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Effects of occlusal attrition and continuous eruption on odontometry of rat molars.

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Effects of Occlusal Attrition and Continuous Eruption on
Odontometry of Rat Molars
Division of Oral Biology, School ofDental and Oral Surgery, and Department of
Anatomy, College off'hysicians and Surgeons, Columbia Uniuersity
KEY WORDS Odontometry Attrition
Rat molars
. Continuous eruption
Recurrent reports by others of posteruptive dimensional increase of the crowns of rat molar teeth were analyzed in the context of our present study of occlusal attrition, continuous eruption and alteration of the occlusal
planes of rat maxillary molar teeth with age. Marked attrition of the anatomical
crowns occurs, together with a considerable continuous eruption that increasingly brings the markedly convex mesial root of the maxillary first molar into the
clinical crown. Further, a slight change in occlusal plane occurs. Previous
workers used standardized planes of orientation and of registration prior to
measurement, which masked the phenomena mentioned above. They thus
mistakenly reported increased coronal dimensions.
Periodically it is reported that the anteroposterior (mesiodistal) dimensions of crowns
of rat (and human) molar teeth increase
posteruptively (Donaldson and French, '27;
Riesenfeld, '70; Lavelle, '74). Since it is absolutely certain that: (1) coronal amelogenesis is complete prior to eruption; (2) amelogenesis occurs only when a functioning
enamel organ is present; and (3) the posteruptive crown is totally devoid of a functioning enamel organ. the periodic reiteration of
the claim for posteruptive coronal dimensional increases in rat molars requires explication.
The data presented below demonstrate that
with age and concurrent wear rat molar
crowns undergo marked occlusal attrition,
continuous eruption and possibly a slight
change in occlusal plane relationship to the
palatal plane. The net effect of these morphological changes is to produce unstable odontometric reference points. If corrections are
not made for these same changes, i t will appear, incorrectly, that rat molars increase dimensionally while in fact they do not.
Boiled and bleached skulls of 22 normal,
male Canon strain rats were used, 40 to 412
AM J. PHYS. ANTHROP., 47. 403-408.
grams body weight. Third maxillary molars
were clinically unerupted in five specimens,
whose mean body weight was 75 grams. All
dentitions were studied stereomicroscopically. Lateral roentgenograms and lateral and
vertical photographs of the maxillary dentition were made of 40,45,80,120,230,327and
412 gram rat skulls. Both roentgenograms
and photographs were uniformly enlarged, the
former five and the latter ten times.
Donaldson and French ('27) measured
horizontally "from the most posterior point of
the crown of the third molar to a constant and
conspicuous shoulder on the anterior surface
of the first molar." They, as well as Riesenfeld
('701, essentially used the technique of Lavelle ('74), i.e., ". . . the mandibles were oriented in a standardized position such that the
occlusal plane of the molars was as near to the
horizontal as possible." Similar horizontal positions were also used for the maxillary
molars by all of these workers. In none of
these measurements is any explicit attention
directed to occlusal attrition, continuous eruption, or to the differences between clinical
Aided, in part, by the Plastic Surgery Research Fund, Diviaiun vf
Plastic Surgery, The Presbyterian Hospital. New York. Dr. George F.
Crikelnir, Director, and by the Polly Annenberg Levee Charitable
Figs. 1 , 2 Lateral (buccal) and occlusal views of the maxillary molar teeth of a 45 g rat The third molar is
unerupted. Note the deeply fissured crown and that, the convex mesial surface of the first, molar i e formed
entirely by the enamel covered anatomical crown.
and anatomical crowns; the former being all
portions of a tooth erupted above (a) the
gingiva in the intact head, and (b) the alveolar bone crest in the prepared skull. The anatomical crown is the enamel covered portions
of the tooth; erupted, unerupted or abraded.
Using both enlarged lateral photographs
and roentgenograms, and after a careful
study of molar coronal attrition patterns, we
traced on acetate paper both the mean occlusal plane of the three maxillary molars, as
well as the mean plane of the external
diastemal contour (the “palatal plane”), and
then determined the angular relationship between these two planes. With the lateral photographs of each first maxillary molar, we
similarly traced the complete clinical crown,
and oriented each tracing upon an arbitrary
horizontal plane (i.e, on a simulated occlusal
plane). The greatest antero-posterior lengths
of each first maxillary molar clinical crown
was then measured parallel to this plane. In
each case the most anterior point of the clinical crown, and of measurement, lily just nbnve
the alveolar bone margin.
In the newly erupted first and second maxillary molars (about 45 gm body weight), the
clinical crown is formed completely by the
anatomical crown (figs. 1, 2). These molars
have a complicated pattern of cusps, forming
a series of transverse convex lophs, between
which similarly directed concave grooves are
found, forming a mean, horizontal occlusal
plane that could alter if the cusp heights, as
Figs. 3, 4 The maxillary molar teeth of a 412 g rat. Marked occlusal attrition is evident, with the consequent loss of coronal fissures. In the lateral view observe that as a result of continuous eruption the markedly
convex mesial ruot of the first molar is now a prominent landmark on the clinical crown. Compare figures 1and
3 t o perceive how a constant method of caliper placement could easily lead to reports of dimensional increase.
visualized, undergo attrition. Moreover, in
norma frontalis, the mean occlusal planes of
the molar teeth face infero-laterally a t about
25"-30" from a horizontal plane.
In the molars of a 412 gm specimen (figs. 3,
4), cuspal heights are markedly reduced, and
in the intercuspal areas occlusal enamel has
been completely abraded, exposing underlying
dentin. While the loph-like structures are still
extant, they now are gently elevated convexities on essentially tabular occlusal surfaces.
