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CLINICAL ARTICLES

J Oral Maxillofac Surg61:417-421, 2003

Panoramic Radiographic Risk Factors for

Inferior Alveolar Nerve Injury After ThirdMolar Extraction

Bart F. Blaeser, DMD, MD,* Meredith A. August, DMD, MD,

R. Bruce Donoff, DMD, MD, Leonard B. Kaban, DMD, MD,

and Thomas B. Dodson, DMD, MPH

Purpose: The purpose of this study was to estimate the association between specific pan-oramic radiographic signs and inferior alveolar nerve (IAN) injury during mandibular third molar

removal.

Patients and Methods: A case-control study design was used; the sample consisted of patientswho underwent removal of impacted mandibular third molars. Cases were defined as patients with

confirmed IAN injury after third molar extraction, whereas controls were defined as patients without

nerve injury. Five surgeons, who were blinded to injury status, independently assessed the preop-

erative panoramic radiographs for the presence of high-risk radiographic signs. Bivariate analyses

were completed to assess the relationship between radiographic findings and IAN injury. The

sensitivity, specificity, and positive and negative predictive values were computed for each radio-

graphic sign.

Results: The sample was composed of 8 cases and 17 controls. Positive radiographic signs werestatistically associated with an IAN injury (P .0001). The presence of radiographic sign(s) had positive

predictive values that ranged from 1.4% to 2.7%, representing a 40% or greater increase over the baseline

likelihood of injury (1%) for the individual patient. Absence of these radiographic findings had a strong

negative (99%) predictive value.

Conclusions: This study confirms previous analyses showing that panoramic findings of diver-sion of the inferior alveolar canal, darkening of the third molar root, and interruption of the cortical

white line are statistically associated with IAN injury. Based on the estimated predictive values, the

absence of positive radiographic findings was associated with a minimal risk of nerve injury,

whereas, the presence of one or more of these findings was associated with an increased risk for

nerve injury.

2003 American Association of Oral and Maxillofacial Surgeons

J Oral Maxillofac Surg 61:417-421 , 2003

Received from the Department of Oral and Maxillofacial Surgery,

Massachusetts General Hospital, Harvard School of Dental Medi-

cine, Boston, MA.

*Clinical Associate.

Assistant Professor.

Professor of Oral and Maxillofacial Surgery and Dean, Harvard

School of Dental Medicine.

Walter C. Guralnick Professor and Chairman.

Associate Professor and Director of Resident Training.

Presented as an abstract at the 78th General Session of the

International Association for Dental Research, Washington, DC,

April 2000.

This project was funded by the Department of Oral and Maxil-

lofacial Surgery Research Fund, Massachusetts General Hospital

(B.F.B., M.A.A., R.B.D., and L.B.K.) and the Mid-Career Investigators

in Patient-Oriented Research award from the NIDCR (K24-

DE00448 (T.B.D.).

Address correspondence and reprint requests to Dr Dodson:

Department of Oral and Maxillofacial Surgery, Massachusetts Gen-

eral Hospital, 55 Fruit St, Warren 1201, Boston, MA 02114; e-mail:

tdodson@partners.org

2003 American Association of Oral and Maxillofacial Surgeons

0278-2391/03/6104-0002$30.00/0

doi:10.1053/joms.2003.50088

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A well-recognized, but uncommon, serious complica-

tion of mandibular third molar extraction is injury to

the inferior alveolar nerve (IAN).1-6 The overall risk of

IAN injury associated with third molar extraction

ranges from 0.5% to 5%. In most cases, the injured

nerve recovers spontaneously. The reported rate of

permanent IAN injury is less than 1%.1 Although the

risk of permanent nerve injury is low, for affected

patients there is considerable dissatisfaction and mor-

bidity.

