Effect of Margin Designs on the Marginal Adaptation of Zirconia Copings.

OBJECTIVE: The aim of this in vitro study was to investigate the effect of Shoulder versus Chamfer margin design on the marginal adaptation of zirconia (Zr) copings.
MATERIALS AND METHODS: 40 extracted molar teeth were mounted in resin and prepared for zirconia crowns with two margin preparation designs (20=Shoulder and 20=Chamfer). The copings were manufactured by Cercon® (DeguDent GmbH, Germany) using the CAD/CAM system for each tooth. They were tried on each tooth, cemented, thermocycled, re-embedded in resin and were subsequently cross sectioned centrally into two equal mesial and distal halves. They were examined under electron microscope at 200 X magnification and the measurements were recorded at 5 predetermined points in micrometers (µm).
RESULTS: The overall mean marginal gap for the two groups was found to be 206.98+42.78µm with Shoulder margin design (Marginal Gap=199.50+40.72µm) having better adaptation compared to Chamfer (Marginal Gap=214.46+44.85µm). The independent-samples t-test showed a statistically non-significant difference (p=.113) between the means of marginal gap for Shoulder and Chamfer margin designs and the measurements were recorded at 5 predetermined points for the two groups.
CONCLUSIONS: The Chamfer margin design appeared to offer the same adaptation results as the Shoulder margin design.

Introduction

All-ceramic reconstructions have become popular as a result of increased patient demands for dental materials without metallic shape, thus allowing excellent esthetics. There are many types of dental ceramics based on different fabrication techniques: the soft-machined techniques such as zirconia (Zr) (Y-TZP) and alumina (Al2O3), the hard-machined techniques such as lithium disilicate ((Li2Si2O5) and leucite (KalSi2O6). Dental ceramics types are also based on their application and crystalline phase. However, because of their poor physical properties, all-ceramic restorations are limited to crowns in anterior regions. In recent years, yttria-stabilized polycrystalline tetragonal zirconia (Y-TZP) has been increasingly applied to the manufacture of all-ceramic restorations. This Zr material, once only used in engineering, combines high esthetics, excellent biocompatibility, low plaque accumulation and low thermal conductivity with high strength gives high importance for dental application. The main advantage of (Y-TZP) is high strength. Also, crack deflection could be an excellent property of Y-TZP to resist crack propagation (-).

The long-term success of ceramic restorations depends on the mechanical and bonding properties of the materials. It is also influenced by the marginal and internal fit. An inaccurate marginal fit is responsible for plaque retention, micro leakage and cement breakdown. The risk of caries, periodontal disease and endodontic inflammation is thus increased, which can have adverse effects on the health of underlying abutments and optical properties (). A poor marginal fit of a coping can increase the thickness of the cement and thus influence the mechanical stability of Zr-based restorations. Several authors have estimated maximal marginal gap (MG) values. In vitro, MG values between 100 and 150μm are considered clinically acceptable. A marginal misfit can be considered acceptable when it is visually imperceptible or cannot be detected using a dental probe. A MG of less than 80μm is proven to be very difficult to detect clinically (-).

Two main fit assessment protocols are described in the literature. One is a destructive protocol, where a specimen or replica is sectioned, followed by microscopic analysis; the second is a nondestructive protocol, where only external gap measurements are performed. The conventional methods aiming to evaluate the marginal fit of restorations can do it only in two dimensions: in different sections, the gaps between the restoration and the die are usually measured by a microscope at 4–24 points (-).

In the literature, there are studies that focused on the effects of preparation angles (), manufacturing processes (), cement used for cementation (), preparation depth differences (), presence or absence of common preparation errors () and effect of occlusal surface preparations for the adaptation of Zr copings (). To the authors’ knowledge, there is still a lack of scientific data on the effect of two most common margin designs (Shoulder and Chamfer) on the marginal fit of Zr copings. The purpose of this study was to investigate the effect of shoulder versus chamfer margin designs on the marginal adaptation of Zr copings. The null hypothesis was that different margin design of preparations does not affect the marginal adaptation of the Zr copings.

