Australian Dental Journal 1997;42:(3):149-52

Inferior alveolar nerve damage following removal ofmandibular third molar teeth. A prospective studyusing panoramic radiography

Andrew C. Smith, BDS, FDSRCS, FDSRCPS, FRACDS(OMS)*Susan E. Barry, BDSc†Allan Y. Chiong, BDSc†Despina Hadzakis, BDSc†Sung-Lac Kha, BDSc†Steven C. Mok, BDSc†Daniel L. Sable, BDSc†

AbstractPermanent alteration of sensation in the lip after theremoval of mandibular third molar teeth is anunusual but important complication. Studies havebeen performed to assess the risk of nerve damagebut most of these have been retrospective andpoorly controlled.This prospective trial predicted the outcome ofaltered sensation prior to surgery based on assess-ment of a panoramic radiograph and correlated thiswith the result postoperatively in the consecutiveremoval of 479 third molar teeth.Results indicated that 5.2 per cent had transientalteration in sensation but only one patient (0.2 percent) had prolonged anaesthesia. As 94.8 per centof teeth extracted had no neurological sequelae thefigures for prediction were skewed and a kappastatistical analysis of 0.27 illustrated a fair level ofagreement between prediction and outcome.This study supports previously reported levels ofneurological damage and confirms that panoramicradiography is the optimum method for radiologicalassessment for mandibular third molar teeth prior totheir removal.

Key words: Inferior alveolar nerve, injury, incidence,recovery, prognosis.

(Received for publication January 1996. AcceptedFebruary 1996.)

IntroductionMany complications can occur from the removal

of mandibular third molar teeth. Damage to theinferior alveolar nerve (IAN) is an unusual butimportant one. The IAN runs in a bony canal withinthe mandible in close proximity to the root tips ofmandibular molar teeth. Damage to the nerve mani-fests itself as a sensory disturbance of the lower lipand chin up to the midline.

Studies of the relationship of the IAN to thirdmolar teeth have been reported.1-3 Rood andNooraldeen Sheehab4 defined seven radiologicalmarkers that suggest an intimate relationshipbetween mandibular third molar teeth and the IAN(Table 1). At present these markers are the stan-dards used for the assessment of the likelihood ofrisk of damage to the IAN and the basis for gaininginformed consent from the patient.

A panoramic radiograph is frequently used as theradiological investigation of choice prior to thirdmolar surgery. The criteria previously mentioned areidentifiable on this projection, but like other conven-tional radiographs it is unable to give completeinformation in three dimensions. The most accuratemethod of prediction with precision of the position

Australian Dental Journal 1997;42:3. 149

*Oral and Maxillofacial Surgery, School of Dental Science, TheUniversity of Melbourne.†Final Year Student Research Group, 1995, School of DentalScience, The University of Melbourne.

Table 1. Radiological markers of proximity oftooth roots to IAN

Root related Canal related

Darkening DiversionNarrowing NarrowingDeflection Loss of lamina duraBifid apex

Scientific papers

Materials and methodsMale and female patients aged 17 to 35 years,

undergoing general anaesthesia for the removal ofmandibular third molar teeth, were recruited for thetrial. Exclusion occurred if they did not meet thesecriteria or if they required the removal of othermandibular teeth, had other associated mandibularpathosis, or any neurological disorder that mightunfairly influence the outcome. Patients who failedto attend for follow-up appointments or were other-wise lost to the study were also excluded. Afterconsultation with a statistician, a sample size of 500third molars was selected.

Prior to surgery, the surgeon was asked to predictfrom a panoramic radiograph any change in sensationto the lower lip that might be present post-operatively. The prediction categories are displayed(Table 4). At the postoperative check, two weekslater, the patient was questioned by another surgeonand any altered sensation scored on the samescheme (Table 4).

To allow for comparison of accuracy and consis-tency of prediction, a sample of thirty radiographsfrom the study population were selected for 15 sitesassessed pre-operatively as no change (Category 1,Table 4) and 15 within the predicted categories ofaltered sensation (Category 2-5, Table 4). Theseradiographs were presented to postgraduate traineeoral and maxillofacial surgeons who were asked topredict alteration in IAN sensation after removal ofmandibular third molar teeth. This procedure wasrepeated twice with a one-month time gap.

Data collection and statistical analysis wereperformed. It was felt inappropriate to applyconventional correlation coefficient and chi-squaredanalyses on the data, as these tests are not designedto assess agreement. After advice a kappa analysiswas chosen. This does have inherent problems inthat the prevalence of the outcome in each categoryis important and if there is a large proportion of oneoutcome then this can skew the kappa result. Advicefrom a statistician was that this analysis was still themost appropriate for this study.

