Annals of the Royal College of Surgeons of England (1979) vol 6i

The radiological assessment of ectopic lowerthird molars

A J MacGregor DDSC MChD FDSRCSDepartment of Dental Surgery, School of Dentistry, University of Leeds

SummaryExperiments using observers ranging in experi-ence from undergraduates to consultantsshowed that the orthopantomograph can beused satisfactorily to predict the time taken toremove those ectopic lower third molars cap-able of being put into Winter's classification.When used for this purpose this radiographcompared well with the lateral oblique viewof the mandible. There were differences withthe intraoral view, but it could not be certainwhich was superior.The value of the orthopantomogram in

predicting impaired postoperative labial sen-sation was tested. It became apparent thatthere were cases in which the nerve was clearlyat risk and others in which it was clearly not atrisk. Variations in the anatomy of the canaland the quality of its image could give rise todoubt, some of which could be resolved bytaking intraoral views.An unsuccessful attempt using analysis of

multiple regression was made to select andquantify those features of the radiograph im-portant for predicting the time taken for re-moval of the teeth. A simple points scoringmethod was evolved which agreed with currentpractice and was capable of being used effec-tively by undergraduates, some of whom hadmade inadequate assessments hitherto.

IntroductionJohn Hunter' was right when he wrote in I 77 Ithat 'this cutting of the dens sapientiae is oftenattended with an inconvenience'. He was alsoaware of the problems of what is now knownas pericoronitis and that 'nothing but the re-moval of the tooth or teeth will remove theevil in many cases'.

This procedure must have been formidablein the eighteenth century and continued to beso until the I92os, after certain technical ad-vances had been made. Radiographs were

recognised as being particularly valuable inplanning the surgery almost as soon as X rayswere discovered by Roentgen in I895, but theexact manner in which they were to be usedwas not codified until 1926, when Winter2produced his well-known classification basedmainly on the position of the third molar inthe jaw. Other systems followed, notablythose of Pell and Gregory3 and Thomas' inthe USA and Warwick-James5 and Ward' inthe United Kingdom. All were influenced byWinter, but it should be said that Clarke7thought Winter's classification of academicvalue and that surgical planning owed mostto practical experience of removing teeth.

Those who believed in the value of radio-graphs almost all based predictions on readingthe intraoral (JO) film, claiming that only thisview provided the necessary detail. Obliquelateral (OL) views of the jaws showing thirdmolars were acceptable only in extremes suchas permanent trismus. When the orthopanto-mogram (OPT) (Fig. i) was introduced in theI970S it quickly replaced the other views. Thiswas determined by the convenience of theradiographer rather than the needs of the sur-geon and it seemed possible that surgicalstandards were being allowed to fall becauseof its introduction.The first question to decide was whether the

OPT could be used to estimate the difficultyof removing wisdom teeth. The possessionof 142 OPTs (Table I) for which the timetaken to remove the wisdom teeth was knownallowed the setting up of a series of psycho-logical experiments based on a classical designof R A Fisher8.

Value of OPT in predicting difficulty ofremoving lower wisdom teethThe time taken from just after raising the flapto just before it being closed ('removal time')was known for each tooth; values ranged from

Fronm a Hunterian Lecture delivered on 24th February 1978

I08 A J MacGregor

FIG. I Orthopantomo-gram showing bilaterallyimpacted wisdom teeth.

TABLE I Distribution of I42 third molars according to time of removal and allocation intocategories of difficulty

A B C D E(very easy) (easy) (average) (difficult) (very difficult)

