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The effect of the consistency of the diet on eruption rates and lengths of incisor teeth in rats
0003-9969/93 $6.00 + 0.00 Copyright 0 1993 Pergamon Press Ltd
Archs oral Bid. Vol.38, No. 8, pp. 699-706, 1993 Printed in Great Britain. All rights reserved
THE EFFECT OF THE CONSISTENCY OF THE DIET ON ERUPTION RATES AND LENGTHS OF INCISOR TEETH IN RATS R.A. BURN-MURDOCH Sherrington
School
of Physiology,
St Thomas’s Hospital (UMDS), London SE1 7EH, U.K. (Accepted
Lambeth
Palace
Road,
2 March 1993)
Summary-It is uncertain whether the consistency of the diet affects the eruption of rodent incisors. One study reported that harder diets made incisors shorter and erupt faster, and suggested that the faster eruption was due to the decreased length. Two other studies found no effect on eruption rates. In the present study the impeded and unimpeded eruption rates of these lower incisors were measured, using ether anaesthesia for all procedures. There were two groups of rats fed in alternate periods on standard laboratory pellets and in the intervening periods on the same material ground to a powder. In each period, one group was fed on pellets and the other on powder. The impeded eruption rate was less when the rats were fed on powder than on pellets and the rate declined with time. The unimpeded incisors were not affected by the diet, so the effects on the impeded incisor were direct effects on the tooth and not systemically mediated. The impeded incisors lengthened during the experiment and the lengthening was greater on powder than on pellets. The ratio of the change in impeded eruption rates to the increase in tooth length on changing from pellets to powder was different from the ratio for going from powder to pellets, so the effect of the diet on the impeded eruption rate was not due to the change in length of the teeth. There was a negative correlation between the decline in impeded eruption rates and the lengthening of the incisor with time, suggesting that tooth length can influence eruption rates. There was a negative correlation between the unimpeded eruption rate and the distance from the incisal edge of the impeded incisor to the end of the unimpeded incisor; its origin was uncertain. Key words:
tooth
eruption,
mastication,
thegosis.
INTRODUCTION
effect is a direct one on the tooth or systemically mediated, and also to see whether the change in eruption rates was produced by the decreased length of the tooth. To prevent function being a variable in studies of tooth eruption, Bryer (1957) introduced the unimpeded incisor: a tooth that is repeatedly shortened to stop it being used; incisors that are not shortened are called impeded. Unimpeded incisors erupt rapidly, so they protrude some way into the mouth before the next shortening and may not achieve their aim. Bryer (1957) reported that unimpeded eruption rates were the same whether the rats were fed on pellets or powder but the effect on the impeded rates was not measured so the change in the diet may not have been large enough to affect eruption. Therefore, a secondary aim of my experiment was to see whether the consistency of the diet affected unimpeded eruption rates.
The continuously erupting incisors of rats are often used as examples of tooth eruption in general. They are similar to teeth of limited eruption, such as those found in man, in that their eruption is affected by function: shortening rat incisor teeth to prevent them from being bitten upon markedly accelerates their eruption (Wetzel, 1927). Rats use their incisor teeth for both eating and non-nutritional uses such as sharpening. Taylor and Butcher (1951) reported that the incisor teeth of rats were shorter and erupted faster when they were fed with intact grains of maize than when they were given powdered maize but the changes were small and not assessed statistically. They suggested that the decreased length of the teeth was responsible for the faster eruption. Other studies have not found an effect of the diet on eruption. Addison and Appleton (1915) stated that giving rats soft food did not alter eruption rates, but gave neither details of their experimental design nor the results. Weinreb, Assif and Michaeli (1967) compared feeding by stomach tubes with conventional diets and found no differences in the eruption rates but the experiment was not well designed for detecting small differences and the rats fed by stomach tube grew less rapidly than the others. Therefore, the present study was undertaken to see whether the consistency of the diet does affect the eruption of rodent incisors and, if so, whether the
MATERIALSANDMETHODS
12 Wistar rats (Tucks, Battlesbridge, Essex, U.K.), weights 415-470 g, were divided into two groups of six. The experiment was divided into four lo-day and a final &day period. In the first and last periods both groups were fed on standard laboratory pellets (Tucks). In the middle three periods, each group was fed in alternate periods on pellets and in the interven699
700
R. A. BURN-MURDOCH
ing periods on the same food given as a powder. In each period one group was given pellets and the other powder (Fig. 1). Two groups were used so that the effects due to the diet could be distinguished from effects caused by time. The rats were caged in pairs and the amount of food eaten was taken as the difference between the amount given at the beginning of each interval and the sum of the amount remaining in the food bowl at the end of the interval plus the food that could be recovered from the floor of the cages. All rats were weighed every 2 days. Eruption rates and lengths of teeth were measured at 2-day intervals by recording the position of marks on the labial surfaces of the lower incisors and their gingival margins using a calibrated graticule in a microscope eyepiece. At each measurement the lower right incisors were shortened to make them unimpeded, the left incisors remaining impeded. Ether anaesthesia was used for all procedures. The method has been described in more detail by Aladdin and Burn-Murdoch (1985). The criteria described in that paper and subsequently expanded (Burn-Murdoch, 1988; Burn-Murdoch and Light, 1992) showed that the gingival margin of the left incisor was the most accurate reference, or stationary, point from which to measure eruption rates. Averages of the various measurements and, for some measurements, the change during each period were calculated for each rat for each of the five periods. The results are given as the mean f SD of the averages for, or changes during, each period. Differences were analysed by paired t-tests when comparisons were made between different periods and by ordinary t-tests when comparisons were made between groups. When the variances of the two groups were significantly different, Satterthwaite’s approximate t-test or the rank-sum test were used. Ratios, which are not normally distributed, are presented as the median and range, two measurements being given for one end of a range if the extreme value is very atypical. Ratios were compared between groups by the ranksum test and within groups by Wilcoxon’s signedrank test. Correlation coefficients were calculated by the linear least-squares method. The level of statistical significance in all tests was taken as 5%. One rat in group 1 died on day 36. The conclusions of this experiment are the same regardless of whether the means for this rat for days 3&36 are taken as the means for days 3&40, whether the means for days 3&40 for this rat are omitted, or if this rat is omitted entirely. Days
IO
0
RESULTS
In the first and last periods when both groups were fed on pellets, the eruption rates were faster in group 2 than in group 1 but this was significant only for the unimpeded rates in the first period (Table 1). Both the impeded and unimpeded rates were significantly slower in the last period than in the first but the slowing was the same in both groups (Table 1). The impeded eruption rates for the middle three periods are given in Fig. 2. For both groups they decreased significantly when changing from pellets to powder but did not alter when changing from powder to pellets. To eliminate any effects of time, the change in eruption rates between period 2 (days l&20) and period 3 (days 20-30) was calculated for each group; the change was significantly different between the two groups (Fig. 2). This corresponds to a change from powder to pellets in group 1 and from pellets to powder in group 2. The change in eruption rate between period 3 (days 20-30) and period 4 (days 30-40) was calculated similarly. The change in diet is the opposite to before and the change in eruption rates in the two groups has been reversed and remains statistically significant (Fig. 2). When all the powderto-pellet changes in periods 2-4 are combined and compared with the pellet-to-powder changes, there is a significant difference (Table 2) with a decline on changing from pellets to powder but no change on going from powder to pellets. The unimpeded eruption rates were analysed in the same way (Fig. 3). In both groups the eruption was slower in period 3 than in period 2 and in period 4 than period 3, but this was not significant for periods 3 and 4 in group 2. The change in eruption rate between period 3 and period 2 in group 1 was not significantly different from the same change in group 2, nor was there a difference between the groups in the change from period 3 to period 4 (Fig. 3). When all the pellet-to-powder changes were compared with the combined powder-to-pellet changes, there was no significant differences between them, both changes being decreases (Table 2). There were no significant differences between the groups in the length of the impeded incisors in the first or last periods but the teeth were longer in the last period than in the first (Table 1). The increase in length during the experiment was the same in both groups. The mean lengths of the impeded incisors for each period were analysed in the same way as the eruption rates (Table 2). The increase in the average lengths for a period on going from pellets to powder was not significantly different from the increase on changing from powder to pellets. The ratios of the 20
30
41)
4x
Group 1
Pellets
Powder
Pellets
Powder
Pellets
Group
Pellets
Pellets
Powder
Pellets
Pellets
2
Fig. 1. A diagram
showing
the food given to the two groups of rats at the different stages of the experiment.
