Effect of Cement Space Settings on the Vertical Marginal gap of Cement Retained - Implant Supported Crowns: An in vitro study

Document Type : Original Article

Authors

Department of Fixed Prosthodontics, College of Dentistry, Tanta University, Tanta, Egypt

Abstract

Background: Several researches on tooth-supported crowns have shown that when cement space is increased, the marginal gap is reduced. However, there is no sufficient data measuring the effect of increasing the cement space on the vertical marginal gap of implant supported crowns.
Materials and methods: Thirty internal connection dummy implants and matching stock titanium abutments were coupled and implanted into auto polymerizing acrylic resin blocks. Three groups (n=10) of zirconia molar crowns with 3 different virtual cement space settings (40, 60, and 100 μm) were designed by using a CAD design software program. A digital microscope was used to measure the mean vertical marginal gap (MG) for each group, where a total of 120 measurements for each of the 3 groups (12 sites per crown and 10 crowns per group) were evaluated. all specimens were collected for thermomechanical aging which in this study simulated 3 months of chewing conditions. marginal gaps were remeasured as described before, One-way variance analysis and the post hoc Tukey pairwise comparison tests were used to analyze the data (α=.05).
Results: A significant difference (P<.001) was found between the MG values of the zirconia implant-supported crowns fabricated by using the 3 cement space settings. The smallest MG was obtained with the 60-μm setting as compared with the 40-μm and 100-μm settings.
Conclusion: The smallest MGs were obtained when a 60-μm cement space value was used (P<.001).

Keywords

Main Subjects

Full Text

 

 

Introduction

Implant-supported fixed dental prostheses (FDP) represent a well-established treatment option that has evolved to become a standard of care in dental medicine over the past four decades. Dental implant systems composed of implant fixtures which implanted into the bone, implant abutment which attach to the fixture and support the final crown and the fixed restoration [1].

There are many types of abutments available for the dentists to choose from. One of these types is the prefabricated metal abutment which became one of the most used abutment types because these types of abutments are relatively easy to use. The dentist places an abutment of the correct size and length on the implant body in the mouth, attaches it with a screw [2].

A successful implant supported restorations should have some distinct properties as: biocompatibility, esthetics, mechanical strength [3], marginal adaptation and passive fit which ensure both mechanical and biological equilibrium and eliminates over loading of the abutment-screw-implant assembly and the supporting bone [4-6].

Marginal adaptation is especially important property for success of restoration as presence of any marginal discrepancies can increase plaque and food accumulation followed by adherence of oral bacteria[7] that may lead to inflammatory reaction in the peri-implant soft tissues (peri-implantities) followed by bone loss around the dental implant then eventual loss of the dental implant.[8-10] Mechanical complications may occur with crowns with marginal holes, in addition to biologic complications. If the strains inside the crown increase, veneering porcelain chipping may occur, particularly with zirconia crowns [11-13].

Computer-aided design/computer-aided manufacturing (CAD/CAM) Restorations are becoming more common because of their efficient manufacturing methods, better reported precision, and decreased laboratory expenses as compared to traditional fabrication processes. Furthermore, as compared to traditional restorations, CAD/CAM-fabricated restorations showed preferable marginal fit and integrity.[14] For fixed restorations, marginal gaps of 50 to 120 µm have been found to be clinically suitable in many studies, with the range narrowing to 50 to 100 µm for CAD/CAM restorations. Various methods of measuring marginal gaps have been identified, with direct microscopy, sectioning, and replica methods being the most widely used. Each approach has its own set of benefits and drawbacks [15, 16].

CAD/CAM systems offer a wide range of options for scanning, designing, and manufacturing restorations. The virtual die spacer (cement space) can be set to the desired thicknesses with these systems.[17-19] Die spacer thickness has been linked to the marginal fit of tooth-supported CAD/CAM restorations in several tests.[14, 17, 20-32] These digital technologies, which depend on precise dimensional predictions, are said to have better marginal adaptation.[26] Some CAD/CAM systems, however, have been confirmed to produce crowns with unacceptable marginal gaps due to poor scan quality and inaccurate design tools [20, 21].

It was shown in several studies concerning tooth supported crowns that the marginal gap is reduced when cement space is increased.[33-35] However marginal gap improvements were not observed for cement space greater than 120 µm, which may also significantly decrease the strength of ceramic restorations due to a large potential inner misfit as well as polymerization shrinkage of the cement.[14, 30-32, 35, 36] However the field of dentistry still remains without a clear agreement regarding the establishment of a clinically acceptable marginal gap value for implant supported crowns due to limitations of studies evaluating it.