Time o - I min 59 s 2 min - 3 min 59 s 4 min - 5 min 59 s 6 min - 7 min 59 s 8 min +Frequency 52 42 20 12 i6

a few seconds to just under I4 min. Difficultyof removal was classified in five categories,A-E, related to the time scale. As shown inTable I, teeth designated 'very easy' (categoryA) were removed in under 2 min, while thoserated 'very difficult' (category E) took over 8min.Two radiographs were selected to represent

each of the five categories, there being thus io

teeth to assess, all of them having been re-

moved by the same surgeon. These radiographswere presented in random order to observerswho did not know the removal times and theywere asked to estimate into which category ofdifficulty the tooth fell. A scoring system was

devised based on the sum of the squared dif-ference between the estimated and known de-

gree of difficulty of the I o teeth. Probabilityvalues were calculated for all the different poss-

ible scores. The system has general value andcan be used to make a rapid and precise esti-mate of ability in matching any I o pairs. Alower value was taken to mean a higher skillin assessing radiographs.An example of the results obtained with this

scoring scheme is shown in Table II. Studentsin the third year of study have not yet at-tended the oral surgery clinic and can beregarded as intelligent laymen. It is clear thatsome of them are quite skilful at assessing theradiographs in this regard. The table also showsthat as the undergraduate course continues an

increasing proportion becomes skilful, but thatjust before qualifying some are still unable to

TABLE II Cross-sectional study of radiological assessment showing frequencydistribution among students with different experience and consultants

Scoreo-6 7-II I2-I 7 i8 +

(P<o.OOi) (P>O.OOI, (P>o.or, (NS, P>o.o5)<O.Oi) <0.05)

3rd year 0 3 I 64th year 0 3 5 2sth year 0 4 4 26th year 2 I 5 2Consultants 2 3 4 I

The radiological assessment of ectopic lower third molars

'T'ABLE iII Comparison of OPTs with OL radiographsP value

Observer Method <0.00I >0.00I, >O.OI, >o.o0 (NS)<O.O0 <0.05

Studenits (i o) Ranking 6 I 2 IConsultants (I 4) Category 7 4 0 3Senior registrars (7) Category 7 0 0 0

perform any better than by chance. Ten con-

sultants from the academic and hospital staffof the Leeds and Sheffield Dental Hospitalsaccepted invitations to read the radiographsand although some achieved almost perfectmatches, it was interesting to see that one couldbe considered inadequate.

Longitudinal studies were made in whichstudents were followed partly through thecourse. In another series it was asked that theradiographs be placed in order and for this a

similar scoring system was devised. Anotherseries of radiographs was assembled relating toanother surgeon's operating times. One mostinteresting finding was that if the groups ofobservers were large enough, their collectiveestimate could be nearly perfect even if noneof them individually was very skilful. All theresults confirmed the view that the OPT con-tained the information necessary to assess thedifficulty, conceived in terms of time, of re-moval of lower wisdom teeth.

Comparison of OPT with OL and IOviewsA direct study of the value of OL and 10

views was not possible because of the shortageof material arising from the popularity of theOPT. Pairs of radiographs were assembled sothat for each of io teeth OPT and OL viewswere available. Observers were presented withthe 20 radiographs in random order and askedto- place them in categories of difficulty. Somewere also asked at different times to place eachseries in order of difficulty, having been pre-sented with them in random order. It will beappreciated that the scoring systems were al-ready available from the direct experiments

with the OPT against time. Similar experi-ments were carried out comparing the OPTand the IO.

Table III shows that the OPT and OL viewscan be matched extremely well by most ob-servers and it must be considered that theyprovide similar information.

Table IV shows that most observers cannot

match the OPT and JO views. However 2

performed at the P<o.oI level and I at

P<O.OOi, and since the total taking the test

was only 20 it must be assumed that for thosepossessing the skill the relevant information ispresent on both films and moreover that it islikely that non-matching is due to the presence

of confusing additional information. It may

be that so far as assessing difficulty is con-

cerned the IO provides too much information,for it has already been shown that the OPTis adequate.

