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Table of Contents   
ORIGINAL RESEARCH  
Year : 2013  |  Volume : 24  |  Issue : 1  |  Page : 48-51
Precision of dimensional measurements of mandible in different positions on orthopantomogram


1 Department of Oral Medicine and Radiology, Subharti Dental College, Meerut, India
2 Department of Oral Medicine and Radiology, ITS institute for Dental Studies, Noida, India
3 Department of Oral Medicine and Radiology, MCODS, Manipal, India
4 Department of Oral Medicine and Radiology, Darshan Dental College, Udaipur, India

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Date of Submission05-Jan-2010
Date of Decision13-Aug-2010
Date of Acceptance05-Oct-2010
Date of Web Publication12-Jul-2013
 

   Abstract 

Objective: To evaluate the precision of dimensional measurements of the mandible in two different positions on orthopantomogram (OPG) (one with mandibular plane parallel to the floor and the other with Frankfort horizontal plane parallel to the floor) and determine the dimensional reliability.
Materials and Methods: Anatomical landmarks were used to denote points for measurements on mandible as well as OPG and respective measurements made. Magnification was hence calculated and compared with magnification factor (1.2) listed by the manufacturer.
Results: Vertical measurements and anterior horizontal and oblique measurements showed minimal magnification. Posterior horizontal and oblique measurements showed increased magnification. The difference in measurements in the two positions and on comparison with the one given by the manufacturer was statistically significant for posterior horizontal measurements but not for posterior oblique. Horizontal measurements crossing the midline were highly magnified and the difference was statistically significant for the two positions and on comparison to the one given by manufacturer.
Conclusion: Magnification factor given by the manufacturer is not uniform in all locations and varies with changes in positioning and hence should not be relied upon when accurate measurements are to be made.

Keywords: Dimensional reliability, panoramic, radiography

How to cite this article:
Arora H, Jain V, Pai KM, Kamboj R. Precision of dimensional measurements of mandible in different positions on orthopantomogram. Indian J Dent Res 2013;24:48-51

How to cite this URL:
Arora H, Jain V, Pai KM, Kamboj R. Precision of dimensional measurements of mandible in different positions on orthopantomogram. Indian J Dent Res [serial online] 2013 [cited 2017 Sep 25];24:48-51. Available from: http://www.ijdr.in/text.asp?2013/24/1/48/114945
It is of great importance to know the magnitude of distortion on a radiograph with respect to morphology of the object. This statement is convincingly valid when the shape of the lesion, e.g., a cyst or tumor is to be assessed using panoramic radiograph. Also, it is necessary to know the exact magnitude of image distortion in congenital malformations, post-traumatic deformation and implant placement. Magnification occurs for various vertical, horizontal and oblique measurements and hence it is necessary to check for shape or form distortion of the image. Distortion arises due to discrepancy existing between horizontal and vertical magnification for all planes other than the central plane of layer.

In vertical dimension,[1]



In the horizontal dimension, there is presence of different foci, and



A varying projection radius during the exposure therefore changes the horizontal dimensions slightly while leaving the vertical dimensions unchanged. [1] Unreliability of the horizontal variables was even seen by Larheim and Svanaes [2] despite the use of head positioner and same radiographer. This is also supported by Welander, Mc David and Tronje [2] who found that horizontal dimensions are unreliable because the distortion effect is influenced not only by a projection factor but also by a "motion factor". This factor has also been previously recognized. The study reinforces vertical magnification of 18-21%. [2] Welander and Wickman [3] showed mathematically that all distortion effects inherent in this specific technique are caused by different magnification factors that are valid for the vertical and horizontal dimensions outside the centre of sharply depicted layer which is called as focal trough. Schiff et al. [4] reported that most frequent errors in panoramic radiography occurred in patient positioning. Tronje [5] showed that imperfectly centered jaws in the sharply depicted layer due to individual anatomy and variations in the adjustment of patient lead to more marked distortion effects and varying magnifications (in both horizontal and vertical dimensions) of different regions of the jaws. This means that distances in the object are not registered with correct proportions on the film. This effect may be defined as linear distortion. Although the vertical magnification factor differs only slightly with displacement from center of the image layer, the resultant variation in horizontal dimension is obvious. In conventional panoramic radiography, [6] the thickness of the image layer is greater in the posterior region of the jaws than in the anterior region. Blurring and distortion are least in the center of the image layer. Magnification factor and thus distortion and displacement of structures vary from one machine to another, and thus, it is essential to use the same type of machine throughout the research. In the present study, we have tried to evaluate the precision of dimensional measurements of the mandible in two different positions on orthopantomogram (OPG).


