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Table of Contents   
ORIGINAL RESEARCH  
Year : 2010  |  Volume : 21  |  Issue : 4  |  Page : 468-473
Predicting the actual length of premolar teeth on the basis of panoramic radiology


1 Department of Orthodontics, School of Dentistry, Shahid Sadoughi University of Medical Sciences, Yazd, Iran
2 Department of Oral and Maxillofacial Radiology, School of Dentistry, Shahid Sadoughi University of Medical Sciences, Yazd, Iran
3 Shahid Sadoughi University of Medical Sciences, Yazd, Iran

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Date of Submission03-Mar-2009
Date of Decision19-Nov-2009
Date of Acceptance16-Jun-2010
Date of Web Publication24-Dec-2010
 

   Abstract 

Background: Panoramic radiography is one of the most common techniques for evaluating the jaw bones and associated structures.
Aim: The aim of the study was to predict the actual length of the premolar teeth, based on measurements taken on a panoramic radiograph.
Materials and Methods: This study was done in two stages. In the first stage, the actual and panoramic lengths of 102 teeth of orthodontic patients were measured and compared. In the next stage, the actual and radiographic vertical lengths of four metal balls placed in the molar and first premolar areas of 27 patients, referred to radiography clinic were also analyzed. Comparison of the mean magnification between the two methods was performed by one-sample T-test and P<0.05 was considered as statistically significant.
Results: The total magnification of the premolar region in the vertical plane in the tooth length measurement method was 17.39%, while the same in the metal markers method was 27.39%. The upper teeth showed larger magnification than the lower teeth in the vertical plane. Similarly, the magnification and distortion of the metal markers in the horizontal plane were significantly more than this in the vertical plane and also their magnification and distortion in the molar region were more than this in the premolar region. The actual length of the premolar teeth can be estimated by using a suitable regression formula.
Conclusion: Panoramic radiography can be used for calculating the actual length of premolar teeth.

Keywords: Distortion, magnification, panoramic

How to cite this article:
Yassaei S, Ezoddini-Ardakani F, Ostovar N. Predicting the actual length of premolar teeth on the basis of panoramic radiology. Indian J Dent Res 2010;21:468-73

How to cite this URL:
Yassaei S, Ezoddini-Ardakani F, Ostovar N. Predicting the actual length of premolar teeth on the basis of panoramic radiology. Indian J Dent Res [serial online] 2010 [cited 2014 Oct 30];21:468-73. Available from: http://www.ijdr.in/text.asp?2010/21/4/468/74207
Panoramic or rotational radiography is an easy and fast technique in which the images of the upper and lower arches along with their surrounding structures are studied in one view with less radiation exposure than the complete series of intra-oral radiographs. [1] But one of the main problems of this method is magnification of anatomical structures, especially teeth, which limits its dimensional evaluation. [2] Estimation of the actual length of teeth from panoramic radiographs can help the orthodontists to review root resorption during treatment. The lengths of the roots and their angulations need to be estimated before orthodontic treatments such as force eruption and orthodontic movement of teeth. [3],[4] Accurate vertical measurements are required during stages of post and core treatment, for the measurement of bony surface next to an implant, root measurements during orthodontic treatment and evaluation of the rate of bony destruction in patients with periodontal problems. [5] Lien and Soh studied the actual and panoramic lengths of permanent premolars that were candidates for extraction in orthodontic treatment and the results showed that the magnification rate in the upper teeth was approximately 22%, while in the lower teeth it was 11%. [6] Schulze et al. placed markers in the premolar region and left mandibular angle of a skull and took 70 panoramic images in seven positions. Repetition of vertical measurements was less than horizontal measurements and horizontal measurements were most reliable linear measurements in this study. [7] Yitschaky et al. studied the actual length of 112 premolar teeth extracted for orthodontic reasons and compared it with their radiographic lengths. Results showed that the vertical magnification of the first upper premolars was significantly more than the magnification of the lower first premolars. [8] Volchansky et al. compared the vertical and horizontal lengths of posterior teeth in 16 skulls using standard intra-oral and panoramic radiographies. They concluded that if the patient is positioned properly in the focal trough, it is possible to use panoramic radiography for making measurements in posterior region. [9]

Therefore, due to the clinical importance of panoramic radiography and its numerous practical uses, many studies regarding magnification of panoramic radiography have been performed, [6],[7],[8],[9] but there is still no specific and comprehensive formula to estimate the actual length of teeth from their radiographic lengths. In the present study, the authors have tried to derive a formula for estimating the actual length of premolar teeth from their panoramic radiographic length by evaluating the relationship between the actual and radiographic length of teeth and the rate of magnification of vertical panoramic views taken with Planmeca machine.


