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. 2024 Jan 24;24(1):95–102. doi: 10.4103/jips.jips_275_23

A comparative evaluation of internal and marginal fits of custom post and core fabricated using conventional and two digital techniques: An in vitro study

Vidhi Himanshu Sheth 1,✉, Nikita Gharat 1, Vishrut Mohan Bhatnagar 1, Shruti Gill 1, Naisargi P Shah 1
PMCID: PMC10896309  PMID: 38263564

Abstract

Aim:

The purpose of this in vitro study was to comparatively evaluate the marginal and internal fits of cobalt–chromium metal custom post and core fabricated using a conventional technique with two digital techniques.

Settings and Design:

The study was designed in an in-vitro study setting.

Materials and Methods:

Five sets of custom post and core restorations were fabricated using the conventional (Group 1) and two semi digital methods (digital scanning of the resin pattern and computer aided additive manufacturing, and digital scanning of the silicone impression and subsequent computer aided designing [CAD] computer aided manufacturing fabrication) (Group 2 and 3). Marginal and internal fits of the posts were evaluated using a micro computed tomography scan at various points.

Statistical Analysis Used:

A one way ANOVA test of the scores was made to evaluate the effect of different methods of custom post and core fabrication on marginal and internal fits. Bonferroni adjusted post hoc tests were conducted for intergroup comparison.

Results:

Least marginal gap was reported in Group 3 (82.5 ± 14.36 μm) followed by Group 1 (110 ± 25.19 μm) and Group 2 (112.5 ± 26.75 μm). Least internal gap at cervical, middle and apical as well as overall values were observed in Group 3 (78 ± 9.25 μm, 72 ± 7.79 μm, 160 ± 15.81 μm, 103.3 ± 4.43 μm) followed by Group 1 (113.5 ± 25.35 μm, 132.5 ± 19.92 μm, 502 ± 74.63 μm, 249.3 ± 25.44 μm) and Group 2 (114.5 ± 21.68 μm, 133.5 ± 19.57 μm, 598 ± 87.86 μm, 282 ± 28.91 μm) respectively. The results of one-way ANOVA and Bonferroni adjusted post hoc tests for marginal gap did not show any statistically significant difference between the three groups (P > 0.05) but revealed statistically significant difference (P = 0.02) in internal gap values at the cervical, middle, and apical regions as well as overall internal gap region between the three groups.

Conclusions:

Better marginal and internal fits were observed in custom post and core fabricated by digital scanning of the silicone impression and subsequent CAD as compared to those fabricated by the other two groups.

Keywords: Cobalt–chromium post and core, computer-aided designing and computer-aided manufacturing, custom post and core, digital method

INTRODUCTION

Endodontically treated teeth have always been a challenge to restore.[1] The difficulty of restoring such teeth drastically increases when the residual tooth structure is unable of supporting a restoration or satisfying the tooth's masticatory and esthetic needs.[1,2] Custom posts and core restorations have become the primary treatment option to restore structurally compromised teeth, teeth that are exposed to high functional loads, or teeth requiring a modification in the emergence profile.[3] Many factors influence the prognosis of such post and core restorations. The adaptation of posts to the root canal anatomy has been acknowledged as an important factor associated with the fracture resistance and survival of such teeth.[4,5,6] Conventionally, custom post and cores are fabricated by either direct or indirect method followed by casting procedures.[3]

The advent of computer-aided designing and computer-aided manufacturing (CAD-CAM) technology has greatly improved the accuracy and speed of prosthetic treatment when compared to conventional methods.[7,8,9,10,11] The application of this technology to custom post and core fabrication definitely seems promising.[12,13,14,15] These digital methods can be broadly classified as fully digital and semi-digital fabrication techniques.[16,17,18,19,20]

The fully digital technique involves a direct digital scan or a dual digital scan with the help of scan posts, followed by computer designing and manufacturing of the custom post and core.[19,20] The semi-digital technique involves a digital scan of a wax or resin pattern or a scan of the final impression of the post space followed by computer designing using specific CAD software and subsequent CAM fabrication.[20,21,22] Thus, clinicians not having access to an intraoral scanner can still utilize the benefits of the digital workflow.[22]

There are very few studies comparing the marginal and internal fits of custom post and core fabricated using semi-digital fabrication techniques to those fabricated by conventional techniques.[17,19,20,22] Currently, there is no clear consensus as to which direct semi-digital fabrication technique provides a custom post and core with better marginal and internal fits.

