Fit accuracy in the rest region of RPDs fabricated by digital technologies and conventional lost-wax casting: a systematic review and meta-analysis

In traditional dental practice, removable partial dentures (RPDs) are fabricated by lost-wax casting [1]. By this collaborative process between dentists and dental technicians, high-quality RPD frameworks can be produced. Nevertheless, it requires a great deal of experience and remains to be labor-intensive [2]. In 1970s, computer-aided design/computer-aided manufacturing (CAD/CAM) was applied into dentistry by Duret and Preston [3]. Since then, digital technology began its dental life.

A CAM system can be categorised into two types: subtractive manufacturing (SM) and additive manufacturing (AM). The most common SM technology used in dentistry is computer numerical controlled (CNC) milling. This method uses a milling machine to produce the object by removing bulk material from solid blocks with all the steps controlled by a computer program [4, 5]. While AM, also known as three-dimensional printing or rapid prototyping (RP), includes a range of different technologies such as stereolithography (SLA), selective laser melting (SLM), selective laser sintering (SLS), direct metal laser-sintering (DMLS), fused deposition modeling (FDM), selective electron beam melting (SEBM) and inkjet printing [4, 5]. SLA is basically used in the manufacture of resin-based structures, for instance temporary crowns, acrylic teeth, dentures, mouth guard and bite plane appliances through deposition of consecutive layers of photosensitive material that is readily polymerized [6, 7]. SLM, SLS and DMLS are laser powder forming techniques that use a high-energy laser beam to fuse material in its powder form and construct 3D objects layer by layer [6]. When processing polymers and ceramic the industry generally refers to this as SLS whereas for metals the terms used are SLM or DMLS [5]. FDM is a filament extrusion-based process that a plastic filament is heated to a semiliquid state and then extruded through a nozzle to deposit on to a platform to create 3D parts directly from a CAD model [5, 8]. SEBM is generally used for forming near-net shaped components of metals by melting metal powder layer per layer with an electron beam in a high vacuum [5, 9]. Inkjet printing works by propelling individual small ink drops toward a substrate and is capable of printing objects using two materials with quite distinctively different properties [5].

Milling(MI) manufacturing is superior in creating a smooth surface, while AM technologies overcome the limitations of subtractive methods, producing complex small shapes layer by layer directly from a computer model without limitations of the size of the smallest cutting tool [5, 10]. And thus hybrid manufacturing(HM) emerged, combining the advantages of the two techniques. Nakata et al. developed a one-process molding machine which integrated repeated laser sintering and MI into a single platform [11, 12]. Due to economic considerations, the indirect digital method consists of milling or printing wax/resin patterns that are then converted into cast-metal frameworks through conventional lost-wax technique (CLW) [13]. All these technologies mentioned above are collectively referred to as digital technologies in this review.

It should be a primary quality of CAD/CAM systems that they can produce accurate fitting prosthetic components [14]. The Aker’s clasp commonly used in RPDs is composed of three parts: clasp arm, counter arm and the rest. Rests affords efficient resistance to functional chewing forces, which are transmitted vertically to the abutment teeth and conducted along the long axes of the teeth. To avoid independent movement or slippage of RPDs under occlusal loading, the rests and the teeth must remain in stable contact. Considering this, it is important for the rests to not only be rigid but also fit accurately to the rest seats [15]. Stern et al. evaluated the adaptation between the occlusal rests and their corresponding rest seats in order to investigate the clinically acceptable in the fit accuracy of RPDs [16]. Fit accuracy of digitally fabricated RPD rests have been evaluated and described in several studies [11‐13, 17‐24], with inconsistent conclusions. In a study by Pelletier et al., frameworks made with SLS were less accurate at rest region than those produced with CLW [23], while Soltanzadeh et al. found that compared to 3D-printed groups, the cast RPD group showed better overall fit and accuracy [24]. Therefore, it is still unknown whether the digital technologies could provide acceptable fit accuracy for the rests in RPDs.

The purpose of this study is to systematically review in vitro and clinical studies comparing the fit accuracy in the rest region of RPDs fabricated by digital technologies and conventional lost-wax technique. The null hypothesis was that no differences would be found between CLW and digital technologies.

The results observed in this study suggested that there were no significant differences in the fit accuracy of the rests of RDPs fabricated by digital technologies and CLW, which means digital technologies can be a viable alternative for the manufacture of RPD frameworks. Subgroup analysis on different types of digital technologies showed that RPDs fabricated by CLW fit significantly better in rest region than those made by AM technologies. Similar result was also found between indirect digital technologies and CLW. However, MI fabricated RPDs presented a significant better fit accuracy in rest region than CLW RPDs. In regard to the effect of finishing and polishing procedure, RPDs before finishing and polishing presented nominally better but not statistically significant fit accuracy in rest region than those after finishing and polishing. With digital relief and heat treatment, HM clasps also presented significantly better fit accuracy in rest region than cast ones [12]. However, this evidence remains to be verified since the HM clasp data was compared to the cast Co-Cr clasp data from their previous study [11] rather than including an in-study control group.

Digital technology developed rapidly in dentistry. With the assistance of the computer, previously manual tasks are becoming faster and easier and the processing costs are reduced as well [5]. However, Takaichi et al. reported that the fitness of the SLM frameworks and clasps was no better than that of cast ones [32]. Moreover, Pordeus et al. reported a similar fit between CAD-CAM technology and the conventional technique [29]. The present results in this study are in agreement with these previous findings [29, 32]. Tan et al. reported that MI is an alternative method of CLW for fabricating titanium RPD clasps [33]. Several other studies have also proved that milled RPDs are comparable to or better than CLW RPDs and thus can be recommended for longer-term clinical use [13, 33, 34]. Results of subgroup analysis for MI vs CLW in this study provides further evidence for this. On the contrary, AM group showed significantly worse fit accuracy compared to CLW group in this review. This finding was corroborated by Pelletier et al. [23] who found that SLS frameworks exhibited significantly worse clinical accuracy as well as higher variability at the rest region than CLW frameworks. Arnold et al. also reported distinct fitting irregularities in the fit of RPDs fabricated with RP techniques [13].

