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Year : 2015  |  Volume : 3  |  Issue : 3  |  Page : 198-203

Evaluation of the degree of taper and convergence angle of full ceramo-metal crown preparations by different specialists centers at Assir Region, Saudi Arabia

1 Department of Prosthodontics, College of Dentistry, Jazan University, Jazan, kingdom of Saudi Arabia
2 Department of Prosthodontics, King Khalid University, Muhayil Assir, kingdom of Saudi Arabia
3 Ministry of Health, Muhayil Assir, kingdom of Saudi Arabia
4 Preventive Dental Science, King Khalid University, Muhayil Assir, kingdom of Saudi Arabia

Date of Web Publication3-Aug-2015

Correspondence Address:
Mohammed M Al-Moaleem
Department of Prosthodontics, College of Dentistry, Jazan University, Jazan
kingdom of Saudi Arabia
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DOI: 10.4103/1658-631X.161996

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Background: Ceramo-metal crowns are commonly indicated for restoration of extensively damaged posterior teeth. Prosthodontists have ability to prepare teeth for retention, resistance, and longevity of the restorations.
Objectives: The aim was to measure the degrees of the taper and convergence angle (CA) of dies prepared by different prosthodontists at Assir region, and the effect of the tooth position on the taper and CAs degrees.
Materials and Methods: Eighty stone dies were obtained from specialist dental clinics, College of Dentistry at King Khalid University (KKU), Muhayil, Kamis-Mushayat and Assir Dental Centers. All the dies were mounted with the occlusal plane of the prepared teeth parallel to the floor. Photographs of buccal and proximal aspects were taken for each die. The photographs were transferred into a personal computer, AutoCAD software program was used to measure the mesio-distal (MD) and bucco-lingual (BL) taper and CA of each preparation. Analysis of variance (ANOVA) was used to test the level of significance difference, which was set at 5%.
Results: The highest values were observed with Kamis-Mushayat Center for BL (31.89) taper and Assir Center for MD (38.21) and overall CA degrees (34.38). The mean values for KKU specialist dental clinics were the lowest for all parameters of CA degrees (22.91). ANOVA showed statistically significant (P < 0.05) differences with the mean CAs between all the four dental centers. The MD, BL taper and overall CA for all the premolar teeth was significantly lower than molar teeth. No statistical difference in the overall CA between maxillary and the mandibular arch (P > 0.05).
Conclusions: The values of MD, BL taper and CA degrees carried-out by specialist at College of Dentistry, KKU are the lowest values of the prepared teeth compared with other dental centers at Assir region. There was no significant difference between maxillary and mandibular arch, while taper and CA are lower at premolars comparing to molars. The recommended taper and CAs are difficult to be achieved clinically.

  Abstract in Arabic 

ملخص البحث :
هدفت الدراسة إلى تقييم جدران وزوايا الميلان وكذلك موقع الأسنان للقوالب الجبسية لأسنان محضرة بواسطة أخصائي التركيبات في مراكز طب الأسنان الاختصاصية بمنطقة عسير. تم جمع 08 قالبًا جبسيا لأسنان محضرة في جامعة الملك خالد ومحايل, عسير وخميس مشيط وأبها بالتساوي. وضعت القوالب أمام كاميرا رقمية بحيث كان مستوى الإطباق موازيا لسطح الأرض ومن ثم التقاط صورتين احداهما جانبية والأخرى أمامية لكل قالب. نقلت الصور إلى جهاز الحاسب وقيست زاوية الميل بواسطة برنامج أوتوكاد للزاويتين الدهليزيتين اللسانية والإنسية الوحشية، ومن ثم جمع قيمها لتكون زاوية الميل لكل قالب. لوحظ أن قيم الميلان الأعلى للزوايا الدهليزية اللسانية قد سجل عند الأخصائيين في مركز خميس مشيط بينما الإنسية الوحشية للاخصائيين في مركز أبها. لم توجد فروق ذات دلالة إحصائية بين المراكز الأٍربعة, ولا توجد فوارق ذات دلالة إحصائية بين أسنان الفك العلوي والسفلي.

