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Table of Contents   
TECHNICAL NOTE  
Year : 2014  |  Volume : 25  |  Issue : 4  |  Page : 521-526
Modified bisecting angle technique in eliminating zygomatic superimposition over apices of maxillary molars


Department of Oral Medicine and Radiology, Mahatma Gandhi Dental College and Hospital, Sitapura, Jaipur, Rajasthan, India

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Date of Web Publication10-Oct-2014
 

   Abstract 

Background: Image superimposition is a part and parcel of two-dimensional radiography. However, some overlappings are a result of radiographic technique itself. The technique induced superimposition of zygomatic buttress on apices of maxillary molars is common in bisecting angle technique (BAT), which affects the endodontic performance. Certain alterations were carried out in BAT to remove these undiagnostic shadows. The aim of this study was to compare two techniques of periapical radiography namely, BAT and modified BAT (MBAT) in preventing zygomatic superimposition over apices of maxillary molars.
Materials and Methods: A total of 62 patients requiring endodontic treatment for at least one maxillary molar were recruited in this cross-sectional study. One tooth from every patient was subjected for two periapical radiographs, one with each BAT and MBAT respectively giving a total of 124 radiographs. Each radiograph was recorded as acceptable or unacceptable, based on zygomatic superimposition and different technical aspects. Chi-square test was used for data analysis. The kappa statistic was used to test intra-observer reliability.
Results: With MBAT, the acceptability was 82.3% (n = 51) compared with 43.5% (n = 27) when BAT was employed. The statistically significant difference was found between these two techniques (P = 0.000). The level of agreement between two oral radiologists in their interpretation was high (kappa index = 0.897).
Conclusion: Modified BAT was more accurate for periapical radiography of maxillary molars in preventing zygomatic superimposition and related technical errors than BAT.

Keywords: Bisecting angle technique, modified bisecting angle technique, paralleling technique, vertical angulation

How to cite this article:
Jamdade AS. Modified bisecting angle technique in eliminating zygomatic superimposition over apices of maxillary molars. Indian J Dent Res 2014;25:521-6

How to cite this URL:
Jamdade AS. Modified bisecting angle technique in eliminating zygomatic superimposition over apices of maxillary molars. Indian J Dent Res [serial online] 2014 [cited 2020 Feb 24];25:521-6. Available from: http://www.ijdr.in/text.asp?2014/25/4/521/142560
Radiographs are considered as third eye to the dentist so is an integral aid in endodontic. Intra-oral periapical radiograph shows 2-4 teeth and the periodontal bone. [1] Intra-oral periapical radiographic techniques despite having limitations and disadvantages have yet not been replaced by superior method. Intra-oral periapical radiography could be performed either by bisecting angle technique (BAT) or paralleling technique (PT). [2] Even though having inherent image distortion, BAT is still popular in dental practice because of its greater flexibility. PT is a standardized method and should always be preferred when dimensional accuracy and reproducibility is paramount.

Ideally, BAT should be employed for patients unable to accommodate and tolerate the positioning of the PT. The radiographic superimposition of apical anatomical structures especially zygomatic buttress on roots of maxillary molars is common in BAT which affects the outcome of dental therapy. Angle reduction modification was carried out in BAT to minimize such image superimposition. This article describes modified BAT (MBAT) and its comparison with BAT in preventing zygomatic superimposition on periapical radiographs of maxillary molars.


   Materials and methods Top


In this cross-sectional comparative study, a total of 62 patients requiring root canal treatment of at least one maxillary molar (either first or second) were randomly recruited. The sample consisted of 33 males (53.2%) and 29 females (46.8%), aged 15-52 (mean 29.11) years. Patients with skull abnormalities and incomplete root apex were not selected. A total of 124 periapical radiographs were taken with BAT and MBAT, from 62 maxillary molars (62 radiographs from each technique). Each patient was subjected for two radiographs on the same tooth, one by each technique. This study followed the ethical standards of the committee on human experimentation of the institution. All patients gave informed written consent to participate in this study.

