Clinical Paper

Oral Surgery

Int. J. Oral Maxillofac. Surg. 2009; 38: 964–971doi:10.1016/j.ijom.2009.06.007, available online at http://www.sciencedirect.com

Position of the impacted thirdmolar in relation to themandibular canal. Diagnosticaccuracy of cone beamcomputed tomographycompared with panoramicradiographyH. Ghaeminia, G. J. Meijer, A. Soehardi, W. A. Borstlap, J. Mulder, S. J. Berge:Position of the impacted third molar in relation to the mandibular canal. Diagnosticaccuracy of cone beam computed tomography compared with panoramicradiography. Int. J. Oral Maxillofac. Surg. 2009; 38: 964–971. # 2009 InternationalAssociation of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rightsreserved.

Abstract. This study investigated the diagnostic accuracy of cone beam computedtomography (CBCT) compared to panoramic radiography in determining theanatomical position of the impacted third molar in relation with the mandibularcanal. The study sample comprised 53 third molars from 40 patients with anincreased risk of inferior alveolar nerve (IAN) injury. The panoramic and CBCTfeatures (predictive variables) were correlated with IAN exposure and injury(outcome variables). Sensitivity and specificity of modalities in predicting IANexposure were compared. The IAN was exposed in 23 cases during third molarremoval and injury occurred in 5 patients. No significant difference in sensitivityand specificity was found between both modalities in predicting IAN exposure. Todate, lingual position of the mandibular canal was significantly associated with IANinjury. CBCT was not more accurate at predicting IAN exposure during third molarremoval, however, did elucidate the 3D relationship of the third molar root to themandibular canal; the coronal sections allowed a bucco-lingual appreciation of themandibular canal to identify cases in which a lingually placed IAN is at risk duringsurgery. This observation dictates the surgical approach how to remove the thirdmolar, so the IAN will not be subjected to pressure.

0901-5027/090964 + 08 $36.00/0 # 2009 International Association of Oral and Maxillofacial Surge

H. Ghaeminia, G. J. Meijer,A. Soehardi, W. A. Borstlap,J. Mulder, S. J. BergeDepartment of Oral and Maxillofacial Surgery,Radboud University Nijmegen MedicalCentre, Nijmegen, The Netherlands

Keywords: diagnostic accuracy; cone beamCT; third molar surgery; panoramic radiogra-phy; inferior alveolar nerve injury; third molarposition.

Accepted for publication 1 June 2009Available online 28 July 2009

ons. Published by Elsevier Ltd. All rights reserved.

Position of impacted third molar in relation to mandibular canal 965

Table 1. i-CATTM 3-D imaging system specifications for mandibular scan.

X-ray source High frequency, constant potential,fixed anode 120 kVp, 3–8 mA (pulse mode)

X-ray beam Cone-beamFocal spot 0.5 mmField of view 6 cmImage detector Amorphous silicon flat panel 20 cm * 25 cmVoxel size 0.25 mmGray scale 14 bitScan time 20 sRadiation dose 32 mSv

Damage to the inferior alveolar nerve(IAN) is a serious complication followingthird molar removal. The overall risk oftemporary IAN injury associated withthird molar removal ranges from 0.4%to 6%4,26. The reported rate of permanentIAN injury, in which the sensory impair-ment lasts longer than 6 months, is lessthan 1%4,26. The overall risk of permanentimpairment during third molar removal islow, but a significant number of patientsare affected because many third molars areremoved.

The most evident risk factor for injury ofthe IAN is the proximity of the root of thethird molar to the mandibular canal4,25,26.When a close relationship between the thirdmolar and the mandibular canal is observedradiographically, the risk of temporary IANinjury increases6,7. It is important to assessthe position, and establish the relationship,of the third molar with the mandibular canalpreoperatively to minimize the risk of nerveinjury. Panoramic radiography is the stan-dard diagnostic tool for this purpose. Clin-icians use various radiographic markers toindicate a close relationship between thethird molar and the mandibular canal7. Ifthe radiological marker on the panoramicradiograph indicates there is a close rela-tionship between the third molar and themandibular canal, additional investigationusing computed tomography (CT) may berecommended to verify the relationship in athree-dimensional (3D) view10,14,18. Thedrawbacks of CT are the higher radiationdose22 and increased financial costs com-pared with panoramic imaging.