The risk of IAN injury after third molar extraction is

a function of the anatomic relationship between the

nerve and the tooth. Panoramic imaging is the stan-

dard radiologic examination to evaluate the anatomic

relationship of third molars and the IAN.2 Several

authors have identified high-risk radiologic signs asso-

ciated with an intimate, anatomic relationship be-

tween third molars and the IAN. These radiologic

signs include 1) darkening of the root where it crosses

the inferior alveolar canal, 2) deflected or hooked

roots around the inferior alveolar canal, 3) narrowing

of the root implying perforation or grooving by the

nerve, 4) a bifid root apex representing intimacy of

the apical periodontal membrane, 5) interruption or

obliteration of either of the radiopaque white (corti-

cal) lines of the inferior alveolar canal, 6) diversion or

bending of the inferior alveolar canal in the region of

the root apices, and 7) narrowing of the inferior

alveolar canal.3-6 Of these 7 signs, Rood and

Nooraldeen Shehab7 identified 3 (radiologic signs 1,

5, and 6) that were significantly associated with IAN

injury. Although the panoramic radiograph is a usefulscreening tool for assessing the anatomic relationship

between third molars and the IAN, it is imperfect. In

Rood and Nooraldeen Shehabs study, the reported

sensitivity for these 3 signs ranged from 24% to 38%,

and the reported specificity ranged from 96% to 98%.

As an oral and maxillofacial surgery group working

in a referral center for the management of IAN inju-

ries, we have the opportunity to document the pres-

ence and nature of such injuries at the time of repar-

ative surgery. In this study, we correlated operative

findings with preoperative radiographic signs to eval-

uate their predictive value in a sample of patients withconfirmed injury to the IAN. Given the variability of

the sensitivity and specificity estimates in the litera-

ture, the purpose of this study was to measure the

association between radiographic signs, noted on

panoramic imaging, and documented IAN injury by

limiting the sample to patients with confirmed nerve

injury. We believe this to be the first study to use a

sample composed of patients with operatively con-

firmed IAN injury to assess the relationship between

panoramic radiographic findings and risk for IAN in-

jury.

Patients and Methods

STUDY DESIGN/SAMPLE

This was a retrospective case-control study com-posed of patients who had 2 impacted mandibularthird molars removed. Cases were defined as patients

with a documented IAN injury, manifested by com-

plaints of persistent sensory deficit or dysesthesiaafter third molar removal, who underwent operativeexploration of the nerve. At the time of exploratorysurgery, the injury was confirmed. The case-controlstudy design is specifically targeted to address situa-tions where the outcome of interest, that is, IANinjury, is a rare event. Controls were defined as pa-tients who had third molars extracted but did notcomplain of a postoperative IAN injury and the neg-

ative complaint was confirmed by a normal neurosen-sory examination. The control patients were ran-domly derived from a population of patients treated in

the Oral and Maxillofacial Surgery Unit, MassachusettsGeneral Hospital, Boston, MA. Data were collected byreviewing charts and panoramic radiographs. The In-stitutional Review Board approved the project.

STUDY VARIABLES

The predictor variable was the presence or absenceof 1 or more preoperative panoramic radiographicfindings: 1) diversion or bending of the canal, 2)darkening of the tooth root, and 3) interruption of thecortical white line of the canal. These 3 radiographic

signs were selected because they have been shown tobe associated with IAN injury.7

Five surgeons who were blinded to the IAN injurystatus independently reviewed the preoperative pan-oramic radiographs and assessed the images for thepresence of the high-risk radiographic signs. The sur-geons also evaluated third molar angulation and thelevel of third molar impaction. Angulation was de-

fined as mesioangular, distoangular, horizontal or ver-

tical. Level of impaction was classified using the Pelland Gregory system.8 The outcome variable was pres-ence (case) or absence (control) of an IAN injury. Inaddition, we abstracted age, gender, and indicationsfor extraction from the patients record.

DATA ANALYSES

A database was constructed using Microsoft Access(Redmond, WA). Data analyses were completed usingSPSS version 8.0 (SPSS, Inc, Chicago, IL). Descriptivestatistics of the sample were computed. Bivariate anal-

yses were used to assess the relationship among pan-oramic signs, IAN injury, and variability of individual

clinicians radiographic interpretations. The sensitivityand specificity of each radiographic sign were com-puted. IfP .05, the results were considered statis-tically significant.

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Results

The study sample was composed of 25 patients, ofwhom 8 had documented evidence of IAN injury(cases). The remaining 17 patients were classified ascontrols. The mean ages of the case and control sam-ples were 34 and 27 years, respectively. Both case and

control samples were predominantly composed of

females: 88% and 65%, respectively. There was nostatistically significant difference between the casesand controls in terms of age or gender. In all of thecases, at operation, there were 1 or more findingsconsistent with IAN injury: nerve compression (n 2), neuroma formation (n 5), and partial (n 2) ortotal (n 1) IAN transection.