The purpose of this study was measuring the marginal gaps of CAD/CAM zirconia crowns constructed using different cement space settings. The null hypothesis will be that there will be no difference in the mean marginal gaps between all the crowns constructed using three cement space settings.

 

 

Materials and Methods

Sample size calculation (power analysis):

The total sample size in this study will be 30 samples (10 in each group). The significance level was 0.05 and the power was 85% using a computer program G power version 3.

where

Z= z value (1.96 for 95% confidence level).

P=percentage picking a choice, expressed as decimal.

C=confidence interval, expressed as decimal.

N= community size.

Thirty implant analogs were vertically embedded in auto polymerizing acrylic resin (Vertex self-curing; Vertex Dental) with the aid of a dental surveyor to verify perpendicular placement. Thirty straight titanium stock implant abutments (s-Clean; Dentis Co, Ltd) were inserted into the implant analogs and tightened to 30 Ncm with the manufacturer’s torque driver Figure (1).

 

Figure (1) attaching the stock abutment to the dummy implant

 

The screw access channels were sealed with polytetrafluoroethylene tape and then covered with a flowable composite resin (Premise Flowable; Kerr Corp). The titanium abutments were sprayed with titanium dioxide spray (SILADENT Dr. Böhme & Schöps GmbH, Goslar, Germany) and scanned with an extraoral scanner (DOF Freedom HD, Seoul, Republic of Korea). Virtual replicas of each abutment were created on the software and used for designing the crowns for each abutment using the CAD/CAM system Figure (2).

 

Figure (2) designing the crowns using exocad software

 

This study involved 3 groups (n=10) according to the cement space setting used during the crown design, namely, group 40, group 60, and group 100, where the 30 specimens were divided according to the designed cement space settings into 3 groups of 10 crowns each.

Group : 10 crowns designed with cement space set to 40 µm and 1mm away from the margins.

Group 60: 10 crowns designed with cement space set to 60 µm and 1 mm away from the margins.

Group 100: 10 crowns designed with cement space set to 100 µm and 1 mm away from the margins.

Virtual model for the crowns were created on the software and transferred to milling CAD/CAM machine (imes-icore, GmbH, Eiterfeld, Germany) to start the milling process of pre-sinterd zirconia discs (Nacera Pearl, DOCERAM Medical Ceramics, Germany).

After milling was performed, the crowns were sintered to full density, cleaned in ultrasonic cleaner for 5 minutes in distilled water and then steam cleaned and were allowed to air dry and then polished with a special polishing kit for all-ceramic restorations available for dental laboratories.

The inner surface of all crowns was air-abraded with 110 µm aluminum oxide particles at a pressure of 2 bars for 5 seconds.

All crowns were collected, numbered and divided. Then they were tried-in onto their corresponding abutment to confirm proper seating and adequate marginal fit. The crowns were then cemented to the abutments with glass ionomer cement (Medicem, Promedica Dental Material GmbH, Domagkstrasse, Neumuenste, Germany)

Before cementation, the top of each abutment was covered with a cotton pellet to protect the abutment screw. The cement was mixed according to the manufacturer’s instructions. A thin uniform layer was applied to all internal surfaces of the crowns by using disposable brushes.

The crowns were then seated with finger pressure onto the abutment and were cemented using a uniform 3-kg load directed down the long axis of the implant for 10 minutes until the cement has set. Excess cement was removed from the margins using a probe and a plastic implant curette then the crowns were allowed to set for 24 hours.

Marginal gap distance:

Each specimen was photographed using USB Digital microscope with a built-in camera (Scope Capture Digital Microscope, Guangdong, China) connected with an IBM compatible personal computer using a fixed magnification of X45.  A digital image analysis system (Image J 1.43U, National Institute of Health, USA) was used to measure and evaluate the gap width.

Thermal aging:

After all marginal gaps were measured all specimens were collected for thermal cycling. In this study the number of cycles used was 2500 cycles to simulate the 3 months clinically. Dwell times were 25 s. in each water bath (Robota automated thermal cycle; BILGE, Turkey) with a lag time 10 s. The low-temperature point was 5 0C, the high temperature point was 55 0C.