Predicting impaired labial sensationSewill9 reported two third molars, one groovedand the other perforated by the neurovascularbundles, to the Odontological Society of Lon--don in i88I. The patient had suffered loss ofsensation in the lower lip and it is evident fromthe discussion on the paper that the compli-cation was well known. La Cronique"0 in1922 is given the credit of being the first to

predict nerve damage from radiographic ap-

pearance. The relationship of nerve and im-pacted wisdom tooth received sporadic atten-tion through case reports. Bowdler Henry"1made a good review of the early literature ina Hunterian Lecture in 1935. The appliedanatomy of the region was described by Milesand West in I95412, using specimens from the

TABLE IV Comparison of OPTs with IO radiographsP value

Observer Method <0.001 >0.00 I, >0.0 I, >0.05 (NS)<0.01 <0.05

Students (20) Ranking I 2 4 I3

log

I 10 A J MacGregor

Odontological Museum. It was not, however,until Howe and Poyton" published their re-

view in I960 of the results of removing 1355

wisdom teeth that the problem was quantifiedaccurately and described in terms allowing in-dependent criticism. They developed criteriabased on alteration of appearance of the toothor inferior dental canal when they were super-imposed which led to a diagnosis called truerelationship. Their findings were confirmed atoperation and apart from slight overdiagnosisin erring on the side of caution they were al-most always correct. If a true relationship oc-curred a patient had a 36 in ioi (35.640/0)chance of getting postoperative labial anae-sthesia and if not then the chance was 34 inI254 (2.700)

Seward14 later emphasised that if the canalwas adjacent to the tooth the nerve was injeopardy. Roberts and Harris"5 drew attentionto the impaired labial sensation resulting fromdamage to the mylohyoid nerve. Neither ofthese possibilities is yet capable of being diag-nosed using conventional two-dimensionalradiographs, so Howe and Poynton's predic-tions must still be considered the best available.

It would have been of interest to have com-

pared the OPT directly with the 10 in regardto their usefulness in predicting impaired labialsensation, but the 10 had almost gone intodisuse. Another complicating problem was thelow occurrence of the disorder. A retrospectivestudy showed only 5 examples in 504 opera-tions. This proportion was confirmed when a

prospective study showed only 2 out of 398teeth with damage to the inferior dental nerveand I with damage to the mylohyoid nerve. Inthese circumstances it was not feasible to makea meaningful study of the relationship of radio-graphic features to impaired sensation.An attempt was made to predict the pres-

ence of a groove on the tooth by reading theOPT. Forty of the 398 teeth were not

fully developed and therefore unsuitable.The remaining 358 were examined in randomorder and independently of the radiographsfor the presence of grooves. Of these, 29 were

considered grooved and 239 not grooved,while go had been so mutilated at operation as

to make opinion uncertain. The radiographswere now examined and grooving of the toothwas predicted if either tooth or canal was

altered just as they overlapped. Positive pre-dictions were made in i cases and negative in294, while in the remaining 45 the relationshipwas not discernible. Only 23i had completedata and here the important finding was thatin I6 cases in which no groove was predicteda groove was found anatomically (Table V).Not all of these grooves would be due to thepresence of the nerve and some were so shallowthat they could not be demonstrated radio-graphically on the extracted tooth, but if Howeand Poyton's findings are accepted it is reason-

able to suppose that the OPT is an inferiorview to the IO for these purposes.

Nevertheless, there are circumstances inwhich the OPT gives clear indications. If thetooth is separate from the canal or if eitheralters in character just as they are super-

imposed the prediction is obvious. Where theyare adjacent, and this must include all whichare superimposed without alteration, Seward'sinjunctions must be obeyed, especially whenthe tooth is mesioangular and the canal is atits apex.

Where there is any doubt and the outcomeis of importance to the patient then it may beconsidered wise to ask for an 10 view.

An analysis of multiple regressionThe fact that some observers could make ac-

curate assessments of the difficulty of removalof the teeth made it seem worth while to seekmeasurable features of the radiograph whichcould be used to predict removal time.

TABLE V Correlation of radiographic and anatomical assessmentsX-Ray assessment

Anatomical + - Not Totalassessment discernibleGrooved 7 I6 6 29Not grooved 8 200 3I 239Mutilated 4 78 8 go

Total I9 294 45 358

The radiological assessment of ectopic lower third molars

Records were made of 22 attributes andvariables relating to the size of the jaws andteeth, their shape, and their relationship. Theymainly concerned such factors as the arnountof bone covering the teeth and the complexityof root form. A full account can be found else-where"6. Data were subjected to stepwise mul-tiple regression as described by Nie et al." Thisprogramme had the facility of dealing withboth metric and parametric data and had theadded advantage of accepting incompletedata. The hope was to derive heuristically theminimum number of factors to be taken intoconsideration and to be able to construct anequation which would allow the clinician tocompute accurately the time of removing atooth by reading the radiograph.

