Introduction

All crowns require acceptable retention and resistance for their clinical success (1). Based on researches, loss of crown retention has been the second leading cause of failure of crowns and fixed partial dentures (2). Factors such as wall parallelism, preparation length, convergence angle and surface texture highly affect retention rate (3). Also, clinical success of fixed restorations is dependent on the type of cement and cementation procedures (4). Dental luting cements influence crown retention but can vary in tensile strength from 5.5 MPa to 45.1 MPa (5). Ideally, luting cements should have physical properties sufficient to resist functional forces, be resistant to degradation in oral environment and adhere to the underlying dentin (1).

Zink phosphate, polycarboxylate, glass ionomer, hybrid glass ionomers and resin cements are five commercially available luting agents for permanent cementation. Zink phosphate cement provides a mechanical interlocking to tooth and casting irregularities (6) and is still the gold standard for many clinicians (7). Glass ionomers are being used with great success and few complications. This cement holds the tooth and restoration together by physicochemical bonding (8, 9).

In recent years, resin cements have gained great reputation among clinicians because of their improved physical properties. Their dislodgement resistance is greater than other cements such as zinc phosphate or conventional glass ionomer (10). Resin cements not only enhance the retention of restorations, but also are less soluble (11) and have less micro leakage compared to zinc phosphate or glass ionomer cements (12, 13). It was found in a study that many non-metallic and
non-retentive restorations can only be cemented by resin cements (4). In another study, resin cements were used to attach crowns to teeth with short clinical crowns and the resultant bond strength was approximately 3 times more than the bond gained by zinc phosphate cement (14). This highlights the fact that resin cements are a great choice when the retentive features of the teeth are compromised (15).

Conventional resin luting cements require pretreatment of prepared dentin including separate etching, priming and bonding. In a study, it was shown that Panavia-F2, which is conventional resin cement, shows higher retentive strength, approximately twice that of zinc phosphate cements (16). Despite their high efficacy, different preparation steps make the use of conventional resin cements sensitive and time consuming (17). Also, micro leakage and polymerization shrinkage can lead to cuspal deformation (18, 19). Therefore, self –adhesive resin cements were introduced and found popularity because they do not need pretreatment of dentin (20). The self-etch property of self-adhesive resin cements eliminates the need for separate etching and priming and reduces technique sensitivity of these cements (21). The number of self-adhesive resin cements is increasing due to their high success rate. Among self-adhesive resin cements, Maxcem, G-CEM and Bifix SE found popularity in dental profession.

Currently, conventional resin luting cements provide the greatest bonding capacity for indirect restorations (22). However, self-adhesive resin cements reduce technique sensitivity and working time. Although some studies have investigated the efficacy of different resin cements, there is no published study to have comparatively investigated these cements regarding their bond strength. Therefore, the present study was designed to comparatively evaluate the retention of metallic copings using different resin luting cements. The null hypothesis was that there would be no difference in the retentive strength of complete metal crowns, luted with different resin cements.

Materials and Method

Sampling

In the present experimental study, 40 human maxillary first premolar teeth of comparable crown length and size, extracted for orthodontic reasons, were included )n=10 , a=0.05,β=0.8 ,d=7.51 Kgf). Teeth with any sign of crack, caries, previous restorations or filled canals were excluded from the study. Teeth were stored in thymol 0.2 % at room temperature for two days and in 4°C distilled water for no more than one week prior to tooth preparation.

