|Ahead of print publication
An In vitro study to evaluate the depth of reduction achieved by three different techniques for porcelain fused to metal crown preparation
TC Devikaa1, Sunil Kumar Mishra2, Prince Kumar2
1 Department of Prosthodontics, Meenakshi Ammal Dental College and Hospital, Chennai, Tamil Nadu, India
2 Department of Prosthodontics, Rama Dental College Hospital and Research Centre, Kanpur, Uttar Pradesh, India
|Date of Submission||03-May-2022|
|Date of Acceptance||19-Jan-2023|
|Date of Web Publication||20-Feb-2023|
Department of Prosthodontics, Meenakshi Ammal Dental College and Hospital, Chennai, Tamil Nadu
Source of Support: None, Conflict of Interest: None
Introduction: A near-ideal preparation lays the foundation and is required for adequate retention and resistance for fixed partial dentures. It is advisable to achieve a near-ideal preparation for the longevity of the prosthesis. Materials and Methods: Thirty maxillary left central incisors were mounted on the acrylic block. The samples were divided into groups with ten specimens: Group A-Free hand, Group B-Index, and Group C-Depth gauge. Group A was prepared using the freehand method, Group B was prepared using the putty index, and Group C was prepared using depth gauge bur. An index was prepared before and placed back on the tooth after preparation, and the gap was filled with low-viscosity elastomer. The thickness of the impression was measured at five different points under the microscope. The statistical analysis was done using one-way analysis of variance and Student's t-tests. Results: The preparations (dimensions) achieved by the freehand method were 2.02 mm, 1.70 mm, 1.11 mm, 0.90 mm, and 0.60 mm respectively at five predetermined points. Similarly, for putty index these were 1.76 mm, 1.21 mm, 0.95 mm, 1.32 mm, 0.71 mm respectively at five predetermined points. Likewise, with depth gauge bur it were 1.88 mm, 1.35 mm, 0.77 mm, 1.09 mm, 0.73 mm, respectively, at five predetermined points. There was a significant difference between the Groups (P < 0.001). Intergroup comparisons between Group A, Group B and Group C showed a significant difference. Group A showed near-ideal preparation in the palatal cervical region compared to Group B and Group C. Conclusion: The freehand technique was the least effective of the three techniques. The use of depth gauge bur had yielded the preparation near the ideal and should be considered during tooth preparation.
Keywords: Depth gauge, finish line, freehand technique, porcelain fused to metal crown, putty index, tooth preparation
|How to cite this URL:|
Devikaa T C, Mishra SK, Kumar P. An In vitro study to evaluate the depth of reduction achieved by three different techniques for porcelain fused to metal crown preparation. Adv Hum Biol [Epub ahead of print] [cited 2023 Mar 31]. Available from: https://www.aihbonline.com/preprintarticle.asp?id=370027
| Introduction|| |
In most dental practices, metal-ceramic crowns are one of the most widely used fixed restorations. This is because of the growing cosmetic demands that challenge dentists today. The restoration consists of a metal substructure veneered with a layer of porcelain to match the tooth colour. Metal ceramic crowns have a combined advantage of a metal substructure with the aesthetics of ceramic material.
Proper replication of the anatomic contour of the tooth leads to a successful restoration. Incisal reduction of at least 2 mm is required to provide good bulk to the restoration to prevent it from fracture. Uniform reduction of the tooth structure to 1.5–2 mm on all surfaces is necessary to make the restoration smooth and aesthetically pleasing. Poor emergence profile may lead to marginal inflammation and poor tooth contour. The most common errors encountered during tooth preparation are an inadequate reduction of axial walls, over reduction of tooth structure, undercuts in the prepared surfaces, sharp line angles, indistinct margins, and excessive gingival extensions.
The most common errors in tooth preparation can be divided into mechanical, biological, and aesthetic failures. Mechanical failures are of many types like debonding the ceramic superstructure, fracture of the abutment, chipping or fracture of ceramic. Over-reduction of tooth structure may lead to increased post-operative sensitivity because of the exposure of dentinal tubules. Under reduction may cause an increase in the bulkiness of the restoration. All the line angles and point angles may be rounded so that the stress concentration areas can be avoided.