   Materials and Methods Top


This is a descriptive, analytical and diagnostic study in which the mean premolar teeth length from the panoramic radiographs was compared with the mean of the actual length of the teeth after extraction and a regression model was used to derive a formula to estimate the actual length from the radiographic length. Panoramic radiographs were taken by Planmeca 2002 machine. The inclusion criteria were existence and having complete root and apex in premolar and molar teeth; and no pathologic lesion in the crown and root area, permanent dentition without any primary tooth. Exclusion criteria were metal restoration in molar and premolar teeth.

To increase the precision of calculating the magnification of panoramic radiography, two methods were used. In the first method, in order to compare the actual and radiographic lengths of the teeth, 29 orthodontic patients in the 15-20 years age group were selected. These cases were healthy and their first premolars were extracted for orthodontic treatment. A total of 22 pairs of lower first premolars and 29 pairs of upper first premolars were included in the study. The roots and their apices were completely formed without any pathology in radiographic studies (for example, decomposition of the roots, decay or repair). Non-traumatic extractions were done. Before undergoing orthodontic treatment, every patient routinely took a panoramic radiography using Planmeca 2002 EC proline (Planmeca Co., Helsinki, Finland) according to standard conditions of 12 mA/80 KVP/18 seconds. The teeth were divided into four groups according to their position in the oral cavity: T44 (right upper first premolar), T34 (left upper first premolar), T24 (right lower first premolar) and T14 (left lower first premolar). The extracted teeth were placed in 1/1 sodium hypochlorite (NaOCL 5%) and normal saline solution.

Vertical height was considered as the maximum distance between the apex and tip of the palatal cusp, both in extracted teeth and radiographs. Measurement of the teeth and the radiographs was performed twice at an interval of 1 week by a trained observer and a maxillofacial radiologist using a Digimatic 500197, CD-8″CS Digital Vernier Caliper (Mitutoyo Co., Kawasaki, Japan) with an accuracy of 0.01 mm by a standard table view box. The mean of measurements was recorded for each case.

In the second method of the study, 27 patients referring to the orthodontic clinic and requiring a panoramic radiograph were included. Oral explanation of the whole study was given to the patients and then informed consent was taken from each participant. These patients had an almost complete dental system to close their teeth over a bite plan. In addition, the presence of 4th and 6th upper and lower teeth on both sides was a compulsion. Two metal balls of 6 mm in diameter were placed in red dentistry wax with the dimensions of 20×40 mm at a distance of 13 mm from each other as a set. The balls were made in China and coated by stainless steel [Figure 1]. The wax was used to prevent the balls from swallowing and it served as a bite plan. Two sets were placed in each side of the patient's mouth in such a way that the first ball was parallel to the crown of the first premolar and the second ball was at the center of the crown of the first molar. The sets were held in position by contact of the upper and lower teeth [Figure 2] and [Figure 3]. The regions were named as S L4, S L6, S R4 and S R6. After testing each patient, the balls were removed from the wax, sterilized in Deconex Solarcept solution for 5 minutes and new wax was used for the next patient. The maximum vertical and horizontal lengths of the balls were measured on the radiographs [Figure 4]. In cases where the 4th or 6th tooth had been repaired and the tooth filling interfered with the metal ball on radiography for measuring the marker, the case was excluded from the study. We did not consider the magnification factor of the unit manufacturing factory because of the possible variations in anthropometric measures of subjects. Differential magnifications on both sides of each panoramic radiograph were calculated and compared. Radiographs with large difference in magnification between right and left sides were ignored. The Ethics Committee of Shahid Sadoughi University of Medical Sciences approved the protocol.
Figure 1: Metal balls

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Figure 2: Method of placement of metal markers in the mouth

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Figure 3 : A panoramic radiography showing the metal balls in the mouth

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Figure 4: Method of measurement of metal markers by Digital Vernier Caliper

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Statistical analysis

Data obtained from two times measurements were entered to a computer and their mean was calculated. The values were then coded, entered in SPSS software program and appropriate statistical tests like single specimen T-test and Pearson's correlation test were used for analysis. A linear regression model was used to estimate the difference between the actual and radiographic length and to produce a mathematical formula. Confidence level of 95% and analytical value of 80% were considered for reporting the results of the study.