Cone beam computed tomography(CBCT) has been introduced to improveconventional CT, because it reduces theradiation dose9, offers high spatial resolu-tion1 and decreases costs. CBCT providesbetter image quality of teeth and theirsurrounding structures compared withconventional CT5,8. CBCT seems to bea more accurate imaging modality fordetermining the relationship of the thirdmolar to the mandibular canal. To justifythe application of CBCT in the preopera-tive assessment of impacted third molars,it is necessary to assess whether it givesthe practitioner a more detailed insightinto the anatomical relationship of thethird molar and the mandibular canal thanconventional imaging techniques. CBCTis a relatively new imaging technique sothere is little literature available concern-ing its diagnostic value. This study aims toinvestigate the potential benefits of CBCTby comparing the diagnostic accuracy ofCBCT and panoramic radiography in pre-dicting IAN exposure and evaluating thereliability of CBCT in determining the

bucco-lingual position of the third molarin relation to the mandibular canal.

Materials and Methods:

Study sample/design

This is a prospective study of consecutivepatients who consulted the department ofOral and Maxillofacial Surgery, for man-dibular third molar removal between Feb-ruary 2007 and September 2007. A poweranalysis was performed based on dataobtained from the literature23,24.

Patients thought to have a close rela-tionship between the mandibular canal andone or both mandibular third molars, diag-nosed from digital panoramic radiographs,underwent additional CBCT imaging. 42patients, with 56 impacted mandibularthird molars (22 women and 20 men) wereenrolled in this study. Patients with radi-ological evidence of a cyst and those forwhom the time interval between imagingand third molar removal exceeded 6months, were excluded from the study.All were informed of possible complica-tions following removal of the third molarand written informed consent wasobtained from all patients.

System specifications

Digital panoramic radiographs were takenwith a Soredex Cranex Tome device (Sor-edex, Helsinki, Finland), operated at81 kV and 10 mA using a photostimulablephosphor plate. The CBCT mandibularscan was acquired using i-CATTM 3-DImaging System (Imaging Sciences Inter-national Inc, Hatfield, PA, USA). Thescanner specifications are listed in Table 1.

Clinical evaluation

All third molars were removed under localanaesthesia by 2 senior oral and maxillo-facial surgeons who had at least 15 years’experience of the procedure. After raisingthe mucoperiosteal flap, with a burr buc-cally and distally, bone was removed. Ifnecessary, the tooth was sectioned one or

more times. Postoperatively, after rinsingand irrigating, the extraction sites wereexamined to monitor if the IAN was visible.

Postoperative surveillance

The patients had a postoperative reviewappointment 2 weeks after surgery. Neu-rosensory disturbances of the lip and chinwere assessed by measuring the functionof the IAN with light touch sensation(large nerve fibres), using Semmes Wein-stein (SW) monofilaments nr. 1.65, 2.83and 3.22, and thermal discrimination(small nerve fibres), by applying an alu-minum rod (cold) and a Perspex rod (dia-meter 4 mm).

A test procedure using two alternativechoices was used, as described by van derGLAS et al.3 The contralateral halves of thelip and chin were taken as control site. Thearea with impaired sensation was drawn onthe skin and recorded photographically.Patients with altered sensation returned 3and 6 months postoperatively and theirrecovery pattern was noted. Patients whorecovered fully within 6 months weredefined as suffering from temporary IANinjury. Altered sensations lasting longerthan 6 months were scored as permanentIAN injury. The neurosensory testing of allpatients was carried out by one investigator.

Evaluation of images

In a darkened room, the CBCT andpanoramic images were shown in a ran-dom order on a 17 inch PC monitor.Evaluation was carried out by two trainedoral and maxillofacial surgeons, but notthose who had surgically removed thewisdom teeth. Both were experienced indiagnosing maxillofacial structures andfamiliar with both imaging modalities.They were blinded for the clinical out-come.