Thus, the purpose of this in vitro study was to comparatively evaluate the marginal and internal fits of cobalt–chromium metal custom post and core fabricated using conventional technique and two semi-digital techniques using direct metal laser sintering (DMLS) for CAM. The null hypothesis was that there would be no difference in the marginal and internal fits of custom metal post and core fabricated using a conventional technique with those fabricated using two digital techniques.

MATERIALS AND METHODS

This comparative in vitro study was conducted in the department of prosthodontics and crown and bridge of a dental college in collaboration with a micro-computed tomography (CT) center. Ethical committee approval TDCEC/45/2019 dated on 01/10/2019.

The sample size was calculated by considering the mean and standard deviation values (0.118 ± 0.066 µm, 0.665 ± 0.189 µm, and 0.294 ± 0.115 µm) obtained from a previous study by Hendi et al.[20] using G*Power software (Version 3.0.10). The level of significance (α error) was set at 5% and the power of the study (1-β) was set at 80% (0.80). The total sample size calculated was 9 (3 per group). A total of 15 samples (5 per group) were included in the present study to account for any loss of specimen.

Five freshly extracted noncarious, single-rooted mandibular first premolar teeth with relatively straight roots and completely formed apex of similar shape and size were selected. The teeth were washed and immersed in hydrogen peroxide solution to remove organic remnants. All remaining organic debris were removed with an ultrasonic scaler (UDS-P; WOODPECKER). Digital radiographs (Xios XG Select RVG Sensor; Dentsply Sirona) were taken to ensure a single canal, closed apex, and relatively straight root canals. Teeth were then decoronated 2 mm above cementoenamel junction with a diamond rotary cutting instrument under water using a high-speed handpiece taking two points of reference (one point on the labial surface and one on the lingual surface).

The teeth were then endodontically treated following protocol. Biomechanical preparation was done using K-files and ProTaper endodontic hand files as per the manufacturer's instructions using the crown-down technique in the sequence of S1, S2, F1, and F2. First, the coronal and middle thirds of the canal were shaped using 10 and 15 No. K-files and S1, S2, and F1 ProTaper hand files, using a reciprocating back-and-forth motion. All pulp tissues were extirpated, and the canals were cleaned and shaped to obtain straight access to the middle and the apical third of all specimens. Apical preparation was done using 15 and 20 No. K-files and F2 ProTaper hand files. The canals were dried with size F2 paper points and were obturated using size F2 gutta-percha points and sealer by the warm vertical compaction technique.

Any excess coronal to the canal orifice was removed with a warm plugger post space prepared after 24 h. Post space length was kept constant at 10 mm for all teeth.[7] Initially, no. 1, 2, and 3 Peeso reamers (MANI Peeso reamer; MANI, Inc.) were used to remove gutta-percha. Final post space preparation was done using ParaPost preparation drill No. 4 (ParaPost System Casting Technique Starter Kit; COLTENE). A tapered round bur (DIA-BURS TR 12; MANI, Inc.) was used to prepare a deep chamfer finish line of 1 mm width. Three groups were formed based on the method of fabrication of custom post and core.

Group 1 custom post and cores were fabricated by direct resin pattern using laboratory burnout post and pattern resin and casting the pattern using cobalt–chromium ingots. The ParaPost laboratory burnout tapered post No. 4 (ParaPost System Casting Technique Starter Kit; COLTENE) was tried in the prepared canal and evaluated for fit. The pattern resin was mixed in a ratio of 1:1 by weight, painted onto the burnout post, and inserted into the post space (GC Pattern Resin; GC). The cylindrical core pattern of 2 mm height was also built. All patterns were spruced and invested with a phosphate-bonded investment (ADENTAvest CB; Adentatec) and cast in an induction casting machine (Ducatron Serie 3; Ugin Dentaire) with Type 4 cobalt–chromium alloy (System NE; Adentatec).

