Evaluation of Effect of Screw Access Filling Materials on the Retention of Cement-Retained Implant Supported Crowns with Surface Modified Abutments – An In Vitro Study

Introduction

Implant-supported fixed partial dentures have become one of the most popular modes of rehabilitating missing dentition. The implant-supported prosthesis can be screwed or cemented to the implant for retention. The choice between the two is determined by the clinical situation and may act as a determinant for the prosthesis survival as it is seen that screw-retained implant restorations are more prone for technical complications while cement-retained implant restorations show more biological complications.¹

Screw-retained implant supported prosthesis was favored over its cement-retained counterpart due to the disadvantages of poor retrievability, unpredictable retention and the biological effect of retained excess cement associated with cement-retained implant supported prostheses.² However, currently there is a shift towards the use of cement-retained implant supported prostheses as they provide various advantages such as passive fit, improved aesthetics, ease of access, axial and progressive loading, less technical complications, reduced cost and better operator comfort.^2-4 This shift has been further aided by the development of various techniques for the retrieval of cement-retained implant supported crowns.⁵

In cement-retained implant supported prosthesis, crowns are cemented using a luting agent onto the implant abutment and the factors that influence the retention of the prosthesis to the abutment are cement or abutment related. The cement related factors are type, film thickness and composition of cement.⁶ The commonly available cements for luting the prosthesis onto the abutment include zinc oxide eugenol, polycarboxylate, zinc phosphate, glass ionomer, resin modified glass ionomer and composite resin cements. These luting agents can be divided into provisional, semi-permanent and permanent cements based on their clinical application.⁷

The abutment related factors include taper, surface area, height, surface roughness and geometry of the abutment.^3,4,6,8 In addition, the screw access filling method and material have also been shown to influence retention.^6,8 The screw access channel is that portion which provides a pathway to the screw that fixes the abutment or the prosthesis to the implant body. Many materials have been used to fill the screw access either individually or in combinations. Some of them are polytetrafluoroethylene tape (Teflon tape), cotton pellets, endofrost pellets, gutta-percha, restorative composite resins, wax, temporary filling materials, polyvinyl siloxane and silicone sealing materials.⁹

One of the problems associated with the use of cementretained implant supported prosthesis is the chance of dislodgement of crowns, especially in cases with short implant abutments. Cement-retained restorations require a minimum crown height space of 8 mm which includes an abutment height of at least 5 mm to provide sufficient retention of the crown and the remaining 3 mm being occupied by the restorative material.³ In clinical situations with inadequate inter-ridge space, short abutments must be used. This makes the area occupied by the access hole significant in relation to the total surface area of the abutment and may influence the retention of the crown.⁶

There are many studies on various abutment surface modifications to improve retention, such as sand-blasting, grooving, creating punches using burs, selective removal of the abutment wall, acid-etching and laser etching.^8,10-12 However, data regarding the advantages of the screw access filling material and its interaction with the luting agent for improving retention is quite minimal. Also, the studies available are on provisional luting agents.⁶ Hence, there is a need to study the interaction of the semi-permanent and permanent luting agents with the screw access filling material.

The purpose of this study was to evaluate the effect of three different screw access filling materials on the retention of cement-retained implant supported crowns with sandblasted implant abutments when the semi-permanent zinc phosphate is used as the luting agent. The null hypothesis being, there will be no effect of different screw access filling materials on the retention of cement-retained implant crowns when luted with zinc phosphate on sandblasted abutments.

Materials and Methods

Thirty implant analogs (Bioline Dental GmbH & Co. KG, Wermeuchen, Germany) and thirty regular platform straight abutments (Bioline Dental GmbH & Co. KG, Wermeuchen, Germany) of diameter 4 mm and height 9 mm were used in this study. This in vitro study did not use any data/samples from human participants and was approved by the Institutional Ethics Committee. The sample size for the study was estimated to be ten samples per group using G*Power software based on values from a previous study.6 The implant abutments were cut down to a height of 5 mm to simulate minimum abutment height using a carborundum disk and a mandrel using a micro-motor handpiece. The outer surface of the abutments was sandblasted using 110 µm aluminium oxide to improve the surface roughness.

The implant abutments were attached to their implant analogs with a 35 Ncm torque as recommended by the manufacturer. A stainless-steel die of dimensions 20 x 20 x 20 mm was fabricated using which a mould was prepared with polyvinyl siloxane putty impression material (Flexceed putty, GC Corporation, Bunkyo-ku, Tokyo, Japan). This mould was used to fabricate the base for the implant analog-abutment assemblies. By means of a dental surveyor (Jintai Machines, Yuyao, Zhejiang, China) for proper vertical positioning, the implant analog-abutment assemblies were lowered into cold cure acrylic (DPI RR Cold Cure, DPI, Mumbai, India) poured into the mould and held in place until set (Figure 1).