Function Table
1. Various
measurements
First period
Group 1 Group 2 Groups compared Group 1 Group 2 Groups compared Group 1 Group 2 Groups compared Group 1 Group 2 Groups compared
Group 1 Group 2 Groups compared
701
and eruption during
Last period
the first and last periods
Periods compared
Impeded Eruption Rates (mm.day -‘) 0.51 + 0.06 0.31 & 0.05 (5) -0.18 * 0.05 (5)
0.57 + 0.07 N.S.
0.35 * 0.07 N.S.
-0.21 + 0.09 N.S. Unimpeded Eruption Rates (mm.day - ’ )
1.12+0.06 0.91 + 0.08 (5) -0.19 f 0.09 (5) 1.20 + 0.06 0.97 & 0.08 -0.22 _+0.06 N.S. N.S. p < 0.05 Length of the Impeded Incisor (mm) 8.49 + 0.88 10.60 + 1.04(5) + 1.93f 0.39 (5) 8.60 + 0.36 10.21 & 0.43 + 1.61 k 0.46 N.S. N.S. N.S. Weights (g) 442.4 + 17.7 451.4 + 26.1 (5) +11.4& 14.8 (5) 450.7 + 12.7 470.3 * 14.7 +19.6+4.1 N.S. N.S. N.S. Distance from the Incisal Edge of the Impeded Incisor to the End of the Unimpeded Incisor (mm) 5.10 + 0.55 5.59 k 0.26 (5) +0.36 k 0.34 (5) 5.05 + 0.63 5.36 k 0.38 +0.31 f 0.38 N.S. N.S. N.S.
p < 0.001 p < 0.01
p < 0.05 p < 0.001
p
N.S. p < 0.001
N.S. N.S.
The averages for each rat for the periods were calculated and the results in the table are the mean I 1 SD of the averages for the rats. n = 6 unless indicated to the contrary in parentheses. The lengths of the impeded incisor in the first period omit the readings for day 0 because of the marked shortening that occurs on unimpeding the adjacent incisor. For
statistical analysis, see the text.
_
change in impeded eruption rates to the changes in length of the impeded incisor were significantly more negative on changing from pellets to powder than from powder to pellets @ < 0.01, Table 2). The ratios for the changes between the first and last periods ( - 0.10, range - 0.06 to - 0.18) were not significantly different from the ratios for the changes between periods 2 and 4 ( - 0.08, range - 0.03 to - 0.14) and neither showed significant differences between the groups. These ratios lay between the ratios for the two changes between the diets and were significantly different from them (p < 0.01 in all cases with the two groups combined) except for the comparison between the ratios for the pellet-to-powder change and for the first and last periods. The changes in length of the impeded incisor within each period are given in Table 3. In each of the middle three periods, the increase in length was greater in the group given powder than in the group on pellets, but this was significant only for period 4. Taking overall means for periods 2 and 4, in both of which group 1 was on powder and group 2 on pellets, the increase in length was significantly greater for powder than for pellets (0.75 + 0.11, 0.47 f O.l4mm/period, p < 0.01). Combining the length changes for periods 2, 3 and 4, there was a greater lengthening on powder than on pellets (Table 4). The results for powder include 11 values from group 1 and six from group 2, while the pellet results have six values from group I and 12 from group 2, but this is unlikely to have caused the difference between powder and pellets because there were no significant differences between the groups either in the first period when both groups were on pellets or in the change in length between the first and last periods. The length change in the last period was significantly larger in group 2 than group 1 (Table 3) but this difference between the groups
would have made the length changes larger on pellets than on powder, contrary to what was obtained. When the results for pellets and powder were combined separately for periods 2 and 3 and for periods 3 and 4 to avoid this bias, the difference between pellets and powder was still present but significant only for periods 3 and 4 (Table 4). There was a significant, positive correlation between the unimpeded and impeded eruption rates, using mean rates over the whole experiment for each rat, in group 1 (r = f0.87, n = 6, p < 0.05) but the correlations were not significant in group 2 or when the two groups were combined. There was no correlation between the decline in the unimpeded rates from the first to the last periods and the corresponding decline in the impeded rates. There were no significant correlations between impeded eruption rates and the lengths of the impeded incisor when means for the whole experiment were used, but there were negative correlations between the decline in impeded rates between the first and last period and the corresponding change in length of the incisor in group 2 (Y = -0.92, n = 6, p < 0.05) and when the two groups were combined (r = -0.61, n = II, p < 0.05). The correlation was not significant in group 1 but the corresponding correlations for the differences between period 1 and both periods 3 and 4 were significant and negative. The distance between the incisal edge of the left incisor and the cut end of the unimpeded incisor in each 2-day interval was taken as the average of the distance immediately after shortening the incisor at one measurement and the distance just before shortening the incisor at the next measurement. Means of the distances were taken for each rat for each period and were analysed in the same way as the eruption rates. There were no significant differences between
R. A. BURN-MURDOCH
702
the groups in any period or any differences due to the consistency of the diet (Table 2). No significant differences were found when the same analyses were made separately on the distances just before and after shortening the incisor. The distance increased during the experiment (Table 1); the difference between the first and last periods was significant when the two groups were combined. There were negative correlations between the unimpeded eruption rate and the distance from the incisal edge to the end of the unimpeded incisor (r = -0.67, n = 12, p < 0.05) when mean values over the whole experiment for each rat were used. The correlations for the distances at the beginning and end of the 2-day intervals were also significant and negative. There were no correlations between the unimpeded eruption rates and the lengths of the impeded incisors. There were no significant differences in the weights of the rats between the two groups in the first and last periods (Table 1). Both groups gained weight during the experiment but this was significant for group 2
only; the difference between the groups in the weight gains was not significant (Table 1). There was not a significant difference between the groups in the weight gains during the first or last periods (Table 3). In each of the periods 24, the weight gain was greater in the group on pellets than in the powder group but this was not significant in period 2. When the weight gain in period 2 was subtracted from the gain in period 3 there was a significant difference between the groups (groups 1, + 18.3 + 21.5; group 2, -4.2 f 11.7 g; p < 0.05). The difference between periods 3 and 4 was reversed and still significant (- 18.0 f 13.6, + 10 k 12.2 g for groups 1 and 2 respectively, p < 0.01). When the weight gains from periods 2, 3 and 4 were combined (Table 4), the gain for pellets was significantly greater than for powder but, as with the length changes, there are unequal numbers of results from the two groups in the pellets and powder results. No significant differences between the groups or diets was found in the amount of food eaten. Wear facets were not seen on the end of the unimpeded
Mean impeded eruption rates
mm-day-’
I
I Periods
Powder
Powder
Pellets
p
N.S.