Group 2 direct resin patterns were fabricated similarly to Group 1. These resin patterns were digitally scanned using a dental table-top scanner (Medit T300 Dental table-top scanner; MEDIT) [Figure 1]. The scanned data were compiled into an STL file for further digital fabrication. The custom posts and cores were fabricated with DMLS technology (EOSINT M 270; EOS GmbH) using a cobalt–chromium alloy powder (Wirobond C+; BEGO GmbH and Co.) at 40 microns layer thickness.

Figure 1.

Figure 1

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STL files of Group 2 custom post and cores

For Group 3, the impression of post space was made using the No. 4 ParaPost plastic impression post (ParaPost System Casting Technique Starter kit; COLTENE) and addition silicone impression material (AVUE GUM; Dental Avenue). The light-bodied material was loaded inside the post space with the help of an intraoral tip. The plastic impression post was also painted with light-bodied material and inserted into the prepared post space. Putty consistency elastomeric impression material was loaded onto an impression material holder and was placed over the coronal portion of each tooth. The impressions were digitally scanned using a dental table-top scanner (Medit T300 Dental table-top scanner; MEDIT) to obtain digital virtual models [Figure 2]. Custom post and core were then designed using a CAD software (exocad 2.3 Matera Dental CAD software; exocad GmbH) (spacer thickness set to 0 microns) [Figure 3]. The custom post and cores were fabricated with DMLS with the same material and technique.

Figure 2.

Figure 2

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Digital virtual models of Group 3 custom post and cores

Figure 3.

Figure 3

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Digital designing of Group 3 custom post and cores using computer-aided designing software

To ensure the standardization of procedure, a single operator performed all the above-mentioned procedures following the same protocol to fabricate the 15 custom posts and cores [Figure 4]. Laboratory-based micro-CT system with sub-micron spatial resolution (high aspect ratio tomography mode at 95 kV and 85 mA with a resolution of 14.5 mm, exposure time of 3 s) was used for scanning and image reconstruction (Carl ZEISS XRadia 520 Versa; ZEISS). Two-dimensional virtual slices were examined coronoapically. A compatible software (Dragonfly Pro Analysis Software) was used to measure linear distances to evaluate the internal fit and marginal fit of custom post and core [Figure 5].

Figure 4.

Figure 4

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Group 1, 2, and 3 custom posts and cores

Figure 5.

Figure 5

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Points of measurement of marginal and internal fits. MG: Marginal gap, IGC: Internal gap (cervical), IGM: Internal gap (middle), IGA: Internal gap (apical)

Marginal fit of custom post and core was evaluated by means of marginal gap, which was taken as the vertical distance between the custom post and core and the tooth at the margin at mesial, distal, buccal, and lingual aspects. Internal fit of custom post and core was evaluated by means of internal gap values measured at 3 levels: cervical, middle, and apical. Internal fit at cervical region was evaluated as the perpendicular distance between the custom post and core and the tooth as measured at the post and core junction at mesial, distal, buccal, and lingual aspects. Internal fit at middle region was evaluated as the perpendicular distance between the custom post and core and the tooth as measured at the mid-point of the post at mesial, distal, buccal, and lingual aspects. Internal fit at apex was evaluated as the vertical gap between the custom post and core and the tooth as measured at the apex of the post space preparation.

The values obtained were entered into a Microsoft Excel spreadsheet and subjected to statistical analysis using the Statistical Package of the Social Sciences (SPSS) Software (IBM SPSS Statistics v20.0; IBM Corp). A one-way ANOVA test of the scores was made to evaluate the effect of different methods of custom post and core fabrication on marginal and internal fits. Bonferroni-adjusted post hoc tests were conducted for intergroup comparison.

RESULTS

The descriptive statistics for the three groups are shown in Table 1 and Figure 6. The results of one-way ANOVA and Bonferroni-adjusted post hoc tests for marginal gap did not show any statistically significant difference between the three groups (P > 0.05) [Tables 2 and 3]. Least marginal gap was reported in Group 3 (82.5 ± 14.36 µm) followed by Group 1 (110 ± 25.19 µm) and Group 2 (112.5 ± 26.75 µm) [Table 2].