150 Australian Dental Journal 1997;42:3.

of the IAN pre-operatively is the use of computer-ized tomography; however, unnecessary radiationdosage and cost/benefit analysis need to beconsidered.

Nerves consist of fasciculi held together by aprotective areolar connective tissue that coalesces toform the nerve sheath. This sheath is strengthenedby linear collagen bands. The outcome of damage toa nerve depends on the nature of the injury. Merrill5

outlined the classification defined by Sneddon andthis is the standard used for neurological assessment(Table 2). Prognosis also depends on other factorsincluding the age of the patient and adequacy of thelocal vascular supply.

The subjective and objective assessment of nervedamage is problematic. It is possible to classifypatterns of sensory loss6 (Table 3).

The exact aetiology of IAN injury is also impreciseand multi-factorial. Kipp, Goldstein and Weiss7

considered that mechanical injury from chisels, bursor elevators was most likely. Howe and Poyton2

concluded that crushing or tearing of the nerve frommovements of the teeth was more likely, particularlyif the IAN grooved or perforated the third molartooth (Fig. 1). Crushing of the roof of the IAN canalonto the IAN has also been implicated.8

Howe and Poyton2 also suggested that there wasan increased risk of IAN damage with advancing ageand difficulty of extraction. There is no substantiationof these factors in the literature.

The incidence of transient IAN damage rangesfrom 0.41 per cent to 8.4 per cent and permanentdamage is reported to occur in 0.014 per cent to 1.5per cent of cases.9 Studies vary in size from less than100 to over 1400 teeth, but most have beenperformed retrospectively, possibly resulting in datacollection inaccuracy.

The aims of this study were to identify theincidence of IAN damage following the removal ofmandibular third molar teeth and to assess whetherthe panoramic radiograph is valuable in predictingoutcome.

Table 3. Patterns of sensory loss

Hypoaesthesia Decreased sensitivity to stimulationHyperaesthesia Increased sensitivity to stimulationParaesthesia Abnormal sensation, spontaneous or evokedDysaesthesia Unpleasant abnormal sensation, spontaneous

or evokedAnaesthesia Total loss of sensation

Table 4. Predictions of alteration in sensation

1 No anticipated change2 Hypoaesthesia3 Hyperaesthesia4 Dysaesthesia or paraesthesia5 Anaesthesia

Table 2. Classification of nerve injury

Injury Type of damage Prognosis

Neuropraxia No axonal degeneration ExcellentAxonotmesis Axonal degeneration and

regeneration FairNeurotmesis Neural separation, healing

with cicatrisation Poor

Table 5. Data and statistical analysis (2)Prevalence of abnormality

OutcomeNormal Abnormal Total

Normal 411 7 418Prediction Abnormal 43 18 61

Total 454 25 479

ResultsFive-hundred sites were entered into the study.

Twelve patients with 21 sites failed to attend forfollow-up.

PrevalenceFrom a total of 479 third molar removals, 25

patients (5.2 per cent) reported having transientalteration in IAN sensation. Within two weeks ofsurgery only one patient (0.2 per cent) had aresidual neurological deficit. This patient’shypoaesthesia has persisted (Table 5).

PredictabilityOf the 454 normal outcomes (94.8 per cent) 411

were predicted from the radiographs to be normal.This shows a 90.5 per cent specificity. Of the 25outcomes with altered sensation, 18 were predictedfrom the radiograph showing a 72.0 per centsensitivity (Table 6).

Kappa valueTable 7 shows the distribution of pre-operative

prediction compared with the actual outcome. Thelast row and column of the chart show the incidenceof IAN damage predicted compared with the inci-dence of IAN damage that resulted. The diagonalson the table represent the agreement between theprediction and the actual outcome. The kappa valuetests the strength of agreement along this diagonaland was 0.27 with a 95 per cent upper and lowerconfidence interval of 0.45 and 0.10 respectively(Table 8). This shows a statistically fair level ofagreement between prediction and actual outcome.

To confirm accuracy and consistency of prediction,the results of repetitive testing of postgraduate oraland maxillofacial surgery trainees show a statisticallygood level of agreement (-0.06) between the testsconducted with a time interval (Table 8).

DiscussionInformation obtained from the Defence

Committee of the Australian Dental Association

Victorian Branch reveals that from 1990-1994 thirdmolar removal was responsible for 18.1 per cent ofall litigation and that 43.7 per cent of this was due toneurological damage.