Materials and Methods

This in-vitro research study was conducted at the Department of Prosthodontics, College of Dentistry, King Saud University Riyadh from October 2014 to March 2015. The research study was planned and approved by the College of Dentistry Research Center (CDRC) at King Saud University (CDRC Reg. # IR0110).

A sample size of 40 sound or minimally restored extracted molar teeth (mostly third molars) from adult patients were collected and stored in hydroxide-diluted water at The Department of Maxillofacial Surgery. The root of each tooth was embedded in self-curing orthodontic resin (Ortho-Resin, DeguDent GmbH, GERMANY) base (2 cm x 2 cm), exposing the anatomic crown and 2 mm of the coronal root. The sample was divided into two groups (A=Shoulder Margin Design & B=Chamfer Margin Design) using a random draw method.

A silicone putty index (Ivoclar, Vivadent Inc., USA) of each tooth was recorded before the preparation of the teeth and was used to provide a mesiodistally sectioned index for verifying the amount and design of preparation reduction.

Each tooth was prepared for an all-ceramic Zr crown according to current guidelines/recommendations by an experienced prosthodontist. All of the teeth were prepared by following the same guidelines which included a 5 to 10 degrees combined convergence angle, a functional cusp bevel, 2 mm of occlusal reduction, 1 mm of axial reduction, an overall rounded and smooth line angles except for the margin design, which for Group A was a 1 to 1.5 mm deep shoulder and for Group B was 1 to 1.5 mm deep heavy chamfer with a smooth continuous gingival finishing line (, ). The preparations for the Group A were performed using long Flat-ended tapered diamond burs (Bur # TF-14, ISO 172/023, Mani Inc., Japan) and for the Group B preparations were performed using long round-ended tapered diamond burs (Bur # TR-14, ISO 198/022, Mani Inc., Japan).

The teeth were then scanned digitally with CERCON EYE®, Digital Scanner (DeguDent GmbH, Germany). Copings were designed using CERCON®ART 3.2 (DeguDent GmbH, Germany). All of the copings were prepared with the same cement gap of 30 μm. They were milled in CERCON®BRAIN (DeguDent GmbH, Germany) and sintered in a sintering device CERCON®HEAT (DeguDent GmbH, Germany) by one technician.

All of the copings were first examined visually for any defects or retained debris and steam cleaned. Subsequently, they were placed on their corresponding prepared teeth. The marginal fit of the copings was examined visually and tactically using a sharp explorer. The internal fit of the copings was verified further with a fit checker (Occlude, Aerosol Indicator Spray, Pascal Company, Inc., Washington USA) and adjustments were made using a high speed round diamond bur (#BR31, Mani, Inc., Tochigi, Japan) under copious water irrigation, if required. This procedure was repeated three times. Subsequently, the internal surfaces of all the copings were cleaned carefully with BegoDousrar using 60 psi sandblasting (Korox 110 & 50 μm, special corundum blasting material of 99.6% aluminum oxide). The copings were then cemented with a Resin Modified Glass Ionomer Cement (RelyX Luting 2; 3M ESPE, St. Paul, MN 5514-1000, USA) under standardized axial force of 30 Newton’s using Drill Press (DREMEL-MOTOTOOL, Model 212, U.S.A) for 10 minutes and excess cement was removed by a small disposable brush, following protocols described by authors of recent studies, evaluating the fit of zirconia copings (, , ). After cementation of the copings, the samples were thermocycled for 24 hours in a thermocycling machine (Huber, SD Mechatronik Thermocycler, Germany) to simulate the oral environment. The copings were again re-embedded with orthodontic resin (Ortho-Resin, DeguDent GmbH, Germany) to avoid any dislodgement of the coping from the teeth during the sectioning of the samples. The samples were then cross sectioned centrally into two equal mesial and distal halves using precision saw (Isomet 2000 Precision Saw, Buehler, USA).

Each of the sectioned specimen was designated a and b for mesial and distal half, respectively. They were placed over a metallic cylinder (Figure 1) and gold coated (using Fine Coat Ion Sputter JFC-1100, Tokyo, Japan) for placement in a Scanning Electron Microscope (JEOL, JSM-6360LV, Tokyo, Japan), keeping the exposed sectioned surfaces parallel to the base of the microscope. The copings were examined at 200X magnification by an expert electron microscope technician at 5 predetermined points for the buccal and lingual margins for each of the sectioned half (Figure 2, 3 and Table 1). The technician was initially trained for the study by studying a pilot sample of 5 sectioned copings. The cement thickness between the fitting surfaces of the copings and the teeth at the margins were measured in micrometers (µm) by the technician.

