Abutment misfit in implant-supported prostheses manufactured by casting technique: An integrative review.

The aim of this study was to perform an integrative review of the literature on the clinically usual prosthesis-abutment misfit over implant-supported structures manufactured by conventional casting technique. The present integrative review used the PRISMA methodology. A bibliographical search was conducted on the following electronic databases: MEDLINE/PubMed (National Library of Medicine), Scopus (Elsevier), ScienceDirect (Elsevier), Web of Science (Thomson Reuters Scientific), Latin American and Caribbean Center on Health Sciences Information (BIREME), and Virtual Health Library (BVS). A total of 11 relevant studies were selected for qualitative analysis. The prosthetic-abutment vertical misfit considered clinically usual ranged from 50 to 160 μm. The vertical misfit depends on several steps during technical manufacturing techniques, which includes the materials and technical procedures. Lower values in misfit are recorded when precious metal or titanium alloys are utilized. Although a vertical misfit mean value of 100 μm has been considered clinically usual, most of the previous studies included in this revision showed lower mean values.

INTRODUCTION

Implant- supported prostheses are frequently used for oral rehabilitation and therefore osseointegrated dental implants do overcome many of the limitations found in conventional fixed and removable prostheses.[1234] Several factors must be considered to improve the long-term performance of implant-supported prostheses such as the distribution of stresses through the structural materials to the alveolar bone, the fitting of the connections, as well as the properties of the materials.[56789]

On implant-supported prostheses, microgaps have been detected between abutment and implant contacting surfaces as well as between prosthetic structures and abutment. Such microgaps vary depending on the processing of the prosthetic and implant structures, and therefore it can affect the mechanical performance of the prosthesis.[6789] Several studies have shown that a certain degree of inaccuracy is inevitable even with the current manufacturing processes such as computer-aided design-computer-aided manufacturing (CAD-CAM).[6789101112] As a result, the penetration of oral fluids (e.g. glycoproteins and acidic substances) and biofilms could cause the decrease of friction between contacting surfaces and induce corrosion by the presence of bacterial metabolites.[1671314]

Implant-supported prostheses produced by conventional casting technique show technical complications caused by misfit between the prosthetic structure and the abutment.[345678] The misfit in implant connections can result in intense oblique loads and concentration of stresses at prosthetic and implant structures.[12671215] That can induce loosening of mechanical integrity of abutment screw, fracture of ceramic prosthetic materials, and bone resorption.[123456789101112151617181920] The selection of materials and design of implant-abutment-prosthesis assemblies is crucial to avoid mechanical issues such as fractures and overloads.[456789101112]

The finite element method (FEM) is a tool that evaluates the distribution of stresses through structural materials by varying occlusal loading, design, and materials' properties.[212223] This method is performed to indicate biomechanical aspects of materials and human tissues that cannot be measured in vivo,[24] allowing to evaluate the effect of several factors before laboratorial or animal studies.[2122] The magnitude of stresses transferred to the surrounding bone plays a key role in the long-term success of dental implants. Some factors that influence the load transfer from implant to bone are loading modality, bone-implant interface, length and diameter of the implants, shape and characteristics of the implant surface, type of prosthesis, and quantity and quality of the surrounding bone.[2324] The three-dimensional geometry of the jaw is important for planning the positioning, number, diameter, and length of dental implants. Virtual models can help the clinician in choosing a correct design for a prosthetic rehabilitation considering stress distribution from occlusal loading.[232526]

The marginal abutment-prosthetic misfit is still an issue in implant and restorative dentistry once the complete connection sealing or cold welding does not occur. Within the issues related to vertical misfit, it is clear the need to achieve a minimum misfit in implant-supported prostheses. Thus, the present study aims to perform an integrative review of the scientific literature on the clinically usual prosthetic-abutment misfit over implant-supported rehabilitation manufactured by conventional casting techniques.

METHODOLOGY

The study followed the methodology proposed by PRISMA (2009)[27] to perform systematic reviews. PRISMA methodology allows to conduct a wide variety of systematic reviews, ensuring reproducibility and traceability. A bibliographical search was conducted on the following electronic databases: MEDLINE/PubMed (through National Library of Medicine), Scopus (Elsevier), ScienceDirect (Elsevier), Web of Science (Thomson Reuters Scientific), Latin American and Caribbean Center on Health Sciences Information (BIREME), and Virtual Health Library (BVS). The following search terms were used: (tw: [”discrepancy” or “variance” or “misfit” or “discrepancies” or “discrepância” or “vertical”]) AND (tw: [”Implante” or “implant” or “implant-supported”]) AND (tw: [”Protético” or “prótese” or “próteses” or “ponte” or “coroa” or “crowns” or “frameworks”]). Laboratory studies with a minimum sample of two abutments were included, in which results were showed in for vertical misfit in micrometer obtained by optical or scanning electron microscopy. Articles from 2005 to 2015 were included. There were no language restrictions. Case–control studies, animal studies, letters, book chapters, reviews, and articles that did not comply with the requirements proposed were discarded. The evaluation of the potentially relevant articles by review of the title and abstract was completed independently by two of the authors. Selected articles were individually read and analyzed considering the purpose of this study. The review article variables considered for this review were authors' names, journal, publication year, marginal misfit on dental-supported fixed prostheses, implant-abutment misfit, or studies evaluating crowns that were not manufactured by the lost-wax technique such as scanners or CAD-CAM technologies.

