The impact of digitization and conventional techniques on the fit of fixed partial dentures FPDs: systematic review and Meta-analysis

Dental impressions are commonly used to record oral structures in various dental fields [1]. An accurate impression is the most essential step in the construction of FPDs [2]. Inaccurate impressions may result in ill-fitting margins around the prostheses, plaque deposition, cement dissolving [3], and an elevated risk of pathogenic bacteria, which may also result in pulpal inflammation and necrosis. This causes abutment teeth to fail in various ways [4, 5].

The accuracy of dental impressions is critical for well-adapted restorations. The parameters that determine the fit of a dental restoration FPD are marginal and internal and contribute to its long-term duration [5, 6]. It refers to the degree of intimacy between the established abutment surface and the prostheses. The accuracy of dental impressions is critical for well-adapted restorations. Holmes et al. [7] defined the marginal gap (MG) as the vertical gap between the interior surface of the restoration and the margin of a prepared tooth, whereas the internal gap is the space from the same measurement to the axial wall. The marginal and internal fit of FPDs is determined by the size of the marginal and internal gaps [8].

Although the marginal discrepancy has various clinically acceptable values, McLean and von Fraunhofer presented a value of 120 μm as clinically acceptable as long as the internal fit is between 200 and 300 μm [9].

There are two methods of obtaining a dental impression, conventional impression technique and digital impression methods [10].

In conventional impression techniques, elastomers, such as polyether or polyvinyl siloxane (PVS), are commonly used to obtain impressions of prepared teeth because of their adequate precision and stability. Although conventional impressions have been the gold standard in the construction of multiple-unit fixed dental prostheses (MFDPs) for decades, inappropriate mold selection, material preparation, impression deformations before pouring, and stone model dimensional variations remain the most obvious drawbacks [10‐12].

Digital dentistry has undergone dramatic improvements, and numerous CAD/CAM systems that induce intraoral scanning and dental prosthesis manufacturing have been widely accepted [13].

The CAD/CAM system includes two procedures [14]: the CAD process for data collection by digitalization with scanners and designing restorations using accurate software, and the CAM process for manufacturing restorations after data processing. Digitalization can be performed directly on the abutment tooth using an intraoral scanner or an extraoral/laboratory scanner on the impression or definitive model.

The most significant benefit of employing CAD/CAM over conventional methods is that it significantly reduces discomfort in patients who are often hesitant to make impressions using traditional techniques [15].

Additional advantages of employing CAD/CAM systems include technological advancements that have made it possible to minimize the shrinking process of the materials to be scanned while simultaneously improving patient convenience [15, 16]. Similarly, the geometry of the intraoral scanner’s light bulb has also been altered and reduced, making it more comfortable for its purpose, that is; the ratio of the apex bulb has been adjusted, allowing the scanner to detect all but the most demanding dental features with this system, notably the posterior teeth [17, 18]. The images acquired from the scan consume significantly less time than analog impressions [3], allowing the dental or technical team to identify errors and limitations at each step and correct them in CAD/CAM systems, where the scanner models, system software, and manufacturing machines are perfectly coordinated [19]. In addition, a digital mock-up provides the patient with an immediate future treatment plan and outcome [16].

Ultimately, this systematic review and meta-analysis aimed to determine whether digital scanning and manufacturing techniques may improve the marginal and internal fit of FPDs compared to conventional techniques. Furthermore, to assess the other variances that improve the marginal and internal fit of FPDs, as well as to evaluate whether intraoral scanners and CAD/CAM technology could be legitimate substitutes for traditional techniques in manufacturing FPDs.

The null hypothesis H0 states that, digital impression techniques produce FPD with similar marginal and internal fit compared to conventional techniques, while the alternative hypothesis H1, the digital impression technique could improve the marginal and internal fit of FPD compared to conventional methods.

Recent data regarding the fit of tooth-supported FPDs that compare digital and conventional workflows are contradictory and heterogeneous, the need for updated results has encouraged this review. According to the relevant literature, this study will be the first review of scientific research discussing FPDs’ marginal and internal fit fabricated by digital and traditional approaches, with a focus on published studies within the last 6 years. The year 2017 was chosen as the cutoff for study inclusion because the rate of innovation in scanner systems and CAD/CAM technologies has drastically progressed over the last 5 years. The earliest studies revealed the greatest mean difference between digital and traditional approaches, which may have affected the analysis results [20].