p-value
I 30-40
20-30
IO-20
0.5 t
Mean impeded eruption rates
mm*day-’
0 I
Periods
I
I
I
1O-20
20-30
30-40
Pellets
p-value Group 1 versus group 2
Pellets
Powder p
p-co.01
Fig. 2. The impeded eruption rates during days l&IO. the results, the mean rate for each rat for each IO-day The lines link the means of the same rat. The p-values of the differences between adjacent periods calculated p-values are the significance of the differences between periods and were calculated
N.S. fKo.01
As paired t-tests within rats were used to analyse period was calculated and represented by a point. immediately under the graphs are the significance by paired r-tests within rats. The bottom row of the two groups for the changes between adjacent by unpaired I-tests.
Function Table 2. The changes
in eruption
and eruption
rates and lengths
of the impeded
Pellets to powder Impeded eruption (mm.day-‘) Unimpeded eruption (mm.day-‘) Lengths of the impeded incisors (mm) Distance from the incisal edge of the impeded incisor to the end of the unimpeded incisor (mm) Ratio of the change in impeded eruption rate to the change in length of the
703 incisor
Powder
between
to pellets
-0.09 -0.13 +0.54
* 0.04 + 0.10 + 0.25
0.00 + 0.03 -0.11 f 0.07 +0.4s * 0.21
+0.21
k 0.27
f0.34
-0.16(-0.01
to -0.41)
periods Difference p < 0.001 N.S. N.S.
* 0.31
-0.01(+2.84, +0.26 to -0.07)
N.S.
Jo < 0.01
impeded incisor (day-‘) The changes on going from pellets to powder and on going from powder to pellets in the middle three periods have been combined; e.g. the pellet-to-powder values include the differences between periods group I and periods 2 and 3 for group 2. The results are given as the mean + 1 SD with n = which are not normally distributed, are given as the median and range, with two values for a range where there was an outlier. Comparisons were by l-tests and the rank-sum test for
incisor at the end of the 2-day intervals except for one rat on day 2.
DlSCUSSlON
The rats gained less weight on the powder than when they ate pellets, though no difference in the amount of food eaten was seen. There was no difference in the composition of the diets, the powder being pellets that were ground before they were given to the rats. The difference in weight gains was about 1 g.day-’ but depriving rats completely of their food for 1 day in a similar experiment (Burn-Murdoch, 1991) made them lose more than 15 g.day-’ In the present experiment there were only three food bowls per group and it is uncertain how complete was the recovery of the food from the floor of the cages at the end of the 2-day intervals, so the difference in weight gains may have been produced by a difference in the amount eaten that was too small to detect. The increase in the length of the impeded incisor in each period was greater than on pellets but the change in mean lengths was no greater on going from pellets to powder than from powder to pellets. These two findings do not contradict each other because the changes in mean lengths will include half a period on pellets and half a period on powder in both cases. On changing from powder to pellets there was no change in impeded eruption rates. The change of diet from pellets to powder decreased the impeded eruption rates. The change in impeded eruption rate on going from pellets to powder was significantly different from the difference in eruption rate on changing from powder to pellets. These findings are consistent with the impeded eruption rate being faster on pellets than powder, this being superimposed on a decline in eruption rates with time, which is evident when the first and last periods are compared. These findings show that the diet does affect eruption rates and lengths of the teeth, contrary to the findings of Addison and Appleton (1915) and Weinreb et al. (1967) and that the small effects reported by Taylor and Butcher (1951) are real. Similarly, the greater increase in length on powder than on pellets is also consistent with the impeded incisor being longer on
3 and 4 for 12. Ratios, one end of the ratios.
powder than on pellets, this being superimposed on a lengthening during the experiment, which is seen when the first and last periods are compared. The unimpeded eruption rate declined with time but was not affected by the consistency of the diet. This suggests that the effect of the diet on the impeded incisor was a local one and not systemically mediated but, as the weight gains were different on the two diets, the possibility that there may have been a systemically mediated effect of the diets on the unimpeded incisor that was masked by the changes in length of the teeth ought to be considered. When the impeded incisor gets longer the distance between its incisal edge and the end of the unimpeded incisor would also tend to increase. If any forces were applied to the unimpeded incisor, they would diminish when the distance increased, so the longer the impeded incisor the faster would be the unimpeded eruption. As the impeded incisor lengthened less on pellets than on powder, the unimpeded rate would tend to be faster on powder than on pellets, which could mask a systemic effect making the eruption slower on powder. However, there is evidence that this did not occur. Though the increase in length on powder was greater than on pellets, this was not large enough to produce any significant differences in the absolute lengths or in the distance between the incisal edge of the impeded incisor and the end of the unimpeded incisor. Also, with time the distance increased while the unimpeded rate decreased, the opposite of what would be expected if the unimpeded incisor were subjected to occlusal forces. Therefore the effect of the diet on the impeded incisor is a direct one on the tooth. Taylor and Butcher (195 1) suggested that the faster eruption when the rats were given a harder diet occurred because the incisors were often broken by the diet so that less sharpening was required and thus less force was applied to the incisor. This would imply that the eruption rate was simply related to the length of the tooth. In the present study, there was no change in eruption rate on going from powder to pellets and a decrease on changing from pellets to powder, though the length increased in both cases. The ratios of the change in eruption rate to the
R. A. BUKN-MUKLXXH
704
Mean unimpeded eruption rates
’
mm-day-
Periods
20-30 Pellets
1O-20 Powder
30-40 Powder
p
y-value
p4.01
I.3
Group
2
I.2
Mean unimpeded eruption rates mm*day-’
I.1
I
.o
Il.‘)
0.X
Periods
20.30
I O-20 f’ellet?
eruption
in length for the two dietary changes were significantly different. There is a decline in impeded eruption rates with time and this could explain the different ratios if the decline occurred independently of the change in length. There was a decline in the eruption rate of the unimpeded incisor but there was
in the lengths
Group
I
Group 2 Groups compared
f0.49 f0.38
I
f 0.36 k 0.27
N.S. -0.8 -4.2
+ 8.4 k 6.7
N.S.
of the impeded
Period
2
N.S.
rates during
change
Group 1 Group 2 Groups compared
N.S.
