandible via the "mandibular foramina". It is now well known that the IAN may have several points of entry. Several "offshoots" which arise higher in the infratemporal fossa either from the IAN itself or V3 travel down and forward and enter at the base of the coronoid process.
In some circumstances, branches of the IAN or the buccal branch will enter into the mandible in the retromolar fossa. These can provide innervation to the molar teeth and complicated achieving adequate anesthesia by a conventional IAN block.
Additionally, branches of the mylohyoid nerve enter the mandible via the "retro mental foramina," located on the lingual cortex of the mandible in the area of the second premolar. Most commonly providing ancillary innervation to the premolar, cuspid and anterior teeth, it further complicates obtaining adequate anesthesia.
Passing through the mandible the IAN forms dental (innervates teeth) and interdental plexus (innervate the alveolar bone, periodontium and gingiva). Its intraosseous course likewise is quite variable. Olivier (1927) and Carter and Keen6,38 (1971) established that the text book description (single nerve, close to the apices of the teeth) occurred in only 6o percent 0f circumstances (Fig. 5).
In 20 percent of cases the canal is placed much more inferiorly and a plexus of offshoots constitutes the dental plexus (Fig. 6). In a third variation (Fig. 7) there is an inferiorly placed canal and the dental plexus originated from two main offshoots. Ollier also pointed out that in 40 percent of the mandibles he studied, there was no discernable canal.
The mental nerve branches within the body of the mandible form the inferior alveolar nerve before emerging at the mental foramen and dividing into two or three branches under the depressor anguli oris; one branch supplies the skin of the chin and the other two supply the skin and mucous membrane of the lower lip.
Terminal branches have been found to re-enter the cortical plate to supply the lower incisors even on the opposite side.7
Clearly for the dentist the most common iatrogenic origin of IAN injury will be related to:
Administration of local
Extractions (were roots approximate the IAN canal).
Lower molar endodontics.
Placement of dental implants.
Chemical agents used in treatment.
The effects of various chemicals (chemical paresthesias) on the nerve are probably one of the least appreciated sources of problems.
Table 4 outlines some of the more common agents employed by the dentist which are known to produce problems.36 Tetracycline was commonly used in the 1960's by Oral surgeons to prevent "Dry Socket".
Leist & Zuniga37 clearly established that when placed into extraction sites that it produced a chronic inflammatory and foreign body giant cell response in the soft tissues. They were able to show further that if placed in areas where the epineurium had been injured a significant epi fascicular inflammation occurred which could be associated with axonal destruction. Surgicel (oxidized regenerated cellulose), owing to it's low pH when placed in contact with a sensory nerve altered neural function for a period of two weeks, as did White Heads varnish, which is sometimes used for controlling hemostasis and the treatment of dry socket.
Carnoy's solution which is employed by surgeons to decrease the potential re-occurrence of Odontogenic Keratocysts has also been shown to affect nerve function for long periods of time and based on its components is directly neurotoxic. In a like fashion many medicaments employed in endodontic therapy (Table 4) can by their pH, alkalinity or direct tissue fixative property injure neural tissues. Other factors involved in endodontics, other than direct chemical injury include:
Trauma from over instrumentation;
Mass compression from overfilling and extrusion;
Inflammatory edema of surrounding tissue;
Combinations of any of the above factors.
The primary cause of damage to the IAN for the dentist is without a doubt the removal of teeth (Table 1), especially impacted lower molars that exhibit a close relationship to this nerve. Although there is a wide range of the reported incidence of its occurrence it is probably safe to say based on published reports, that permanent dysfunction occurs less than one percent, and is much more likely to occur if there is a true relationship of the nerve to the roots of the teeth.
In a landmark article by Howe & Poyton10 in 1960 it was determined after evaluating 1,355 impacted mandibula molars clinically at the time of extraction and radiographically that a true relationship existed in approximately 7.5 percent. A "true relationship" was defined as the visualization of the neurovascular bundle at the time of tooth removal. An "apparent" relationship was defined by radiographs as a circumstance in which the roots of the teeth appeared to be in an intimate relationship to the IAN. This occurred in 61.7 percent of the teeth.