Keywords: Ceramo-metal, convergence angle, prosthodontics specialist, taper, tooth preparation

How to cite this article:
Al-Moaleem MM, Shariff M, Porwal A, AlMakhloti EA, Tikare S. Evaluation of the degree of taper and convergence angle of full ceramo-metal crown preparations by different specialists centers at Assir Region, Saudi Arabia. Saudi J Med Med Sci 2015;3:198-203

How to cite this URL:
Al-Moaleem MM, Shariff M, Porwal A, AlMakhloti EA, Tikare S. Evaluation of the degree of taper and convergence angle of full ceramo-metal crown preparations by different specialists centers at Assir Region, Saudi Arabia. Saudi J Med Med Sci [serial online] 2015 [cited 2022 Jan 26];3:198-203. Available from: https://www.sjmms.net/text.asp?2015/3/3/198/161996

  Introduction Top

Complete crowns are commonly recommended for the restorations of extensively damaged posterior teeth. The ability of dentists to adequately prepare teeth is fundamental to success of these restorations. [1]

One of the fundamental principles of tooth preparations is the retention and resistance form. Retention features prevent the dislodgment of the prosthesis along the path of insertion, whereas resistance features prevent prosthesis dislodgment when oblique, nonaxial forces act on the tooth. [2]

The angle formed between opposing walls of the tooth preparation is called taper or convergence angle (CA). [3] Retention of castings decreases with increasing taper and has been shown to be inversely proportional to taper or CA. [4]

The ideal taper recommended by fixed prosthodontics textbooks and different dental schools is 2°-7° per axial wall or 4°-14° total CA. [2],[5],[6]

Among the different degrees of tapers used, initial and recommendation retentive strength of tooth preparations with 5° taper is significantly greater than those with 12° and 25° with optimal retention occurring between 5° and 12°. [7] Some studies on specialists and general practitioners have found mean tapers in the range 14°-20°. [8] Despite higher than ideal tapers found in some studies, tapers of up to 20Ί have been shown to be clinically acceptable with few crowns reported having loosened or dislodged. [9]

However, Kent et al. [10] studied the taper of 418 dies prepared by Shillingburg during a period of 12 years. CAs of 15.8° between mesial and distal walls and 13.4° between facial and lingual walls with an overall mean of 14.3° were observed.

Nordlander et al., [11] compared dies prepared by residents, with those of prosthodontics and found no statistically significant difference in the axial wall convergence.

Christensen [12] prefer to make tooth preparations taper >10-15° from the long axis of the tooth and to make the tooth preparation at least 4 mm from the gingival margin to the occlusal surface.

Al Ali et al., [13] reported that, the mean CA value was 16.7°, with the mean bucco-lingual (BL) taper values less than the mean mesio-distal (MD) taper values for teeth prepared by specialist at Riyadh private sectors.

Ghafoor et al., [14] concluded that, greater CA and taper values for molars teeth were greater than premolars preperaed by specialist. Also reported CA values were greater in BL than MD dimension. [15]

Shillingburg et al. [2] recommended CAs of 14°, 19°, and 22° for premolars, maxillary, and mandibular molars teeth, respectively.

Several techniques have been described for evaluating CAs of preparations. Devices such as photocopy machines, [16] overhead projectors, [11] goniometric microscopes, [1] three-dimensional laser scanners, [17] diamond rotary cutting instruments [18] and metrology equipment [19] have been used to measure the CA of working dies. A new technological method is using the AutoCAD software (Sony Corporation, Tokyo, Japan) to measure the CA, which is reliable and with a high degree of accuracy that can be used as an educational tool for clinical assessment. [13],[15]

Many specialist dental centers are offering dental services all over Assir region, among these are prosthetic services, which were introduced by dental specialists from different countries. The present cross-sectional study was undertaken to measure the clinically achieved MD, BL taper and CA with respect to its theoretical guidelines. Also to assess the effect of the tooth position (maxillary or mandibular posterior teeth) on degrees of taper and CA of the prepared teeth.