   Materials and Methods Top


Ten randomly selected dry human mandibles of average size were acquired from Department of Anatomy, Kasturba Medical College, Manipal. Dental status varied from fully dentate to partially edentulous with left first molar and right third molar present in all the mandibles. Anatomical landmarks in the mandible were used as markers. Metal markers were not used in the study to avoid formation of ghost image if a part of the jaw lay outside the center of sharply depicted layer. Measurements were made on dry mandible with the help of divider and ruler. Each mandible was exposed in Planmeca 2002CC (Planmeca Co, Helsinki, Finland) OPG machine in two different positions. First time, the radiographs were obtained with mandible positioned with mandibular plane parallel to the floor, and the second time, the mandible was positioned with the frankfurt plane parallel to the floor by tilting down the mandible to 20° (mandible stabilized in this position by keeping modeling wax below the angle region). A listed magnification factor for the machine by the manufacturer was ×1.2.

Mandible was placed on a holder designed for positioning in focal trough. Exposure was made at 60 kVp, 4 mA current with four aluminum plates (1.2 mm thick) kept in the path of radiation. This was done to simulate the reduction in radiation, usually done by presence of soft tissue. Processing was done in Promax automatic processor (Eastman Kodak, New York, USA) and OPG obtained was traced and various measurements were made as shown in [Figure 1]. The same observer repeated the various measurements hence made, after a period of 2 months, and intraobserver reliability [6] was calculated for full sample.
Figure 1: Radiographic tracing of the mandible obtained from OPG taken with Frankfort plane parallel to the floor. The various measurements include: V1: Height of the mandible in mid-saggital line, V2: Height of the mandible at mesial border of 36, H1: Distance between mid-sagittal line and distal of 48, H2: Width of left ramus at the level of sigmoid notch, H3: Intercoronoid distance, H4: Intercondylar measurements, O1: Oblique measurement from right coronoid to superior point of alveolus in mid-sagittal line, O2: Measurement made from the left coronoid to left condyle

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Magnification index was calculated for each variable as

  • Distance on the radiograph
  • Distance on dry mandible.



   Results Top


Descriptive statistics were calculated (arithmetic mean, standard deviation, standard error, and 95% confidence interval). One sample t test was used to compare the difference between the calculated magnification factors in the two different positions and the magnification factor given by the manufacturer (SPSS version 10). Statistical significance was set at P = 0.05. [Table 1] and [Table 2] represent the results in tabular form. Z-test was done to compare the effect of different mandibular positions on dimensions of various variables as shown in [Table 3].
Table 1: Calculated magnification index in two positions

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Table 2: Significance of difference between the calculated magnification indices for various variables in position 1 and position 2 and the one given by the manufacturer (×1.2)

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Table 3: Significance of difference among different mandibular positions in various variables

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Intraobserver reliability varied from 0.91 to 0.99 for radiographic measurements and from 0.78 to 0.99 for measurements made on the mandible. [7]

Vertical measurements and anterior horizontal and anterior oblique measurements showed very minimal magnification. More magnification occurred when the mandible was positioned with the Frankfurt horizontal plane parallel to the floor, though the difference was not statistically significant. However, the difference in the two positions showed statistically significant different as compared to the magnification factor given by the manufacturer.

For posterior horizontal and posterior oblique measurements, magnification was high with the position of mandible with mandibular plane parallel to the floor and low for the position of the mandible with the Frankfurt plane parallel to the floor w.r.t. the magnification factor given by the manufacturer. Magnification in the two different mandibular positions on comparison to the magnification factor given by the manufacturer showed statistically significant difference for posterior horizontal measurements but not for posterior oblique.

Horizontal measurements crossing the midline were highly magnified in both the positions with more magnification with mandibular plane parallel to the floor. The difference in measurements was statistically significant for the two positions and also on comparison to the one given by manufacturer.


   Discussion Top


An interesting observation made in the study was that in the measurements, whether vertical, horizontal or oblique, magnification progressively increases posteriorly. This can be attributed to the laterally turned focal trough in the posterior region [Figure 2] which would have led posterior structures to come closer to the rotation center and hence showing more magnification. Little information is available regarding magnification of oblique measurements. However, based on a small sample size used in this study, the results are difficult to be justified.
Figure 2: Focal trough for jaw of average size in Planmeca 2002CC

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Horizontal measurements crossing the midline showed high magnification. This can be explained by the fact that OPG is a two-dimensional image like other radiographs and mandible is a three-dimensional structure. So, a long distance is travelled from posterior end of one rotation center to the other posterior end before image is formed on the film. Moreover, the film cassette is flat and not curved. This can be interpreted as the change in the angulation of central beam to compensate for the curvature of the mandible, which makes horizontal distances unreliable. [1]

Increased magnification for oblique and horizontal measurements can also be explained as partly due to difficulty in determining the anatomic point in a projection using oblique beam direction. This observation was made by Kjellberg et al. [8] in his study where condylion on the mandible when correlated with OPG was found to be positioned slightly inferior.