In the event the programme selected i9factors as being significant, and when theywere used merely to compute the removaltimes of the ioo teeth from which the originalmeasurements were derived the results werehopelessly inaccurate. It was perhaps just aswell because if it had been otherwise dentalsurgeons might have been faced with findingan excuse not to make a large number ofsophisticated measurements before removingsuch teeth.The programme could not even predict with

any accuracy the removal times of the io teethused in experiments with clinical observers.This failure was thought to be due to the in-adequacy of measuring lines and angles to de-scribe shapes. It would appear that such topo-logical problems are not yet amenable tomathematical analysis despite the ease withwhich they can be solved by the human brain.The matter could have been left there, but

it was so tantalising that some preclinical stu-dents could be so skilful in this matter that itseemed that a further effort ought to be madeto find a solution. It was possible that despitethe sophistication of the method we had triedto deal with too many data. An attempt wasnow made to select a few factors, easy to mea-sure, not inter-related, and important to theregression. The factors were selected by refer-ence to the correlation matrix. It was known,for example, that the size of the jaws was sig-nificantly related to the length of the secondand third molars and to their widths, andsince the OPT was being used the height of

the jaw was used rather than the dimensionsof the teeth. At the same time as the factorswere selected a crude scoring system was de-vised in which increasing points were givenwhen, for example, the jaw increased in sizeor root shape increased in complexity. A tedi-ous process of trial and error eventually re-sulted in a system (see Appendix) which couldmake acceptable predictions concerning the IOradiographs used in the first experiment.When this process was extended to another

IOO radiographs the results were found to havesome clinical value. These results are shownin Table VI. It is apparent that there is a trendfor the more difficult teeth to be given thehigher scores. Perusal will show that a scoreof 5 or less is not often associated with moredifficulty than average. Examination of theradiographs of 50 impacted teeth successivelyremoved under local anaesthesia from out-patients at the Leeds Dental Hospital showedthat indeed the scoring system conformed tocurrent practice. Only one tooth had a scorehigher than 5.The next question to be settled was the

value to the undergraduate of the scoring sys-tem. All 32 students in the final year wereasked to classify the io radiographs referred toin the first section. These students had almostcompleted their training and had some experi-ence of third molar surgery. They were al-lowed to use whatever system they chose to esti-mate the removal time they thought necessary.They were then asked to assess the radiographsusing the new system. The results shown in

TABLE VI Frequency distribution of categoriesof difficulty and scores for Ioo OPTs.Score Category of difficulty (see Table I)

A B C D E10 - - - I 3

9 - I 2 -

8 - 4 I7 5 5 3 I 36 2 4 I 3

5 6 8 5 I -4 9 3 5 -3 3 2 I I2 4 3 II 4 I -

0 I I -

-I I - - - -

I I I

I 1 2 A J MacGregor

TABLE VII Comparison of results of testingby method of choice (Test i) and new scoringsystem (Test 2) (32 students).

P value<0.00I >0.00I, >0.0I, >0.05 (NS)

<0.01 <0.05

Test i 3 8 I3 8Test 2 2 i8 Io 2

Table VII indicate that the method greatly re-duced the number of incompetents, from 8 to2, and also increased the number showing skillat the P<o.oi level, from 8 to i8. Alas, thenumber of really excellent assessors was re-duced from 3 to 2 ! This falling off in stand-ards of the brilliant when they are overcoachedis a fault in many disciplines, but so far as thisparticular exercise was concerned it wasthought to be more than compensated for bythe decrease in the number of patients likelyto be put to severe inconvenience. The methodis one which is therefore taught to undergradu-ates and recommended to the admissions de-partment of the hospital, whence patients maybe referred to oral surgery departments byrelatively inexperienced staff.