The 5 surgeons independently evaluated 2 third

molars for each of the 25 patients, resulting in a totalof 250 third molar observations. There were 40 ob-servations of third molars associated with IAN injuryand 210 observations of third molars not associated

with IAN injury. The clinicians evaluations of theradiographs were consistent as evidenced by the ab-sence of a statistically significant difference in classi-fication of third molar angulation, level of impaction,or identification of the high-risk radiographic signs(P .31). Neither angulation nor level of impaction

was associated with IAN injury (P .72).

Table 1 summarizes the association of high-riskradiographic signs and IAN injury. Consistent with theresults of Rood and Nooraldeen Shehab,7 diversion ofthe canal, darkening of the root, and interruption ofthe white line were each statistically associated withIAN injury (P .0001). The sensitivity and specificityof the radiographic signs, however, were variable.Depending on the radiographic sign, the sensitivityranged from 50% to 80%, and the specificity ranged

from 52% to 82% (Table 2). The sensitivity and spec-ificity for one or more positive findings were 100%and 33%, respectively.

Discussion

IAN injury after third molar extraction is a well-recognized complication with a reported prevalenceranging from 0.5% to 5%.9,10 Various contributing fac-tors, including age, history of infection, use of rotaryinstruments, use of concomitant general anesthesia,and level of impaction, have been reported.11-13 Themost important factor contributing to IAN injury maybe the anatomic proximity of the IAN to the third

molar root. This anatomic intimacy is well known by

oral and maxillofacial surgeons; however, direct con-tact is only rarely and anecdotally discussed. At theMassachusetts General Hospital, many patients (1 to 3per month) undergo operative treatment for postex-traction nerve injuries. As such, the existence of ananatomic injury to the inferior alveolar or lingualnerves can be definitively documented by direct ex-amination at the time of surgery. Given this unique

opportunity, the authors thought it was an excellentopportunity to reevaluate the usefulness of panoramicimaging to establish the relationship between the IANcanal and third molars and the risk for associated

Table 1. RELATIONSHIP OF RADIOGRAPHICFINDINGS TO IAN INJURY

IAN Injury

Yes No Totals

Diversion of the canal

Yes 20 38 58No 20 172 193Total 40 210 250

Darkening of rootYes 26 56 82No 14 154 192Total 40 210 250

Interruption of cortical lineYes 32 96 128No 8 114 122Total 40 210 250

Any positive radiographicfindingYes 40 140 180No 0 70 70Total 40 210 250

NOTE. For each category, the difference between presence andabsence of IAN injury was significant (P .0001).

Abbreviation: IAN, inferior alveolar nerve.

Table 2. ESTIMATES OF SENSITIVITY, SPECIFICITY, AND PPF AND NPV

RadiographicFinding

Sensitivity(%)

Specificity(%)

16% Prevalence ofIAN Injury

1% Prevalence ofIAN Injury

PPV

(%)

NPV

(%)

PPV

(%)

NPV

(%)

Diversion 50 82 34 89 2.7 99Darkening 65 73 31 93 2.3 99Interruption 80 54 25 93 1.7 99Anyfinding 100 33 22 100 1.4 100

Abbreviations: IAN, inferior alveolar nerve; PPV, positive predictive value; NPV, negative predictive value.

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nerve injury. Needless to say, this rigorous definitionfor nerve injury limited the number of patients eligi-ble for inclusion as cases. In the setting where theoutcome variable of interest is rare, such as IAN in-

jury, the case-control study design is indicated.In this study, third molar angulation was not asso-

ciated with risk for IAN injury. This finding contrasts

with other reports in which third molar angulationwas associated with IAN injury.6,14 Given this studyssmall sample size, we caution against weighting thenegativefindings heavily because there is a significantrisk for type II errors (reporting no difference when adifference exists).

Currently, panoramic radiographic imaging is thestandard technique for assessing the risk of IAN injuryafter third molar extraction.15-17We hypothesized that

by limiting the sample to patients with confirmed IANinjury, we would identify radiographic signs associ-ated with nerve injuries with a high level of sensitivity

and specificity. The results of this study showed thatthe high-risk radiographic signs had fair to poor sen-sitivity and specificity. These findings contrast withthe study reported by Rood and Nooraldeen Shehab,7

in which the high-risk radiographic signs had weaksensitivity but high specificity. The discrepancy in the

results may be due to the difference in case selection.In Rood and Nooraldeen Shehabs7 study, a case wasdefined as any IAN injury after third molar removal. Inthe current study, a case was limited to those patientsin whom the surgeon visualized nerve injury at thetime of nerve exploration and repair.