N.S.
Fig. 3. The unimpeded
Period
Pellets
p<0.001
p-value Group 1 versus group 2
Table 3. Changes
30-41)
Powder
days
1040,
arranged
as in Fig. 2.
also an increase in the length of the impeded incisor. The unimpeded and impeded rates were positively correlated when means for the whole experiment were taken only in group 1, and the change in unimpeded rates with time was not correlated with the change in impeded rates. The lack of correlation between unim-
incisors
Period
and weights
3
Period
of the rats in each period 4
Period
Length of rhe Impeded Incisor (mm) + 1.03 f 0.28 +0.29 + 0.25 f0.60 + 0.20 (5) f0.26 + 0.14 f0.68 k 0.28 f0.44 + 0.28
-0.10 +0.12
5
f 0.05 (5) + 0.20
p < 0.05
p < 0.05
-2.5 + 14.4 +5.8 k 5.3
N.S. Weight Q) + 15.8 + 8.9 + 1.7 + 7.5
- I.0 f 8.0 (5) + 11.7 & 6.2
-2.0 + 5.1 (5) +2.5 * 7.5
N.S.
p < 0.05
p < 0.05
N.S.
N.S.
The results are given as the mean + 1 SD with n = 6 except parentheses. For the length of the impeded incisor, period marked shortening of impeded incisors that occurs after statistical tests. see the text.
where it is stated
to the contrary
in
1 was taken as days 2-10 to omit the shortening
the adjacent
incisor.
For
Function Table 4. Changes Periods Pellets Powder Difference Pellets Powder Difference
705
and eruption
in various measurements according to diet
2 and 3
Periods
3 and 4
Periods
2, 3 and 4
Change in the L.ength of the Impeded Incisor (mm) +0.41 * 0.30 (18) f0.49 + 0.33 (12) +0.28 f 0.20 (12) +0.70 + 0.36 (17) +0.74 5 0.41 (12) +0.51 + 0.26 (11) N.S. p < 0.05 p < 0.05 Change in Weight (g) + 10.8 * 8.9 (12) + 13.8 * 7.9 (12) +ll.l k8.1 (18) -0.4* 11.6 (12) +OS + 7.8 (11) -0.6 + 10.7 (17) p < 0.05 p < 0.001 p < 0.001
The results for each diet from both groups have been combined, e.g. the pellets’ results for periods 2 and 3 contains the results for group 1 during period 3 and for group 2 in period 2. The results are given as mean + 1 SD (n). Comparisons between diets was by t-tests.
peded and impeded eruption rates has been noted before (Ness, 1956). Conversely the change in impeded eruption rates showed significant, negative correlations with the change in length of the impeded incisor. The ratios of the change in impeded eruption rate to the change in length using the means of periods 2 and 4 were not significantly different from the ratios using the first and last periods. In the first and last periods both groups were on pellets and in both of periods 2 and 4, group 1 was on powder and group 2 on pellets, so these ratios reflect changes due to time not the diet. The ratios for time lie between the ratios for the two changes in diet, so the changes with time cannot explain the different ratios for the dietary changes. The eruption rate on pellets is faster than predicted from the length of the incisor and the eruption on powder is slower than the length predicts. Therefore the effect of the diet on the eruption rate is not due to the change in length of the incisors but is independent of it. Only one lower incisor was in occlusion here, and so would have been exposed to more function than normal, which produces changes in eruption rates, lengths of the incisors and dimensions of the mandible (Brin, Steigman and Michaeli, 1990). In the study of Taylor and Butcher (1951) both lower incisors were in occlusion. As the response of the incisors in my study and theirs was similar, it is unlikely that the different amounts of function had a major effect on the results. The slowing of the impeded eruption rate with time is associated with the increased length of the tooth. A slowing of eruption would not lengthen the incisor and, though both changes could be consequences of a third factor, it is possible that an increased length could slow eruption. Increased sharpening (Taylor and Butcher, 1951) could slow the impeded rate but there would have to be additional factors to explain why the tooth lengthens despite increased sharpening. Changes in the length of the incisor could alter the magnitude (Boos, 1940) direction or leverage of the forces applied to the tooth, and the different lengths may also alter biting behaviour, which would change the forces applied to the tooth. The previous studies concluded from the smallness or lack of effect of the consistency of the diet that sharpening rather than eating is the principal influence on eruption rates. In the studies of Addison and Appleton (1915) and of Taylor and Butcher (1951) the smallness of the effect of the diet could have arisen
because the differences in the forces exerted on the incisor were slight, but in the study of Weinreb et al. (1967) one group was fed by stomach tubes, so would have done no normal eating. However, it is possible that the rats in all these studies may have responded to decreased wear on the soft or absent diets by increased sharpening, so that the effect of the diet may have been underestimated. The absence of an effect of the diet on the unimpeded eruption rate is consistent with these incisors achieving their aim of eliminating function as a variable from studies of tooth eruption (Bryer, 1957). The absence of an effect is not because conflicting local and systemic effects cancel out, as discussed earlier. Large rats were used in my study, giving large distances between the incisal edge and the end of the unimpeded incisor, and the incisors were shortened every 2 days so that a wear facet was seen on the end of the unimpeded incisor on only one occasion. In other studies (e.g. Ness, 1956), incisors are often shortened less frequently and wear facets may occur more often, so unimpeded eruption may not be as successful under those conditions. In conclusion, my study resolved the discrepancy between the earlier studies and showed that the consistency of the diet does affect eruption rates and lengths of rodent incisors but the effect is slight, though real. Also the effect is a local one on the tooth and not systemically mediated and the change in eruption rates was not due to the changes in lengths, as was suggested in a previous study, even though it appears that tooth length might be able to affect eruption rates. Unimpeded eruption rates are not affected by the consistency of the diet and an unexpected negative correlation was found between the distance from the incisal edge of the impeded incisor to the end of the unimpeded incisor and the unimpeded eruption rate. Acknowledgement-l am grateful for typing this manuscript.
to Miss L.M.
Forrester
REFERENCES Addison W. H. F. and Appleton J. L. (1915) The structure and growth of the incisor teeth of the albino rat. J. Morph. 26, 43-96. Aladdin Q. I. and Burn-Murdoch R. A. (1985) Techniques for choosing reference points from which to measure eruption rates of rat incisors. Archs oral Biol. 30,531L537.