Many experiments have been conducted on the accuracy of tooth preparation in porcelain laminate veneer crowns with a smaller sample size. They have concluded that tooth reduction by the freehand approach has a tendency for excessive reduction; hence, depth gauge burs and the index have been used for accurate tooth reduction. However, there are many limitations in previous experiments due to smaller sample sizes, and only three predetermined points were used for measurements to conclude.
Hence, the purpose of this study was to compare the accuracy of tooth preparation for ceramic metal crowns by using an index, depth gauge bur and freehand approach at five predetermined points. The null hypothesis was that there would be no difference in the tooth preparation with the index method, depth gauge bur and freehand approach.
| Materials and Methods|| |
This in vitro study was carried out in the Department of Prosthodontics with the aim to evaluate the accuracy of tooth preparation for the metal-ceramic crown by three different methods. The institutional ethical committee approved the proposal of the study vide reference no. 02/IEC/RDCHRC/2021–22/054. The sample size was determined following the previously published research article. A sample size of 30 maxillary left central incisor typodont teeth was divided into three groups of 10 each (Group A-freehand method, Group B-index and Group C-depth gauge bur). These 30 teeth were mounted on an acrylic block of standard dimension.
Preparation of master block
The block was first prepared using a mould of modelling wax of dimension 5 cm in breadth and 3 cm in length. The mould space created was filled with self-cure acrylic resin. The space for mounting the tooth was provided at the centre of the block, V-shaped notches were given on the mesial and distal side of the block, and “U” shaped notches were given at the labial and lingual aspect of the block for stabilising the putty impression. Once the resin was set, the master block was finished and polished. The master block was duplicated with additional silicone heavy body impression material so that all the blocks were of the exact dimensions. By this method, 30 blocks of self-cure acrylic resin were fabricated. The left maxillary central incisors were mounted at the centre of the individual blocks [Figure 1].
Preparation of the samples
A prosthodontist, unaware of the study, had done the tooth preparation to eliminate the operator bias. The tooth was prepared according to the standard protocol by giving a shoulder finish line on the labial side and a chamfer finish line on the lingual side. The teeth in Group A were prepared using a freehand approach. Before the preparation, an index of an unprepared tooth is made using an additional silicone putty impression material [Figure 2]. Then an incisal reduction of 2 mm, and a labial reduction of 1.5 mm with a shoulder finish line of 1 mm width was given labially. The labial reduction is done in two planes; the incisal plane and the cervical plane. The lingual concavity was given using a football-shaped bur with a lingual reduction of 1 mm, and a chamfer finish line of 0.5 mm was given on the lingual aspect. The axial walls are given a taper of 6 degrees with a smooth transition from the facial shoulder finish line to the lingual chamfer finish line. Once the preparation was done, the index was seated on the tooth with the help of grooves and sectioned in the midline. Then a light body impression material was injected in the gap between the index and the prepared tooth. Once the material had been set, a thin section of the left-hand side of the index was taken and viewed under the microscope [Figure 3].
|Figure 3: Measurement of putty index of prepared tooth at "5"predetermined areas.|
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Teeth in Group B were prepared using an index made of additional silicone impression material. First, an index of an unprepared tooth was made. The index was sectioned in the midline to provide enough guidance to the operator to facilitate the reduction. The tooth preparation was done using the index as a guide and by following the standard metal ceramic crown preparation protocol described for Group A. Light body impression material was injected in the gap between the prepared tooth and the index. A thin section of the left-hand side of the index was taken and viewed under the microscope.
The teeth in Group C were prepared by using depth gauge burs. Initially, an index of an unprepared tooth was made using an additional silicone putty impression material. Depth orientation grooves were given on the incisal half and cervical half of the tooth with the help of a depth gauge bur of 1.5 mm on the labial aspect. These grooves were also given on the incisal region. Tooth preparation on the incisal and labial aspect was done by using these grooves as a guide, this way, there is no chance of over-reduction of the tooth structure and lingual side reduction was done by freehand technique as described in group A. After the completion of tooth preparation, the index was sectioned in the midline, and light body impression material was injected in the gap between the prepared tooth and the index. Once the material had been set, a thin section of the left-hand side of the index was taken and viewed under the microscope.