Figure 1

Mounted Sectioned Copings.

Figure 2

Scanning electron microscope examinations at 5 measurement areas.

Figure 3

Cross sectional diagrammatic representation of the 5 measuring areas.

Table 1

Details of the 5 points of measurements for each coping

Point	Abbreviation	Description
1.	AMP	Absolute Marginal Gap. The distance from the edge of the coping to the edge of the finish line.
2.	MG	Marginal Gap. The perpendicular distance from the surface of the finish line to the coping’s margin.
3.	MMG	Mid Marginal Gap. The perpendicular distance from the internal surface of the coping to the mid-point of the floor of the margin.
4.	AG	Angle Gap. The perpendicular distance from the internal surface of the coping to angle between floor and axial wall of the finish line.
5.	AWG	Axial wall Gap. The perpendicular distance from the internal surface of the coping to the axial wall.

The data were analyzed using SPSS Version 22 software package (SPSS, Inc., Chicago, IL, USA). The buccal and lingual marginal readings for each half were recorded and then the mean of the readings for the two halves was calculated and considered as the final reading for each sample. Analyses included the mean values and standard deviations for each of the 5 gap sites and for each group (A and B) of 20 teeth using column statistics; and comparison of these latter means (95% CIs) using T-test. The probability for statistical significance was set at α = 0.05.

Results

The overall mean gap values for the two groups are presented in Table 2. The mean marginal gap for Group A (Shoulder Margin Design) was lower compared to Group B (Chamfer Margin Design). Assessment by T-test showed a statistically non-significant difference (p = .113) between the Shoulder and Chamfer margin designs (Table 2).

Table 2

Mean values (standard deviation) of the marginal gap for the experimental groups measured by the electron microscope (n=40)

Group	Margin Design	*Mean gap of two halves	Overall mean	Standard Deviation	T Testp value
A(n=20)	Shoulder	**a = 201.74	199.50	40.72	.113
		***b = 197.26
B(n=20)	Chamfer	a = 212.90	214.46	44.85
		b = 216.02
Total			206.98	42.78
*Mean gap was measured in micrometers (µm)**Sectioned mesial half of the specimens.***Sectioned distal half of the specimens.

Table 3 shows the mean values for the Group A and B at 5 areas of measurements. The area number 2 (Marginal Gap) showed the lowest mean gap values and area number 4 (Angle Gap) showed the highest mean gap values measured (Table 3). The results with Independent Sample T-test showed a statistically non-significant difference between the two groups for all the 5 areas of measurements (Table 3).

Table 3

Descriptive statistics plus T test results for 5 points of measurements

Points	Groups	Mean (Std. D.)(n=20)	***Mean (Std. D)(n=40)	Independent Samples test
1.	*A	155.30 (88.25)	170.55 (100.08)	.167
	**B	185.80 (111.92)
2.	A	152.18 (66.85)	157.21 (66.67)	.491
	B	162.25 (66.50)
3.	A	236.36 (55.72)	248.70 (59.89)	.063
	B	261.05 (64.07)
4.	A	268.06 (73.42)	266.54 (62.67)	.829
	B	265.02 (51.92)
5.	A	185.61 (78.93)	191.9 (68.18)	.410
	B	198.19 (57.44)
* Shoulder Margin, **Chamfer, ***Mean gap was measured in micrometers (µm)

Figure 4 shows the differences in the mean values for the two groups at 5 evaluated regions obtained with electron microscope. The highest difference among the groups was found in the Area 1 (Absolute Marginal Gap) and the least difference was found in the Area 4 (Angle Gap).

Figure 4

Comparison of the mean values of gaps for the two groups at 5 measured areas.

