Key Words

Arch width changes, extraction Vs non-extraction

Introduction

Correction of malocclusion is one of the major objectives of orthodontics. Placing the teeth in their appropriate antero-posterior positions is an essential aspect of all orthodontic cases. In orthodontic treatment with the extraction of bicuspids, based on the anchorage considerations, either retraction of incisors or protraction of molars or a combination has been done. This can be achieved by using various appliances of which Pre-adjusted Edgewise Appliance system has been the mainstay nowadays. One of the stable reference landmarks in the oral cavity is the “palatal rugae” [1][2]. These palatal rugae are a series of mucosal ridges composed of dense connective tissues covered by squamous epithelium, present on either side of the median palatal raphe behind the incisive papilla of the anterior palate. They play an important role in orthodontics to assess the antero-posterior and transverse movements of the teeth. Lysell in 1957 devised a classification of rugae based on size, shape, angulation and number [3]. It was further modified by Thomas & Kotze in 1983 [4]. Based on their classification,3 categories were formed as 1st, 2nd and 3rd primary rugae. Various methods have been utilized in the past to record and interpret rugae pattern which included direct vision, photographs and palatal print using bubble gum and dental stone. Thomas & Kotze (1983) indicated that rugae are unique to each individual and don’t change with age and of constant shape throughout life. Rugae patterns by virtue of their position are protected from extreme trauma and are insulated from heat by teeth, tongue and fatty tissues beneath them. With extraction of premolars & space closure using fixed appliances, movement of the teeth occur in the antero-posterior and transverse direction aided by alternating bone resorption and apposition at various levels [5]. Routinely, treatment progress and post treatment changes can be evaluated using cephalometric superimposition. Limitations using the cephalometric analysis are radiation hazards, errors due to operator skills and difficulty in differentiating between right and left sides of the arch. Previously dental casts were evaluated using Symmetrography, Stereophotography etc., [3]. Two dimensional photography have been used successfully by Huddart in the early 1970’s. Following this, 3-D measurements have been utilized to study the dental casts which include Optocom by Vanderlinden [6] in 1978, stereophotogrammetry by Savara [7] in 1965. Image analysis by Brook & Pitts [8] in 1983 and Reflex metrograph by Almedia [1] in 1995 & Bailey [2] et al. in 1996 had been used to study the rugae pattern in dental casts. Recent advances in Bio-technology had led to the analysis of tooth movements that are relative to rugae points with acceptable method of computers in routine orthodontic practice [7]. The present study is aimed to evaluate and assess the antero-posterior tooth movements as well as to determine the transverse arch dimensional changes in the intercanine and intermolar region using third primary rugae as a stable reference landmark in the dental casts in bicuspid extraction and non-extraction cases before and after orthodontic treatment using AUTO-CAD.

Materials and Methods

A total of 100 dento-alveolar malocclusion cases treated with bicuspid extraction and non extraction using PEA system with good treatment records were selected. Surgical and dento-facial deformity cases were excluded from the study. Models with poor clarity of the rugae pattern were also eliminated. Selected cases were in the age group of 19-25yrs. Of these,50 were males & 50 were females. 50 cases were ( Group 1) bicuspid extraction and the other 50 were of (Group 2) non-extraction category. The photographs of the occlusal view of the study models were taken by using a digital camera, Nikon cool pix 2000, mounted on a tripod at a standardized height of about 20 cm from the table parallel to the floor. Study models were oriented by placing it on the template parallel to the floor, so that orientation of the pre & post treated models did not change. All the 100 pre and post treatment study models were successfully photographed by a professional photographer. The outline of the third primary rugae were traced using 0.5mm graphite marker. The medial and lateral ends of the rugae were marked as MR, LR. The mesio-incisal tip of the most proclined incisor was marked as R1, L1 for the respective sides. The mesial contact points of the first molars were marked as R6, L6. The above landmarks were used for measuring the antero-posterior arch dimensions. The cusp tip of the canines and the buccal cusp tip of the first molars on both sides were also marked for measuring the transverse arch dimensional changes. The digital images of the models were then fed into the computer using USB port and flash card reader. The landmarks were then digitized on the computer using on-screen digitization technique. After on-screen digitization, the specified distances were calculated using AUTOCAD between the points plotted, so that changes in the antero-posterior and transverse dimensions can be assessed accurately. The data obtained were subjected to statistical analyses using Paired t-test, unpaired t-test, Levene’s test and Gain score measurement test.