706
R. A. BURN-MURDOCH
Boos R. H. (1940) Intermaxillary relation established by biting power. J. Am. dent. Ass. 27, 1192-1199. Brin I., Steigman S. and Michaeli Y. (1990) Effect of occlusal functional forces on incisor socket morphology and location in the rat mandible. Anat. Res. 226,367-372. Bryer L. W. (1957) An experimental evaluation of the physiology of tooth eruption. Inf. Dent. J. 7, 432478. Burn-Murdoch R. A. (1988) The effect of corticosteroids and cyclophosphamide on the eruption of resected incisor teeth in the rat. Archs oral Biol. 33, 661467. Burn-Murdoch R. A. (1991) The effect of different oral functions on eruption of rat incisor teeth. J. dent. Res. 70, 699. Burn-Murdoch R. A. and Light L. C. H. (1992) Assessing the stability of reference points used for recording move-
ments by discontinuous measurements of position. J. Physiol. 446, 477P. Ness A. R. (1956) The response of the rabbit mandibular incisor to experimental shortening and to the prevention of its eruption. Proc. R. Sot. B146, 129-154. Taylor A. C. and Butcher E. 0. (1951) The regulation of eruption rate in the incisor teeth of the white rat. J. exp. Zool. 117, 165-188. Weinreb M. M., Assif D. and Michaeli Y. (1967) Role of attrition in the physiology of the rat incisor. I. The relative value of different components of attrition and their effect on eruption. J. dent. Res. 46, 527-531. Wetzel G. (1927) Die Regulationen der Nagetierschneiderzihne. Archs. Enrwick. Mech. 112, 445479.
Archs oral Bid. Vol.38, No. 8, pp. 699-706, 1993 Printed in Great Britain. All rights reserved
THE EFFECT OF THE CONSISTENCY OF THE DIET ON ERUPTION RATES AND LENGTHS OF INCISOR TEETH IN RATS R.A. BURN-MURDOCH Sherrington
School
of Physiology,
St Thomas’s Hospital (UMDS), London SE1 7EH, U.K. (Accepted
Lambeth
Palace
Road,
2 March 1993)
Summary-It is uncertain whether the consistency of the diet affects the eruption of rodent incisors. One study reported that harder diets made incisors shorter and erupt faster, and suggested that the faster eruption was due to the decreased length. Two other studies found no effect on eruption rates. In the present study the impeded and unimpeded eruption rates of these lower incisors were measured, using ether anaesthesia for all procedures. There were two groups of rats fed in alternate periods on standard laboratory pellets and in the intervening periods on the same material ground to a powder. In each period, one group was fed on pellets and the other on powder. The impeded eruption rate was less when the rats were fed on powder than on pellets and the rate declined with time. The unimpeded incisors were not affected by the diet, so the effects on the impeded incisor were direct effects on the tooth and not systemically mediated. The impeded incisors lengthened during the experiment and the lengthening was greater on powder than on pellets. The ratio of the change in impeded eruption rates to the increase in tooth length on changing from pellets to powder was different from the ratio for going from powder to pellets, so the effect of the diet on the impeded eruption rate was not due to the change in length of the teeth. There was a negative correlation between the decline in impeded eruption rates and the lengthening of the incisor with time, suggesting that tooth length can influence eruption rates. There was a negative correlation between the unimpeded eruption rate and the distance from the incisal edge of the impeded incisor to the end of the unimpeded incisor; its origin was uncertain. Key words:
tooth
eruption,
mastication,
thegosis.
INTRODUCTION
effect is a direct one on the tooth or systemically mediated, and also to see whether the change in eruption rates was produced by the decreased length of the tooth. To prevent function being a variable in studies of tooth eruption, Bryer (1957) introduced the unimpeded incisor: a tooth that is repeatedly shortened to stop it being used; incisors that are not shortened are called impeded. Unimpeded incisors erupt rapidly, so they protrude some way into the mouth before the next shortening and may not achieve their aim. Bryer (1957) reported that unimpeded eruption rates were the same whether the rats were fed on pellets or powder but the effect on the impeded rates was not measured so the change in the diet may not have been large enough to affect eruption. Therefore, a secondary aim of my experiment was to see whether the consistency of the diet affected unimpeded eruption rates.
The continuously erupting incisors of rats are often used as examples of tooth eruption in general. They are similar to teeth of limited eruption, such as those found in man, in that their eruption is affected by function: shortening rat incisor teeth to prevent them from being bitten upon markedly accelerates their eruption (Wetzel, 1927). Rats use their incisor teeth for both eating and non-nutritional uses such as sharpening. Taylor and Butcher (1951) reported that the incisor teeth of rats were shorter and erupted faster when they were fed with intact grains of maize than when they were given powdered maize but the changes were small and not assessed statistically. They suggested that the decreased length of the teeth was responsible for the faster eruption. Other studies have not found an effect of the diet on eruption. Addison and Appleton (1915) stated that giving rats soft food did not alter eruption rates, but gave neither details of their experimental design nor the results. Weinreb, Assif and Michaeli (1967) compared feeding by stomach tubes with conventional diets and found no differences in the eruption rates but the experiment was not well designed for detecting small differences and the rats fed by stomach tube grew less rapidly than the others. Therefore, the present study was undertaken to see whether the consistency of the diet does affect the eruption of rodent incisors and, if so, whether the
MATERIALSANDMETHODS
12 Wistar rats (Tucks, Battlesbridge, Essex, U.K.), weights 415-470 g, were divided into two groups of six. The experiment was divided into four lo-day and a final &day period. In the first and last periods both groups were fed on standard laboratory pellets (Tucks). In the middle three periods, each group was fed in alternate periods on pellets and in the interven699
700
R. A. BURN-MURDOCH
ing periods on the same food given as a powder. In each period one group was given pellets and the other powder (Fig. 1). Two groups were used so that the effects due to the diet could be distinguished from effects caused by time. The rats were caged in pairs and the amount of food eaten was taken as the difference between the amount given at the beginning of each interval and the sum of the amount remaining in the food bowl at the end of the interval plus the food that could be recovered from the floor of the cages. All rats were weighed every 2 days. Eruption rates and lengths of teeth were measured at 2-day intervals by recording the position of marks on the labial surfaces of the lower incisors and their gingival margins using a calibrated graticule in a microscope eyepiece. At each measurement the lower right incisors were shortened to make them unimpeded, the left incisors remaining impeded. Ether anaesthesia was used for all procedures. The method has been described in more detail by Aladdin and Burn-Murdoch (1985). The criteria described in that paper and subsequently expanded (Burn-Murdoch, 1988; Burn-Murdoch and Light, 1992) showed that the gingival margin of the left incisor was the most accurate reference, or stationary, point from which to measure eruption rates. Averages of the various measurements and, for some measurements, the change during each period were calculated for each rat for each of the five periods. The results are given as the mean f SD of the averages for, or changes during, each period. Differences were analysed by paired t-tests when comparisons were made between different periods and by ordinary t-tests when comparisons were made between groups. When the variances of the two groups were significantly different, Satterthwaite’s approximate t-test or the rank-sum test were used. Ratios, which are not normally distributed, are presented as the median and range, two measurements being given for one end of a range if the extreme value is very atypical. Ratios were compared between groups by the ranksum test and within groups by Wilcoxon’s signedrank test. Correlation coefficients were calculated by the linear least-squares method. The level of statistical significance in all tests was taken as 5%. One rat in group 1 died on day 36. The conclusions of this experiment are the same regardless of whether the means for this rat for days 3&36 are taken as the means for days 3&40, whether the means for days 3&40 for this rat are omitted, or if this rat is omitted entirely. Days
IO
0
RESULTS
