|Year : 2016 | Volume
| Issue : 3 | Page : 139-144
Evaluation of the effectiveness of auxiliary features on resistance with decreased occluso-cervical height: An In Vitro study
Aman Arora, Viram Upadhyaya, Sheen J Arora, Ritu Sangwan
Department of Prosthodontics, DAV Dental College, Yamunanagar, Haryana, India
|Date of Web Publication||7-Oct-2016|
Dr. Ritu Sangwan
Department of Prosthodontics, DAV Dental College, Yamunanagar, Haryana
Source of Support: None, Conflict of Interest: None
Background: Fewer studies were conducted on resistance form. This study evaluated the effect of different auxiliary features on inadequate resistance form. Aim: The aim of the study is to evaluate the resistance at 22° taper with reduced occluso-cervical height with different auxiliary features. Methodology: An ivorine tooth was prepared with computer-aided design-computer-aided manufacturing with total occlusal convergence (TOC) of 22°, shoulder finish line 0.9 mm wide, reduced occluso-cervical height, i.e., 2.5 mm, and reduced diameter. The crown preparation was subsequently modified to include interproximal grooves, interproximal boxes, and reduced TOC in the axial wall from 22° to 8° in the cervical 1.5 mm of the axial wall. A total of four groups with ten standardized metal dies were prepared for each design with the computer-aided milling machine. Standardized complete metal crowns using silicon mold were fabricated and cemented on metal dies with glass ionomer cement. The resistance of each specimen was evaluated when force was applied at a 45° angulation to the long axis of the die in a lingual to buccal direction by a universal testing machine. The values were then analyzed using one-way analysis of variance test and post hoc Bonferroni test. Results: The comparison of the mean resistance test values was done among all the groups, and there was a significant (P ≤ 0.001) difference found among the groups. Conclusion: The most effective method of enhancing resistance form preparation is to decrease the TOC of the cervical portion of the prepared axial walls. Two interproximal boxes significantly increased the resistance form. However, two interproximal grooves did not significantly increase the resistance form.
Keywords: Grooves, resistance form, tooth preparation, total occlusal convergence
|How to cite this article:|
Arora A, Upadhyaya V, Arora SJ, Sangwan R. Evaluation of the effectiveness of auxiliary features on resistance with decreased occluso-cervical height: An In Vitro study. Indian J Dent Sci 2016;8:139-44
|How to cite this URL:|
Arora A, Upadhyaya V, Arora SJ, Sangwan R. Evaluation of the effectiveness of auxiliary features on resistance with decreased occluso-cervical height: An In Vitro study. Indian J Dent Sci [serial online] 2016 [cited 2022 May 17];8:139-44. Available from: http://www.ijds.in/text.asp?2016/8/3/139/191730
| Introduction|| |
In the past, most studies were related to the effect of different auxiliary features on retention form. Based on the current literature, taper that can be achieved experimentally and clinically is not 2°–5° but 12°–22°. Resistance form has been regarded as one of the key principles in dictating the success of crowns and fixed partial dentures., Resistance form is the feature of a tooth preparation that increases the stability of a restoration and resists dislodgement along an axis other than the path of placement.
Adequate resistance to dislodgement depends on three factors: (a) Magnitude and direction of the dislodging forces. (b) Geometry of tooth preparation. (c) Physical properties of luting cements. Of these three factors, the magnitude and direction of the dislodging forces are an inherent patient factor that the dentist may not be able to control adequately.
Resistance is a function of the relationship between axial wall taper, preparation diameter, and preparation height. It decreases as the taper or diameter increases or preparation height is reduced. All the above-mentioned aspects are closely interrelated and should be considered in together when determining adequate resistance form. The taper is the convergence of two opposing external walls of a tooth preparation as viewed in a given plane. Total occlusal convergence (TOC) is the angle of convergence between the two opposing prepared axial surfaces. According to Tiu et al. TOC is the most important preparation parameter.
Tooth location is a critical factor in achieving adequate resistance form in preparation taper. The molar region frequently presents the most inadequate TOC. The resistance form of a crown preparation can be improved by re-preparing the apical portion of the axial walls at a reduced or more “ideal” taper.