Mechanical aging:

Mechanical aging was performed using a programmable logic-controlled equipment; the newly developed four stations multimodal ROBOTA chewing simulator (Model ACH-09075DC-T, AD-TECH TECHNOLOGY CO., LTD., GERMANY) integrated with thermo-cyclic protocol operated on servomotor

The specimens were embedded in chemical cured acrylic mold which in turn fixed by tightening screw to Teflon holder in the lower part of simulator. A weight of 5 kg, comparable to 49 N of chewing force was exerted. The test was repeated 37500 times to clinically simulate the 3 months chewing condition.

 

 

Results

The results were analyzed using Graph Pad Instat (Graph Pad, Inc., USA) software for windows. A value of P < 0.05 was considered statistically significant. Continuous variables were expressed as the mean and standard deviation. After homogeneity of variance and normal distribution of errors had been confirmed, one-way ANOVA was done for compared cement spacegroups followed by Tukey’s pairwise if showed significant results. Student t-test was done between groups before and after thermo-mechanical aging. Two-way analysis of variance was performed for total effect of variables on gap results. Sample size (n=10/group) was large enough to detect large effect sizes for main effects and pair-wise comparisons, with the satisfactory level of power set at 80% and a 95% confidence level.

Marginal gap

Descriptive statistics showing mean values and standard deviation of Marginal gap test results measured in micrometer (µm) for all cement space groups as function of thermo-mechanical aging are summarized in table (1) and figure (3).

 

Table (1) Marginal gaptest results for all cement space groups as function of thermo-mechanical aging

 

Variables

Thermo-mechanical aging

Statistics

Before

After

t-test

Mean

SD

95% CI

Mean

SD

95% CI

t value

P

value

 

Lower

Upper

Lower

Upper

 

 

 

Cement space groups

40

36.45Ab

3.61

31.36

39.60

56.61Aa

2.79

54.61

58.59

25.1

<0.0001*

 

60

26.95Bb

7.64

20.12

28.72

44.36Ba

2.79

42.36

46.35

7.8

<0.0001*

 

100

57.76Cb

4.09

50.56

56.88

76.13Ca

2.88

74.07

78.18

5.7

<0.0001*

 

Statistics

ANOVA

F value

13.09

F value

276.9

 

 

P value

<0.001*

P value

<0.001*

 

 

 

Different superscript capital letter in the same column indicating statistically significant difference between cement space groups (p < 0.05)

 Different subscript small letter in the same row indicating statistically significant difference between aged subgroups (p < 0.05)

 CI; confidence intervals                 *; significant (p < 0.05)              ns; non-significant (p>0.05)  

 

Figure (3): Bar chart of the mean values of marginal gap for all cement space groups before and after thermo-mechanical aging.

 

Before thermo-mechanical aging

The highest mean ± SD values of marginal gap were recorded for 100 group (55.78±3.09 µm) followed by 40 group mean ± SD values (34.43±4.61 µm) meanwhile the lowest mean ± SD value was recorded with 60 group (24.97±7.84 µm). The difference between groups was statistically significant as indicated by one-way ANOVA test followed by Tukey’s pair-wise post-hoc test (F=13.09, P<0.001).

After thermo-mechanical aging

The highest mean ± SD values of Marginal gap were recorded for 100 group (76.13±2.88 µm) followed by 40 group mean ± SD values (56.61±2.79 µm) meanwhile the lowest mean ± SD value was recorded with 60 group (44.36±2.79 µm). The difference between groups was statistically significant as indicated by one-way ANOVA followed by Tukey’s pair-wise post-hoc test (F=276.9, P<0.001).

Before vs. after thermo-mechanical aging

With 40 group; it was found that 40 group recorded higher mean ± SD value of marginal gap after aging (56.61±2.79 µm) than before aging group mean ± SD value (36.45±3.61 µm). This was statistically significant as indicated by paired t-test (t value =25.1, P <0.001)

With 60 group; it was found that 60 group recorded higher mean ± SD value of marginal gap after aging (44.36±2.79 µm) than before aging group mean ± SD value (26.95±7.64 µm). This was statistically significant as indicated by paired t-test (t value =7.8, P <0.001)

With 100 group; it was found that 100 group recorded higher mean ± SD value of marginal gap after aging (76.13±2.88 µm) than before aging group mean ± SD value (57.76±4.09 µm). This was statistically significant as indicated by paired t-test (t value =5.7, P <0.001).