   Results Top


The comparison of the actual and radiographic lengths of the teeth according to the different jaw regions is shown in [Table 1]. The difference between the magnification of the upper and lower premolars was significant (P=0.000). In addition, the overall horizontal magnification of panoramic radiography in the area of first premolars was calculated to be 17.39%.
Table 1: Mean and standard deviation of actual and radiographic lengths of premolars and their vertical magnification in various groups

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The comparison of the actual and radiographic length and width of the markers according to the different jaw regions is shown in [Table 2] and [Table 3]. According to the data, there was no significant difference between the vertical and horizontal magnifications in the S R6 and S L6 groups. Similarly, there was no significant difference between the vertical and horizontal magnifications in the S R4 and S L4 groups. The difference in magnification of the markers in the molar and premolar regions in the vertical dimension was significant (P=0.000), while the difference in magnification of the markers in the molar and premolar regions in the horizontal dimension was not significant. The magnification of panoramic radiographic images in the premolar region by using markers was significantly different from the actual dental length measurement method (P=0.000).
Table 2: Mean and standard deviation of the vertical plane of markers and their vertical magnification in various groups

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Table 3: Mean and standard deviation of the horizontal plane of the markers and their horizontal magnification in various groups

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At the end, the X and Y coefficients and the regression formula:

T A *=(T R **×X)+Y were used to calculate the actual premolar dental length from the panoramic radiographic lengths on the basis of statistical analysis of the data. This formula in the different jaw regions is shown separately in [Table 4].
Table 4: Presentation of fixed coefficients and regression formulas in various groups

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The regression formulas for different groups are as following:

T14 group: T A =(T R×0.13)+18.14

T24 group: T A =(T R×0.082)+19.41

T34 group: T A =(T R×0.205)+17.86

T44 group: T A =(T R×0.084)+20.9


   Discussion Top


The present study was conducted in two ways so that the conditions and factors effective in clinical measurement of teeth could be considered and in addition to being just a clinical study, it could be more of a practical use. As healthy teeth free from any pathology were required for the study, teeth extracted for the purpose of orthodontic treatment were the best choices. In the first stage of the study, since factors like root curvature, deviation of dental axis, growth position and tooth size have effects on the measurements, [2] in the second part of the study, metal ball markers with similar size, dimension and suitable contrast were used. [7] Therefore, use of these metal balls as markers reduced the effects of factors like dental anatomical differences, overlapping or haziness of the root ends, problem in developing and fixing the films, and distortion of angles, on the results of the study. [10] The accuracy of measurement in panoramic radiography is affected by magnification and distortion. Distortion is due to the asymmetrical magnification in different subject dimensions. [11] Factors affecting both vertical and horizontal magnifications include the distance of the film from the source of radiation which is constant in each panoramic radiography machine, and the distance of the film from subject which is variable depending on the position of the patient. [12] If the patient is positioned properly in the machine, it is assumed that the lower jaw is in the image layer. As a general role, placement of subject in the central plane has an ideal vertical and horizontal magnification (nodistortion). [13] With the increase in the distance between subject and central plane image layer, the distortion and haziness also increase which is more prominent in the horizontal than the vertical plane because distortion is due to the horizontal movement of film and radiation source and haziness also occurs in the direction of the movement of the radiation group (transversely). [12] If an object is not in an ideal position, but within the image layer limit and it is moving away from the film, magnification in the horizontal plane is more than in the vertical plane, but if it is moving toward the film, reduction in its dimensions in the horizontal plane is much more than in the vertical plane. Anyway, the result of both conditions is distortion of the image. [13]