Proussaefs et al. explained the effectiveness of mesiodistal grooves, mesiodistal boxes, and TOC in enhancing resistance form. They concluded that grooves and boxes did not increase the resistance form significantly. Roudsari and Satterthwaite  suggested interproximal grooves and reduction in TOC to enhance resistance form. However, they found that reduction in cervical TOC was more effective than interproximal grooves. Farshad et al. recommended mesio-occlusal distal isthmus, occlusal inclined plane, and reduced TOC to enhance resistance form. Tiu et al. recommended that TOC values have increased over the past four decades from an unachievable 2°–5° taper to a more realistic 10°–22°.
This study was done to re-evaluate the past conflicts about the effectiveness of different auxiliary features in resistance form.
The aim of the present study is to evaluate the resistance at 22° taper with reduced occluso-cervical height with different auxiliary features.
| Methodology|| |
An ivorine tooth (Columbia, Long Island, NY, USA) was mounted in dental stone (Gypstone, PREVEST DenPro) using Ney surveyor (Unident). The tooth was prepared with increased TOC (22°), shoulder finish line 0.9 mm wide, reduced occluso-cervical height, i.e., 2.5 mm, and reduced diameter to resemble inadequate resistance form., For this, a computer-aided software (SOLIDWORKS) was used and tooth preparation was done using computer-aided milling machine (CNC MACHINE SPECS ECO 321).
This definitive tooth preparation was scanned with a dental scanner (Delcam iMetric scanners) and transformed into ten metal dies (nickel chrome) with the milling machine and was considered as “Control group or Group I” [Figure 1] (I) and [Figure 2] (I)].
|Figure 1: Occlusal view of tooth preparation – (I) Group I, (II) Group II, (III) Group III, (IV) Group IV.|
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|Figure 2: Occlusal view of metal die – (I) Group I, (II) Group II, (III) Group III, (IV) Group IV.|
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The tooth preparation was then modified by preparing two interproximal grooves which were centered on mesial and distal surfaces of the tooth extending from the occlusal surface of the preparation to the level of the finish line. The dimension of grooves was 1 mm mesiodistally and 1 mm faciolingually. They were prepared in milling machine (Milling Unit BF2, Bredent, UK) with the help of a flat end tapered carbide bur (171 L, SS White). It was scanned and ten metal dies were fabricated. This was considered as “Group II” [Figure 1] (II) and [Figure 2] (II)].
The grooves were transformed to interproximal boxes using the same carbide bur and milling machine, extending buccally and lingually resulting in proximal boxes with 3 mm faciolingual dimension at the gingival floor and 1 mm mesiodistal depth. Ten metal dies were fabricated and it was considered as “Group III” [Figure 1] (III) and [Figure 2] (III)].
The boxes were then blocked with inlay wax (Bego USA), and the taper was reduced in cervical half of the prepared axial surfaces. For this, computer-aided software (SOLIDWORKS) was used to get the taper of 4° per wall in the cervical half to get the TOC of 8°. This resulted in increase in width of the finish line from 0.9 to 1.09 mm. This preparation was then transformed to ten metal dies and considered as “Group IV” [Figure 1] (IV) and [Figure 2] (IV)].
The wax pattern for cast metal crown was fabricated on the control group die using blue inlay wax (BEGO USA). A recipient site for the tip of the universal testing machine (UTM) was carved at the lingual incline of buccal cusp of the pattern. A mold of duplicating silicon was made over the wax pattern of prepared tooth which was used to fabricate the wax pattern for all the groups.
The crowns of base metal alloy (Nickel Chromium, DFS, Germany) were then fabricated using Lost Wax Casting Technique. They were cemented on metal dies with glass inomer luting cement (Gc, Gc Corporation Tokyo, Japan). The crowns were seated on the metal dies and loaded with 5 kg compressive load in UTM (Autograph, Ag-Is, Shimadzu) for 5 min and cement was allowed to set for 24 h.