In the first and last periods when both groups were fed on pellets, the eruption rates were faster in group 2 than in group 1 but this was significant only for the unimpeded rates in the first period (Table 1). Both the impeded and unimpeded rates were significantly slower in the last period than in the first but the slowing was the same in both groups (Table 1). The impeded eruption rates for the middle three periods are given in Fig. 2. For both groups they decreased significantly when changing from pellets to powder but did not alter when changing from powder to pellets. To eliminate any effects of time, the change in eruption rates between period 2 (days l&20) and period 3 (days 20-30) was calculated for each group; the change was significantly different between the two groups (Fig. 2). This corresponds to a change from powder to pellets in group 1 and from pellets to powder in group 2. The change in eruption rate between period 3 (days 20-30) and period 4 (days 30-40) was calculated similarly. The change in diet is the opposite to before and the change in eruption rates in the two groups has been reversed and remains statistically significant (Fig. 2). When all the powderto-pellet changes in periods 2-4 are combined and compared with the pellet-to-powder changes, there is a significant difference (Table 2) with a decline on changing from pellets to powder but no change on going from powder to pellets. The unimpeded eruption rates were analysed in the same way (Fig. 3). In both groups the eruption was slower in period 3 than in period 2 and in period 4 than period 3, but this was not significant for periods 3 and 4 in group 2. The change in eruption rate between period 3 and period 2 in group 1 was not significantly different from the same change in group 2, nor was there a difference between the groups in the change from period 3 to period 4 (Fig. 3). When all the pellet-to-powder changes were compared with the combined powder-to-pellet changes, there was no significant differences between them, both changes being decreases (Table 2). There were no significant differences between the groups in the length of the impeded incisors in the first or last periods but the teeth were longer in the last period than in the first (Table 1). The increase in length during the experiment was the same in both groups. The mean lengths of the impeded incisors for each period were analysed in the same way as the eruption rates (Table 2). The increase in the average lengths for a period on going from pellets to powder was not significantly different from the increase on changing from powder to pellets. The ratios of the 20
30
41)
4x
Group 1
Pellets
Powder
Pellets
Powder
Pellets
Group
Pellets
Pellets
Powder
Pellets
Pellets
2
Fig. 1. A diagram
showing
the food given to the two groups of rats at the different stages of the experiment.
Function Table
1. Various
measurements
First period
Group 1 Group 2 Groups compared Group 1 Group 2 Groups compared Group 1 Group 2 Groups compared Group 1 Group 2 Groups compared
Group 1 Group 2 Groups compared
701
and eruption during
Last period
the first and last periods
Periods compared
Impeded Eruption Rates (mm.day -‘) 0.51 + 0.06 0.31 & 0.05 (5) -0.18 * 0.05 (5)
0.57 + 0.07 N.S.
0.35 * 0.07 N.S.
-0.21 + 0.09 N.S. Unimpeded Eruption Rates (mm.day - ’ )
1.12+0.06 0.91 + 0.08 (5) -0.19 f 0.09 (5) 1.20 + 0.06 0.97 & 0.08 -0.22 _+0.06 N.S. N.S. p < 0.05 Length of the Impeded Incisor (mm) 8.49 + 0.88 10.60 + 1.04(5) + 1.93f 0.39 (5) 8.60 + 0.36 10.21 & 0.43 + 1.61 k 0.46 N.S. N.S. N.S. Weights (g) 442.4 + 17.7 451.4 + 26.1 (5) +11.4& 14.8 (5) 450.7 + 12.7 470.3 * 14.7 +19.6+4.1 N.S. N.S. N.S. Distance from the Incisal Edge of the Impeded Incisor to the End of the Unimpeded Incisor (mm) 5.10 + 0.55 5.59 k 0.26 (5) +0.36 k 0.34 (5) 5.05 + 0.63 5.36 k 0.38 +0.31 f 0.38 N.S. N.S. N.S.
p < 0.001 p < 0.01
p < 0.05 p < 0.001
p
N.S. p < 0.001
N.S. N.S.
The averages for each rat for the periods were calculated and the results in the table are the mean I 1 SD of the averages for the rats. n = 6 unless indicated to the contrary in parentheses. The lengths of the impeded incisor in the first period omit the readings for day 0 because of the marked shortening that occurs on unimpeding the adjacent incisor. For
statistical analysis, see the text.
_
change in impeded eruption rates to the changes in length of the impeded incisor were significantly more negative on changing from pellets to powder than from powder to pellets @ < 0.01, Table 2). The ratios for the changes between the first and last periods ( - 0.10, range - 0.06 to - 0.18) were not significantly different from the ratios for the changes between periods 2 and 4 ( - 0.08, range - 0.03 to - 0.14) and neither showed significant differences between the groups. These ratios lay between the ratios for the two changes between the diets and were significantly different from them (p < 0.01 in all cases with the two groups combined) except for the comparison between the ratios for the pellet-to-powder change and for the first and last periods. The changes in length of the impeded incisor within each period are given in Table 3. In each of the middle three periods, the increase in length was greater in the group given powder than in the group on pellets, but this was significant only for period 4. Taking overall means for periods 2 and 4, in both of which group 1 was on powder and group 2 on pellets, the increase in length was significantly greater for powder than for pellets (0.75 + 0.11, 0.47 f O.l4mm/period, p < 0.01). Combining the length changes for periods 2, 3 and 4, there was a greater lengthening on powder than on pellets (Table 4). The results for powder include 11 values from group 1 and six from group 2, while the pellet results have six values from group I and 12 from group 2, but this is unlikely to have caused the difference between powder and pellets because there were no significant differences between the groups either in the first period when both groups were on pellets or in the change in length between the first and last periods. The length change in the last period was significantly larger in group 2 than group 1 (Table 3) but this difference between the groups
would have made the length changes larger on pellets than on powder, contrary to what was obtained. When the results for pellets and powder were combined separately for periods 2 and 3 and for periods 3 and 4 to avoid this bias, the difference between pellets and powder was still present but significant only for periods 3 and 4 (Table 4). There was a significant, positive correlation between the unimpeded and impeded eruption rates, using mean rates over the whole experiment for each rat, in group 1 (r = f0.87, n = 6, p < 0.05) but the correlations were not significant in group 2 or when the two groups were combined. There was no correlation between the decline in the unimpeded rates from the first to the last periods and the corresponding decline in the impeded rates. There were no significant correlations between impeded eruption rates and the lengths of the impeded incisor when means for the whole experiment were used, but there were negative correlations between the decline in impeded rates between the first and last period and the corresponding change in length of the incisor in group 2 (Y = -0.92, n = 6, p < 0.05) and when the two groups were combined (r = -0.61, n = II, p < 0.05). The correlation was not significant in group 1 but the corresponding correlations for the differences between period 1 and both periods 3 and 4 were significant and negative. The distance between the incisal edge of the left incisor and the cut end of the unimpeded incisor in each 2-day interval was taken as the average of the distance immediately after shortening the incisor at one measurement and the distance just before shortening the incisor at the next measurement. Means of the distances were taken for each rat for each period and were analysed in the same way as the eruption rates. There were no significant differences between
R. A. BURN-MURDOCH
702
the groups in any period or any differences due to the consistency of the diet (Table 2). No significant differences were found when the same analyses were made separately on the distances just before and after shortening the incisor. The distance increased during the experiment (Table 1); the difference between the first and last periods was significant when the two groups were combined. There were negative correlations between the unimpeded eruption rate and the distance from the incisal edge to the end of the unimpeded incisor (r = -0.67, n = 12, p < 0.05) when mean values over the whole experiment for each rat were used. The correlations for the distances at the beginning and end of the 2-day intervals were also significant and negative. There were no correlations between the unimpeded eruption rates and the lengths of the impeded incisors. There were no significant differences in the weights of the rats between the two groups in the first and last periods (Table 1). Both groups gained weight during the experiment but this was significant for group 2
only; the difference between the groups in the weight gains was not significant (Table 1). There was not a significant difference between the groups in the weight gains during the first or last periods (Table 3). In each of the periods 24, the weight gain was greater in the group on pellets than in the powder group but this was not significant in period 2. When the weight gain in period 2 was subtracted from the gain in period 3 there was a significant difference between the groups (groups 1, + 18.3 + 21.5; group 2, -4.2 f 11.7 g; p < 0.05). The difference between periods 3 and 4 was reversed and still significant (- 18.0 f 13.6, + 10 k 12.2 g for groups 1 and 2 respectively, p < 0.01). When the weight gains from periods 2, 3 and 4 were combined (Table 4), the gain for pellets was significantly greater than for powder but, as with the length changes, there are unequal numbers of results from the two groups in the pellets and powder results. No significant differences between the groups or diets was found in the amount of food eaten. Wear facets were not seen on the end of the unimpeded
Mean impeded eruption rates
mm-day-’
I
I Periods
Powder
Powder
Pellets
p
N.S.