Tooth preparation

Shallow notches were prepared on the outer surface of the roots of the teeth which were vertically mounted in self-cure acrylic resin (Meliodent, Heraeus Kulzer, Hanau, Germeny) within 2 mm of cemento-enamel junction (CEJ). A dental surveyor (Degussa-Ney, Yucaipa, CA, USA) was used to vertically align the long axis of each tooth. Mounted teeth were stored in an atmosphere of 100% humidity. Tooth preparation was initiated by vertically reducing occlusal surfaces in order to expose the dentin. Any remaining enamel was removed to expose the dentin. The reduced occlusal surfaces were examined using a ×19 stereomicroscope (SMZ-1; Nicon, Inc, Garden City, NY). A high-speed hand piece (KaVo America, Lake Zurich, IL, USA) and water spray mounted on a milling machine was used to control the tooth-bur angle and standardize axial reduction. The method used to estimate the axial surface area was similar to that of Felton et al (23). By comparing the foil strip for each sample with strips of known surface area, the appropriate axial surface area was obtained. The teeth were prepared to receive complete cast copings with a 1 mm diameter chamfer bur (Henry Schein Rexodent, Southall, UK) at high speed, cooled with an air/water spray. We made the occlusal surface flat, perpendicular to the long axis of the root at a standardized height of 4 mm from the gingival chamfer finish line with a 20 degree consistent taper of axial walls (12). Putty-wash impressions of the finished preparation were made (Panasil; Kettenbach GmbH & Co KG, Eschenburg, Germany) in prefabricated trays. The impressions were then poured in high-strength stone (GC Fujirock EP, GC Corp, Leuven, Belgium) to construct dies.

Wax patterns with 0.5 mm thickness and flat occlusal surface were made using type I blue inlay wax (Kerr/Sybron, Orange, Calif, USA). A ring like wax pattern was added to the occlusal portion to facilitate tensile testing after cementation as described by Tjan and Sarkissian (24).

Casting the crowns and cementation

The wax patterns were poured and invested using a gypsum-bonded investment (Rema-Exakt; Dentaurum, Ispringen, Germany). Each pattern was cast with ADA predominantly base metal, Ni-Cr-Be alloy (Rexillium III, Pentron, Wallingford, CT). A pilot study was done to produce crowns that seated well with minimal force on the stone dies and tooth preparations. The accuracy of the copings was measured using wash impression materials (Fit Checker, GC Co., Tokyo, Japan). The internal surface of each casting was inspected with a stereomicroscope mentioned above and any minute nodules were removed using a half-round bur in a slow speed straight handpiece. Then, the inner surfaces of the copings were air abraded at 60 psi from a distance of 3 cm using 50 µm alumina powders (Rocatec  Pre, 3M  ESPE,USA) for 15 seconds and ultrasonically (Tecna3 ,Technogaz, Parma, Italy) cleaned in deionized water for 10 minutes.

Completely seated, uncemented copings were randomly assigned to the four following groups (table 1) each including 10 specimens:

Group 1: copings luted with Panavia-F2 cement (Kuraray, Osaka, Japan)

Group 2: copings luted with Maxcem cement (Kerr, Orange, CA, USA)

Group 3: copings luted with G-CEM cements (GC America, Alsip, IL)

Group 4: copings luted with Bifix SE (Voco GmbH, Cuxhaven, Germany)

Before filling the castings with cement, the castings and teeth were dried with an air syringe. Cements were prepared and applied in accordance with the manufactures' instructions. Then the castings were seated under a 25 Kg compressive load, applied for 10 minutes using a force gauge (Chatillon model DPP; Ametek US Gauge Division, Largo, Fla) through a 100-mm-length × 8-mm-diameter wood stick placed horizontally on the occlusal surface of the crown. Then the excess cement was removed. The cemented copings were stored in an environment of 100% humidity for 24 hours before tensile testing.

Measuring crown retention

The tensile load required to dislodge the copings was measured using Instron Universal Testing Machine (Instron Ltd., High Wycombe, UK) in a path parallel to the axis of withdrawal for each sample with a crosshead speed of 1mm/min.

The data were analyzed with Kruskal-wallis test and SPSS (22.0) software program. (α=0.05)

Table1. Materials used in study

Results

The means and standard deviations of crown retention in different groups are shown in Table 2. Pavania-F2 cement showed the highest mean retention values among other resin cements (242.30 Kg) and Maxcem cement showed the lowest (148.90 Kg). Also, the maximum and minimum amounts of crown retention in samples were seen in G-CEM (450.00 Kg) and Maxcem (76.00 Kg) cements respectively. Despite this different retention values in cements, Kruskal- wallis disclosed no significant difference between groups in the mean amount of crown retention.
(P-Value>0.05)

Table 2. The means and standard deviations of complete metal crown retentive strength (Kgf) in different groups