Discussion

In the present in vitro study, the marginal adaptation of the single unit of zirconia copings, manufactured from semi sintered zirconia blocks using Cercon® (DeguDent GmbH, Germany) system and cemented on prepared extracted teeth with two different types of margin designs, was investigated. To the best of the authors' knowledge, there are no such studies in the literature comparing the marginal adaptation of zirconia copings in Shoulder and Chamfer margin designs which are the most commonly used margin designs when preparing the teeth for all ceramic zirconia crowns (). This study was an attempt to find the margin design that will result in least marginal gap between the finishing lines and the coping margin using natural teeth instead of using artificial dies. There were some limitations of the study which may have affected the results such as standardization of the preparation of teeth, individual designing of the coping for each tooth, trial fitting of the coping on the teeth, effect of cementation procedure, sectioning of the coping with the saw, too many persons involved in the complete process and finally the measurements recorded with the electron microscope. However, an attempt was made to address each of the abovementioned issues.

According to the data obtained in this study, the null hypothesis stating that the margin design does not affect the marginal adaptation of the zirconia copings could be accepted. The results showed a difference that is not statistically significant (p>.113) between the means of the overall gap between the two groups and at 5 points measured for the Shoulder and Chamfer margin design preparations. The mean gap measurement for the Chamfer margin preparation of 214.46+44.85µm was found to be higher than for the Shoulder margin preparation of 199.50+40.72µm. However, this difference is negligible in terms of clinical significance. The results of the study revealed that 4 out of the 5 measured gap points showed Shoulder margin design to have better marginal adaptation compared to the Chamfer margin design.

In the literature, many studies have evaluated the fit of zirconia copings using various systems and materials and only one study carried out by Habib SR et al () has reported the results similar to this study with respect to single tooth zirconia copings, fabricated on prepared extracted natural teeth. The results of all these studies are difficult to interpret because of the variations in the sample size, measurements of the specimens and different methods used for the measurements. Several authors have considered marginal discrepancies between 100 and 150 µm to be in a range of clinical acceptance (-). In the present study, the mean of the buccal marginal gap and lingual marginal gap was found to be 206.98+42.78µm which is higher compared to the results of other studies. This variation in the results obtained in the present study could be explained by the difference in the CAD/CAM system and use of individual teeth for the fabrication of each coping. Another reason for higher marginal gap values may be explained by the fact that the mean value recorded at 5 different points were considered and used as the final value, while calculating the marginal gap values in the study samples compared to the vertical marginal gaps were measured only buccally or lingually in other studies. With this variation we can also predict that a certain percentage of variation in the fit of Zr copings exists in clinical cases and is difficult to predict with the visual examination alone.

In the current study, the buccal and lingual marginal readings for each half were recorded and then the mean of the readings for the two halves was calculated and considered as the final reading for each sample. The limitation of using this technique is that it gives a two dimensional view for measuring the thickness of gap in a single section and does not examine the 3D (three dimensional) adaptation (). In a study by Wakabayashi et al () the adaptation of the all ceramic crowns was analyzed in a non-destructive technique using microfocus X-ray CT system. Although they reported several advantages of using the technique, the overall mean gap thickness measured (119+7µm) by this technique was found to be not more significantly different than that reported by other studies (, , , ).

The current study followed the guidelines recommended by the manufacturers of Shoulder and Chamfer types of margin designs for the zirconia crowns which in fact are the most commonly used types of margin designs prepared by the dentists (). Researchers have reported the stress levels, fracture resistance, adequacy and biomechanical performance between the Shoulder and Chamfer types of margin designs (-). However, there is still lack of scientific data in the literature about the type of the margin design that will result in least marginal discrepancy. The results of the current study showed that there is not a statistically significant difference between the Shoulder and Chamfer margin designs which indicates that none of the two margin designs is superior to the other in terms of marginal fit. On the basis of these findings we hypothesize that, depending on the clinical requirement, the clinician has freedom to choose the proper type of margin design. Although, clinical decisions regarding the choice of margin design and the ultimate outcome are dictated by many factors, the clinician’s experience and preferences play an important role during the preparation of teeth for zirconia crowns.

Conclusions

Within the limitations of this study, it was concluded that marginal adaptation of zirconia copings with either Shoulder or Chamfer margin design is the same. No statistically significant differences were found between the Shoulder and Chamfer types of margin design.