Radiographic technique

The basic principle of MBAT is same as the conventional technique of bisecting the angle, that is, Cieszynski's rule of isometry. The angle formed between the plane of the film and the long axis of the tooth is mentally bisected. The central rays of the beam are at right angle to this imaginary bisector [Figure 1]. Using the geometrical principle of isometric triangles, the actual length of the tooth will be equal to its image length. The main differences between MBAT and BAT are film position and vertical angulation.
Figure 1: In bisecting angle technique, vertical angulation is not low enough to pass between root apices and zygomatic buttress

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  1. Positioning of the patient is same as in BAT, that is, patient's head should be upright with sagittal plane vertical and occlusal plane horizontal
  2. Snap-A ray type film holder was used to hold and stabilize the film. The film was held 2-4 mm apical to its occlusal edge to prevent its bending
  3. The apical edge of the film is at same position as in routine method but the occlusal edge is placed around 5 or 10 mm away from the occluso-palatal line angle of the teeth to be radiographed as shown in diagrammatic representation [Figure 2]
  4. Figure 2: In modified bisecting angle technique, vertical angulation is reduced by decreasing the angle between the film and the teeth

    Click here to view
  5. Vertical angulations given to the X-ray beam for maxillary molars were approximately + 10°-+20°. If the distance between the occlusal edge of the film and the occluso-lingual angle of the teeth is 5 mm, then the vertical angulation will be approximately 5° lesser than what required in BAT. And for the distance of 10 mm, it will be reduced around 10°[Table 1] and [Table 2]
  6. Table 1: Average vertical angulations required in MBAT for periapical projection of maxillary molars if angle between film and teeth is 40°


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    Table 2: Average vertical angulations required in MBAT for periapical projection of maxillary molars if angle between film and tooth is 30°


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  7. Horizontal angulation of the central beam should be aimed through the interproximal contact areas to avoid horizontal image overlapping
  8. Centric point is 0.5-1 cm below the meeting point of perpendicular drawn from outer canthus of eye to ala tragus line.


Data collection

Diagnostic radiographs were taken using both the radiographic techniques, namely BAT and MBAT, on the same tooth utilizing periapical films (Speed E, Eastman Kodak co., Rochester, NY, USA). A single researcher took all the radiographs using the Snap-A ray film holder (Dentsply Rinn Co., India) for both techniques. Radiographs were made with a GNATUS (Gnatus Equipamentos Medico-Odontologicos Ltd., Brazil) operated at 70 kVp, 9 mA at 0.6 s. Total filtration was 3.81 mm aluminum equivalent. All exposed films were processed in an automatic processor with similar standards

(MR-C31, China).

Data analysis

Each radiograph was recorded as acceptable (A) or unacceptable (UA), based zygomatic superimposition and technical aspects. Radiographs covering the entire tooth free from zygomatic superimposition and technical errors and at least 2 mm of periradicular bone were considered A. If a film was rated UA, the reasons for errors were recorded. Zygomatic superimposition, periapical cutoff (apex not seen) and image distortion were the three reasons for unacceptability, applicable for both techniques. Each patient was asked about tolerance of both techniques and recorded as tolerable or

intolerable.

All radiographs were assessed by two oral radiologists using a standard illumination source and a viewing box. Two techniques were compared in preventing zygomatic superimposition, concerned technical errors and patient's tolerability.

Statistical analysis

Statistical comparison of two techniques in terms of acceptability and unacceptability was performed using Chi-square test and P value was recorded. The kappa statistics was used to test intra-observer

reliability.


   Results Top


With MBAT (n = 62), the retake (UA) was 17.7% (n = 11), while 35 out of 62 radiographs (56.5%) were repeated when BAT was employed, as shown in [Table 3]. Periapical radiographs taken by BAT showed zygomatic interference with root apices [Figure 3], [Figure 4], [Figure 5]. However, radiograph taken on same teeth by MBAT gave an unobstructed periapical view [Figure 6], [Figure 7], [Figure 8].
Figure 3: Periapical radiograph taken by bisecting angle technique shows first molar apices are obscured by anatomical noise of zygomatic superimposition

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Figure 4: Periapical radiograph taken by bisecting angle technique shows interference of zygomatic process and lower border of zygomatic arch over molars apices

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Figure 5: Periapical radiograph taken by bisecting angle technique shows zygomatic superimposition over elongated palatal root

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Figure 6: Periapical radiograph of same region as shown in figure 3 taken by modified bisecting angle technique, as a result of better irradiation geometry shows no zygomatic superimposition

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Figure 7: Periapical radiograph of same regions as shown in figure 4 taken by modified bisecting angle technique shows teeth apices free from zygomatic interference

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Figure 8: Periapical radiograph of same regions as shown in figure 5 taken by modified bisecting angle technique shows undistorted image and no zygomatic interference

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Table 3: Comparison of MBAT and BAT in preventing zygomatic superimposition and concerned technical errors

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Modified BAT had lesser zygomatic superimposition 9.6% (n = 6) and image distortion 4.8% (n = 3) than BAT 37.1% (n = 23) and 17.7% (n = 11) respectively. However, the incidence of incomplete periapical image was little higher in MBAT, that is, 3.2% (n = 2) compared with BAT 1.6% (n = 1). Patient tolerated both techniques comfortably in all cases.