Of the 70 cases that developed post surgical nerve impairment over 50 percent of them had a true relationship which represented 35.64 percent incidence. This was a 13 times greater incidence than that occurring with those teeth exhibiting an apparent one. They further noted an increased incidence in older patients, teeth that were deeply impacted, those which exhibited grooving, notching, or perforation, as well as a three and four time increase in mesial and horizontally impacted teeth with linguoversion.
Based on further correlations with the periapical x-rays of the teeth they developed criteria to aide in determining the presence or absence of a true relationship (Table 3). Employing these criteria they determined an error of 1.075 percent. Even today these criteria established over four decades ago are the most commonly employed determinates of the potential for establishing the individual risk for a patient to sustain injury.
Rood and Noraldeen Shahab39 (1990) in a similar analysis established that these criteria were reasonably valid. Purely on the incidence of nerve impairment three criteria were found to be significant, two unrelated but potentially significant and two insignificant. The significant and highly related included:
Diversion of the canal;
Darkening of the root;
Interruption of the white lines.
The latter two are certainly in keeping with the findings of Howe & Poyton, as well as those of Kipp and Rud.
Of questionable importance were:
Narrowing of the roots;
Deflection of the root.
Bifid roots or narrowing of the canal were as found by Kipp, Howe & Poyton and Rud to be of no predictive value. More recently, Bell (2004) has attempted to provide similar information when using a panographic x-ray.40 He pointed out the significant concern of the resolution of the Panorex as compared to a peri-apical x-ray and therefore the former's ability to provide the same diagnostic detail.
Radiographic findings in the Panorex having the highest correlation with a true relationship to the IAN included:
1. Superimposition of canal on root with radiolucent area (darkening) and loss of one or both white lines;
2. Root apex just touched top of the outline of the IAN;
3. Darkening of the root combined with Deflection of root Narrowing of root Narrowing of canal +/- Deflection of canal.
Although some inconsistency exists, it is also important to remember that not only do positive radiographic findings not correlate 100 percent to the development of nerve impairment, absence of radiographic signs does not ensure that injury will not occur.
Many of the studies that have been undertaken to assess the incidence of IAN dysfunction post surgically have also attempted to assess the variables that maybe responsible. Table 4 summarizes those most commonly felt to be of significance. Some studies documented are surveys, others represent the incidence as a percent of the number of patients treated as apposed to the number of teeth removed, while others report the incidence of any nerve impairment (IAN, mental, buccal, lingual). Such reports confuse the issue.
If one eliminates these as well as those employing the "split bone" technique for tooth removal then a more representative estimate of the incidence of temporary dysfunction for the IAN would be 0.6 percent - 8.0 percent. Similarly, the assessment of the risk of a permanent impairment is further complicated by studies which additionally are retrospective vs prospective, fail to objectively assess neurologic function or are of inadequate duration.
Eliminating these reports provides an incidence of 0.15 percent - 1.0 percent for permanent impairment. Both these figures fall into the range which requires disclosure to the patient, as established by the NIH consensus report with respect to the removal of third molars carried out in 1979.
Indeed this report strongly recommends that surgical complications which might be permanent at an incidence greater than 0.5 percent, and those temporary, with an incidence of 5.0 percent or greater be discussed with the patient.
Other nerves (buccal, mylohyoid, postero superior alveolar) may also be impaired in the process of removing wisdom teeth. The incidences of their involvement have seldom been addressed.
The significance of the variables that may contribute to the development of IAN impairment as outlined in Table 4 have been evaluated by a number of clinicians and there has been no uniformity of the results. This obviously may be related to the methodology of the clinical study implemented.
For every study that contends the importance of each of them, there exists another which did not come to the same conclusion. Yet despite the reported incongruities the listed factors repeatedly appear as important. The least important, in the hands of an experienced and skillful clinician would appear to be the specific technique used (buccal vs spit bone).43,44
The most infrequent, yet most litigated nerve impairment injury to the IAN or lingual nerve occurs as a result of anesthetic injections (most commonly the inferior alveolar nerve block). Few articles exits on this subject, but an excellent review has been provide by Pogrel and Schmidt41 (2001). It's incidence as reported, has ranged from 1 in 30.00 nerve blocks to 1 in 750,000 or higher (Fig. 11).
In 1994 Haas & Lenon,42 conducted a 21-year retrospective review of reported cases of injuries to nerves as a result if injection, that had been reported that had been reported to the Ontario Professional Liability Association of the province of Ontario. From this study a projected incidence of occurrence was estimated to be one per 785,000 mandibular nerve blocks.