RESULTS AND DISCUSSION

The flowchart of the search strategy is shown in Figure 1, and the selected articles within content details are listed in Table 1. A total of 11 studies were identified for qualitative analysis.

Figure 1

Flowchart of the search strategy

Table 1

Articles organized by year of publication

The largest vertical misfit considered as clinically usual in the sample analyzed was at 150–160 μm.[28293031] Still, Bayramoǧlu et al.[32] reported values of up to 120 μm and Barros et al.[5] considered misfit values at 100 μm. The lowest values reported as clinically usual were at 70[33] and 50 μm.[34] Oyagüe et al.[35] do not discuss an acceptable range for vertical misfit.

There was a difference between the values considered acceptable and the values found by the authors as shown in Figure 2, although only two articles showed higher misfit values in the experiment than those expected.[534] The study of Gomes [20] was not included since it was a computational simulation of marginal discrepancy.

Figure 2

Vertical misfit expected mean (or clinically accepted) and effective mean found in each selected study

The lost-wax casting technique is still one of the most used procedures in manufacturing prosthetic crowns although poorer marginal adaptation has been recorded when compared to CAD/CAM systems. Barros et al.[5] found discrepancy mean values of 108.63 μm for cast bars and 9.65 μm for the milled bar. All selected articles in the present study found that the misfit mean values regarding CAD/CAM technique were always lower when compared to those for casting.[303132]

Marginal misfit is one of the factors to be analyzed for clinical success, always seeking the lowest values depending on the materials, casting techniques, cementation, torque, etc. The lowest marginal misfit was found for lost-wax method, but this result was not statistically significant when compared to burn-up and impression cap.[34] Regarding the comparison among modified caps, calcinable capsule, and the non-use of caps, Eliopoulos et al.[28] reported more favorable results and reduced precision for vertical discrepancy when modified prefabricated plastic components were used.

Several alloys are used to produce the prosthetic crowns, and it is necessary to evaluate the cost effective. Precious metal alloys have better properties but higher costs. NiCr-based alloys are cheaper and have properties considered acceptable.[28] Oyagüe et al.[35] showed a significant difference when comparing alloys of CoCr-based, titanium, and AuPd-based. CoCr-based alloy revealed higher vertical misfit values, whereas Titanium and AuPd-based alloys were similar. Although a significant difference has been found among the alloys, all values are considered acceptable.

The cements utilized may also influence misfit of prosthesis-abutment and microleakage of oral fluids and biofilm. Previous studies performed [3031] have shown better results regarding marginal sealing for resinous glass ionomer (~68 μm) and urethane-based cements (~67 μm) than those recorded on dual-core resin cements (95 μm). However, dual-core resin cements have better stability regarding microleakage since it is slightly soluble. Despite these results, there was no statistically difference among them.

For screw-retained implant-supported prostheses, it is hypothesized that the marginal gap contributes to a poor distribution of stresses. The study of Gomes [20] simulated marginal and angular misfit, and the results showed that each discrepancy contributes to overloading at a specific region of the implant. Unilateral vertical misfit damages the infrastructure and total vertical misfit negatively affects the implant hexagon. This evinces a greater overload according to the discrepancy between abutment and prosthesis, even when the gap is below as 100 μm. However, the other articles examined in this study suggest that the magnitude of the marginal misfit is not a reliable indicator in the prognosis of the stresses generation, once the stress distribution is more related to other factors such as screw tightening.[293336]

In most studies, the marginal discrepancy considered clinically usual was around 100 μm; however, Ghanbarzadeh et al.[33] and Siadat et al.[34] considered lower values of 70 and 50 μm, respectively. The average of 100 μm was only exceeded in three studies: Barros et al.:[5] 108 μm; Rodrigues et al., 2014:[36] 110 μm; and Tramontino et al.:[29] 118 μm. The study of Siadat et al.[34] reported a clinically discrepancy at 50 μm although the laboratory tests have revealed slightly higher values. Three studies revealed results that are suitable for this standard, obtaining a misfit mean values of 45,[33] 45.1,[35] and 50 μm.[28]

It was not possible to notice evolution in the reduction of the abutment misfit over the years, nor statistically significant values among materials or steps since the misfit is considered tolerable by clinicians. It is known that there is a biological tolerance for marginal discrepancy, but the maximum value of this misfit still should be reviewed, once it is based on empirical studies and there is a lack of standard protocols among the studies.

CONCLUSION

The vertical prosthetic-abutment misfit depends on several steps during prosthetic procedures, which includes the materials and technical procedures. Lower values of prosthetic-abutment misfit are obtained when precious metal or titanium alloys are used. Although a vertical misfit mean value of 100 μm has been considered clinically usual, most of the previous studies included in this revision showed lower mean values.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.