Panoramic radiographs were scored forthe presence or absence of the followingradiographic signs, all of which had beenreported to be suggestive of a close rela-tionship between the mandibular canal andthe third molar:21,23 interruption of the

966 Ghaeminia et al.

Fig. 1. Classification of the position and relationship of the third molar root to the mandibular canal, as seen on CBCT images.

white line of the mandibular canal wall;darkening of the root; diversion of themandibular canal; narrowing of the man-dibular canal; narrowing of the roots; anddeflection of the roots. Using these marks,the investigators aimed to find the optimaldiagnostic criteria for predicting IANexposure from panoramic radiographs.

The CBCT images were assessedthrough the i-CAT Vision1 software pro-gram. The implant planning screen and themultiplanar reconstruction (MPR) screenwere used to scroll through the axial,sagittal and coronal planes. The slicethickness was 1 mm. The images wereevaluated in all three dimensions to estab-lish if the cortical layer of the mandibularcanal between the third molar and IANwas still intact. The position of the man-dibular canal with respect to the thirdmolar was classified as lingual, buccal,interradicular or inferior (Fig. 1).

Statistical analysis

The panoramic and CBCT features (pre-dictive variables) were correlated withthe intra-operative finding of IAN expo-sure and the postoperative occurrence ofIAN injury (outcome variables). The X2

and Fisher’s exact test were used to assessthe association between the predictor andoutcome variables. Sensitivity, specifi-city, positive predictive value, negativepredictive value and accuracy of eachimaging modality in predicting IANexposure were calculated. The differencebetween the sensitivity and specificity ofpanoramic radiography and CBCT weretested with a X2 test. Probability values

less than 0.05 were considered statisti-cally significant.

Possible predictors of radiographicsigns for IAN exposure were tested withX2. After selection of significant predic-tors a stepwise logistic regression wascarried out to obtain the effect after cor-rection for the other predictors.

To judge the inter-observer agreementKappa (k) values were calculated. A k

value <0.40 was considered poor agree-ment, 0.40–0.59 was fair agreement, 0.60–0.74 was good agreement and 0.75–1.00was excellent agreement.

All statistical analyses were performedusing the SAS program (SAS InstituteInc., Cary, NC, USA), version 9.1.

Results

3 of the 56 impacted third molars wereexcluded because digital panoramic radio-graphs were not available. The study sam-ple consisted of 53 impacted third molarsfrom 40 patients (20 women and 20 men)with an average age of 27.6 years (rangingfrom 20 to 62 years).

Following removal of the 53 mandib-ular third molars, the IAN was exposedin 23 cases (43%). Based on neurosen-sory testing, temporary IAN injuryoccurred in 5 patients (9%). In 4 of these5 patients the inferior alveolar nerve wasnoted as exposed following the extrac-tion. The frequency of temporary IANinjury after visualization of the IANwas 17%. After 6 months, 3 patients(6%) continued to have some sensoryimpairment, although in one patientthis could not be verified using light

touch sensation and thermal discrimina-tion.

No significant correlation was observedbetween IAN exposure and postoperativesensory impairment compared with gen-der, site of extraction or angulation of thethird molar.

The inter-observer agreement forCBCT, represented as the k-value in theassessment of the bucco-lingual positionof the mandibular canal, was 0.80. The k-value in the assessment of contact betweenthe third molar root and the mandibularcanal on CBCT images was 0.78. Owingto these excellent inter-observer agree-ments, the results achieved from oneobserver were used for further analysis.

In the assessment of panoramic radio-graphs the agreement was poor: k-valueranged from 0.35 (darkening of the root) to0.52 (interruption of the white line).Owing to this poor agreement the panora-mic images were assessed again by bothobservers and consensus was reached bydiscussion. The results obtained from theconsensus were used for further analyses.

Three of the panoramic radiographicsigns were statistically associated withIAN exposure: interruption of the whiteline, darkening of the root, and diversionof the mandibular canal (Table 2). Step-wise logistic regression analysis of thesepredictor variables was performed andonly one radiographic sign, darkening ofthe tooth root, was taken into the modeland showed a significant association withIAN exposure (P=0.007) with an odds-ratio of 0.204 (95% CI 0.062–0.672).

As determined on CBCT images, themandibular canal was positioned lingual

Position of impacted third molar in relation to mandibular canal 967

Table 2. Sensitivity, specificity and predictive values of the radiographic signs on panoramicradiographs.