Results

Table 1 shows comparison of the mean values of respective measurements on right side for Group 1. The mean values of pre &post treated R1-MR values show significant amount of antero-posterior tooth movements & space closure, since ‘p’ value is less than 0.001 which is statistically highly significant for extraction cases. Likewise, mean values of R1-LR, R6-MR, R6-LR were statistically highly significant, since p-value is 0.000 showing statistical significance at 100% level.

Table 2 shows comparison of the mean values of respective measurements on left side for Group 1. L1-MR, L1-LR, L6-MR, L6-LR mean values were statistically highly significant showing significant amount of antero-posterior tooth movements occurred after orthodontic treatment in bicuspid extraction cases (Group 1).

Table 1

Comparison of the mean values of respective measurements on right side for Group

Measurement Mean(mms) S.D t-value p-value Correlation
R1-MR
Pre 33.385 5.672 11.10 0.000 0.715
Post 27.155 4.164
R1-LR
Pre 37.578 6.346 11.30 0.000 0.764
Post 31.015 4.587
R6-MR
Pre 30.821 4.070 4.41 0.000 0.019
Post 28.529 4.330
R6-LR
Pre 19.721 3.785 4.30 0.000 0.703
Post 18.016 3.440

Table 2

Comparison of the mean values of respective measurements on left side for Group 1

Measurement Mean(mms) S.D t-value p-value Correlation
L1-MR
Pre 36.831 10.359 6.97 0.000 .595
Post 28.494 4.625
L1-LR
Pre 36.599 6.402 14.63 0.000 .865
Post 29.886 5.112
L6-MR
Pre 31.129 4.335 4.55 0.000 .764
Post 29.192 4.409
L6-LR
Pre 19.644 2.954 6.07 0.000 .559
Post 17.240 3.007

Table 3

Comparison of the mean values of respective measurements on right side for Group 2

Measurement Mean(mms) S.D t-value p-value Correlation
R1-MR
Pre 30.857 5.382 3.15 0.003 .761
Post 29.019 6.285
R1-LR
Pre 34.727 5.824 3.68 0.001 .698
Post 32.344 5.949
R6-MR
Pre 30.119 4.364 0.37 0.714 .717
Post 30.296 4.648
R6-LR
Pre 19.082 3.683 0.91 0.367 .720
Post 18.712 3.968

Table 3 shows comparison of the mean values of respective measurements on right side for Group 2. Statistical analysis reveals minimal changes between the mean values of pre & post treated cases showing minimal significant p- values for R6-MR, R6-LR and significant p-values for R1-MR & R1-LR.

Table 4 shows comparison of the mean values of respective measurements on left side for Group 2. Statistical values reveal that L1-MR & L1-LR values were highly significant and minimal significance in L6-MR & L6-LR values.

Table 5 shows comparison of the mean values of right side for Group I and II. It can be inferred that significant antero-posterior tooth movements occurred in extraction cases when compared to non extraction cases as the p-values were statistically highly significant for all measurements, except R6-LR value, which is significant at 5% level (not significant).

Table 6 shows comparison of the mean values of left side for Group I and II. It can be inferred that significant changes occurred in the antero-posterior direction in bicuspid extraction and minimal changes in non extraction cases, since p-values were statistically highly significant for all measurements.

On the basis of statistical analysis, arch dimensional changes were having statistically highly significant values for extraction cases when compared to non-extraction cases.

Table 7 shows comparison mean values of pre and post treatment intermolar width (I.M.W) and intercanine width (I.C.W) in Group 1. A significant reduction in the intermolar width after premolar extractions occurred as statistically highly significant p-value was obtained, but only minimal changes in the intercanine width showing significance at 5% level. This shows that there was more mesial migration of the molars and anchorage loss in bicuspid extraction cases.