Table 1.

Descriptive statistics

Group 1
 Group 2
 Group 3
Mesial Distal Buccal Lingual Mean Mesial Distal Buccal Lingual Mean Mesial Distal Buccal Lingual Mean
Marginal gap
100 60 90 80 82.5 70 80 100 60 77.5 80 90 70 80 80
110 160 130 120 130 130 120 120 160 132.5 100 70 110 80 90
100 80 110 90 95 110 140 100 80 107.5 80 110 120 100 102.5
70 120 90 120 100 90 120 70 120 100 40 120 60 80 75
110 160 140 160 142.5 130 160 110 180 145 70 60 90 40 65
Internal gap: Cervical
70 80 110 60 80 100 110 90 80 95 90 70 80 90 82.5
120 120 120 160 130 110 150 110 130 125 40 90 60 80 67.5
110 150 110 80 112.5 110 80 120 90 100 80 90 80 70 80
90 120 70 120 100 90 120 90 120 105 90 80 80 110 90
130 160 110 180 145 110 160 140 180 147.5 80 70 40 90 70
Internal gap: Middle
100 190 190 140 155 180 140 100 200 155 110 40 80 90 80
120 150 130 150 137.5 130 150 120 170 142.5 40 90 40 80 62.5
190 120 100 90 125 90 90 190 150 130 80 90 80 70 80
110 110 160 190 142.5 160 190 90 110 137.5 40 80 110 40 67.5
90 90 120 110 102.5 120 110 90 90 102.5 80 70 40 90 70
Internal gap: Apical
Groups
Group 1 Group 2 Group 3
450 590 170
520 650 180
620 670 140
430 630 150
490 450 160
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Figure 6.

Figure 6

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Results summary (in microns)

Table 2.

Summary statistics and one-way ANOVA

Dependent variable Groups Mean±SD Minimum Maximum F P
Marginal gap (microns) Group 1 110±25.19 82.5 142.5 2.67 0.11
Group 2 112.5±26.75 77.5 145
Group 3 82.5±14.36 65 102.5
Internal gap: Cervical (microns) Group 1 113.5±25.35 80 145 5.41 0.02*
Group 2 114.5±21.68 95 147.5
Group 3 78±9.25 67.5 90
Internal gap: Middle (microns) Group 1 132.5±19.92 102.5 155 22.14 <0.001*
Group 2 133.5±19.57 102.5 155
Group 3 72±7.79 62.5 80
Internal gap: Apical (microns) Group 1 502±74.63 430 620 58.72 <0.001*
Group 2 598±87.86 450 670
Group 3 160±15.81 140 180
Internal gap: Overall (microns) Group 1 249.3±25.44 224.2 285.8 90.32 <0.001*
Group 2 282±28.91 233.3 305.8
Group 3 103.3±4.43 100 110.8
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*Significant factor. SD: Standard deviation

Table 3.

Bonferroni-adjusted post hoc tests: Intergroup comparison

Dependent variable Group-wise comparison Mean difference SE P 95% CI
Lower bound Upper bound
Marginal gap Group 1-Group 2 −2.5 14.40 1 −42.54 37.54
Group 2-Group 3 30 14.40 0.18 −10.04 70.04
Group 3-Group 1 −27.5 14.40 0.24 −67.54 12.54
Internal gap: Cervical Group 1-Group 2 −1 12.64 1 −36.13 34.13
Group 2-Group 3 36.50* 12.64 0.04 1.37 71.63
Group 3-Group 1 −35.5* 12.64 0.05 −70.63 −0.37
Internal gap: Middle Group 1-Group 2 −1 10.59 1 −30.43 28.43
Group 2-Group 3 61.50* 10.59 <0.001 32.07 90.93
Group 3-Group 1 −60.50* 10.59 <0.001 −89.93 −31.07
Internal gap: Apical Group 1-Group 2 −96 42.49 0.13 −214.1 22.1
Group 2-Group 3 438.00* 42.49 <0.001 319.9 556.1
Group 3-Group 1 −342.00* 42.49 <0.001 −460.1 −223.90
Internal gap: Overall Group 1-Group 2 −32.66 14.16 0.12 −72.01 6.69
Group 2-Group 3 178.66* 14.16 <0.001 139.31 218.01
Group 3-Group 1 −146.00* 14.16 <0.001 −185.35 −106.65
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*Significant factor. SE: Standard error, CI: Confidence interval