Reliability was quite high except for the measurement made from condylion (0.78). This could be due to an error in identifying the condylion as pin-point metal markers were not used in this study.

This study showed varying range of magnification in the central plane itself; however, it is usually thought that the central plane of the layer is free of distortion. This statement is strictly valid for very small objects positioned in a plane perpendicular to the X-ray beam, i.e., the angle between the central ray of the beam and the layer is important for the projection of the jaws in panoramic radiography. Also, to obtain the ideal orthoradial projection, this angle should not deviate from 90°. In the part of excursion where the teeth are scanned, the deviation from 90° between the central ray of beam and central plane of the layer is moderate. In most lateral parts of the image, the effect is more severe and as a result, the image of ascending ramus is slightly compressed in horizontal dimension as compared to vertical. [1] These deviations are inherent in the panoramic technique because it is impossible to intercept all parts of the jaws at right angles with conventional continuous patterns without creating undesirable side effects. In the position where mandibular plane is kept parallel to the floor, this compression of ascending ramus is however less. [9]

Our results are in agreement with those of Amir et al., [10] though they did not find any correlation between the anterior and posterior measurements. Thus, it can be stated that significant disparity is present between the mean magnification factor given by the manufacturer and the calculated ones. On the other hand, highest agreement for magnification factor has been found to be for Planmeca 2002CC among the various other machines compared by Kjellberg et al. [10]

It is well known that Planmeca consists of adjustable focal troughs. but the present study used a single focal trough of an average jaw for uniformity. More studies with increased sample size are required to conclude on which panoramic machine is the best for images with minimal distortion of a jaw of average size.

In conclusion, vertical and anterior horizontal and anterior oblique measurements are reliable, as slight change in sagittal tilting does not significantly affect the measurements. Followed by this are posterior oblique measurements. Posterior horizontal and horizontal measurements crossing the midline lead to statistically significant difference on sagittal tilting and hence are highly unreliable. Magnification factor given by the manufacturer varies in different anatomical locations and in similar location with varied position and hence should not be relied upon.

 
   References Top

1.Welander U, Tronje G, McDavid WD. Theory of rotational panoramic radiography. In: Langland OE, Langlais RP, McDavid WD, Delbalso AM, editors. Principles and practice of panoramic radiology. 2 nd ed. Philadelphia: Lea and Febinger; 1989. p. 37-68.  Back to cited text no. 1
    
2.Larheim TA, Svanaes DB. Reproducibility of rotational panoramic radiography: mandibular linear dimensions and angles. Am J Orthod Dentofac Orthop 1986;90:45-51.  Back to cited text no. 2
    
3.Welander U, Wickman G: Image distortion in narrow beam rotation radiography. Acta Radiol [Diagn] 1978;19:507-12.  Back to cited text no. 3
    
4.Sciff T, D'Ambrasio J, Glass BJ, Langlias RP, McDavid WD. Common positioning and technical errors in panoramic radiography. J Am Dent Assoc 1986;113:442-6.  Back to cited text no. 4
    
5.Tronje G, Eliasson S, Julin P, Welander U. Image distortion in rotational panoramic radiography, II: vertical distances, Acta Radiologica: Diagnosis 1981;22:449-55.  Back to cited text no. 5
    
6.Welander U, Wickman G. Blurring and layer thickness in narrow beam rotational radiography. Acta Radiologica Diag 1977;18:705-14.  Back to cited text no. 6
    
7.Slakter MJ, Juliana DB, Fischman SL. Estimating examiner consistency with DMFS measures. J Dent Res 1976;55:930-4.  Back to cited text no. 7
    
8.Kjellberg H, Ekestubbe A, Kiliaridis S, Thilander B. Condylar height on panoramic radiographs: A methodology study with a clinical application. Acta Odontol Scand 1994;52:43-50.  Back to cited text no. 8
[PUBMED]    
9.Christen AG, Segreto VA. Distortion and artifacts encountered in Panorex radiography. J Acad Dent Assoc 1986;77:1096-101.  Back to cited text no. 9
    
10.Amir C, Asja C, Melita VP, Adnan C, Vjekoslav J, Muretic I. Evaluation of precision of dimensional measurements of mandible on orthopantomogram. Oral Surg Oral Med Oral Pathol 1998;86:242-8.  Back to cited text no. 10
    

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Correspondence Address:
Honey Arora
Department of Oral Medicine and Radiology, Subharti Dental College, Meerut
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0970-9290.114945

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    Figures

  [Figure 1], [Figure 2]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]

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