Indentations were made on the light body impression material for all 30 samples corresponding to the five predetermined points on the tooth structure. These points were incisal, midlabial, labial cervical, midpalatal and palatal cervical. All the 30 sectioned index with light body impression was viewed under the optical stereomicroscope at ×20 magnification and 1.0 mm zoom. The reduction depth was measured by measuring the thickness of light body impression material under the stereo microscope. The readings obtained in Group A, B and C have been statistically analysed by one-way Analysis of Variance (ANOVA) and Student's t-test using MATLAB software.
| Results|| |
The mean and standard deviation of the thickness of light body impression obtained at five predetermined points: incisal, mid labial, labial cervical, mid palatal and palatal cervical of the three groups are presented in [Table 1]. Comparison of tooth preparation at the incisal region of different groups using one-way ANOVA showed a significant difference in all three techniques (P < 0.001) [Table 2].
|Table 1: Mean and standard deviation of thickness of light body impression obtained at 5 pre-determined points: incisal, mid labial, labial cervical, mid palatal and palatal cervical of the three groups in mm|
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|Table 2: Comparison of tooth preparation at the incisal region of different groups using one way ANOVA|
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Intergroup comparison at the incisal region using the Student's t-test showed a significant difference between the groups (P < 0.001) [Table 3]. Comparison of tooth preparation by one-way ANOVA test at the mid-labial region showed a significant difference with all three techniques (P < 0.001) [Table 4]. Intergroup comparison at the midlabial region showed a significant difference between the groups (P < 0.001) [Table 5]. Comparison of tooth preparation at the labial cervical of different groups using one-way ANOVA showed a significant difference in all three techniques (P < 0.001) [Table 6]. Intergroup comparison at the labial cervical region showed a significant difference between the groups (P < 0.001) [Table 7].
|Table 3: Intergroup comparison at incisal region (mm) using Student's t-test|
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|Table 4: Comparison of Groups A, B and C by one way ANOVA test at mid labial region of tooth|
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|Table 5: Intergroup comparison at midlabial region (mm) using Student's t-test|
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|Table 6: Comparison of tooth preparation at the labial cervical of different groups using one-way ANOVA|
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|Table 7: Intergroup comparison at labial cervical region (mm) using Student's t-test|
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Comparison of tooth preparation at the midpalatal region of different groups using one-way ANOVA showed a significant difference in all three techniques (P < 0.001) [Table 8]. Intergroup comparison at the midpalatal region showed a significant difference between the groups (P < 0.001) [Table 9]. Comparison of tooth preparation at the palatal cervical region of different groups using one-way ANOVA showed a significant difference in all three techniques (P < 0.001) [Table 10]. Intergroup comparison at the palatal cervical region showed a significant difference between the groups (P < 0.001) [Table 11].
|Table 8: Comparison of tooth preparation at the midpalatal region of different groups using one way ANOVA|
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|Table 9: Intergroup comparison at midpalatal region (mm) using Student's t-test|
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|Table 10: Comparison of tooth preparation at the palatal cervical region of different groups using one-way ANOVA|
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|Table 11: Intergroup comparison at palatal cervical region (mm) using Student's t-test|
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| Discussion|| |
Different methods of tooth preparation must be selected according to the clinical scenario. They may also be selected according to the presence or absence of remaining tooth structure. Mostly freehand techniques are practised in the clinics since other techniques may warrant time. Even replacement of failed fixed partial dentures may widely depend on the existing preparation. The use of an index would be more suitable in cases where tooth structure is to be reduced for a metal-ceramic restoration so proper anatomic contour can be maintained and uniform reduction can be achieved. Also, depth gauge burs and digital post-operative scanning would be used when an adequate amount of pre-existing tooth structure is remaining so consistent preparation can be maintained.
In the present study, the null hypothesis that there would be no difference in the tooth preparation with the index method, depth gauge bur and freehand was rejected as a significant difference was obtained (P < 0.001). Brunton et al. conducted a study to assess depths produced by three preparation techniques: Freehand, index, and depth gauge burs for porcelain laminate veneers. They assessed the depths of preparation with these different techniques at five different points of preparation; incisal, midlabial, labial cervical, palatal and midpalatal region. They concluded that preparation by depth gauge bur showed consistent results, and the freehand approach was shown to be least effective and over preparation.
Ram et al. conducted a study to assess the amount of preparation achieved by the freehand method, silicone putty index and depth gauge bur for the ceramic metal crown. They compared three regions among the groups; incisal, labial and palatal. The result showed under preparation in the incisal aspect (1.72 mm) with a freehand approach. The contradictory result obtained in the present study with over reduction in incisal aspect (2.02 mm) with the freehand approach. The difference in result may be because Ram et al. have prepared the tooth mounted on a working model with the adjacent and opposing tooth present; hence there is a visual tendency to under-prepare when preparation is compared in accordance with the opposing tooth. In the present study, the typodont tooth has been mounted on individual blocks to prepare each tooth accurately.
