Third molar agenesis, spacing and crowding of teeth, and tooth size in caries-resistant naval recruits

AGESESIS (congenital absence) of one or more third molars occurs with vari- able frequency in different human populations.l-j Although most studies of the third molar have been concerned with its presence, absence, size, and morphologyv. recent inrestigations hare shown that third molar agenesis is not completely aI1 isolated anomaly. Garn and associates”-‘” reported an increased incidence 01’ agenesis of other teeth, delay in posterior t,ooth formation, differences in P&I, I&Z’, erupt,ion order, and tooth-size reduction when third molars were con- genitally missing. On t,heorctical grounds, one might also expect to find ;I grclatcr dcgrcc of spacing in the dental arches due to tooth-size reduction when third molar agenesis has occurrrtl. The present report deals with thca association betwren third molar agencsis. sl)acin p and crowding of the teeth. alld tooth size.

METHODS .\PL’D RIATERIAIS

The material available for analysis consisted of artificial stone casts++ mado from alginate impressions of the maxillary and mandibular dental arches of 195 white male naval recruits, 17 to 25 years of age (mean age, 18 years). Each oi the men had a full complement of permanent teeth ($I? to $I,), with no clinica or roentgenographic evidence of dental caries or restorations.-f

F’rom the Dental Research Facility, Naval Administrative Command, T‘niteft 1Jtatw Naval Training Center, Great Lakes, 111.

The opinions or assertions contained herein arc those of tlte author and are nr~l- to I)e construed as official or reflecting the views of the Department of the ziavy, or the Naval Service at large.

“From a collection started in 1961 11y W. R. Shillw, TAieutenant Comman~lrr (l)(‘r ITRN.

tThe caries-free naval recruit comprises approximately 0.2 per cent of total rwruit population at Great Lakes.

446 KEe)l,! .4irr. .I. ol~thudolltr~,s .Jrc~c I !I61

Kane of the men had rcccivcd orthodontic treatment, and thrrc was no (lri- dcnce Of gross deformity of the dentition resulting from disease, habit, or trauma. Men who had congenitally missing or rudimentary teeth other tball third molars were not included in this study.

The stone models were examined for evidence of crowding or spacing of the teeth, and each maxillary and mandibular arch was classified into one of three categories :

1. Spacing. The teeth were regularly arranged, and one or more spaces or diastemas appeared between the proximal surfaces of the teeth because of an excess of space in the dental arch.

2. Crowding. The teeth were irregularly arranged, as evidenced by overlapping of proximal surfaces or buccolingual displacement of one or more teeth because of insufficient space in the dental arch.

3. No spacing OT crowding. The teeth were regularly arranged in the dental arch with all proximal surfaces in contact and no evidence of spacing or crowding. Mesiodistal crown diameters of the mandibular right first molar were dc-

termined by means of sliding vernier calipers with sharpened points, according to the method of Moorrees.ll Since it has been shown by several investigator,+ I3 that the dimensions of right and left teeth are not significantly different, mea- surements were taken on only one side. The mandibular first molar was selected because of its known low coefficient of variation.*’ The experimental error of measurement calculat,ed from a series of repeated determinations (N = 40) was found to be 0.07 mm. (standard deviation of the differences).

Third molar status was determined by clinical observation, intraoral roent- genographic examination, and dental history (extractions).

Table I. Third molar agenesis and spacing and crowding of teeth in 195 caries- resistant naval recruits

A. n.* c.* No third molar Agenesis of one or more Agenesis of four

Arrangement of agenesis third molars third molars

teeth Xen / Per cent Men Per cent Nen Per cent

A4axilla

Spacing 66 45.2 28 -7 ai.2 11 91.6

No spacing, no crowding 67 39.0 13 26.5 1 8.3

Crowding 23 15.7 8 16.3 0 0

Totals 146 99.9 49 100.0 12 99.9 -

lllandible

Spacing 22 15.0 23 4i.o 8 66.6 No spacing,

no crowding 54 37.0 11 22.4 1 8.3 Crowding i0 47.9 15 30.6 3 25.0

Totals 146 99.9 49 100.0 12 99.9

++~roup B includes Group C.

Third, molw agenesis nod tooth spacin,g nn,d crowding 417

The data on third molar agenesis, spacing and crowding of teeth, and mesio- distal crown diameters of mandibular first molars are summarized in Tables 1, IT. and III.

Table IT. Third molar agenesis and mean mesiodistal C’TOWL diameters of wrdibular fibst molars in 195 caries-resistant naval recruits

Third molar agenesis (max~lla/mandible)

i4. o/o R. l/O

(‘. o/ 1 I). l/l E:. 2/O F. n/2 G. n/1

11. I/% I. 2/2

I i

YiZesiodistnl crown diwmter of m~nndib~ulnr

first molar -. -- -_-~-.-.

Mean j Bange i A)‘. Il.”

&fen Per cent (mm.) / (mm.) (mm.)