  Materials and methods Top

A cross-sectional study was conducted with total number of 80 dies of crown preparations, which were collected from four specialist dental centers at Assir regions (20 samples from each center). The dies were divided into 4 groups according to the specialist dental center collected from, which were King Khalid University (KKU) specialist clinics, Muhail, Kamis-Mushayat and Assir Specialist Dental Centers.

Data were collected respectively using convenient sampling technique from die trimmed after cementation of crowns in the patient mouth. The dies of the prepared crowns of molars and premolars from both arches (maxillary and mandibular posterior teeth), were prepared by different specialists in fixed prosthodontics under normal conditions. Anterior teeth were excluded from the study because of the deficiency in the numbers of mandibular anterior teeth.

A digital camera (Cyber-shot ® S750 Digital Camera DSCS750, Sony, Japan) with 12.1 mega pixel was mounted on a tripod stand (Benro Tripod T-600 Ex, Copyright Beniro Industrial Inc., China) perpendicular to the long axis of the die to take the photographs. Each die was placed at 20 cm distance away from the camera and with the position of the occlusal surface parallel to the floor. For each die, two different aspects, buccal and proximal, were photographed. Single trained investigator took photographs of the stone dies under standardized condition. The photographs were then transferred into a personal computer and AutoCAD 2007 software program was used to measure the CA of each die.

For each photograph, lines were drawn over the right and left contours of the axial walls of the die, mid-mesial and mid-distal for the buccal view or mid-buccal and mid-lingual of the proximal view. The lines were drawn from the finish line extended coronally. Another line was drawn parallel to the long axis of the tooth contingent with the internal finish line. The angle created by the two lines (the line parallel to the long axis of the tooth and a line parallel to the axial walls) was measured to determine the taper or axial angle of this side MD and BL. The CA of the preparation in each view was computed or calculated by adding the two sides angles [Figure 1]. Overall CA is the sum of CA bucco-lingually and mesio-distally.
Figure 1: Stone die photograph for measuring the mesio-distal taper and convergence angle degrees

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The data on the taper and CA were recorded, and subjected to statistical analysis by using one-way analysis of variance (ANOVA), Newman-Keuls multiple post-hoc procedure, and two-way ANOVA. Statistical software SPSS (Statistical Package for the Social Sciences, IBM) version 21 was used for the analysis. The level of significance was set at 5%.

  Results Top

[Table 1] shows the distribution of mean MD, BL taper and CA degrees of all four centers. The highest values were observed with Kamis-Mushayat Center for BL (31.89) taper and Assir Center for MD (38.21) and overall CA degrees (34.38). The mean values for KKU specialist dental clinics were consistently the lowest for all parameters of CA degrees (22.91). The results of ANOVA showed statistically significant (P < 0.05) differences with the mean CAs between all the dental centers [Table 2]. The distribution of all taper and CA degrees with the mean and standard deviation can be better appreciated in [Figure 2].
Figure 2: The comparison of mesio-distal, bucco-lingual tapers and convergence angle by dental centers

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Table 1: Summary statistics of MD, BL tapers and CA degrees by dental centers

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Table 2: Comparison of mean of MD, BL tapers and CA degrees by dental centers by one-way ANOVA

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Furthermore, Newman-Keuls multiple post-hoc procedure was used to make pairwise comparison of dental centers with respect to overall CA degrees. The results suggest that the mean overall CA degrees for KKU center was statistically least as compared with other centers [Table 3].
Table 3: Pair wise comparison of dental centers with respect to overall CA values scores by Newman– Keuls multiple post-hoc procedures

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[Table 4] shows a significant difference between both the teeth and arches.
Table 4: Comparison of mean MD, BL and overall CA degrees by arch and tooth types by two-way ANOVA

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The MD, BL taper and overall CA degrees for all the premolar teeth were significantly lower than molar teeth (P < 0.05) [Table 5].
Table 5: Comparisons of MD, BL tapers and overall CA degrees by all dental centers with respect to tooth type and arch type

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  Discussion Top

The primary objective of this study was to determine the taper and CA of the preparations made on teeth. Although there being confusion in the usage of the word taper and CA, as and when described in the previous articles, the usage of CA (which includes both sides taper) is preferred.