The cemented crowns were loaded at the recipient site with a gradually increased external force (1 mm/min) applied at 45° angulation from lingual to buccal direction by UTM [Figure 3] and [Figure 4]. For this, a custom made metal jig was fabricated to hold the crown at 45° angulation. The force was applied at the lingual inclined plane of the buccal cusp. The force (kg) required to dislodge the crown from the metal die was recorded in kilograms.
| Results|| |
The force values/resistance test values (in kg) required for the first micromovement and dislodgement of the crowns of all the four Groups - Group I, Group II (interproximal groove), Group III (interproximal box), and Group IV (reduced taper in cervical) obtained from UTM [Table 1] and [Table 2].
|Table 1: One-way analysis of variance test for the first micromovement (in kg)|
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The software used for the statistical analysis in the present study was SPSS (Statistical Package for Social Sciences) version 21.0 and Epi-info version 3.0. The statistical tests used were analysis of variance (ANOVA) test and post hoc test.
When evaluating the data obtained through the UTM, the post hoc Bonferroni test demonstrated that the values noticed by reducing the TOC of the original preparation design at the cervical area were significantly higher than other modifications. There was also a significant difference between interproximal boxes and control group. But interestingly, no significant difference was observed between the interproximal grooves and Group III [Table 3] and [Table 4].
|Table 3: One-way analysis of variance test for dislodgement of crowns (in kg)|
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[Table 1] and [Graph 1] depict the comparison of the mean resistance test values of first micromovement among all the four groups, i.e. Group I, Group II (interproximal grove), Group III (interproximal box), and Group IV (reduced taper in cervical) using the one-way ANOVA test. It was evaluated that there was a significant (P ≤ 0.001) difference among the groups.
[Table 2] depicts the inter-group comparison of the mean resistance test values among Group I, Group II, Group III, and Group IV using the post hoc Bonferroni test. The comparison of the control group with the other three groups showed a significant (P ≤ 0.001) difference with Groups III and IV (due to increase in surface area in boxes and increase in parallelism in opposing walls in Group IV) and nonsignificant (P > 0.05) difference with Group II. Group II when compared with the other Groups also showed a significant (P ≤ 0.001) difference with Groups III and IV but nonsignificant (P > 0.05) difference with the Group I. Group III showed a significant (P ≤ 0.001) difference with all the groups. Similarly, Group III also showed a significant (P ≤ 0.001) difference with all the groups.
[Table 3] and [Graph 2] depict the comparison of the mean resistance test values of the complete dislodgement of the crown among Group I, Group II, Group III, and Group IV using the one-way ANOVA test. There was a significant (P ≤ 0.001) difference among the groups.
[Table 4] depicts the inter-group comparison of the mean loosening of crown values among Group I, Group II, Group III, and Group IV using the post hoc Bonferroni test. The comparison of the control group with the other three groups showed a significant (P ≤ 0.001) difference with Groups III and IV and nonsignificant (P > 0.05) difference with Group II. Group II, when compared with the other groups, also showed a significant (P ≤ 0.001) difference with Groups III and IV but nonsignificant (P > 0.05) difference with the Group I. Group III showed a significant (P ≤ 0.001) difference with all the groups. Similarly, Group IV also showed a significant (P ≤ 0.001) difference with all the groups.
| Discussion|| |
Tooth preparations with inadequate resistance form often contribute to dislodgement of complete cast crowns. Therefore, before crown preparations, factors such as length, diameter, and occlusal convergence angle must be evaluated. The present study evaluated resistance form with different auxiliary features with 22° taper and decreased cervico-occlusal convergence.
This study demonstrated that in a laboratory simulation of a clinically compromised complete-coverage tooth preparation with reduced occluso-cervical dimension and with 22° TOC, the crown preparation modification which provided the greatest resistance form was the preparation of the die with 8° of TOC at the cervical 1.5 mm of the axial wall. The reduced taper enhances the resistance form by providing more parallel opposing walls.
The findings in the present study agree with the work conducted by Farshad et al. who also evaluated the resistance form of different preparation features on mandibular molars and concluded that reducing the taper of occluso-cervical dimension is the most effective way of increasing the resistance form. Zuckerman  also concluded that reduce TOC reduces buccolingual diameter at the cervical part of teeth, and this area acts as a resistance zone. Proussaefs et al. demonstrated only crown preparation that significantly enhanced the resistance form was reducing the TOC at the cervical area of the axial walls.