Most of these are transient with most (85 percent) recovering in eight weeks or less. For those that fail to recover at this time period, their is evidence that that only one third of the remainder will recover at a later time.
Most reports concur that the lingual nerve is affected more than double that of the IAN. Of the 143 reported incidences in the study by Haas as an example, 42 involved the IAN only, 92 the lingual only and nine both nerves. This type of injury also appears to have a greater tendency to develop dysesthesia or neuropathic symptoms. Three hypotheses have been suggested as a mechanism for this problem:
1. Injury from direct trauma;
2. Intrafascicular bleeding;
3. Neurotoxicity of the local.
None of these theories adequately explain its occurrence. There is no way to predict or prevent this type of injury. Similarly there is no predictable medical or surgical intervention for the non-neuropathic form.
Many studies on this subject are less than statistically ideal, due to inherent biases, their retrospective nature, and or comparisons between complication rates from different centres where surgery was performed in non-randomized fashion by many different surgeons. Furthermore, studies that use surveys are known to report unreliable responses, since some responders may have estimated their total number of extractions and those of associates in their practice.
It is also scientifically unacceptable to compare results reported by different clinicians using different methods and different sample sizes with different assessments and analyses. In spite of these drawbacks it seems clear that the skill of the operator is of great importance. Skill, coupled with anatomical, dental and patient factors will ultimately determine the potential for IAN nerve injury. OH
Dr. Holmes is assistant head, director of Undergraduate Education &, Surgical Director of the Surgical Orthodontic Treatment Program, Department of Oral & Maxillofacial Surgery, Faculty of Dentistry, University of Toronto.
Dr. McQuire is a resident, oral and maxillofacial surgery also pursuing a Msc involving nerve injuries Faculty of Dentistry, University of Toronto.
Dr. Lam is resident, Oral and Maxillofacial Surgery; PhD. Harron Scholar, in the Collaborative Program in Neuroscience, Faculty of Dentistry, University of Toronto.
Dr. Giolious is resident, Oral and Maxillofacial Surgery also pursuing a Msc involving nerve injuries Faculty of Dentistry, University of Toronto.
Oral Health welcomes this original article.
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SYSTEMIC & LOCAL FACTORS AFFECTING THE INFERIOR NERVE
Systemic Diseases - Local Factors
Multiple Sclerosis - Nerve blocks
Sarcoidosis - Fractures
Metastatic tumors - Bone infections
Drug induced - Local tumors
Blood diseases - Perapical odonrogenic lesions
Viral & bacterial - Various oral surgical diseases procedures including removal 3rd molars, Endodontic surgery, Conventional endodontic procedures
INCIDENCE OF LOCAL CAUSES
Facial Trauma - 3
Surgical orthodontics - 2
Third molar removal - 1
Dental implants - 4
TMJ procedures Treating pathology - 5
Administration of local
CHEMICALS CAUSING NERVE INJURY
Whiteheads varnish (iodoform)
Phenols (eugenol, cresol, thymol)
Sodium Hypochlorite >0.5%
Calcium Hydroxide paste
Locally injected steroids
FACTORS AFFECTING THE RISK OF IAN DAMAGE
Expertise of Surgeon
Difficulty Depth: Crown at CE junction of 2nd Molar, Mesial & Horizontall impaction in linguversion, Time taken to perform procedure, Having to section the tooth several times
The surgical technigue emplyed
Wheather the roots completely formed or not
Roots close to NV Bundle as defined on X-ray
Increasing age of the patient
Distal bone removal
Intra operative visualixation of the mandibular canal
Ensure that you properly inform and consent your patient prior to surgery of the normal sequelae, complications and risks of surgery including the attendant risk of both temporary and permanent nerve impairment.Enure that you have taken and performed a proper radiographic assessment prior to the onset of surgery.Ensure that you make proper and adequate surgical procedure notes.Remember that your experience and skill as well as anatomical and patient related factors may determine the potential for nerve impairment. You may wish to refer or wish you had!Remember once having undertaken the procedure the expectation is that you will be capable of handling most of the ensuing post-operative care and treatment.Nerve impairment if it develops should be properly documented and followed to its resolution and if it persists or develops neuropathic characteristics, it should be quickly referred to a specialist who deals with nerve injuries.