It is therefore important to be able to predict andassess the risk of nerve damage. Previous studieshave reported a large variability in outcome whichmay be due to operative technique dependent on orinfluenced by the design of the investigation. Thisstudy is prospective and overcomes these problems.There are difficulties in statistical analysis, however,and it would be inaccurate to derive any furtherstatistical significance than those figures presentedin the results section. As there was a large number ofnormal outcomes, the data are skewed; however, theresults do show that panoramic radiography is usefulin predicting a normal outcome from surgery.

Operating surgeons would be aware of the lowincidence of neurological damage and so their

Australian Dental Journal 1997;42:3. 151

Table 6. Data and statistical analysisPredictions Correct %

Sensitivity 72Specificity 90.5

Fig. 1. – Mandibular third molar with root perforated by IAN. Thetooth was sectioned during surgery to preserve the contents of theneurovascular bundle and the tooth has been repaired with acrylic for

this illustration.

Table 7. Data distribution comparing prediction with actual outcomeNo change Hypoaesthesia Hyperaesthesia Para/Dysaesthesia Anaesthesia Total

No change 411 6 1 418Hypoaesthesia 31 9 1 5 46Hypaesthesia 1 1Para/Dysaesthesia 7 2 1 10Anaesthesia 4 4

Total 454 17 1 6 1 479

estimations might be biased towards a prediction ofnormal outcome. Patients who failed to attend forfollow-up might be assumed to have a normaloutcome, but it was felt that in terms of accuracythese patients should be excluded from the study.

It is clear that panoramic radiography, beingreadily available and relatively low in radiation dose,provides the optimum method of predicting neuro-logical damage. At present coronal computerizedtomographic scans are the only way to image, withprecision, the relationship of the tooth root to theIAN. Radiation dosage and cost to the communitypreclude this investigation as a routine, particularlyas the outcome of neurological deficit does not justnecessarily depend on direct contact of IAN withtooth root. Surgical techniques of bone crushing orthe use of neurotoxic materials during surgery maycause damage to the IAN even if it is distant fromtooth root. This enhances the unpredictable natureof the certainty of outcome in mandibular thirdmolar surgery.

ConclusionDespite many studies and reports in the literature

there is still debate about the aetiology, incidenceand outcome of neurological damage during thirdmolar surgery. This study confirms the previouslyreported prevalence rates.

In order to ensure adequate informed consentprior to the removal of mandibular third molar teeththe patient should be educated in the risks andbenefits of surgery. It has been shown by this studythat the panoramic radiograph is a valuable but by

no means infallible guide to the prediction of asuccessful outcome with no neurological deficit insensation to the lower lip.

AcknowledgementsThanks are due to the Statistics Department of

the University of Melbourne and the Common-wealth Scientific and Industrial ResearchOrganization for valuable advice in the design of thisstudy.

References01. Main J. Further roentgenographic study of mandibular third

molars. J Am Dent Assoc 1938;25:1993.

02. Howe GL, Poyton HG. Prevention of damage to the inferiordental nerve during the extraction of mandibular third molars.Br Dent J 1960;109:355-63.

03. Merrill RG. Prevention, treatment and prognosis for nerve injuryrelated to the difficult impaction. Dent Clin North Am1979;23:471-3.

04. Rood JP, Nooraldeen Shehab BAA. The radiological predictionof inferior alveolar nerve injury during third molar surgery. Br JOral Maxillofac Surg 1990;28:20-5.

05. Merrill RG. Decompression for inferior alveolar nerve injury. JOral Surg 1964;22:291-30.

06. Klineberg I. Craniomandibular disorders and orofacial pain:Diagnosis and management. London: Wright, 1991:144-9.

07. Kipp DP, Goldstein BH, Weiss WW Jr. Dysesthesia aftermandibular third molar surgery: a retrospective study andanalysis of 1377 surgical procedures. J Am Dent Assoc1980;100:185-90.

08. Rud J. Third molar surgery: relationship of root to mandibularcanal and injuries to inferior dental nerve. Danish Dent J1983;87:619-31.

09. Carmichael FA, McGowan DA. Incidence of nerve damagefollowing third molar removal: a West Scotland oral surgeryresearch group study. Br J Oral Surg 1992-30:78-82.

Address for correspondence/reprints:A. C. Smith,

School of Dental Science,The University of Melbourne,

711 Elizabeth Street,Melbourne, Victoria 3000.

Table 8. Data and statistical analysis (3)Kappa value A BKappa 0.27 -0.06Upper confidence interval 0.45 0.10Lower confidence interval 0.10 -0.17

A=Agreement between prediction and postoperative assessment includingconfidence intervals.B=Agreement between first trial and second trial on panoramic radiographsincluding confidence intervals.

152 Australian Dental Journal 1997;42:3.