| Abstract|| |
Context: Accuracy of fit of cast metal posts fabricated with direct and indirect techniques. Aims: To evaluate the accuracy of fit of cast metal posts fabricated with direct and indirect techniques using micro-CT. Materials and Methods: Sixty extracted human maxillary central incisors were used. The teeth were decoronated leaving 2 mm at the cementoenamel junction. Four patterns for each tooth were made including one direct and indirect wax pattern and one direct and indirect resin pattern. In total, 240 patterns and castings were made. Each tooth was scanned using microcomputed tomography (μCT) system with a resolution of 14.5 μm. μCT analyzer Sky scanner software was used to calculate the volume of space between the post and the canals. Statistical analysis used: The student paired t-test and unpaired t-test were used to determine the significant differences between groups. Results: The overall space between the canal walls and posts made with the direct wax and resin technique had an average mean value of 1.10 mm3, whereas with the indirect technique resin and wax pattern, the average mean was 1.30 mm3 which was significant. Conclusion: Within the limitations of the study, it was found that the accuracy of fit is better in direct technique using wax pattern.
Keywords: Cast post, direct pattern, indirect pattern, inlay wax, micro-CT, resin pattern
|How to cite this article:|
Antony S, Venkatesh KV, Vahanaan DS. Evaluation of accuracy of fit in cast metal post fabricated with direct and indirect techniques using micro-CT - An in vitro study. Indian J Dent Res 2021;32:323-9
|How to cite this URL:|
Antony S, Venkatesh KV, Vahanaan DS. Evaluation of accuracy of fit in cast metal post fabricated with direct and indirect techniques using micro-CT - An in vitro study. Indian J Dent Res [serial online] 2021 [cited 2022 May 26];32:323-9. Available from: https://www.ijdr.in/text.asp?2021/32/3/323/338125
| Introduction|| |
Successful restoration of endodontically treated teeth ultimately determines the prognosis of the teeth. There are wide ranges of treatment options available for restoring teeth, which are treated endodontically with varying degrees of complexity. A good clinician must be able to predict the outcome and probability of restoring such teeth. The higher incidence of fractures could be due to the structural changes in dentin, major changes in tooth biomechanics due to loss of hard tissue by decay, fracture, or cavity preparation.
The teeth which are treated for root canal have different mechanical and chemical properties when compared with the teeth which are vital. Current understanding is that teeth, which are treated endodontically are weaker due to the loss of sound dentinal tissues. The remaining tooth structure determines the treatment option considered. Various studies provide us with a clear indication for the placement of post where there is greater than 50% of the tooth structure that is lost. Endodontic posts retain a core when the coronal tooth structure is missing.
Many factors such as direction and magnitude of occlusal load, dowel design, remaining dentin thickness, effect of the ferrule, quality of the cement layer, and the fit of the post influences long-term survival of post and core restorations. Posts can be prefabricated or custom made. For teeth which were treated endodontically, the gold standard for restoring and rehabilitation is customized cast post and core when there is extensive loss of coronal structure. They are anatomic and conform better to the shape of the canal and provide support and maximum retention for the coronal restoration.
For the fabrication of custom post and core, a pattern is fabricated as the first step. When the pattern is fabricated in the mouth, it is known as the direct technique, when the same is done outside the mouth, it is indirect technique of pattern fabrication. Patterns can be made of inlay wax and pattern resin. Pattern resin is more rigid as compared with wax pattern. Based on the clinician's preference and clinical situation, the technique of pattern fabrication is chosen.
Conventional radiography provides only a 2D record and not 3D information of a tooth. Micro-CT is a noninvasive method used to visualize morphological characteristics in a detailed and precise manner, without irreversible changes to the tooth. Micro-CT is used in studies to assess enamel thickness, measure the tooth, analyse the root canal morphology, evaluate the root canal preparation, and evaluate craniofacial skeletal development and structure. To date, there are few studies that compare the fit of the custom-made post and core restorations using micro-CT technique when the patterns are fabricated using either the direct or indirect technique.
| Methodology|| |
The following inclusion and exclusion criteria were used for the selection of the specimens for the study.
Inclusion criteria are maxillary central incisors and periodontally involved central incisors, and exclusion criteria are teeth with developmental anomalies, calcifications, open apex, fractures, caries, and external cracks.
Sixty extracted human maxillary central incisors were collected. They were cleaned of calculus and debris by hand scaling and they were stored in saline as a storage medium [Figure 1]. A preoperative radiovisiograph (RVG) was taken. This was followed by sectioning of the teeth with a diamond disk and slow speed, leaving 2 mm of coronal tooth structure above cementoenamel junction. The length of the teeth was standardized to 17 mm. The tooth was mounted on the wax mould [Figure 2].
No. 2 round carbide bur (ORO ISO 001/008, BR-41, Vietnam) with high-speed handpiece was used for access opening. A drop effect was felt while penetrating into the root or pulp chamber of the tooth. The orifice was flared using a no. 2 Gates Glidden drill (Mani, Vietnam), this drill was used in a circumferential filling motion 2 mm apical to the orifice.