 

 

Discussion

There are several factors that affect marginal adaptation of the fixed restoration, one of them is the cement space that was set during designing the restoration. Several studies concerning tooth supported fixed restorations showed that the marginal adaptation of the restoration improved as the cement space increased. [37]

In this in vitro study ready-made titanium abutment were used to study the effect of the cement space settings on the marginal gap of implant supported fixed restorations as they ensure standardization for all samples, easy to use, widely available. These abutments were attached to dummy implants inserted straightly in acrylic blocks, their straight placement were checked using dental surveyor. This straight placement of the implants and the abutment attached to it allow ease of designing and insertion of the restoration and accurate measurement of the marginal gap.

Digital impression technique was used to scan the specimens as it provides similar accuracy as conventional impression techniques but more time efficient, simple and more comfortable for the patients [38-39], using the resulted images, zirconia crowns were designed using CAD/CAM software as it has the following advantages over the conventional techniques: application of new materials, reduced labor, cost-effectiveness, and quality control.[40]

In this study the material chosen for manufacturing the crowns is zirconia which was selected as it is becoming increasingly popular due to its excellent aesthetic properties and good color matching natural teeth, their biocompatibility and wear resistance. In addition, patients ' fears about the potential side effects of metal restoration on their health have increased demand for metal-free restorations. [41]

During designing the crowns using the software they were divided into 3 groups according to the cement space settings as follow:

Group 40: crowns designed with cement space of 40 µm, as the textbook rule for the best cement space dimensions ranging from 20 to 40 µm [42] and it is within the range of resin cements film thickness [43].

Group 60:crowns designed with cement space of 60 µm, the value of 60 µm was recommended by the authors of a study that evaluated the MG of CAD-CAM tooth-supported ceramic crowns.

Group 100: crowns designed with cement space of 100 µm, the value of 100 µm was selected as the median of the gap settings (0 to 200 µm) provided by the EXOCAD software program.

To standardize the forces applied to each crown during cementation uniform load of 3 kg applied to the crowns using cementation device until the cement was set, for this study glass ionomer cement was used for cementation of the crowns as it is widely available, has lower marginal discrepancy when used for cementation than other conventional cements and suitable for a semi-permanent cementation for implant supported crowns.

Regarding the marginal gap evaluation method, there is also no consensus on the best method for evaluation [44] the direct microscopy [45-46] cross sectioning and replica methods [47-48] are the most used.

The direct microscopy method was selected in this study because it is the most easily repeated, straightforward and time saving. The sectioning method was not used because it may be difficult to do cutting through titanium abutments. Although microcomputed tomography (micro-CT), is a powerful tool in evaluating the marginal gaps of restorations, one of its limitations is that the results may be affected by the radio opacity of the luting cement so it was not used in this study as the marginal gap was measured after cementation.

Laboratory simulations of clinical service are often performed because clinical trials are costly and time consuming. Thermal cycling is an in vivo process often represented in these simulations, but the regimens used vary considerably and, with few exceptions, are always proposed without reference to in vivo observations. Standardization of conditions is necessary to allow comparison of reports.

Temperature regimens previously used for in vitro tests. Thermal cycling is common in tracer penetration (leakage), shear bond strength and tensile bond strength tests of dental materials. The mean low-temperature point was 6.60C (range 0–360C, median 5.00C). The mean high-temperature point was 55.50C (range 40– 100 0C, median 550C). The majority of reports quoted used just hot and cold temperature points. The number of cycles used varied from 1 to 1 000 000 cycles, with a mean of about 10 000 and median of 500 cycles. Dwell times were sometimes not stated, but the mean stated dwell time was 53 s, the median 30 s, with a range of 4 s to 20 min. Sometimes a longer dwell time 23 s was used with an intermediate temperature of 37C, and a shorter dwell time 4 s for the temperature extremes, presumably in an attempt to mimic expected intraoral timings. [49]

In this study the number of cycles used was 2500 cycles to simulate the 3 months clinically. Dwell times were 25 s. in each water bath* with a lag time 10 s. The low-temperature point was 5 0C). The high temperature point was 55 0C. [50]

The results of the current study showed significant differences for all the groups tested, and so the null hypothesis was rejected.