Panoramic radiographic signs Sensitivity Specificity PPV NPV P value

Interruption of the white line 1.0 0.17 0.48 1.0 0.040Darkening of the roots 0.74 0.63 0.61 0.76 0.007Diversion of the canal 0.22 0.97 0.83 0.62 0.036Narrowing of the canal 0.13 0.87 0.43 0.57 0.975Narrowing of the roots –Deflection of the roots 0 0.93 0 0.95 0.207

PPV, positive predictive value; NPV, negative predictive value.

Table 3. Relationship between the buccolingual position of the mandibular canal with IANexposure during third molar removal and postoperative sensory disturbances.

Position CBCT Contact CBCT IAN visibleIAN

not visiblePostoperative

sensory impairment

Lingual * 26 26 15 11 5*

Interradicular 8 8 5 3 0Buccal 9 7 1 8 0Inferior 10 4 2 8 0Total 53 45 23 30 5

* p< 0.05.

to the third molar in 49% of cases, 17%were buccal, 19% inferior and 15% inter-radicular. The rates of IAN exposure andIAN injury following third molar removalwere significantly correlated with the posi-tion of the mandibular canal as seen onCBCT images (Table 3). The IAN wasmore frequently exposed following thirdmolar extraction when the mandibularcanal was situated lingually than in abuccal position (P <0.02). In all patientswith postoperative sensory impairmentsthe mandibular canal was positioned lin-gual to the third molar roots as seen onCBCT images (P<0.02).

In 48 cases in which one or both whitelines of the mandibular canal were inter-rupted, as scored on panoramic radio-graphs, 42 (88%)showed contactbetween the third molar roots and themandibular canal on CBCT.

The diagnostic accuracy of panoramicradiography and CBCT in predicting IANexposure is given in Table 4. No signifi-cant differences in sensitivity and specifi-city between the cone beam CT andpanoramic radiography in predictinginferior alveolar nerve exposure wereseen. As example, two cases are shownin Figs. 2 and 3.

Discussion

A well-described risk factor that is sig-nificantly correlated with IAN injury fol-

Table 4. Diagnostic accuracy of CBCT images

TP TN

Panoramic radiography 23 1CBCT 22 7

TP, true positive; TN, true negative; FN, false n

lowing third molar removal is exposure ofthe neurovascular bundle during extrac-tion6,7,24,25. In this study postoperativesensory disturbances occurred in 17% ofcases of IAN exposure during removal,which is in agreement with other stu-dies4,23,24,25. An accurate preoperativeprediction of IAN exposure is importantto determine the risk of IAN injury. Thisinformation can be helpful in decidingwhether to remove a symptomless thirdmolar and can be used to obtain correctinformed consent.

The panoramic radiograph is the stan-dard diagnostic tool in the preoperativeassessment of mandibular third molars andtheir relationship with the mandibularcanal. Clinical studies have identifiedradiographic signs on panoramic radio-graphs that indicate a high risk of IANexposure or IAN injury following thirdmolar removal. In this study, the panora-mic signs, interruption of the white line,darkening of the root and deviation of themandibular canal were significantly asso-ciated with IAN exposure. This results arein agreement with a study by ROOD andSHEHAB

21, who analysed the association ofpanoramic signs of 1560 third molars andIAN injury. In most cases a combinationof these signs is present on the panoramicradiograph, so the authors performed alogistic regression analysis to obtain theoptimal independent radiographic signthat could predict IAN exposure. Only

and panoramic radiographs in predicting IAN e

FN FP Sensitivity Specifi

0 29 1,0 0,01 23 0,96 0,2

egative; FP, false positive; PPV, positive predic

one radiographic sign, darkening of theroot, was significantly associated withIAN exposure. This result corroboratesother reports, that darkening of the rootis one of the most significant radiographicsigns in predicting IAN exposure23, andIAN injury6,21.

The usefulness of CBCT has beendescribed in endodontology19, implantol-ogy15, periodontology13 and oral sur-gery17, but few systematic validationstudies are available. One study hasreported the diagnostic accuracy of CBCTin predicting IAN exposure followingthird molar removal24.