Rosenstiel et al. explained that reducing TOC will cause paralleling of axial walls and therefore, increases the resistance form.
The second highest resistance form after TOC was found in Group III, i.e., interproximal boxes. The interproximal boxes increase the resistance form by preventing rotation of the restoration and increasing the surface area of the preparation. The boxes should be placed within a sound bulk of tooth tissue or core without any weak surrounding areas which are liable to fracture. Reisbick and Shillingburg  also concluded that resistance value increases significantly with the addition of boxes in the preparation.
The addition of interproximal grooves in the preparation was not found as effective in increasing the resistance form as TOC and interproximal box design. The resistance values of the interproximal groove design and control group were not significantly different. Proussaefs et al. also found in his study that grooves were not effective at increasing resistance form for a short tooth preparation with 20° TOC.
The present study is different from the past in the following aspects. In the present study, a standardized procedure was followed for the tooth preparation taper. The taper was decided based on conclusions of a systemic review by Tiu et al. which states that taper that can be achieved experimentally and clinically is not 2°–5° but 12°–22°. These specifications for the 22° of TOC were verified using a computer-aided software (SOLID WORKS) and then these specifications were fed in the computer-aided milling machine. This machine was used for the tooth preparation, whereas in the past studies, tooth preparation was done using the manually operated milling machine and no standardized procedure was followed to get the desired TOC. In the present study, the definitive tooth preparation was then scanned with a dental scanner (DELCAM Imetric scanner) and transformed into ten metal dies with the computer-aided milling machine, whereas in the past studies, the metal dies were milled using the operator controlled milling machine.
To make the impression of the prepared tooth, silicone mold was used in the present study which enhanced the accuracy of the impression. Moreover, as silicone is elastic, the chances of distortion were reduced, whereas in the past studies, polyvinyl siloxane impressions were made in the acrylic custom tray for duplicating the wax patterns.
The resistance form could have been improved with other factors like use of computer-aided design-computer-aided manufacturing (CAD-CAM) technology instead of lost wax technique used in the present study for crown fabrication. CAD-CAM technology would have improved the internal adaptation and fit of the crowns and would have resulted in improved force values.
Second, the use of luting cement with relatively higher compressive strength (example-resin modified glass ionomer cement) instead of glass ionomer luting cement as used in the present study would have significantly affected the force required for the dislodgement of the crowns.
Other modifications that can also be incorporated to enhance resistance to a nonresistance crown preparation are buccolingual grooves, occlusal inclined planes, occlusal isthmus, gingival extension of the preparation, and pins or posts.
This study is limited by the following factors that include the use of a single tooth type, no inclusion of the factors of the periodontal ligament and use of a single type of alloy. The rationale for using base metal alloy in this study was the high compressive strength of base metal, which allows it to resist deformation if excessive forces are applied. However, Eden et al. have shown that base metal alloys have inferior fit on a metal die.
Within the limitations of this study, it can be concluded that decreasing the TOC from 22° to 8° in the apical 1.5 mm of the reduced axial surfaces significantly enhanced the resistance form. However, decreasing the TOC is limited to the root canal treated teeth with inadequate resistance. It is not recommended in vital teeth or in teeth in which the remaining tooth structure after reduction is closer to pulp.
| Conclusion|| |
Within the limitations of the study, following conclusions were made relative to the effectiveness of several tooth preparation modifications on the resistance form of teeth prepared with TOC = 22°, shoulder finish line 0.9 mm wide, reduced occluso-cervical height, i.e., 2.5 mm, and reduced diameter.
- Decreasing the TOC from 22° to 8° in the cervical 1.5 mm of the reduced axial surfaces significantly increased the resistance form
- Two interproximal boxes that followed the existing convergence of the axial walls (22°) significantly increased the resistance form
- Two interproximal grooves that followed the existing convergence of the axial walls (22°) did not significantly increased the resistance form
- The most effective method of enhancing resistance form is to decrease the TOC of the cervical portion of the prepared axial walls.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2], [Table 3], [Table 4]