Table 4

Comparison of the mean values of respective measurements on left side for Group 2

Measurement Mean(mms) S.D t-value p-value Correlation
L1-MR
Pre 33.0722 5.637 3.52 0.001 .704
Post 30.7614 6.315
L1-LR
Pre 34.5006 5.800 3.96 0.000 .752
Post 32.1284 6.166
L6-MR
Pre 30.6766 5.225 0.76 0.452 .841
Post 31.0028 5.521
L6-LR
Pre 19.0332 3.902 1.10 0.276 .793
Post 19.4262 3.942

Table 5

Comparison of the mean values of right side for Group 1 and 2

Measurement Mean(mms) S.D t-value p-value
R1-mr
Extn 6.230 3.968 5.44 0.000
Non extn 1.832 4.121
R1-lr
Extn 6.502 4.068 4.76 0.000
Non extn 2.383 4.579
R6-mr
Extn 2.291 3.673 3.49 0.001
Non extn 0.177 3.401
R6-lr
Extn 1.704 2.803 2.35 0.021
Non extn 0.370 2.874

Table 6

Comparison of the mean values of left side for Group 1 and 2

Measurement Mean(mms) S.D t-value p-value
L1-MR
Extn 8.337 8.464 4.42 0.000
Non-EXTN 2.310 4.639
L1-LR
Extn 6.713 3.245 5.76 0.000
Non-EXTN 2.372 4.232
L6-MR
Extn 1.936 3.007 3.74 0.000
Non-EXTN 0.326 3.040
L1-MR
Extn 2.404 2.799 5.25 0.000
Non-EXTN 0.393 2.525

Table 7

Comparison of mean values of pre and post treatment intermolar width (I.M.W) and intercanine width (I.C.W) in Group 1

Measurement Mean (mms) S.D t-value p-value Correlation
I.M.W
Pre 65.384 3.873 7.50 0.000 .778
Post 62.632 3.911
I.C.W
Pre 34.922 2.816 2.30 0.026 .785
Post 35.530 2.885

Table 8

Comparison of mean values of pre and post treatment intermolar width (I.M.W) and intercanine width (I.C.W) in Group 2

Measurement Mean (mms) S.D t-value p-value Correlation
I.M.W 63.624 5.628 0.36 0.720 .613
Pre 63.343 6.702
Post
I.C.W 34.787 2.618 0.73 0.467 .761
Pre 34.980 2.768
Post

Table 9

Comparison of mean values of pre and post treatment intermolar width (I.M.W) and intercanine width (I.C.W) between Group 1 and 2

Measurement Mean (mms) S.D t-value p-value
I.M.W 2.752 2.593 2.870 0.005
Extraction 0.280 5.511
Non Extraction
I.C.W 0.608 1.869 1.110 0.270
Extraction 0.193 1.866
Non Extraction

Table 8 shows comparison of mean values of pre and post treatment intermolar width (I.M.W) and intercanine width (I.C.W) in Group 2. There were no significant arch dimensional changes after non-extraction orthodontic treatment in both intermolar and intercanine regions.

Table 9 shows comparison of mean values of pre and post treatment intermolar width (I.M.W) and intercanine width (I.C.W) between Group 1 and 2. There were significant differences between pre & post treatment values for measurements in the intermolar region and insignificant in the intercanine region.