The distribution of various aspects of two techniques was shown in [Figure 9]. A statistically significant difference (P = 0.000) in preventing zygomatic superimposition was found between MBAT and BAT. The level of agreement between interpreters was high (kappa index = 0.897).
Figure 9: Distribution of different techniqual aspects of unacceptability and patient's tolerability in two techniques

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   Discussion Top


Periapical radiography in practice is not as easy as it appears in theory. However, a sound theoretical knowledge helps clinicians in modifying routine techniques to suit individual patient. The anatomy of the mouth does not always allow rules of geometric projections to be satisfied. PT produces more accurate images than BAT. [2],[3],[4],[5],[6],[7],[8],[9],[10],[11],[12] This may be due to satisfying the four of the five projection rules and by using beam-guiding device. Although PT should be considered as the technique of choice, practically it is not always possible. Oral anatomy and patients' intolerance sometimes makes this technique almost impossible.

On the other hand, BAT is routinely used in dental practice. [4],[13],[14] It is relatively simple, quick and patient comfortable, but has an inherent drawback of image distortion. However, some investigators concluded that correctly adjusted BAT and PT provide similar diagnostic results. [15] No significant difference was found between PT and BAT when both techniques used Rinn XCP film holders. [16] However, the image interference of zygomatic arch with the apical third of maxillary first and second molars has made BAT diagnostically unreliable in this region. [17],[18] Therefore, certain alterations were carried out in BAT, and then both techniques were compared in avoiding zygomatic superimposition.

This undiagnostic image superimposition is either technique induced or because of anatomical proximity. In our study, BAT was unable to separate roots and zygomatic arch in 37.1% of cases. In one research, this separation was only 40% in BAT. [18] However, in same study PT had prevented zygomatic image superimposition in 73.8% cases. The reason is central rays in BAT are inferiorly directed and not perpendicular to film and object. Whereas the beam in PT is perpendicular to both film and object, and therefore usually passes between these structures. The excessive vertical angle to beam will definitely cause vertical image overlapping. Even correctly angulated BAT can results in such superimposition because of structural proximity in vertical dimension. This indicates beam angulation in BAT is usually not low enough to pass between these structures. Just by reducing vertical angulation without reducing the angle between the film and the teeth will lead to excessive elongation of the image.

Actually, the vertical angulation used in BAT is directly proportional to the angle between the long axis of the teeth and the plane of the receptor. For that reason, the angle between the film and the teeth in modified technique was reduced by keeping the occlusal edge of the film little away from the occluso-palatal line angle of teeth to be radiographed. This film positioning was achieved by placing the bite platform of film holder little lingually. Consequently, the required vertical angulation will be lesser than the conventional method. Now, the X-ray beam can pass between the zygomatic buttress and molar apex. Therefore, MBAT had successfully nullified similar image overlapping and obtained an A radiograph in 82.3% of our cases.

Film in MBAT can be stabilized with any type of film holder used in BAT. The beam-guiding film holders produced more dimensionally accurate images than simple film holders. [16] Rinn bisected angle instrument directs the beam in right vertical and horizontal angulations, which can negate image distortion and also prevent cone-cut. Even with conventional film holder, we found image distortion only 4.8% in MBAT. This may be because minimum range of vertical angulation. That's why, image distortion was higher (17.7%) in BAT. None of our radiograph by any technique was with partial image (cone-cut) indicating all films were kept in front of central rays.

The angle has to be formed between the receptor and the teeth in modified method and it should be lower than the routine technique. Palatal anatomy guides the distance between the film and the teeth. In case of deep palate, the distance will be less, around 5 mm and for shallow palatal vault; it will be more, around 10 mm. The distance beyond 1 cm usually aligns the film parallel to the teeth.