ReferencesI Hunter, J (177I) The Natural History of theHuman Teeth: Explaining their Structure, Use,Formation, Growth and Diseases. London, John-son.

2 Winter, G B (I926) Principles of Exodontia asApplied to the Impacted Third Molar. St Louis,American Medical Book Co.

3 Pell, G J, and Gregory, G T (I933) Dental Digest,39, 330.

4 Thomas, K H (1932) Journal of Dental Research,12, 175.

5 Warwick-James W (I939) British Dental Journal,67, 217.

6 Ward, T G (I955) Dental Delineator, 6, 3.7 Clarke, H B (I965) Practical Oral Surgery, 3rd

ed. London, Henry Kimpton.8 Fisher, R A (I960) The Design of Experiments.

Edinburgh, Oliver and Boyd.9 Sewill, H (i88i) Transactions of the Odonto-

logical Society, I3, I I0.io La Cronique, Dr (1922) Revue de Stomatologie,

24, 704.

i IHenry, C B (I935) Lancet I, 313.

12 Miles, A E W, and West, W H (1954) DentalPractitioner, 4, 370.

13 Howe, G L, and Poyton, H G (IG96o) BritishDental Journal, I09, 355.

14 Seward, G R (I963) British Dental Journal, Ii5,7.

I5 Roberts, G D D, and Harris, M (I 973) Britishjournal of Oral Surgery, ii, I I0.

i6 MacGregor, A J (1976) The Radiological Assess-ment of Ectopic Lower Third Molars, DDScThesis. University of Leeds.

17 Nie, N N, et al (1970) Statistical Package for theSocial Sciences. New York, McGraw-Hill.

APPENDIXSystem of scoring for assessment of difficulty of removal

The six factors chosen have been placed insuch an order as to make the acronymWHARFE. This order does not reflect the im-portance of the contribution of each factor tothe regression. As a matter of fact W, which isfirst, is derived from Winter's classification,which provides an almost insignificant amountof information and was hardly worth includ-ing. It was kept not only for traditional rea-sons but also because if the tooth cannot be soclassified this scoring system is inappropriateas it deals with impacted teeth for which therelationship of the occlusal plane is capable ofbeing determined. Bizarre impactions and im-

pactions in edentulous jaws are thereforeamong those excluded.One further word of caution is necessary. It

is clear from the factors studied that their com-plete manifestation is not taken into account.For example, the follicle can be obliterated byresorption of the crown and enostosis and thiswould make the tooth extremely difficult to re-move. Extremely deep teeth, those with floridroot shapes will also be excluded. Such ex-tremes are also outside this classification. Themethod does not lose much in its clinical valueby these reservations since such teeth are notonly rare but are easy to assess as likely to bevery difficult to remove.

The radiological assessment of ectopic lower third molars

i ) Winter's classificatioin

2) Height of mandible (mm) (Fig. 2)

3) Angle of second molar (degrees) (Fig. 2)

4) Root shape and developmenta) Less than 3 completeb) - to 2 completec) More than 2 complete:

5) Follicle

ComplexUnfavourable curveFavourable curve

NormalPossibly enlargedEnlargedImpaction relieved

SpaceDistal cusp coveredMesial cusp coveredAll covered

6) Exit path (Fig. 3)

Vert ic a I

Horizontal

FIG. 2 The height of the mandible is measuredfrom the distal profile of the amelocementaljunction to the nearest point on the lower bor-der of the jaw. The angle of the second molaris that made by the long axis of that toothto a fiducial horizontal line.

FIG. 3 Exit path. The tooth is rotated aboutthe midpoint of the amelocemental junctionand the point at which the shadow of the coro-

noid process crosses the crown is noted. In theexample shown a mesioangular tooth when ro-

tated is seen to have only the distal cusp com-

pletely covered.

CategoryHorizontalMesioangularVerticalDistoangular

Score2

0

2

I-30

30-3435-39

'-596-6970-798o-89Qo+

I2

0I2

34

2I

32I

0

-I-2

-3

0

2

3

I I';