The results of one-way ANOVA revealed a statistically significant difference (P = 0.02) in internal gap values at the cervical, middle, and apical regions as well as overall internal gap region between the three groups [Table 2]. Least internal gap at cervical, middle and apical region was observed in Group 3 (78 ± 9.25 μm, 72 ± 7.79 μm, 160 ± 15.81 μm) followed by Group 1 (113.5 ± 25.35 μm, 132.5 ± 19.92 μm, 502 ± 74.63 μm) and Group 2 (114.5 ± 21.68 μm, 133.5 ± 19.57 μm, 598 ± 87.86 μm) respectively [Table 2]. Least overall internal gap values were observed in Group 3 (103.3 ± 4.43 μm) followed by Group 1 (249.3 ± 25.44 μm) and Group 2 (282 ± 28.91 μm) respectively. Bonferroni-adjusted post hoc tests showed a statistically significant difference in internal gap values between Group 1 and Group 3 (P = 0.04) and Group 2 and Group 3 (P = 0.04), and a statistically insignificant difference in internal gap values between Group 1 and Group 2 (P = 1) [Table 3].

DISCUSSION

As seen in the results of this study, the null hypothesis was partly accepted and partly rejected. The null hypothesis that there would be no difference in the marginal fit of custom post and core fabricated using conventional and two digital techniques was accepted. This study reported no significant difference in the marginal fit of the custom post and cores fabricated using the conventional technique and the two digital techniques. An apt reasoning to justify this result could be that there was the least distortion of both pattern resin and elastomeric impression material at the margin.[3,19] This region was also relatively easy to record accurately as compared to the internal post space.[3,19]

The null hypothesis that there would be no difference in the internal fit of custom post and core fabricated using conventional and two digital techniques was rejected. The custom post and core fabricated by scanning the elastomeric impressions reported better internal fit as compared to those fabricated by the conventional direct method and by scanning the pattern resin custom post and cores. Better dimensional accuracy of addition silicone elastomeric impression materials could have led to better recording of the post space anatomy.[20,22] The table-top laboratory scanner was able to better record the surface anatomy and details of the elastomeric impression as compared to pattern resin.[20] The pattern resin custom post and cores could have distorted due to restricted flow, polymerization shrinkage while recording the internal anatomy of the post space leading to shorter posts and greater internal gap values.[3]

Custom metal post and cores could be conventionally fabricated by direct and indirect techniques.[3,6] Rayyan et al. and de Moraes et al. reported that custom post and cores fabricated with the direct technique presented better marginal and internal fits as compared to those fabricated using the indirect technique.[3,6] The indirect technique presented a greater apical gap, probably due to increased laboratory steps and the material distortions.[3,6] The present study has included the conventional direct technique of custom metal post and core fabrication as the control group (Group 1) in consideration of its better performance.

Digitalization has many advantages such as ease, reduction in chairside and prosthesis fabrication time, patient comfort, reduced storage requirements, easy access to diagnostic information, and easy transfer of digital data.[10,11,12,20] The application of this technology to custom post and core fabrication could possibly improve its marginal and internal adaptation mainly by better reproduction of surface detail, reduced cement film thickness, and negligible void formation, leading to better fracture resistance and overall prognosis of such restorations.[13,14] Elimination of multiple laboratory steps could further improve the accuracy and reduce fabrication time.[12,13,14]