Advantages of BAT are justifiable in MBAT as both techniques based on same principles. In addition, MBAT can minimize disadvantages of BAT like foreshortening of the buccal roots of maxillary molars and shadowing of zygomatic buttress on roots of maxillary molars. There is also maximum superimposition of buccal and lingual parts of the teeth and surrounding bone much similar to PT, enabling better detection of approximal caries and periodontal bone levels. This happens because film is near parallel to the long axis of the teeth. Both techniques were very well tolerated by all patients. This may be because of plain film holder, simple and short procedures. Therefore, this modification can be comfortably used in cases unsuitable for PT like shallow palate, gagging and discomfort. It can be applied in radiographing other teeth.

Disadvantages of modified technique are apices of the teeth can sometimes appear near the apical edge of the film. Periapical cutoff (incomplete apical image) was little higher in MBAT (3.2%) than BAT (1.6%) [Figure 10]. It happened because film was kept parallel to object like in PT and closer to object like in BAT. However, apical cutoff is a rare chance in BAT because of more inferiorly directed rays. Otherwise, it is higher in PT, because of divergent rays between object and receptor and insufficient palatal depth.
Figure 10: Periapical radiograph taken by modified bisecting angle technique shows periapical image cutoff in first and second molar region

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Some investigators [9],[16] demonstrated the superiority of PT over BAT on single rooted teeth. However, when PT is used on maxillary molars, radiographic distortions occur because beam is perpendicular to the long axis of the tooth and not to the buccal or palatal roots. The best radiographic technique for endodontic measurement of maxillary molar root canals is 10° BAT than PT. [19] For the buccal roots of maxillary molars, the BAT using the Rinn XCP film holder produced the least mean difference between radiographic image and tooth length. [16] This indicates MABT can give accurate endodontic dimensions of maxillary first premolar and molars.

The vertical angulations required in MBAT were geometrically calculated and applied. The tooth-film angles in BAT in all areas of the mouth were found to be close to 30° except the mandibular posterior regions. [20] In practice, the clinician's goal is to aim the central beam perpendicular to the imaginary bisector. Vertical angulation used in MBAT, that is, minimum + 10° to maximum + 20° are only approximate, but definitely lesser than what required in BAT. However, each case should be assessed independently and the vertical angulations suggested should be taken only as a general guide.


   Conclusion Top


Modified BAT periapical radiography has high accuracy in preventing zygomatic superimposition over roots of maxillary molars than BAT radiography. Although the patient tolerance of BAT was similar to that of MBAT, the latter provided undistorted images and also eliminated foreshortening of buccal roots because of more ortho-radial projection. Periapical cutoff was little higher in MBAT but statistically insignificant. Based on this information, MBAT is recommended for periapical radiography of maxillary molars. Patient outcome can be improved by avoiding retakes and unnecessary radiations.

Future aspects

The comparison between MBAT and PT should be carried out in terms of zygomatic superimposition, concerned technical aspects and patient's comfortness.

 
   References Top

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12.Huh KH, Lee SS, Jeon IS, Yi WJ, Heo MS, Choi SC. Quantitative analysis of errors in alveolar crest level caused by discrepant projection geometry in digital subtraction radiography: An in vivo study. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2005;100:750-5.  Back to cited text no. 12
    
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15.Forsberg J, Halse A. Periapical radiolucencies as evaluated by bisecting-angle and paralleling radiographic techniques. Int Endod J 1997;30:115-23.  Back to cited text no. 15
    
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17.Tamse A, Kaffe I, Fishel D. Zygomatic arch interference with correct radiographic diagnosis in maxillary molar endodontics. Oral Surg Oral Med Oral Pathol 1980;50:563-6.  Back to cited text no. 17
    
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19.Suwadee K, Kanitha A, Gultida T, Supak C. Effect of radiographic techniques on the endodontic measurement in an upper molar. Khon Kaen Dent J 1999;2:75-8.  Back to cited text no. 19
    
20.Manson-Hing LR. What's the angle: A study of the angle bisected in intraoral radiography. Oral Surg Oral Med Oral Pathol 1980;49:86-9.  Back to cited text no. 20
    

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Correspondence Address:
Anshuman S Jamdade
Department of Oral Medicine and Radiology, Mahatma Gandhi Dental College and Hospital, Sitapura, Jaipur, Rajasthan
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0970-9290.142560

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