In a previous study, Michael Braian found out that among the five AM units namely Arcam®, Concept laser®, EOS®, SLM Solutions® and EOS®(Co-Cr), the highest overall fabrication precision was achieved by EOS (CoCr) which was below 0.050 mm, close to that of SM system (Mikron®) [4]. While the other AM machines presented just acceptable precision (< 0.150 mm) on all axes except for the z-axis, which was even worse (> 0.5 mm) [4]. It can be inferred that different AM machines as well as different parameters can affect the fit accuracy of the end product [4], which may also be a possible explanation for the high heterogeneity in the pooled result and in AM subgroup. These results demonstrate that AM techniques should be further improved and standardized in RPD fabrication to make sure that every framework is produced with consistent accuracy [4]. Nonetheless, with higher fabrication speed and better accuracy, additive manufacturing will seriously compete with traditional manufacturing in creating good end-use products [1, 5].

Except for the 11 included studies, many other studies evaluated the fit accuracy of digitally fabricated RPDs [34‐39]. These studies, because they set no control group [35‐37] or performed indirect digital technologies as control groups [34, 38, 39] were excluded after screening. As far as indirect digital technologies are concerned, before conventional investment casting, digital model is obtained by scanning and computer-aided design is performed followed by printing or milling of wax or resin pattern [34, 38, 40]. Therefore, in this study, these indirect technologies were not included within the scope of conventional method and were taken as digital technology for RPD fabrication. A comparison between indirect digital technologies and CLW reflects the difference between digital scanning and conventional method of impression-taking and working cast fabrication, while a comparison between indirect digital technologies and fully digital workflow represents the processing tolerance produced from investment to finishing. However, what is really significant in clinical practice is a summation of the errors involved in all the steps from the scanning to the post-treatment process and also in all the stages of CLW from impression taking to finishing and polishing. For these reasons, a universal classification of RPD fabrication technologies is suggested, especially for indirect methods.

Sensitivity analyses showed that another possible cause of heterogeneity was the measurement method. Similar result was reported by Alabdullah et al., who compared the different approaches to evaluating the fit of RPD frameworks and concluded that the discrepancies in the gap distance values are likely to be caused by different registration methods [41]. There is no gold standard for assessing the fit accuracy of RPD. Quantitative methods include silicon film method and surface-matching, and the latter can be carried out whether by matching the surface from the master model and the master model with the silicone registration attached [22, 42] or superimposing the intaglio surfaces of RPD frameworks onto the STL file of the master model [24]. Besides, the direct optical observation was also used to analyze the fit accuracy by light microscopy [13, 43, 44]. Silicone film method is commonly performed by inserting silicone impression material between the RPD and intraoral dentition or master model under a retentive force that is maintained through the setting time. The the silicone replica of the gap may be sectioned afterwards and its thickness directly revealing the gap was measured with stereomicroscope, digital microscope, electronic calipers or profile projector [18, 35, 45, 46]. However, Yoon et al. reported that the number of measuring points have effect on the average thickness of the silicon replicas, and that the accuracy of silicone film method was not sufficiently reliable [47]. In contrast, three-dimensional surface-matching can be used to assess the fit accuracy of RPDs more comprehensively and effectively than silicone film method [47]. Consequently, for silicone film method, the adequate force applied during the setting time, the type of silicone replica, as well as the number and site of measuring points need to be clarified, which is necessary to insure the reliability and reproducibility of the outcomes of individual studies in the future.

In addition, up to now there is no consensus about the clinical acceptable gap distance of RPDs. Stern et al. reported that a gap of 0 to 50 μm was deemed to be close contact [16]. In a clinical study conducted by Dunham et al., the average gap distance between the rests and the rest seats was 193 ± 203 μm, ranging from 0 to 828 μm [48]. Li et al. fabricated 13 one-piece PEEK RPDs and the gap distance was 84.3 ± 23.6 µm in rest region [37]. Among the present 11 included studies, the mean average gap distances in rest region ranged from 30 μm to 365 μm in digitally fabricated RPDs, and 20 μm to 279.61 μm in CLW groups. Several studies compared the overall fit accuracy of RPD frameworks fabricated by digital and conventional technologies [19, 24, 35, 42], and some only evaluated the fit accuracy of clasps [11, 12, 34, 40, 43]. However, the low overall internal discrepancy value is not equivalent to better fit, as it is the compounded result of individual components. The RPD rests could be the ideal reference for the fit evaluation, which is easy for measurement and important for functional loading of the overall framework [45]. The fit accuracy of other RPD components, namely the connectors, the clasp arms as well as denture base should be further investigated for both digital and conventional fabrication technologies.

One of the factors that is most influential for fit accuracy is the finishing and polishing procedures on the tissue surface of the frameworks, especially for the rests [20]. In other words, to improve fit accuracy, finishing procedures in the laboratory should be well-controlled and excessive removing of metal from the intaglio surface should be avoided [16]. Most of the RPD samples included in this review were polished [13, 17, 18, 20‐23], except in 3 studies [11, 12, 24]. Different methods and extent of manual polishing are likely to affect the interpretation of the final results, while meta-analysis in this review showed no significant influence of finishing and polishing on the fit accuracy in rest region. Hence, further studies considering finishing and polishing procedure with a larger sample size are needed to validate this conclusion.