There was no statistically significant difference between the magnification of the left and right jaws in both methods. Coordination of the magnifications of the right and left sides depends on correct positioning of the patient in the sagittal plane in the panoramic radiology machine. [14] During measurement of the teeth, vertical magnification of the upper premolars was 1.84 times more than the lower premolars and this difference was statistical significant. This finding is similar to the results of the studies by Yitschaky et al. [8] and Lien and Soh. [6] The difference between the vertical magnification of the upper and lower premolars could be due to the bucco-lingual angle of the teeth with the vertical plane of the film. [8] According to a study by McDavid et al., there are different angles between the central radiation rays and central plane of image layer. Therefore, in order to estimate the length more accurately for nonvertical subjects, other factors like angle of the object should be considered. [15] Similarly, Stramotas et al. believe that even if an angular object is placed firmly in the image layer of the machine, different magnifications will be obtained due to the angle of placement of that object. [14]

The angle of radiation also affects the rate of magnification. Due to the negative angle of the ray tube in panoramic radiography, the mandible is placed vertically over the central rays. Therefore, the angle of radiation to the upper jaw is more than this to the lower jaw. Thus, the effect of magnification factors on the teeth of the lower jaw is reduced and the magnification and distortion in them are less than the upper teeth. [12]

Overall, collection of factors including relative position of the radiation rays and its 4°-7° negative angle, relative position of the occlusal plane and the distance between the radiation rays and teeth have an effect on the vertical distortion. [8] The vertical magnification of the premolar region for the Planmeca machine was 17.39% and with a discrepancy of ±3.5, was similar to the studies by Lien and Soh [6] and Yitschaky et al. [8]

Overall, magnification of the markers in the premolar region was significantly more than that of the teeth (P=0.000). This could be due to the difference in method of study, or shape, size and position of subjects. The markers were placed between the teeth and due to their spherical shape, even during movement or shift in position, there was no change in the angle of their placement with respect to the vertical plane of the film. Therefore, they were perpendicular both to the film and central rays in all positions. But the teeth had different bucco-lingual angles and this angle is an important factor in vertical magnification. [14],[15]

In addition to this, the physical shape of objects also has an effect on the rate of magnification and on the basis of this law of physics, the rate of distortion of the external edges of an object in panoramic radiography depends on the shape of that object. When an object is more spherical, the measurement is also more accurate and distortion is also less. [3] The mean vertical magnification of the metal markers in the first molar region was significantly more than premolar region (P=0.000). In the horizontal plane too, the magnification of the metal markers in the first molar region was more than premolar region, but this difference was not significant.

On comparing the vertical and horizontal magnifications of the metal markers, the horizontal magnification was significantly more than the vertical magnification (P= 0.005). This implies that the depth of an object (distance from the clear layer) has less effect on vertical magnification. Distortion in measurement occurs because of the horizontal movement of the film and ray source, while the formation of the image in the vertical plane is due to normal radiation of rays and is not affected by horizontal rotation of rays. [1] These findings are in accordance with the findings of Wyatt et al. [16] and Catic et al., [17] but not in line with the findings of Schulze et al. [7] In a study by Park and Kim, displacement of the mandible in acentric positions resulted in increased magnification errors in the horizontal plane, but the vertical magnification errors in all positions were relatively the same. [18]

Based on the results of the present study, the following regression model formula with X and Y variables is proposed for each quadrant of the jaw:

T A =(T R×X)+Y.

This formula is capable of estimating the length of the first premolar tooth on the basis of its radiographic length on Planmeca 2002 machine. This is practically possible under special conditions including proper placement of the patient and adjustment of the machine. But further studies are required for use of this formula in different X-Ray machines.

 
   References Top

1.White SC, Pharoah MJ. Oral radiology: p0 rinciples and interpretation. 5 th ed. St Louis: Mosby Inc; 2004. p. 196.  Back to cited text no. 1
    
2.Gomez-Roman G, Lukas D, Beniashvili R, Schulte W. Area dependent enlargement rations of panoramic tomography on orthograde patient positioning and its significance for implant dentistry. Int J Oral Maxillofac Implants 1999;14:248-57.  Back to cited text no. 2
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Correspondence Address:
S Yassaei
Department of Orthodontics, School of Dentistry, Shahid Sadoughi University of Medical Sciences, Yazd
Iran
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DOI: 10.4103/0970-9290.74207

PMID: 21187607

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