A 15 k endodontic file (Mani, Vietnam) was used to determine the working length. Canal length was measured by inserting a K-file into the canal until the tip of the file was exposed through the major foramen. Canal length is the distance between the coronal reference point and the file tip. From this length, 1 mm was subtracted and used as the working length. Apical enlargement was completed till 55 k file and a stepback technique was followed, by stepwise reduction in the working length for larger files, typically in 1 mm. Irrigation was done with 0.5% of sodium hypochlorite (PRIME dental, India) followed by saline.
Once cleaning and shaping was done, the next step was obturation. Obturation was completed using lateral compaction technique and RVG was taken [Figure 3]. By using heat, the excess Gutta-Percha was removed and with a plugger the coronal mass were compacted. After obturation, Gutta-Percha removal of 12 mm was done using peeso-reamer (Mani, Vietnam) starting from size 3, 2, and 1. The remaining gutta percha was 5 mm at the apex for apical seal and confirmed by RVG [Figure 4]. Sixty specimens were prepared in the same manner.
|Figure 3: Obturation was done using lateral compaction technique and RVG taken|
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All the 60 teeth were subjected to preoperative scanning using micro-CT imaging. The scanner is set up and the parameters were set according to the sample size versus the resolution. Then, the scanning was done using a high resolution of Sky scan 1173 scanner at 95 kV and with a resolution of 85 μA using a filter brass of 0.25 mm thick filter. The teeth were embedded in polymer mould on the rotating stage. Then, the rotating stage moves into the scanner and the shutter is closed. The preoperative volume of the space is calculated using Sky Scan CT Vox software [Figure 5].
|Figure 5: The preoperative space of the volume is calculated using sky scan CT vox software|
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For direct technique, patterns for post and core were made using inlay wax [Figure 6]a, [Figure 6]b (GC corporation, Tokyo, Japan) and low shrinkage acrylic resin (pattern resin) (GC corporation, Tokyo, Japan). Using endodontic K file, the petroleum jelly (Petroleum jelly, India) used as lubricant was applied to the space prepared for post. Wax pattern were taken using inlay wax that was attached to metal sprue (Konark Dental, India). The resin, which is supplied as a powder and liquid, was then mixed and inserted into the post space using a syringe with a small nozzle. A plastic post was placed to support the resin in the canal and the core was completed using the same pattern resin material. A 2 mm core height was made with a flat occlusal table. In total, 60 resin patterns and 60 wax patterns were fabricated, i.e. one resin pattern and one wax pattern for each tooth.
|Figure 6: (a) Direct patterns taken using inlay wax (b) Direct pattern taken using pattern wax|
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Castings were completed using NiCr alloy (WIRON light BEGO, dental Ni-based metal ceramic alloy, Germany). After the casting was done, the metal castings were cleaned and polished [Figure 7].
For indirect technique, the polyvinyl siloxane (putty) (Aquasil, Dentsply, York, PA, USA) impression material was loaded on to a sectional plastic tray (GDC, India) and into the prepared post space, a light body consistency of polyvinylsiloxane was injected. The impression was made by fitting a plastic dowel into the post space [Figure 8].
After 1 hour, using Type IV dental stone (Kalabhai Karson Pvt. Ltd, Ultrarock, India), the cast was prepared. For each cast, two patterns were prepared; one using inlay wax [Figure 9] and [Figure 10] and the another with pattern resin. About 2 mm of standardized height was made for the cores with a flat occlusal table. The investment was done by using phosphate-bonded investment material and casting was done in the induction casting machine with Nickel chromium alloy.
|Figure 9: (a) Indirect pattern prepared using inlay wax. (b) Indirect pattern prepared using pattern resin|
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The same procedure was followed for all the 60 specimens to fabricate 240 castings. Each specimen received four castings (direct wax pattern, direct resin pattern, indirect wax pattern, indirect resin pattern). Post fit was verified by using RVG for any casting failures.
The postoperative scanning is done using micro-CT imaging [Figure 10]. Each tooth was scanned four times for each technique for both direct and indirect techniques. All the 2D image projections were obtained and the 3D volume was reconstructed. The reconstructed images are loaded. ROI (region of interest) created and then the 3D analysis is done. Corono-apically, each slice was examined to the analysis area in the root canal before and after placement of the posts. The represented 3D images and density mapping of each group was carried out using Sky Scan CT Vox software.
The values obtained were subjected to normality tests, since the data showed normal distribution. The following analyses were employed: paired t-test and unpaired t-test. Paired t-test was used to analyze the fit of the casting. Unpaired t-test was used to compare between the techniques. The study shows that they are significant statistically.
| Results|| |
The mean value of the preoperative space is 3.980 mm3, the mean postoperative space in direct wax pattern is 1.10 mm3, the mean postoperative space of direct resin pattern is 1.20 mm3, the mean postoperative space of indirect wax pattern is 1.12 mm3, and the mean postoperative space of indirect resin pattern is 1.30 mm3.