Discussion

Dental study models being an essential diagnostic criteria serve as a three dimensional record of the dental arch in diagnosis, treatment planning, assessment of the growth as well treatment progress. One aspect of the study using study model is morphometrics of a single model and the other aspect is comparative morphometrics between two study models. The third aspect of the study is comparative morphometrics using stable reference landmark against a variable landmark. A review of literature reveals that the third primary rugae is one of the stable reference landmarks in the oral cavity which does not change with age or orthodontic treatment [1][2][6]. The reliability and validity of palatal rugae for assessing treatment changes on study models has been studied earlier. The results, however, were inconsistent due to the conventional manual methods applied for evaluating the data. Therefore, cephalometric superimposition method has been used as the most reliable method for assessment of treatment progress and post treatment changes. But radiation hazards, inter-operator variations and difficulty in differentiating between the right and left side shadows of the arch are few of its limitations. In the present study, we had attempted to pursue the recent interest to revive the usage of study models for assessing the arch dimensional changes before and after orthodontic treatment by the use of computers for better accuracy. Various techniques have been advocated in the past for measuring the study models like using flexible rulers and dividers. Dental casts were measured by devices like Symmetrograph, Stereophotometry by Savara (1965) etc., [3][7]. But recently more sophisticated 3-D method of analyzing the dental casts by digitization methods are being used [6][7][8]. Peavy & Kendrick (1967) using Symmetrograph evaluated the effect of tooth movements on the palatal rugae and concluded that only slight morphological alterations were seen in 127 rugae studied as a result of tooth movements, indicating that the rugae patterns maybe clinically acceptable [9]. Vanderlinden (1978) using Optocom evaluated the tooth movements relative to the rugae points in non-orthodontically treated individuals and concluded that the medial & lateral rugae points can be used for the evaluation of changes in the sagittal direction of the posterior teeth [6]. Whereas, Bailey (1996) & Almedia et al.(1996) using Reflex Metrography made a comparative analysis to evaluate the stability of the rugae in extraction and non extraction orthodontically treated cases and concluded that the medial & lateral points of the third rugae appear to be stable for construction of anatomical reference points for longitudinal cast analysis [1][2]. Shearn & Woods et al. (2000), Ong & Woods et al., (2001) used WESTCEF program for cephalometric measurements and digital caliper for study cast measurements to determine arch dimensional changes with different premolar extraction patterns [5]. They found that there was evidence of greater intermolar width reduction after 2nd premolar extraction than 1st premolar extractions. They also found that a large amount of individual variation in incisal and molar changes accompanied orthodontic treatment involving different premolar extraction patterns. But their study was based on mandibular arch dimensional variations. Boley & Sachdeva (2003) studied Class I bicuspid extraction patterns using both cephalometric measurements and standardized one to one occlusal photographs of the study models to evaluate the tooth movement [10]. Their results showed that maxillary molar width remained unchanged and there is a reduction in the maxillary arch length during treatment due to molar protraction & anterior retraction. Gianelly et al. (2003) studied the arch dimensional changes after extraction of first premolars and non-extraction orthodontically treated cases on study models using electronic calipers [11]. He measured the intercanine and intermolar width in the maxillary and mandibular arches and compared them statistically to determine whether the dental arches were narrower after extraction treatment. They found that in both the groups, anterior & posterior arch width remained the same except for the mandibular intercanine width which was increased in the extraction groups. They concluded that extraction of first premolars does not result in the narrowing of the dental arches. Recent advances in bio-technology have led to the use of computers for morphometric evaluation of study models in routine orthodontic practice. In the present study, a comparative morphometry on study models was undertaken to assess the arch dimensional changes due to orthodontic treatment. Transverse arch dimensional changes were studied between the cusp tip of canines and buccal cusp tip of first molars (variable landmark) for intercanine and intermolar width respectively. Antero-posterior arch dimensional changes were assessed between the medial and lateral ends of the third primary rugae (stable landmark) to the mesio-incisal tip of the most proclined tooth (variable point). 100 study models were photographed using Nikon cool pix 2000 digital camera, which was mounted on a tripod at a standardized height of 20cm from the table, with optical zoom standardized to 3x and using max. pixel size of 2 Mega pixels, so that the image size captured will be of actual object size. After recording the photographs of the pre & post orthodontically treated study models, they were fed into the computer using specialized device (USB port &flashcard reader), so that the captured image appears on-screen on the monitor and further measurements can be derived accurately using AUTOCAD, a computer based software which has high degree of precision & accuracy for measuring the distances which are useful in engineering & architecture [12]. Using the CAD software, a series of points are plotted on an object and the distance between them can be measured which will appear in certain specified units (usually in inches which is converted into mm). The major advantage claimed is the precision & accuracy of the CAD system of measurements (accuracy up to 0.000000001mm). Limitations of the system is that it is a 2-dimensional system of measuring a 3-D object. Also, the image captured by the camera in one format (jpeg) will be converted to a different format (dwg/bmp) and stored by the CAD software for study. Hence, the actual resolution of the stored image will be reduced. But, this does not have any effect upon the accuracy of the measurements obtained by using AUTOCAD. Results of the study were statistically analyzed using paired and unpaired ‘t’tests as well as gain score measurements.

From Table 1, mean values of R1-MR, R1-LR show a mean of 6 mm anterior retraction and space closure in upper premolar extractions cases. Mean values of R6-MR, R6-LR show a mean of 2-2.5 mm of molar mesial movement and anchorage loss in extraction cases.