146 75.0 10.87 CJ.5.12.4 1.29 (*A)

6 3.0 11.08 10.2-l L.9 .- .-_- -.- .~

10 5.2 10.85 10.3-11.3 05” . - (13 7 (Ii 1 3 1.5 11.23 1O.S12.3 3 1.5 10.93 10.4-11.3 9 4.6 10.93 10.4-11.3 052 I I), k:, E’ ! 2 1.0 10.65 10.6-10.7 .~-.~__-_-.-.

0.39 II 1 4 2.0 10.55 10.3-11.3 ~.--- -..-.. (a, ~~.

12 6.2 10.55 9.x-11.4 0.53 (l’r

Totals 195 100.0 10.86 9512.4

Experimental error of measurement expressed as the standard deviation of the differences between a series of repeated measurements was 0.07 mm. (n = 40).

“Arranged into five groups according to third molar agencsis (O-41.)

Table III. Spacing and crowding of mandibular teeth, third molar agenesis, ant1 mesiodistal crown diameters of mandibular first molars in 1.95 caries-resistnn t nrrt!al recruits

Agenesis of one or mor( Jlcsiodi~stul crown diameter of third molars mnndi~bulnr .first m,oltrr

,<. Spacing

R No spacing, no crowding

C’. Crowding

45 23.1 23 50.1 61 1.4 46 10.65 (Ai 9.8-11.6 0.4:

65 33.3 11 Ii.0 21 0.3 65 10.86 (Ii! 9.6-12.2 0.49

85 43.6 15 17.6 30 0.4 85 10.9i (0) !).5-12.4 0.6:1

Totals 195 100.0 49 25.2 t 112 0.6t 195 10.86t 9.5-12/&t

“The difference between the means of Groups A and C wap statistically significant 10.05>P>0.01).

t Calculated on the basis of 195 men.

In w-m-al fcrms, the maxillary tchcth wer(’ spaced more frttquently than the! WW~ ~~mdxl and the mandibular teeth wt’rc crowded more frequently than thc~- were spaced. The percentage of men with spacing in tither dental arch increased when one or more third molars were congenitally missing and wan greatest when there was agenesis of all third molars. The percentage of men \vitll crowded mandibular teeth diminished when third molars were absent. This was also true to some extent for the maxillary teeth, which showed no evidence of crowding when all four third molars were congenitally missing. A chi-squared analysis of these differences (Table I) indicated that there was a stronger relationship between third molar agcnesis and spacing in the mandible than in the maxilla. When one or more third molars were congenitally missing, the following P values were obtained: maxilla = > 0.10; mandible = < 0.001. 1Vhen agenesis of all third molars occurred, the I’ values were as follows: maxilla = 0.0%; mandible = 0.001. Only twenty-two men (11.3 per cent) had neither spacing nor crowd- ing of teeth in either arch, and twcnt,y of these men had no third molar agenesis.

In 25 per cent, of the recruits (forty-nine men) one or more third molars were congenitally missing. The total number of congenitally missing third molars was 112 (forty-seven maxillary and sixty-five mandibular). The various colt]- binations of third molar agenesis and mcsiodistal crown diameters of the lo~(~la

right first molar are shown in Table Il. There was a 3 per cent reduction in size of this toot,h when all third molars were congenitally absent.

The relationships between spacing and crowding of the teeth, third molar agenesis, and tooth size are shown in Table III. Only 23 per cent of t,he 195 men had spacing of the mandibular teeth, but this small group accounted for 55 per cent of the total number of congenitally missing third molars. In addition, the mean mesiodistal crown diameter of the lower right first molar in these men was 3 per ccrlt smaller than in the men who had crowded mandibular t&h (0.05 > P > 0.01).

DISCUSSION

Other studies14l lh of crowding and spacing of the teeth suggest a tendency toward crowding of both dental arches in certain white populations. Lundstriiml’ investigated the maxillary arches of 111 13-pear-old boys, and Moorrees and Keed’” studied t,he mandibular dentitions of 72 American females, 18 to 20 years old, of European stock.

It is of interest to note that in the present material the over-all trend in the maxilla was for t,he teeth to be spaced more frequently than crowded and in the mandible for the teeth to be crowded more frequently than spaced. When third molar agrnesis occurred, however, the mandible was less frequently crowded and showed more tendency to be spaced, and the maxilla showed a greater tendency toward spacing.

The preliminary measurements of the mandibular right first molar in the present study suggest a reduction in mean mesiodistal crown diameter when third molars are congenitally missing. This finding is consistent with the reports of other investigators’-‘3 I”, W-B who also found reductions in tooth size associated with tooth bud agenesis. According to current biologic theory,“2 a greater degree

I-ollLme 50 .vI’111 be, 6

Third, ntolnr agenesis and tooth spacing rind crowdinfi 1-i:)

of reduction should be observed when the less stable teeth (I,, I’,, &) a~ studied.

Cram, Lewis, and KerewskylO recently showed that “reduction in size of all the teeth rather than the molars alone charartcrizes individuals with confirmed third molar agenesis.” If this relationship also applies to the present material, one might postulate that third molar agenesis predisposes to greater spacing and 1~s crowding of the dentition and could account for some of the diffr~rcnecs notcct.