The mean postoperative space in wax pattern was 1.10 mm3 in direct and 1.12 mm3 in indirect pattern [Table 1] and [Chart 1]. The difference between the pattern was found to be significant because the P value is <0.05. This shows that direct wax pattern technique produced more accurate castings when compared with that of the indirect wax pattern technique
The mean postoperative space in resin pattern was 1.20 mm3 in direct and 1.30 mm3 in indirect pattern [Table 2] and [Chart 2]. The difference between the patterns was found to be significant because direct wax technique was better than indirect wax pattern.
The mean postoperative space in direct pattern was 1.10 mm3 in wax and 1.20 mm3 in direct resin pattern [Table 3] and [Chart 3]. The P value is 0.0001, which is less than 0.05, which shows the difference between the wax and resin patterns is significant. We can, therefore, infer that direct wax pattern proved accurate fitting than resin pattern.
Indirect wax pattern proved accurate fitting (P = 0.0001) than resin pattern [Table 4], [Chart 4]. The mean postoperative space in indirect pattern was 1.12 mm3 in wax and 1.30 mm3 in resin [Table 5] and [Table 6].
|Table 5: The mean postoperative space between direct and indirect wax pattern|
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|Table 6: The mean postoperative space between direct and indirect resin pattern|
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The best fit was direct wax pattern followed by indirect wax pattern followed by direct resin pattern and finally indirect resin pattern [Chart 5] and [Chart 6].
| Discussion|| |
The importance of post is to retain a core in the tooth when there is extensive loss of tooth structure. Post and core is a foundation restoration that replaces lost dentin and provides retention for prosthetic rehabilitation.
There are different types of post available categorized by modulus of elasticity, composition, shape, and fabrication process. The post systems include prefabricated, custom fabricated, and zirconia posts. They are made up of precious or nonprecious casting alloys, whereas prefabricated posts are typically made up of stainless steel, nickel chromium alloy, or titanium alloy. Nonrigid post systems include carbon fiber, glass fiber, quartz fiber, and silicon fiber.
Anterior teeth must resist lateral and shearing types of forces, and the pulp chambers are too small to provide adequate retention and resistance without a post. The amount of remaining coronal tooth structure and the functional requirements of the tooth determine whether an anterior tooth requires a post.,,
The purpose of the study was to evaluate the accuracy of fit in cast metal post fabricated with direct and indirect techniques using micro-CT imaging.
Different methods have been used to evaluate the accuracy of fit of cast post. One of the recently adopted methods for a higher accuracy is the micro-CT imaging. It has a higher resolution range to produce three-dimensional reconstructed imaging. In this study, two main techniques have been used to evaluate the fabrication of cast post and core under micro-CT imaging: direct technique and indirect techniques.
Few studies have compared the accuracy of two techniques in post retention and root fracture resistance. Al-Omari and Zagibeh evaluated the retention in custom cast post and core under direct and indirect techniques and showed no significant difference between them. Another study by Pitigoi-Aron et al. reported that the indirect technique showed better accuracy and fit when compared with the direct technique.,
The results of this study show that the direct technique using wax pattern showed better adaptation to the walls of the canals, volume of 1.10 mm3. The mean volume of space with indirect (is it wax) pattern (P = 0.0001) was 1.12 mm3. When the resin patterns were compared, direct technique produced a mean space of 1.20 mm3 and indirect technique (P = 0.0001) resulted in a mean space of 1.30 mm3. An intertechnique comparison between direct wax and direct resin pattern revealed a significant difference (P = 0.0001) with wax pattern performing better than resin pattern. An intertechnique comparison between the indirect wax and indirect resin pattern revealed a significant difference (P = 0.0001) with indirect wax pattern providing better fit than indirect resin pattern.
Hence, an overall comparison between the materials showed that wax patterns were better than resin patterns and direct techniques provided better fitting castings than indirect technique. The overall ranking in terms of fit was direct wax pattern, indirect wax pattern, direct resin pattern, and indirect resin pattern.
Wax patterns have proved better fit probably because wax flow was better than acrylic resin (pattern resin), and hence, it must have adapted better than resin. While comparing the techniques, direct technique was better than the indirect technique. This can be explained by the fact that direct technique involves lesser number of steps and hence the errors due to impressions, cast pouring were minimized.
There has been no reported literature on evaluation of fit of a post, fabricated by direct and indirect techniques, using micro-CT analysis. However, the limitations of this study include that it is an in vitro study design since micro-CT evaluation can involve only a laboratory study.
Within the limitations of this in vitro study, it can be concluded that
- Accuracy of fit in cast post was found to be better for direct technique using wax pattern
- The advantages of direct technique include better reproduction of root canal anatomy, minimal chairside time, and elimination of laboratory errors
- Inlay wax produces better adaptation and reproduction of the canal space owing to its better flow and plasticity in oral temperature
- The use of micro-CT to assess the adaptation and accuracy of the fit of root canal post was shown to be reliable.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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Dr. Sujana Antony
11/1, Ramagiri nagar, AKS Garden, M Block, F-1, Velachery, Chennai - 60042, Tamil Nadu
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]