From Table 2, L1-MR, L1-LR values show a mean of 8 mm anterior retraction & space closure in bicuspid extraction cases in the left quadrant. Mean values of L6-MR, L6-MR show a mean of 1.5 – 2 mm mesial molar movement & anchorage loss in the left quadrant. From Table 1 & Table 2, it may be construed that 1/3rd -1/4th of the bicuspid extraction space has been found to be consumed as anchorage loss even in PEA system as in Begg’s technique. At this juncture, it is necessary to mention that the precise anchorage consideration and mode of anchorage conservation, if any used, couldn’t be obtained in most of the cases studied.

Table 3 & Table 4 show the mean values of pre & post orthodontically treated non-extraction cases, both of which show minimal anterior retraction & space closure (less than 2 mm) as well as minimal molar mesial movement & anchorage loss.

From Table 5 & Table 6, it is clearly evident that statistically significant changes can be observed in the antero-posterior dimensions in bicuspid extraction cases when compared to non extraction cases. The above findings of the present study were consistent with the results of Bailey et al. & Almedia et al. [1][2]. Also, the present findings were consistent with those of Boley & Sachdeva, where there was a greater reduction in the arch length due to anterior retraction as well as molar protraction [10].

Table 7 & Table 8 show that the arch dimensional changes in transverse dimension were highly statistically significant between the pre & post treated models in extraction cases (a mean of 3 mm intermolar width reduction & 0.5 mm increase in intercanine width) and insignificant in non-extraction cases (0.2 – 0.3 mm reduction in the corresponding values). Also, from Table 9, comparing extraction and non-extraction intermolar width values, a mean of 2.5mm of intermolar width reduction occurred in extraction cases and less than 0.5 mm in non-extraction cases showing statistically insignificant changes in non-extraction cases. In the intercanine region, a mean of 0.5 mm increase in the I.C.W value in extraction cases and less than 0.2 mm increase in non-extraction cases showing statistically insignificant change in both cases. Gain score values from Table 9 indicate that there were significant changes between pre & post treatment values for measurements in the intermolar region and insignificant in intercanine region. Thus, it can be inferred that transverse arch dimensional values also showed significant changes in extraction cases when compared to non-extraction cases. All these findings were similar to the findings of the study by Shearns & Woods (1999),where there was evidence of greater reduction in the intermolar width after bicuspid extractions [5]. However, our results did not correspond to the findings of Boley & Sachdeva (2003) as well as that of Giannely (2003) et al. with respect to the changes in the arch width, since their results showed that maxillary arch width remained unchanged after orthodontic treatment in both extraction & non-extraction cases [10][11]. Minimal increase in the intercanine width of 0.5 mm in extraction groups from our study correspond to the previous study of Giannely et al. [11] Therefore, from Table 1, Table 2 & Table 7, it can be inferred that molar had moved mesially into the narrower part of the upper arch thus contributing to the reduction in the intermolar width and contraction of the maxillary arch after orthodontic treatment in bicuspid extraction cases.

To conclude, there were significant arch dimensional changes in extraction cases & minimal arch dimensional changes in non-extraction cases after orthodontic treatment.

Significant antero-posterior tooth movements including correction of proclination by retraction of anteriors & space closure, mesial migration of molars into the narrower part of the arch occurred in extraction cases. Also, there was a considerable amount of anchorage loss in the posterior segment in bicuspid extraction cases and minimal loss in non-extraction cases. Transverse arch dimensional changes include reduction in the intermolar width and contraction of the maxillary arch as well as minimal change in the intercanine width in extraction cases and minimal transverse arch dimensional changes in non-extraction cases after orthodontic treatment.

Summary and Conclusions

The present computer based AUTOCAD study reveals that maximum antero-posterior arch dimensional changes can be seen in orthodontically treated bicuspid extraction cases and non-extraction cases show minimal changes in the antero-posterior dimensions even after orthodontic treatment. Significant amount of anchorage loss can be seen in upper bicuspid extraction cases when compared to non-extraction cases. Also, significant reduction in the intermolar width and contraction of maxillary arch occurred in upper bicuspid extraction cases because of the mesial migration of the molars into the narrower part of the upper arch as well as minimal changes in the intercanine width in extraction cases and minimal transverse arch dimensional changes in non-extraction cases after orthodontic treatment.