Hcgg”:’ reports that, “the average size of the upper permanent central anti lateral incisor teeth in jaws with crowded irregular teeth is significantly large1 thau the average size of these teeth when they arc in jaws with regularly alignt~ci tWth.”

The data on tooth size in this report arc of additional interest from a dental caries-nutritional status point of view. Holloway, Shaw, and Sw(Lency’* report that, in the rat, a low-protein maternal diet rrsulted in offspring with teeth ot reduced size. Wallenius’” found that the width of teeth tended to be 1.7 pera WIII larger in children whose drinking water contained more than 0.8 ppm Auoridt>. Shill@” reported a greater tendency t,oward reduc$ion in thtL nwmhc~ of cws~~s in caries-susceptible teeth as compared with caric>s-resistaut tocth.

(Grahn&n and IngervallY’ found no significant tooth&x diffcrcnc~cs betwrc~n caries-resistant and caries-susceptible men in Sweden. KeanZb found incrcasctl mesiodistal diameters of the permanent mandibular first molars of Ivew %eala~d boys who drank “ground” water rather than rain watclr collected iu a “tank.” 11~ also suggested that there was a “tcndenqv for lareel* mandibular molars tcf be more susceptible to pit and fissure caries.”

The mean mesiodistal diameter of the mantlibular right first molar for thcb ?arics-free mm in this report. was 10.86 1111n. \Yhrn all third molars wl:(~ (:OII- r, Wnitallr missing, the mean value was 10.55 mm., a reduction of 0.31 mm. (:: pet’ cacnt 1. This reduction represents only 0.13 mm. less than thrl scs diffcrenc~~ of 1.4 mnl. reported by Moorrees and his coll~~~~g~~cs~~ for the mean incsiodista I diamct,(~r of the lower first molars of 160 Xorth American white rhildrt>rr.

If thr mean mcsiodistal measurcmcnts of the mandibular first molar in t&l* prcscnt study arca compared with those of th(> males in RIoorr(~c~s’“!’ study ( 1 1. i 3 mm.). it IWCOI~CS apparent that the molars of tlw (*al’itxs-fr(>(b group are :3 p(~t’ cant stuallcr (0.32 mm.). The first molars of the twl\-(1 caarics-free> mc~l witlr agc~n&s of all third molars wcrc 5.6 per ecnt smaller (0.6 trim.) I l1a11 tht, AIoorretbs malts. I-nfortunately, neither the caries status nor the t’tyqll(qlc~y OI thirgl molar agenesis was reported in the latter proul).

The findings reported in the present study su pgrst that data rcga rding cai*ics$ silscrptibilit>- or sex, species, and racial tliff(~rcncc~s which arc‘ basetl 011 o(lont 0. mctrica analysis sl~ould also be accompanied b>, ;I dctcl,lrlirlatiorl of the 1’tq11t~n(*> oi’ thiytl molar agenesis.

X0 definite conclusions regarding tooth size and tlcntal caries suaccptibilit~~ (fan bc drawn from the available data, As&l from the possibility of tliffcrcltt ratcts of third molar agencsis in the groups compared, the age distribution o I’ the persons whose teeth were measured is different. The children studityl I):,-

Moorrees and amociateP’ were observed longitudinally from the ages of 3 to 15 years, and no mention was made of age at the time of measurement. The latter is important because of the possibility of obtaining a larger, more absolute mesio- distal measurement of the first molar when its anterior or posterior companion is not present in the arch. This is a condition which occurs frequently in the young child but not in the adult with a full complement of teeth (M, to Mr,). This source of error” could quite easily account for some of the differences in size noted between the two groups.

The present data also suggest that the presence or absence of third molar agenesis in a dentition may play an important role in the alignment and spacing relationships of the teeth in the dental arches.

1. Third molar agenesis, spacing and crowding of the dental arches, and mesiodistal crown diameters of the lower right first molars were investigated in 195 caries-free white male naval recruits, 17 to 25 years of age, by means of stone study models and roentgenograms.

2. In 25 per cent, of the men one or more third molass were congenitally missing.

3. In general, the maxillary teeth were more frequently spaced and less frequently crowded than the mandibular teeth. Only 11.3 per cent of all the men studied had perfectly aligned dental arches (no spacing or crowding in either arch).

4. When third molars were congenitally missing, the teeth in both arches were more frequently spaced and less frequently crowded, and the mean mesio- distal crown diameter of the lower right first molar was smaller than when no third molar agenesis had occurred.

5. The foregoing relationships were most pronounced when all four third molars were congenitally missing.

6. The relationships of third molar agenesis to reduction in tooth number and tooth size were discussed with a view toward their possible association with spacing and crowding of the teeth.

The author wishes to thank Mr. Willard E. Pierce and Mr. Edward Scheiner, Biometrics Division, NAMRU #i, for their assistance with the statistical analysis of the data.

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Third molar agenesis and tooth spacing und crowding 151

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