TANTANAPORNKUL et al.24 concluded thatthe 3DX CBCT (Morita Corp.) was sig-nificantly more accurate compared withpanoramic radiography in predicting IANexposure during third molar removal witha sensitivity of 93% and a specificity of77%. In the present study, a comparablehigh sensitivity (96%), but a lower speci-ficity (23%) for the i-CAT CBCT wasscored. Owing to the low specificity, nosignificant difference in the diagnosticaccuracy between the i-CAT CBCT andpanoramic radiography in the predictionof IAN exposure was measured. Theauthors found the same positive predictivevalue (0.49) for the i-CAT CBCT as TAN-

TANAPORNKUL et al. for the 3DX CBCT.This means that in the absence of corticalbone between the mandibular canal andthe third molar root as seen on CBCTimages, the IAN was visible during extrac-tion in almost half of the cases. The nega-tive predictive value for CBCT found inthis study (0.88) was also comparable withthat found in the study of TANTANAPORNKUL

et al. (0.90). The only factor that couldexplain the lower specificity is the rela-tively high prevalence of positive testresults and the low prevalence of negativetest results in this study, due to the morestrict selection criteria. In the presentstudy sample, 98% of cases had one ormore radiographic signs on the panoramicradiograph that suggested a close relation-ship between the third molar root and themandibular canal and 85% of the casesshowed contact on the CBCT. In the studyof TANTANAPORNKUL et al. these valueswere 44% and 36%, respectively. In casesin which one or more signs of an intimaterelationship between the mandibular canaland the third molar roots is present on

xposure during third molar removal.

city PPV NPV Accuracy

3 0,44 1,0 0,453 0,49 0,88 0,55

tive value; NPV, negative predictive value.

968 Ghaeminia et al.

Fig. 2. Panoramic radiograph showing darkening of the roots and interruption of the white lines of the mandibular canal (A). Transverse (B) andcoronal (C) CBCT images show a flattened mandibular canal between the root and buccal cortex with disappearance of the cortical layer of themandibular canal. The IAN was exposed during removal as anticipated from the CBCT images.

panoramic radiographs, the CBCT is notsignificantly more accurate in predictingIAN exposure compared with panoramicradiographs. This is mainly because if thesesigns are present on the panoramic radio-graphs, the third molar root is in contactwith the mandibular canal on the CBCTimages as well. The association of one ofthese radiographic signs, interruption of thewhite line, with CBCT images is confirmedin a study by NAKAGAWA et al.16 Theyconcluded that in 86% of cases in whichthe superior white line of the mandibularcanal was interrupted on panoramic radio-graphs, theCBCTimages (PSR 9000, AsahiRoentgen) also showed contact between thethird molar root and the mandibular canal.In the present study, the authors found acomparable high rate of 88%.

CBCT has a relative low accuracy inpredicting IAN exposure in those highlyselected cases where there is a close rela-tionship between the mandibular canal andthe third molar, however it is highly reli-able in determining the bucco-lingualposition of the mandibular canal withrespect to the third molar. TANTANAPORN-

KUL et al. also found a high inter-observeragreement in the assessment of the bucco-lingual position of the mandibular canalusing the 3DX Accuitomo CBCT.

The mandibular canal was more oftenpositioned lingually to the third molar rootthan buccally. This is in accordance withsome studies using volumetric ima-ging12,18,24, while others found more man-dibular canals positioned buccally to thethird molar root (Table 5)10,11,14.

The position of the third molar in rela-tion to the mandibular canal was a sig-nificant risk factor in the occurrence ofIAN exposure. The IAN was more fre-quently exposed during third molarremoval when the mandibular canal waspositioned at the lingual side or interradi-cular to the third molar root rather thanbuccally. This result was in agreementwith other studies10.

To the authors’ knowledge, this is theonly study in which a significant associa-tion has been found between the positionof the mandibular canal in relation to thethird molar and the occurrence of IANinjury. Patients are at higher risk of IANinjury in cases where the mandibular canalis positioned lingually to the third molarroot. This could be because the surgeon

Position of impacted third molar in relation to mandibular canal 969

Fig. 3. Panoramic radiograph showing darkening of the distal root and interruption of the superior white line of the mandibular canal (A).Transverse (B) and coronal (C) CBCT images show the presence of bone tissue between the mandibular canal and third molar root (arrows). Theneurovascular bundle was not exposed during removal as was expected from the CBCT images.

always starts his surgical approach, evenin case of a lingually positioned IAN, atthe buccal side of the wisdom tooth, gen-erating unfavourable lingually directedforces.