p-value
I 30-40
20-30
IO-20
0.5 t
Mean impeded eruption rates
mm*day-’
0 I
Periods
I
I
I
1O-20
20-30
30-40
Pellets
p-value Group 1 versus group 2
Pellets
Powder p
p-co.01
Fig. 2. The impeded eruption rates during days l&IO. the results, the mean rate for each rat for each IO-day The lines link the means of the same rat. The p-values of the differences between adjacent periods calculated p-values are the significance of the differences between periods and were calculated
N.S. fKo.01
As paired t-tests within rats were used to analyse period was calculated and represented by a point. immediately under the graphs are the significance by paired r-tests within rats. The bottom row of the two groups for the changes between adjacent by unpaired I-tests.
Function Table 2. The changes
in eruption
and eruption
rates and lengths
of the impeded
Pellets to powder Impeded eruption (mm.day-‘) Unimpeded eruption (mm.day-‘) Lengths of the impeded incisors (mm) Distance from the incisal edge of the impeded incisor to the end of the unimpeded incisor (mm) Ratio of the change in impeded eruption rate to the change in length of the
703 incisor
Powder
between
to pellets
-0.09 -0.13 +0.54
* 0.04 + 0.10 + 0.25
0.00 + 0.03 -0.11 f 0.07 +0.4s * 0.21
+0.21
k 0.27
f0.34
-0.16(-0.01
to -0.41)
periods Difference p < 0.001 N.S. N.S.
* 0.31
-0.01(+2.84, +0.26 to -0.07)
N.S.
Jo < 0.01
impeded incisor (day-‘) The changes on going from pellets to powder and on going from powder to pellets in the middle three periods have been combined; e.g. the pellet-to-powder values include the differences between periods group I and periods 2 and 3 for group 2. The results are given as the mean + 1 SD with n = which are not normally distributed, are given as the median and range, with two values for a range where there was an outlier. Comparisons were by l-tests and the rank-sum test for
incisor at the end of the 2-day intervals except for one rat on day 2.
DlSCUSSlON
The rats gained less weight on the powder than when they ate pellets, though no difference in the amount of food eaten was seen. There was no difference in the composition of the diets, the powder being pellets that were ground before they were given to the rats. The difference in weight gains was about 1 g.day-’ but depriving rats completely of their food for 1 day in a similar experiment (Burn-Murdoch, 1991) made them lose more than 15 g.day-’ In the present experiment there were only three food bowls per group and it is uncertain how complete was the recovery of the food from the floor of the cages at the end of the 2-day intervals, so the difference in weight gains may have been produced by a difference in the amount eaten that was too small to detect. The increase in the length of the impeded incisor in each period was greater than on pellets but the change in mean lengths was no greater on going from pellets to powder than from powder to pellets. These two findings do not contradict each other because the changes in mean lengths will include half a period on pellets and half a period on powder in both cases. On changing from powder to pellets there was no change in impeded eruption rates. The change of diet from pellets to powder decreased the impeded eruption rates. The change in impeded eruption rate on going from pellets to powder was significantly different from the difference in eruption rate on changing from powder to pellets. These findings are consistent with the impeded eruption rate being faster on pellets than powder, this being superimposed on a decline in eruption rates with time, which is evident when the first and last periods are compared. These findings show that the diet does affect eruption rates and lengths of the teeth, contrary to the findings of Addison and Appleton (1915) and Weinreb et al. (1967) and that the small effects reported by Taylor and Butcher (1951) are real. Similarly, the greater increase in length on powder than on pellets is also consistent with the impeded incisor being longer on
3 and 4 for 12. Ratios, one end of the ratios.
powder than on pellets, this being superimposed on a lengthening during the experiment, which is seen when the first and last periods are compared. The unimpeded eruption rate declined with time but was not affected by the consistency of the diet. This suggests that the effect of the diet on the impeded incisor was a local one and not systemically mediated but, as the weight gains were different on the two diets, the possibility that there may have been a systemically mediated effect of the diets on the unimpeded incisor that was masked by the changes in length of the teeth ought to be considered. When the impeded incisor gets longer the distance between its incisal edge and the end of the unimpeded incisor would also tend to increase. If any forces were applied to the unimpeded incisor, they would diminish when the distance increased, so the longer the impeded incisor the faster would be the unimpeded eruption. As the impeded incisor lengthened less on pellets than on powder, the unimpeded rate would tend to be faster on powder than on pellets, which could mask a systemic effect making the eruption slower on powder. However, there is evidence that this did not occur. Though the increase in length on powder was greater than on pellets, this was not large enough to produce any significant differences in the absolute lengths or in the distance between the incisal edge of the impeded incisor and the end of the unimpeded incisor. Also, with time the distance increased while the unimpeded rate decreased, the opposite of what would be expected if the unimpeded incisor were subjected to occlusal forces. Therefore the effect of the diet on the impeded incisor is a direct one on the tooth. Taylor and Butcher (195 1) suggested that the faster eruption when the rats were given a harder diet occurred because the incisors were often broken by the diet so that less sharpening was required and thus less force was applied to the incisor. This would imply that the eruption rate was simply related to the length of the tooth. In the present study, there was no change in eruption rate on going from powder to pellets and a decrease on changing from pellets to powder, though the length increased in both cases. The ratios of the change in eruption rate to the
R. A. BUKN-MUKLXXH
704
Mean unimpeded eruption rates
’
mm-day-
Periods
20-30 Pellets
1O-20 Powder
30-40 Powder
p
y-value
p4.01
I.3
Group
2
I.2
Mean unimpeded eruption rates mm*day-’
I.1
I
.o
Il.‘)
0.X
Periods
20.30
I O-20 f’ellet?