The implant abutment surface was scanned using a dental 3D scanner (AutoScan-DS-EX, SHINING 3D®, Zhejiang, China) for the fabrication of metal copings to simulate a metal ceramic crown. The copings were designed in the form of maxillary central incisor using the exocad software. The thickness of the copings was maintained as 0.5 mm with a cement space setting of 25 µm. A ring of thickness 2 mm and internal diameter of 5 mm was designed and added at the incisal region of the coping design to accommodate the universal testing machine during tensile testing. This design was given as the input for the direct metal laser sintering machine (EOS M100), using which 30 cobalt-chromium metal copings were prepared (Figure 2). The intaglio surface of all the 30 copings were sandblasted using 110 µm aluminium oxide.

The 30 samples prepared were divided into three groups with ten samples each. A small cotton pellet (Jaycot Industries, Hyderabad, India) was placed to protect the screw head and the remainder of the screw access channel was filled as follows:

Group A: Universal restorative composite resin (Te-Econom Plus®, Ivoclar Vivadent AG, Schaan, Liechtenstein) was filled using a hand instrument and light cured for 20 seconds.

Group B: Cavit temporary filling material (Cavit™ G, 3M ESPE, Neuss, Germany) was compacted using a hand instrument.

Group C: A piece of Teflon tape (Mexiko GP, Hyderabad, India) was cut and twisted for easy handling. This was then compacted into the screw access channel using a hand instrument.

All 30 samples (Figure 3) were luted using zinc phosphate cement (Harvard Cement, Harvard dental international, Hoppegarten, Germany) mixed according to the manufacturer’s instructions. The copings of the crowns were seated over the abutments with finger pressure for the first five seconds and then subjected to a constant load of 5 kg for five minutes with the universal testing machine. The excess cement from the margins was removed using an explorer. The cementation procedure was carried out by a single operator to ensure that there was no inter-operator variability across the samples. After cementation, all the samples were stored in artificial saliva (Fusayama-Meyer composition) at 37°C for 14 days in a constant temperature water bath for hydro-stress aging.

After aging, the uniaxial retentive force required to remove the copings from their abutments was measured in Newton using a universal testing machine (UTES-HGFL, Fuel Instruments & Engineers Pvt. Ltd, Maharashtra, India) with a cross-head speed of 5 mm/min (Figure 4). The mode of cement failure was observed by visually examining the intaglio surface of the copings and the external surface of the abutments (Figure 5).

The collected data was entered in Microsoft Excel 2019 and subjected to statistical analysis using Statistical Package for Social Sciences (SPSS Version 20.0). The normality of the obtained data was assessed using Kolmogorov-Smirnov and Shapiro-Wilk tests and subjected to One-way ANOVA to determine the difference between the groups.

Results

The descriptive statistics of the uniaxial retentive force measured in Newton (N) for the three groups are shown in Table 1. Although the mean uniaxial retentive force was found to be highest for Group B (Cavit) (Figure 1), followed by Group A (Composite Resin) and Group C (Teflon Tape), the results of One-way ANOVA test in Table 2 showed no statistically significant difference in uniaxial retentive force between the three groups with p > 0.05.

With regard to the failure mode of the cement, it was observed that the cement predominantly remained adherent to the intaglio surface of the copings depicting adhesive failure at the abutment-cement interface in all the three groups (Figure 5). In terms of its interaction with the screw access filling material, in the composite resin and teflon group the cement remained on the intaglio surface of the coping with nothing on the screw access filling material surface. Whereas in the Cavit group, some of the Cavit material was found adherent to the cement on the intaglio surface of the coping.

Discussion

The results of this study showed that there is no statistically significant difference in the uniaxial retentive force between three different screw access filling materials when zinc phosphate cement was used as luting agent, thus accepting the null hypothesis.

The controversy between the use of screw retained and cement retained prosthesis is an ongoing one and is dependent on weighing the pros and cons between the two types. In either case, the retention of the prosthetic super structure is one of the factors that ultimately determines the success of the implant prosthesis. Previous studies have shown that there are various factors that influence the retention of the cement-retained implant supported prosthesis.^3,4,6,8 This study was designed to evaluate the effect of different screw access filling materials on retention when all other factors are kept constant.