Ideally, 2 mm of incisal preparation is required to maintain a proper aesthetic profile while fabricating the restoration. In the present study, preparation on the labial aspect showed over reduction in the midlabial region (1.70 mm) and in the labial cervical region (1.11 mm), which is not in accordance with the results by Ram et al., which they have obtained under preparation of the entire labial aspect (1.23 mm). There must be a minimum of 1.5 mm reduction in the labial aspect so that the restoration appears smooth and appealing. In the labial cervical reduction under preparation may damage the gingival health of the tooth and also affect the emergence profile of the restoration. In the midpalatal aspect, it showed under reduction (0.90 mm). The palatal cervical aspect showed adequate reduction (0.6 mm), which is in favour of the results by Ram et al., in which they achieved a reduction of 0.5 mm for the palatal aspect. Adequate reduction in the palatal aspect helps to avoid premature contact during occlusion. Sufficient amount of porcelain and metal in the restoration also increases the longevity of the restoration.
In the present study, tooth preparation with the index method showed adequate preparation in the incisal region (1.76 mm) and in the midlabial region (1.21 mm), which is in favour with results by Ram et al. in which they obtained 1.96 mm of incisal preparation and 1.45 mm of labial preparation. The present study had shown overpreparation (0.95 mm) which is not in favour of Ram et al. Overpreparation in the cervical aspect might lead to post-operative sensitivity after cementation of restoration. Present study showed an adequate reduction in the midpalatal region (1.32 mm) and over reduction in the palatal cervical region (0.71 mm), which is in favour with the findings by Ram et al. where they have achieved a palatal reduction of 0.68 mm. The over-reduction in the palatal aspect may be due to the potential limitation of visual assessment. By using an index as a guide, there is little tendency for over/under reduction of the tooth structure except in the palatal cervical region, which may be due to visual error.
In the present study, tooth preparation using the depth gauge showed ideal preparation in the incisal region (1.88 mm), in the mid-labial region (1.35 mm), in the labial cervical region (0.77 mm) and in the midpalatal region (1.09 mm) which is also in accordance to the results by Ram et al. in which they have obtained an incisal reduction of 1.95 mm, labial reduction of 1.47 mm. There is an over-reduction in the palatal cervical region (0.73 mm), which is also in favour of Ram et al. This may be due to angulation of the depth gauge bur while preparing the cervical portion below the cingulum. There may be the tendency of the bur to create deeper grooves which might have resulted in over-preparation of the cervical aspect. Depth gauge bur creates orientation grooves which help as a guide for achieving adequate preparation except in the cervical aspect, which may be due to angulations of bur or due to visual error.
From these results, we can infer that the depth gauge bur method showed consistent results except in the palatal cervical region, followed by the index method. The freehand method showed overpreparation and proved to be the least effective. Dunne has stated that constancy in visual perception is not perfect, and objects may be perceived as more minor and more significant than they usually are depending on the size of the image formed in the retina. The same limitation was observed in the present study. However, the operator exhibited consistency over a mean range within the number of samples, but when it comes to between the groups, a difference was observed.
| Conclusion|| |
Within the limitations of this study following conclusions were drawn:
- The freehand technique was the least effective, as it resulted in significant preparation, whereas the use of depth gauge burs was the most effective technique
- Though no single technique could achieve results close to the recommended ideal, the use of depth gauge bur provides the most consistent results
- In clinical situations where a good amount of tooth structure is remaining, using a depth gauge might be a recommended technique as it provides uniform tooth reduction.
This study had certain limitations, such as visual examination being the main element to assess the depth of the preparation; studies have shown that visual perception is subject to variation. Depth gauge bur removes the tooth structure in the sagittal and transverse plane, and depth of reduction in the sagittal section was the only analysis point. Further research is required to compare the depth reduction achieved by depth gauge bur and index on sagittal planes other than midline. In this study, only preparation depth is considered a criterion to achieve the accuracy and consistency of tooth preparation. Still, the authors should consider other factors like retention, resistance, marginal integrity and geometry of complete preparation to achieve the results.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7], [Table 8], [Table 9], [Table 10], [Table 11]