The ideal and recommended tapers and CA degrees mentioned in fixed prosthodontics textbooks and the dental literatures are difficult to achieve. In clinical practice, axial wall taper and CA of the prepared teeth varied from tooth to tooth in different dimensions and depends upon operator skills, experience and the position of the prepared tooth. [15]

Adequate resistance and retention can be achieved during tooth preparation if a systemic approach as outlined is applied. [20] The complex inter-relationships of clinical, theoretical, and mechanical factors determine the retention and resistance characteristics. [15]

Taper and convergence angle degrees with respect to the dental center

With respect to all the dental centers when taper and CAs were checked, the difference was found to be significant between and within the dental centers. The KKU specialist clinics recorded the lowest CA (22.91°), these could be explained on the basis that the specialists daily evaluate the students' preclinical and clinical works during the whole academic year. Although the specialists at all the centers are with different nationality, graduation schools, experience level and certificate, in this academic center the standard textbooks are recommended and on the basis of that the students are evaluated.

Our finding coincided with the findings of Ghafoor et al., [14] regarding the degrees of MD, BL taper and overall CA. Although we agree with them in that, effect of operator clinical experience did not show significant differences in CA and taper values. [21] Al Ali et al. [13] measured the CA degrees with the prosthodontists at Riyadh private sector and found the CA degrees (17.8 ± 8.4), which agrees with our CA at the specialist clinics of KKU.

Comparing the CA degrees of this study, with those by Shillingburg et al. [2] and Dorriz et al., [21] there is a significant difference between posterior teeth prepared by them and our results, which could be related to the accessibility problems in the posterior part of the mouth due to which proper placement of the diamond points at the correct angulation was not possible or might be due to anatomical variations of the crowns.

Regarding the MD taper, the lowest degree is for KKU specialist clinics, while it is high at Assir Dental Center, and for the BL taper, the lowest degrees are for KKU specialist clinic (22.78), while the highest values are for both Muhayil (31.78) and Kamis-Mushayat (31.89) respectively. This could be explained on the basis that the academicians who are teaching the students regarding the principles of tooth preparations, help themselves also as they are consistently reminded of the taper. However, the specialists who are working in other dental centers are working only in the clinics and are away from the literature, and the consistent reminding effect is not there.

We disagree with Ghafoor et al. [15] and Al Ali et al. [13] regarding the MD taper values, which are greater than that registered by our values; this could be due to limited access and visibility in the posterior region and the distal aspect of molars.

For the BL taper, the results disagree with the finding of Ghafoor et al., [15] because the buccal wall taper could be affected by the clinical operator to reduce the thickness in order to achieve good esthetics outcome while we agreed with the Al Ali et al., [13] who concluded that the BL taper of the preparation is less than MD, which coincided with our finding.

Taper and convergence angle degrees with respect to the position and type of teeth

On the arch types, there are no significant differences between maxillary and mandibular prepared teeth, this is in agreement with Dorriz et al., [21] but it does not agree with them regarding the taper (MD, BL) between molars and premolars. This could be explained by the ability of specialists at different centers to control the position of both dentist, patient's head and overall the hand-piece burs alignment with the long axis of the existing teeth at the different positions.

The CA is lower at the mandibular arch [Table 5] in the premolars due to the better accessibility than the molars. While it is greater on the mandibular molars, the reason can be attributed to anatomical position of the teeth that is, severe lingual inclination or to avoid a possible injury to the tongue as well as to the cheek. This is in agreement with Ghafoor et al. [14],[15] Poor accessibility on posterior teeth may force the specialist to an awkward instrument position or motion that may compromise wall inclination.

Overall this disparity in the results of this study regarding MD, BL and CA values may be probably due to small sample size from all the centers or random selection of crown preparation models from the involved dental specialist centers.