eruption
in length for the two dietary changes were significantly different. There is a decline in impeded eruption rates with time and this could explain the different ratios if the decline occurred independently of the change in length. There was a decline in the eruption rate of the unimpeded incisor but there was
in the lengths
Group
I
Group 2 Groups compared
f0.49 f0.38
I
f 0.36 k 0.27
N.S. -0.8 -4.2
+ 8.4 k 6.7
N.S.
of the impeded
Period
2
N.S.
rates during
change
Group 1 Group 2 Groups compared
N.S.
N.S.
Fig. 3. The unimpeded
Period
Pellets
p<0.001
p-value Group 1 versus group 2
Table 3. Changes
30-41)
Powder
days
1040,
arranged
as in Fig. 2.
also an increase in the length of the impeded incisor. The unimpeded and impeded rates were positively correlated when means for the whole experiment were taken only in group 1, and the change in unimpeded rates with time was not correlated with the change in impeded rates. The lack of correlation between unim-
incisors
Period
and weights
3
Period
of the rats in each period 4
Period
Length of rhe Impeded Incisor (mm) + 1.03 f 0.28 +0.29 + 0.25 f0.60 + 0.20 (5) f0.26 + 0.14 f0.68 k 0.28 f0.44 + 0.28
-0.10 +0.12
5
f 0.05 (5) + 0.20
p < 0.05
p < 0.05
-2.5 + 14.4 +5.8 k 5.3
N.S. Weight Q) + 15.8 + 8.9 + 1.7 + 7.5
- I.0 f 8.0 (5) + 11.7 & 6.2
-2.0 + 5.1 (5) +2.5 * 7.5
N.S.
p < 0.05
p < 0.05
N.S.
N.S.
The results are given as the mean + 1 SD with n = 6 except parentheses. For the length of the impeded incisor, period marked shortening of impeded incisors that occurs after statistical tests. see the text.
where it is stated
to the contrary
in
1 was taken as days 2-10 to omit the shortening
the adjacent
incisor.
For
Function Table 4. Changes Periods Pellets Powder Difference Pellets Powder Difference
705
and eruption
in various measurements according to diet
2 and 3
Periods
3 and 4
Periods
2, 3 and 4
Change in the L.ength of the Impeded Incisor (mm) +0.41 * 0.30 (18) f0.49 + 0.33 (12) +0.28 f 0.20 (12) +0.70 + 0.36 (17) +0.74 5 0.41 (12) +0.51 + 0.26 (11) N.S. p < 0.05 p < 0.05 Change in Weight (g) + 10.8 * 8.9 (12) + 13.8 * 7.9 (12) +ll.l k8.1 (18) -0.4* 11.6 (12) +OS + 7.8 (11) -0.6 + 10.7 (17) p < 0.05 p < 0.001 p < 0.001
The results for each diet from both groups have been combined, e.g. the pellets’ results for periods 2 and 3 contains the results for group 1 during period 3 and for group 2 in period 2. The results are given as mean + 1 SD (n). Comparisons between diets was by t-tests.
peded and impeded eruption rates has been noted before (Ness, 1956). Conversely the change in impeded eruption rates showed significant, negative correlations with the change in length of the impeded incisor. The ratios of the change in impeded eruption rate to the change in length using the means of periods 2 and 4 were not significantly different from the ratios using the first and last periods. In the first and last periods both groups were on pellets and in both of periods 2 and 4, group 1 was on powder and group 2 on pellets, so these ratios reflect changes due to time not the diet. The ratios for time lie between the ratios for the two changes in diet, so the changes with time cannot explain the different ratios for the dietary changes. The eruption rate on pellets is faster than predicted from the length of the incisor and the eruption on powder is slower than the length predicts. Therefore the effect of the diet on the eruption rate is not due to the change in length of the incisors but is independent of it. Only one lower incisor was in occlusion here, and so would have been exposed to more function than normal, which produces changes in eruption rates, lengths of the incisors and dimensions of the mandible (Brin, Steigman and Michaeli, 1990). In the study of Taylor and Butcher (1951) both lower incisors were in occlusion. As the response of the incisors in my study and theirs was similar, it is unlikely that the different amounts of function had a major effect on the results. The slowing of the impeded eruption rate with time is associated with the increased length of the tooth. A slowing of eruption would not lengthen the incisor and, though both changes could be consequences of a third factor, it is possible that an increased length could slow eruption. Increased sharpening (Taylor and Butcher, 1951) could slow the impeded rate but there would have to be additional factors to explain why the tooth lengthens despite increased sharpening. Changes in the length of the incisor could alter the magnitude (Boos, 1940) direction or leverage of the forces applied to the tooth, and the different lengths may also alter biting behaviour, which would change the forces applied to the tooth. The previous studies concluded from the smallness or lack of effect of the consistency of the diet that sharpening rather than eating is the principal influence on eruption rates. In the studies of Addison and Appleton (1915) and of Taylor and Butcher (1951) the smallness of the effect of the diet could have arisen
because the differences in the forces exerted on the incisor were slight, but in the study of Weinreb et al. (1967) one group was fed by stomach tubes, so would have done no normal eating. However, it is possible that the rats in all these studies may have responded to decreased wear on the soft or absent diets by increased sharpening, so that the effect of the diet may have been underestimated. The absence of an effect of the diet on the unimpeded eruption rate is consistent with these incisors achieving their aim of eliminating function as a variable from studies of tooth eruption (Bryer, 1957). The absence of an effect is not because conflicting local and systemic effects cancel out, as discussed earlier. Large rats were used in my study, giving large distances between the incisal edge and the end of the unimpeded incisor, and the incisors were shortened every 2 days so that a wear facet was seen on the end of the unimpeded incisor on only one occasion. In other studies (e.g. Ness, 1956), incisors are often shortened less frequently and wear facets may occur more often, so unimpeded eruption may not be as successful under those conditions. In conclusion, my study resolved the discrepancy between the earlier studies and showed that the consistency of the diet does affect eruption rates and lengths of rodent incisors but the effect is slight, though real. Also the effect is a local one on the tooth and not systemically mediated and the change in eruption rates was not due to the changes in lengths, as was suggested in a previous study, even though it appears that tooth length might be able to affect eruption rates. Unimpeded eruption rates are not affected by the consistency of the diet and an unexpected negative correlation was found between the distance from the incisal edge of the impeded incisor to the end of the unimpeded incisor and the unimpeded eruption rate. Acknowledgement-l am grateful for typing this manuscript.
to Miss L.M.
Forrester
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