The lowest mean MG of the CAD-CAM implant-supported zirconia crowns examined in this study was observed in group 60 (26.95 µm), and this value is within the range of MGs reported by Mohammed et al [51] and Wilson. However, this result is slightly less than half the mean MG value (56.09 µm) reported by Hassan Zadeh et al, [52] who used the same restoration material but a different software program to fabricate tooth-supported crowns. Shim et al [43] concluded that the MG value of CAD-CAM fabricated crowns may be influenced by the software program type and spacer settings.

The mean MG in group 40 (36.45 µm) was within the range of values reported by Shim et al [43] and Rahme et al,[53] and the mean MG in group 100 (57.76 µm) was within the range of the minimum vertical MG values reported by Carrilho et al. [54] The lower MG values recorded in this study, as compared with other studies, may be the result of using different restorative materials, which has been confirmed in a recent systematic review. [55]

The inverse relation between MG and the cement gap is supported by the results of group 40 and group 60 only, where the MG decreased from 36.45 µm to 26.95 µm when the cement gap increased from 40 to 60 µm. [56-57] However, the MG increased again to 57.76 µm when the cement gap was increased to 100 µm. Most of the studies reporting this inverse relationship were conducted on natural or artificial tooth-supported crowns that may have different taper and finish line configurations than implant abutments.

Moreover, unlike crown seating on a stock implant abutment, which is mainly circular and only has one minor flat area on one of its axial surfaces, crown seating on a natural tooth is usually well handled by tooth preparation features and non-circular circumference. As a result, growing the cement space above a certain point would result in a larger restoration that may or may not be properly seated on the implant abutment, resulting in an inaccurate MG measurement. The higher MG in group 100 may be attributed to a looser internal fit than in groups 40 and 60, allowing the crowns' virtual CAD design to change along their respective abutments.

Since the cement gap affects not only the MG but also the retention and fracture resistance of restorations, particularly with ceramics, the inverse relationship between the cement gap or die spacer and MG can only be valid to a level. [58] As a result, additional researches is needed to determine how various cement gap settings affect ceramic crown retention, fracture resistance, and marginal adaptation.

The current study just focused on the vertical marginal fit of crowns since, under the right laboratory conditions, the horizontal misfit can be corrected by reshaping. If the discrepancy is a vertical misfit, however, it can only be corrected with a new restoration. [59] All the MG values in this study were below 110 µm, which has been the highest reported MG value in CAD-CAM restorations. [60]

In this study, significant increase in the marginal gap after artificial aging as the degradative effect of thermocycling in an aqueous atmosphere on dental ceramics has been reported. [61] Reasonable explanation of this increase in marginal discrepancy suggests that it is related to the luting cement. Some portions of the cement film were washed out during the aging procedures, resulting in a clearer image under microscope and, thus, creating the possibility for increased measurements of the marginal discrepancy particularly, when using water-soluble cements such as glass-ionomer which deteriorate over time due to the deleterious effects of thermocycling. On contrary, found no significant effect on marginal discrepancy after aging was reported by others. This different finding can be due to the different ceramic systems and luting agents being evaluated as the insoluble resin cement they used decreased the potential of interfacial failure of the luting agent during thermocycling.

Limitations

  • The evaluation of internal fit has been neglected.
  • The possible collapse of the luting cement layer and fracture or loosening of crowns may be increased by poor internal fit. [62]
  • Because of the use of a laboratory scanner, the results may not reflect the outcome in clinical situations, where the accuracy of CAD-CAM crowns may be adversely influenced by gingival bleeding, moisture, and limited mouth opening during intraoral scanning.
  • Differences in luting cement, materials for restoration, and CAD/CAM system used could also affect the results. So further research under different conditions is required.
  • The MG values reported in this study may be smaller than most of the studies in the literature, so a future study may be needed to verify these results by two different evaluation methods.

 

 

Conclusions

Within the limitations of this in vitro study, the following conclusions were drawn:

  1. The marginal fit of implant supported CAD/CAM zirconia crowns was significantly affected by the 3 cement space settings used.
  2. Up to 60 µm, the marginal discrepancy values decreased when the cement space increased.
  3. The optimal mean marginal adaptation of implant supported CAD/CAM zirconia crowns was obtained when the digital cement gap was set at 60 µm. 

 

 

Conflict of Interest

The authors declare that there is no conflict of interest

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

 

 

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Advanced Dental Journal
Volume 4, Issue 2
April 2022
Pages 110-122

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