Practitioners have difficulty in meeting the recommended degrees of taper and CA values during clinical practice in respect to the values recommended in the dental literatures and fixed prosthodontics text book. So more long-term clinical studies with larger samples size are needed to fully assess the effect of CA on the longevity of the cemented crowns.

  Conclusions Top

From this cross-sectional study, the following conclusions can be drawn:

  1. The specialists at the College of Dentistry, KKU recorded the lowest MD, BL tapers and CA. Assir Dental Center recorded the highest CA as well as MD taper. Kamis-Mushayat Dental Center recorded the highest degrees of taper.
  2. The MD, BL taper and overall CA degree for all the premolars were lower than molar teeth.
  3. There is need for exerting extra effort to reduce the taper and CA degrees during teeth preparations which is possible only if regular monitoring of the work is done.

  References Top

Ayad MF, Maghrabi AA, Rosenstiel SF. Assessment of convergence angles of tooth preparations for complete crowns among dental students. J Dent 2005;33:633-8.  Back to cited text no. 1
Shillingburg H, Hobo S, Whitsett LD, Jacobi, R, and Brackett, SE. Fundamentals of Fixed Prosthodontics. Chicago: Quintessence; 1997. p. 119-38.  Back to cited text no. 2
Chan DC, Wilson AH Jr, Barbe P, Cronin RJ Jr, Chung C, Chung K. Effect of preparation convergence on retention and seating discrepancy of complete veneer crowns. J Oral Rehabil 2004;31:1007-13.  Back to cited text no. 3
Zidan O, Ferguson GC. The retention of complete crowns prepared with three different tapers and luted with four different cements. J Prosthet Dent 2003;89:565-71.  Back to cited text no. 4
Rosenstiel SF, Land MF, Fujimoto J. Contemporary Fixed Prosthodontics. 4 th ed. St. Louis (MO): Mosby Elsevier; 2006. p. 336-75.  Back to cited text no. 5
Malone WF, Koth DL. Tylman's Theory and Practice of Fixed Prosthodontics. 8 th ed. St. Louis: CV Mosby Co.; 1989. p. 2-45.  Back to cited text no. 6
Ayad MF, Johnston WM, Rosenstiel SF. Influence of tooth preparation taper and cement type on recementation strength of complete metal crowns. J Prosthet Dent 2009;102:354-61.  Back to cited text no. 7
Patel PB, Wildgoose DG, Winstanley RB. Comparison of convergence angles achieved in posterior teeth prepared for full veneer crowns. Eur J Prosthodont Restor Dent 2005;13:100-4.  Back to cited text no. 8
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Kent WA, Shillingburg HT Jr, Duncanson MG Jr. Taper of clinical preparations for cast restorations. Quintessence Int 1988;19:339-45.  Back to cited text no. 10
Nordlander J, Weir D, Stoffer W, Ochi S. The taper of clinical preparations for fixed prosthodontics. J Prosthet Dent 1988;60:148-51.  Back to cited text no. 11
Christensen GJ. Frequently encountered errors in tooth preparations for crowns. J Am Dent Assoc 2007;138:1373-5.  Back to cited text no. 12
Al Ali K, Al Wazzan KA, Al Amri MD, Al-Shahrani AM, Al-Shahrani M, Al-Qahtani H. Assessment of convergence angle of full veneer preparations carried out by practitioners with different levels of experience. Saudi Dent J 2009;21:37-44.  Back to cited text no. 13
Ghafoor R, Rahman M, Siddiqui AA. Comparison of convergence angle of prepared teeth for full veneer metal ceramic crowns. J Coll Physicians Surg Pak 2011;21:15-8.  Back to cited text no. 14
Ghafoor R, Siddiqui AA, Rahman M. Assessment of convergence angle of full-coverage porcelain fused to metal crowns in clinical practice. Indian J Dent Res 2012;23:241-6.  Back to cited text no. 15
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Dorriz H, Nokar S, Naini RB, Madadi A. The convergence angle of full-coverage crown preparations made by dental students. J Dent Med Tehran Univ Med Sci 2008;5:37-41.  Back to cited text no. 21


  [Figure 1], [Figure 2]

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]

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