Clinical studies were excluded as there were few published in vitro experiments and even fewer in vivo studies evaluating the fit of FPDs in terms of marginal and internal fit using digital and conventional workflows [22]. Ethical concerns aside, intraoral environmental challenges that restrict the scanning procedure, the swallowing movements, the existence of blood or saliva, and involuntary tongue movement may jeopardize the digitalization procedure [18]. Additionally, the results were variable, and the majority of studies confirmed the presence of many confounding factors that may have affected the analysis results. Moreover, no study has evaluated the survival and follow-up of full-coverage restorations or fixed partial dentures [23]. Furthermore, two recent meta-analyses were based on in vivo studies, obviating the need for a second analysis [22, 24].

The outcome of this analysis indicated that tooth-supported FPDs fabricated by digital techniques significantly enhance the internal fit but it didn’t influence the marginal fit compared to fabrications using conventional methods.

These results are compared to a meta-analysis by Russo et al. [25], a greater marginal gap value was observed in MFDPs fabricated by scanning systems than in those fabricated by conventional techniques, however, the difference was not statistically significant. In contrast, Morsy et al. [26] revealed that the marginal and internal fit of FPDs were significantly enhanced by digital scanning. In this study, a single clinical study and eight experiments were selected for the meta-analysis. Nevertheless, the clinical and experimental values may conflict with and mislead the outcome. Furthermore, the analysis did not fully evaluate the direct and indirect scanning technique results, and the publication period of the included studies is another factor that could influence the results of this analysis.

Regarding the method of digitalization, the results using extraoral and intraoral scanners showed no significant difference in marginal and internal fit, however, intraoral scanners could replace conventional impressions for the fabrication of FPDs because they minimize the operating time and remove patient pain. They also reduce fabrication processes, which may lead to errors due to less coordination between the clinic and dental laboratory.

Three different fabrication methods were observed in some of the included studies, and the results were comparable to those of different combinations of the fully conventional method, full digitalization, and partially digital groups. Conventional impressions and/or stone casts were scanned using an extraoral scanner. Even though the included studies preferred intraoral scanners over conventional or partial techniques, the statistical difference was insignificant.

A comprehensive review of intraoral scanner precision has been performed [18]. For short-span FPD impressions, the accuracy of the IOS was still similar to that of PVS and polyether impressions. As the span increased, the accuracy of the impression of the PVS became evident. Hasanzade et al. [21] observed that a fully digital workflow is superior to conventional techniques in terms of marginal fit. The authors suggested that the majority of inconsistencies in conventional or partial workflows were induced by stone-cast fabrication. However, in a fully digitalized group, the scanner systems, design software, and milling machines are appropriately surpassed, and the errors in each process can be corrected.

When evaluating the effect of the span length /number of units based on the outcome, digital techniques can significantly enhance the internal fit of three and five-unit FPD, however the difference in the marginal fit between the digital and conventional workflows for three-unit and five-unit FPDs is statistically non-significant. Another study revealed that marginal and internal fit were significantly affected by the edentulous span length of three- and four-unit fixed partial dentures, the study also discovered that the digitalization system produces fewer marginal and internal discrepancies than traditional techniques in up to 4-unit zirconia FPDs [21]. This significant difference is probably due to the marginal and internal gap values in the experimental findings in early published literature (2017–2012).

Regarding the cement spacer thickness, the results revealed a significantly better marginal and internal fit with digital workflow than a conventional workflow. The internal gap of the three-unit FPDs was significantly smaller when the spacer thickness decreased [26].

As the number of clinical studies is limited, the clinical significance of this study is that the digital scanning technique is a developing technology, and it is essential to test in standard situations while eliminating confounding factors. The results of this study will help make an initial judgment about the superiority of digital and conventional methods before making a conclusive decision about their clinical performance.

There are many reasons could explain the heterogeneity between the studies; the small number of included studies, increased bias in most of the selected studies, and experimental differences such as dissimilar scanner models utilized among experiments, study sample size, impression materials, preparation design, measuring method, fabrication machines and techniques, milling system, and data analysis tests used in each study.

The possible source of bias in marginal fit results between the digital and conventional group is mainly due to methodological difference between the included studies, however in internal fit analysis a publication bias was noticed due to small effect size.

This systematic review and meta-analysis produced specific conclusions based on these findings:

The results should be interpreted cautiously, as they were conducted on a limited number of studies throughout a limited period. Besides, the findings focused mostly on experiments conducted in laboratory settings.

Fewer clinical studies in the published literature mean less conclusive results; thus, more updated clinical studies with success and survival rates are needed to provide a stronger evidence.