The current technological limitation of intraoral scanners to fully record the depth of the post space and the inability of scan posts to record the internal anatomy of post space has dissuaded us from evaluating this method in the current study.[16,20] On the other hand, the semi-digital methods can be easily adapted into clinical practice and can also be used by clinicians not having access to an intraoral scanner.[20] The two semi-digital direct techniques of custom post and core fabrication evaluated in the current study were digital scanning of the resin pattern (Group 2) and digital scanning of the elastomeric impression (Group 3). Both these semi-digital techniques could help eliminate many laboratory steps and thereby improve the ease, accuracy, and speed of manufacturing.[20]

Different materials have been documented in literature for manufacturing of custom post and core such as zirconia, composite, glass fiber, and cobalt–chromium metal alloy.[8,15,20,22] Most authors have used subtractive methods of CAD CAM manufacturing.[19,20] This study has used DMLS which is an additive method of CAD CAM manufacturing for preparation of custom post and cores in both the semi-digital direct technique groups (Groups 2 and 3). The custom post and cores fabricated using DMLS method have been reported to provide similar fracture resistance and internal and marginal fits when compared to those manufactured using milling and conventional casting techniques.[21] DMLS reports advantages such as better corrosion resistance and surface properties, less material waste, and fewer microporosities.[21] Hence, this method was selected for fabrication of the custom post and cores.

Good marginal and internal adaptation and passive fit of post and cores to the root canal anatomy have been reported to be extremely essential for long-term success of custom post and core restorations.[2,3,4] The apical portion of post and cores should contact the residual gutta-percha to prevent the ingress of saliva and bacteria.[3] There are currently no clear guidelines to evaluate the adequacy of fit of posts.[6] Therefore, this study has followed a previous study by de Moraes et al. to evaluate marginal and internal fits.[6] Furthermore, an apical gap of >1 to 2 mm was considered unacceptable and categorized as poor fit and was associated with clinical complications.[6]

This study utilized micro-CT images reconstructed in three planes to accurately view and measure the marginal and internal gap values at various points. Micro-CT scans have been known to exhibit high accuracy, better resolution, and detailed imaging with minimized metal scatter.[23,24] The micro-CT system uses micro-focal spot X-ray sources and detectors with high resolution to produce three-dimensional reconstructed images with higher spatial resolution than CT imaging.[21,25]

A lack of similarly conducted studies on custom metal post and cores fabricated using semi-digital techniques does not permit for a direct comparison of the results. A lack of standardized guidelines and techniques for assessing the internal and marginal fits of custom post and core as well as the plethora of material and scanner options used further prevents correlation of results.

The present study does have a few limitations. The in-vitro study design does not evaluate the impact of clinical factors such as saliva, limited mouth opening, and accessibility, on the accuracy of custom post and core fabrication. Hence, the results should be interpreted with caution. A lack of similarly conducted studies does not permit for a direct comparison of the results. Techniques to directly digitalize the post space have not been evaluated in the current study.

Evaluation of fully digital and semi-digital methods of custom post and core fabrication with respect to different materials and methods of CAD CAM manufacturing should be considered in future studies. A research lacuna was observed in this regard. Furthermore, standardization of point of measurement to evaluate the internal and marginal fits can help compare different materials and techniques accurately.

Overall, the advent of digital dentistry in the manufacturing of custom post and core has definitely ensured ease, accuracy, and faster fabrication of the custom post and core. The CAM technology eliminated various laboratory steps and helped speed up the fabrication process. The direct scanning of the elastomeric impression followed by digital design and DMLS technique for custom post and core fabrication does seem to provide better internal and marginal fits among the three groups tested in the current study. However, the custom post and core fabricated in all the three groups showed internal and marginal fit values in the acceptable range.

CONCLUSIONS

  1. Better internal fit was observed in custom post and core fabricated by digital scanning of the silicone impression and subsequent CAD as compared to those fabricated by casting the direct resin pattern and digital scanning of the direct resin pattern

  2. This study reported no significant difference in the marginal fit of custom post and core fabricated by the conventional and two semi-digital techniques

  3. Custom post and core fabricated in all the three groups showed internal and marginal fit values in the acceptable range

  4. DMLS technology can be used successfully for manufacturing custom post and core restorations when using digital methods of CAD CAM fabrication.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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