The mean frontal occlusal surfaces continue
to face infero-laterally a t about the same
angulation as previously noted in unabraded
Posteruptive and post-attritional elongation of the molar teeth occurred, and significant portions of the clinical crowns are now
formed by root structures; the mesial (anterior) slope of the first molar is formed by the
root (fig. 3).
The mean antero-posterior maxillary molar
occlusal plane declined downward and forward relative to the palatal plane with age.
Comparing the individual tracings of the five
skulls (of 45-gm mean body weight) with the
single 412-gm specimen, showed that this
angle decreased between 5" and 7", a declination range observed identically when compared with the 327-gm specimen.
Superimposition of the lateral tracings of
the first molar clinical crowns of the five
youngest skulls, and of the 412-gm specimen,
on a constant arbitrary horizontal occlusal
plane produced a 6% increase in greatest
antero-posterior length of the line of greatest
crown length, parallel to that horizontal
plane. I t was noted that in the newly erupted
specimens the most mesial, anterior point of
the clinical crown lay on the enamel-covered
cervical convexity of the mesial surface o f the
crown, while in the 412-gm body weight specimen this same point lay 011 a noticeable
shoulder of the anterior (mesial),and markedly exposed, root.
The first claim of rat molar dimensional increase with age by Donaldson and French
(’27) was refuted effectively by Wood and
Wood (’31) who showed, by longitudinal odontometry, no increase in anatomical crown dimensions of the rat.
Recognition of the differences between the
clinical and anatomical crowns of rat molar
teeth is important. With age the mesial root
of the rat first maxillary molar becomes increasingly visible, and subject to being mistaken for a portion of the anatomical crown.
Both Riesenfeld (’70) and Lavelle (’741 consider this same non-enamel covered surface as
a very apparent “landmark,” ascertained by
inspection. The rat molar undergoes considerable posteruptive occlusal attrition and
constant compensatory eruption. If the points
between which r a t molar tooth lengths are
measured include such a “landmark,” this
same “landmark” will increasingly shift downward on the constantly curved and widening cement-covered root surfaces.
The molars of laboratory rats and mice are
teeth of limited growth and have closed roots,
covered with cementum. Continued radicular
deposition of cementa1 tissues, often exceedingly massive in amount, normally accompanies continuing occlusal attrition and eruption (Cohn, ’571, effectively maintaining functional root lengths.
The excellent data of Lavelle (’74) are representative of all workers claiming posteruptive coronal diameter increase. He reports
that the combined mesio-distal width “increase” of r a t M1-M3is 0.38 mm (6.45 mm at
20 days; 6.83 mm a t 360 days), a 5.9% increment. It has been noted previously that the
presumed increase in rat molar crown anteroposterior diameters is correlated with marked
occlusal attrition (Donaldson and French,
This “increase” can be accounted for in two
ways. First, if we assume a change in angula-
tion of either 5” or 7” did occur in the molar
occlusal planes, and using the absolute data
of Lavelle (’74), trigonometric computat,ion
shows that a 5” change in occlusal angulation
can account for 7.9% of the total reported absolute change in molar row length, while a 7X
change can account. for 15.8%of this same r e
ported “increase” in molar crown dimensions.
That is, the systematic repositioning of the
unabraded and abraded, occlueal planes upon
single external and artificial horizontal reference plane could introduce the amount of
error noted above.
Second, the confusion of anatomical and
clinical crowns, by itself, is sufficient to completely account for the reported increase in
crown diameters. The 6%increment we noted
in compared superimposed tracings, constantly using the most mesial point of the
clinical crown, whether formed by coronal or
radicular tissues, is essentially equal to the
5.9% increase noted by Lavelle (’74).
Our data suggest the failure to distinguish between anatomical and clinical crown
structures was the major cause of the report,ed dimensional changes. Absolute angular changes in antero-posterior orientation of
abraded occlusal planes, coupled with a n
odontometric technique using a constant external registration plane could possibly provide an additional minor cause of error.
Apparently finding the consensually agreed
upon data base of mammalian amelogenesis
insufficient to explain their findings of continued rat molar coronal growth, Lavelle (’74)
suggests that intradental slippage occurs
along incremental lines in both enamel and
dentin, while Riesenfeld (’70) tentatively suggests some process of posteruptive enamel expansion. Experimental data to support either
suggestion do not exist a t this time.
Thus, the report of posteruptive dimensional increase of rat molar crowns rests on
several, often cumulative errors:
(a) apparent confusion of the anatomical
and clinical crown in the face of marked occlusal attrition;
(b) the use of constant external registration planes together with mensurational techniques which included significant portions of
the anterior root of the first molar in the coronal length determinations of older rats;
(c) possible errors caused by absolute alteration in mean molar occlusal planes. In
this case there would be a consistent overestimation since
lengths were
measured rather than the
Cohn, S.A. 1957 Development of the molar teeth in the
albino mouse. Am. J. A n a t , 101' 295-320.
Dnnaldson, H. H., and H.E. French 1927 On the growth
in the diameters of the molar teeth of the albino r a t after
eruption. Anat. Rec., 34: 277-299.
Lavelle, C. L. B. 1974 The effect of age on human third
molar and rat molar teeth. Acta anat., 87: 110-118.
Riesenfeld, A. 1970 The effect of environmental factors
on tooth development: a n experimental investigation.
Acta anat., 77: 168-215.
Wood, H. E.. and F D. Wood 1931 Refutation of the
alleged diametric growth of erupted r a t molars. Anat.
Rec., 48. 169-183
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effect, occlusal, molar, eruption, attrition, rat, odontometric, continuous
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