MAEGAWA et al.10 reported the samefinding using medical CT, although theirresults were not significant. HOWE andPOYTON

6 reported that grooving of thetooth root mainly occurs at the lingualsite, indicating a high risk of IAN injury.OHMAN et al.18 also found that in mostteeth that showed grooving of the root onmedical CT images, the mandibular canalwas positioned lingually to the thirdmolar root. In the present study, 4 patientshad sensory impairment after exposure ofthe neurovascular bundle during thirdmolar removal. In one case, the neuro-vascular bundle was damaged directly bythe burr, due to a sudden movement by thepatient. After reviewing the CBCTimages of the other 3 patients, the authorsfound grooving of the tooth root at thelingual side in all cases (Fig. 4). Thesecases also showed the radiographic sign,darkening of the tooth root, on the panora-mic radiographs. Other studies have cor-roborated the association betweendarkening of the third molar root onpanoramic radiographs with grooving ofthe root6,18 and IAN injury6,7,21.

Table 5. The bucco-lingual position of the man

no

GHAEMINIA et al., 2009 53TANTANAPORNKUL et al. 2007 142dE MELO ALBERT et al. 2006 29OHMAN et al. 2006 90MONACO et al. 2004 73MAEGAWA et al. 2003 47MILLER et al. 1990 31

CBCT scanners produce a lower radia-tion dose than medical CT scanners.According to the European Guidelineson radiation protection in dental radiol-ogy2, an effective dose in the range of364–1200 mSv is provided by a medicalCT scan of the mandible. The radiationdose of CBCT scanners depends on theapparatus used. According to LUDLOW

et al.,9 the i-CAT produces an effectivedose of approximately 135 mSv for a fullfield of view scan, which is in accordancewith the data provided by the manufac-turer. The same data give an effective doseof 32 mSv for a mandible i-CAT scan.With an approximate dose of 32 mSv fora mandible scan, the effective dose wouldbe reduced by a factor 11–37 comparedwith a medical CT scan. The CBCT scangives a higher radiation dose than conven-tional panoramic radiographs, which arein the range 4–30 mSv. A sectionalpanoramic view, capable of imaging themandible alone, would reduce the doseeven further. It is important to weigh upthe potential benefits of using CBCTimages against the risk of extra exposureto ionizing radiation.

The results of this study show thatCBCT is not better than panoramic radio-graphy in predicting IAN exposure inpatients who are at high risk of IAN

dibular canal with the third molar root as report

Buccal Lingual

17% 49%25% 26%45% 48%31% 33%25% 19%51% 26%45% 39%

injury. CBCT images provide a reliableinsight in the bucco-lingual relationshipbetween the third molar root and themandibular canal, which cannot beachieved with panoramic radiography.This information is important when plan-ning and carrying out the surgicalremoval, to avoid subjecting the mandib-ular canal to pressure from movements ofthe roots or the careless use of burrs andelevators. Knowing the bucco-lingualposition of the mandibular canal in rela-tion to the third molar root is valuablebecause it identifies cases that are athigher risk of IAN injury: patients witha lingually positioned mandibular canaland grooving of the third molar root. Inthese cases the information could help todecide whether to extract the tooth or toprovide a coronectomy20 to prevent IANinjury. The patient can also be more ade-quately informed about his or her riskprofile. In the authors’ view, a CBCTimage is specifically indicated when thepanoramic radiograph shows that the apexof the third molar root touches or crossesthe inferior border of the mandibularcanal. Additional multicentre studies arerequired to determine cases in whichpanoramic radiographs are sufficient toprevent injury of the IAN or when addi-tional CBCT imaging is needed.

ed in the literature.

Between roots Inferior

15% 19%4% 45%

7%10% 26%

5% 51%4% 19%

16%

970 Ghaeminia et al.

Fig. 4. Three cases are shown with darkening of the tooth root on panoramic radiographs (A).The corresponding coronal CBCT images show grooving of the tooth root (arrows) at the lingualside (B). In all these cases the IAN was exposed during surgery and postoperative sensoryimpairments occurred in all these patients. One patient continued to have some sensoryimpairment after 6 months.

Competing interests

None declared.

Funding

None

Ethical approval

Not required

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