Despite the numerical differences between the cur-rent study and that of Rood and Nooraldeen Shehab,7

the clinical implications are similar. In both studies,

the radiographic signs were associated with nerveinjury with high statistical significance. However,

when the surgeon applies these findings in a clinicalsetting, it is important to estimate the overall predic-tive value of the diagnostic test. Positive predictive

value (PPV) is the probability that the disease ispresent when the test or sign is present. In this case,PPV is the probability that an IAN injury will occur

when a high-risk radiographic sign is present. Nega-

tive predictive value (NPV) is the probability that the

disease is absent if the sign is absent. Specifically forthis study, the NPV is the probability that an IANinjury will not occur if high-risk radiographic signs areabsent.

The predictive value of any diagnostic test or sign isa function of the sensitivity and specificity of the testor sign and the underlying prevalence of the diseaseor disorder. In this study, the prevalence of IAN injury

(16%) was artificially elevated due to the selectioncriteria used. Given the sensitivity and specificity inthis study and an IAN injury prevalence of 16%, thePPV of the various radiographic signs ranged from

25% to 34%. The NPV ranged from 89% to 93%. If we

decrease the prevalence of IAN injury after third mo-lar removal to 1%, the PPV and NPV change dramati-cally. The PPV ranges from 1.4% to 2.7%, and the NPV

ranges from 99% to 100% (Table 2).Table 3 summarizes the sensitivity and specificity

for the various high-risk signs reported by Rood andNooraldeen Shehab7 and estimates the PPV and NPVassuming a 1% rate of IAN injury. The PPVs rangefrom 6% to 12%, and the NPV is 99%. In the setting of

an unlikely event such as IAN injury, the presence ofa positive radiographicfinding suggests that there is aslight increase in the absolute risk for injury. Specifi-cally, given a positive radiographic finding, the riskincreases from 1% (background prevalence) to 1.7%to 12% based on the results of our data and those of

Rood and Nooraldeen Shehab.7

Conversely, the ab-sence of positive radiographic findings suggests thatthe risk for IAN injury is remote (1%).

In summary, a patient with 1 or more high-riskradiographicfindings has a significantly increased risk(70% to 1,200%) for nerve injury, although overall thisis still an uncommon event. Conversely, in the ab-sence of any positivefindings, the risk for nerve injuryis negligible. In the absence of high-risk radiographicsigns, there is little preventive value in obtaining ad-ditional imaging. However, when 1 or more high-riskradiographic signs are present, additional imaging,

such as computed tomographic imaging in the axial,

coronal, and sagittal planes, may be indicated to bet-ter establish the anatomic relationship between theIAN canal and third molars.18-20 More accurate riskassessment may help the surgeon and patient in mak-ing the decision for or against removal of the tooth inquestion and in discussing alternative managementstrategies.21-23

The results of this study confirm other reports of a

statistically significant association between specificpanoramic radiographic signs and the risk for IANinjury during third molar removal. To apply thesefindings in a clinical setting, one should consider the

Table 3. ESTIMATES OF PPV AND NPV BASED ONROOD AND NOORALDEEN SHEHAB7

Radiographicfinding Sensitivity(%) Specificity(%)

1%Prevalence

of IANInjury

PPV(%)

NPV(%)

Diversion 29 98 12 99Darkening 38 95 7 99Interruption 24 96 6 99

Abbreviations: IAN, inferior alveolar nerve; PPV, positive predic-tive value; NPV, negative predictive value.

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PPV and NPV of these radiographic signs. The resultsof this study indicate that in the absence of predictiveradiographic signs, the risk of IAN injury is negligible.In the presence of a high-risk radiographic finding,however, additional patient assessment may be indi-cated.

Acknowledgments

We would like to thank Drs Edward Seldin, Walter C. Guralnick,Kenneth A. MacAfee, II, John A. Buehler, and Carole A. Lorente for

volunteering to review the radiographs.

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