Although several materials and their combinations have been in use to fill the screw access channel, in a survey conducted by Tarica et al., the usage of cotton pellet and composite remained one of the top combinations.⁹ The common availability of the material and its easy handling properties make it a sought-after material for filling the screw access channel. In addition, it was shown that using composite resin material for filling the screw access channel improved the retention of cement-retained crowns in case of short abutments (Cakan et al.).⁶ In a study by Emms et al., it was reported that a partially filled screw access channel provided more retention than a completely filled screw access channel.⁸ However, most practitioners back the complete filling of the access channel.⁹

Cavit is a non-eugenol temporary restorative material composed of zinc oxide, calcium sulfate, zinc sulfate, glycol acetate, polyvinyl acetate, polyvinyl chloride-acetate, triethanolamine, and red pigment. It sets by reaction with water making it a hydrophilic material that provides a good seal on setting. In addition, the nature of this material is such that it is easy to manipulate and condense into cavities.¹³

Teflon tape or polytetrafluoroethylene tape is a hydrophobic polymeric material that has a resistance to high temperature, solvents and acids. It is capable of being stretched and closely adapted to different surfaces.¹⁴

It is colloquially called ‘Plumbers tape’ and commonly used for hardware and industrial applications. However, this material being easy to manipulate and remove if necessary, also found many applications in implant dentistry such as a barrier to remove and limit excess cement flow into the gingival sulcus,¹⁵ a technique to limit screw loosening,¹⁶ and as a screw access filling material.¹⁷ Because of its non-filamentous nature and its ability to undergo good compaction into the screw access channel, Teflon tape has less chance of undergoing bacterial colonization.14 The semi-permanent cement zinc phosphate was used as the luting agent in this study to provide a compromise between retention and retrievability as both these qualities have significance in implant prostheses.⁷ In addition, zinc phosphate is radiopaque which enables the detection of excess cement in the radiograph. This is clinically significant as residual excess cement is one of the most common problems of cement retained implant prostheses.²

The results of one-way ANOVA in the present study failed to demonstrate significant results which was contradictory to the results obtained by Cakan et al., where the mean uniaxial retentive force for the composite resin group was significantly higher than those for the other filling material groups.⁶ The difference in the results between the two studies may be explained by the fact that non-eugenol provisional cement was used in the previous study, while the semi-permanent zinc phosphate cement was used in the current study. Covey DA et al., have shown that the retention forces of zinc phosphate was 2.5 to 4.7 times more than zinc oxide eugenol.¹⁸ So, the difference in the retention offered by different screw access materials maybe masked by the greater retentive force offered by the semi-permanent luting agent in the current study. The interaction of the luting agent and the screw access material may also have influenced the results. In addition, the abutments were reused five times in the study done by Cakan et al.,⁶ which may have influenced the retentive force values, while in the current study each abutment was used only once.

The mode of failure of the cement when inspected visually was found to be an adhesive failure at the cement-abutment junction as most of the cement remained in the intaglio surface of the copings. Similar adhesive mode of failure was seen in a previous study by Al Hamad et al., which used zinc phosphate as one of the luting agents.¹⁹ Similar to the results reported by Sahu et al., since both the intaglio surface of the copings and the outer surface of the abutments were sand blasted with 110 µm alumina, this mode of failure may be attributed to the difference in the bonding of zinc phosphate to titanium and cobalt-chromium base metal alloy.²⁰

Regarding the interaction between the luting agent and screw access filling material, in contrast to the study conducted by Cakan et al.,⁶ there was no luting agent adherent to the screw access filling material surface in the composite resin and Teflon tape groups. This may be explained by the fact that an oil-based luting agent was used in the previous study and a water based luting agent, zinc phosphate was used in the current study. Of interest is the adherence of a portion of Cavit to the luting agent on the intaglio surface of the coping. Both these materials being water based may have resulted in such an interaction. In addition, this may also explain the higher mean uniaxial retentive force values seen in this group as greater force would have been required to cause a cohesive failure in the Cavit material present in the screw access channel.

As the results of one-way ANOVA test failed to show any significant difference in the retention between the three groups, it may be inferred that the usage of composite resin, Cavit and Teflon tape as screw access filling materials did not affect the retention of the cement-retained implant crowns cemented with zinc phosphate cement on sandblasted abutments. However, as the results are contradictory to the previous study,6 despite the materials tested being different, further research is required in the subject matter using different combinations of materials.

In conditions where improved retention of the crown is required, the operator can achieve this by other methods such as using a permanent luting agent, modification of the abutment by milling, grooving, sand blasting, surface treatments or any combinations of these methods.

Limitations & future study prospects

1. Only three of the commonly used screw access filling materials and one luting agent was assessed in this study. The testing of other materials and their combinations in future studies may yield different results.

2. The uniaxial retentive force measured in this study may not be similar to the dynamic intra-oral force factors that cause decementation in a clinical scenario.

3. The factors that influence retention other than the screw access filling materials were not evaluated in this study.

Conclusion

Within the limitations of the present study, it can be concluded that the retention of cement-retained implant supported crowns did not vary considerably when composite resin, Cavit and Teflon tape were used as screw access filling materials and zinc phosphate was used as the luting agent on sandblasted abutments.

Conflict of Interests

Nil