Fit accuracy in the rest region of RPDs fabricated by digital technologies and conventional lost-wax casting: a systematic review and meta-analysis
BMC Oral Health volume 23, Article number: 667 (2023)
Digital technologies have recently been introduced into the fabrication of removable partial dentures (RPDs). However, it is still unclear whether the digitally fabricated RPDs fit better than conventionally cast ones in the rest region. The aim of this study was to evaluate the fit accuracy in the rest region of RPDs fabricated by digital technologies and compare it to those made by conventional lost-wax (CLW) technique.
A comprehensive search was conducted in Cochrane Library, PubMed, EMbase, Web of Science and SpringerLink. Studies published up to August 2022 were collected. Two authors analyzed the studies independently and assessed the risk of bias on the modified methodological index for non-randomized studies (MINORS) scale. The mean values of gap distance between rests and corresponding rest seats of each study were extracted as outcome. A random-effects model at a significance level of P < 0.05 was used in the global comparison and subgroup analysis was carried out.
Overall, 11 articles out of 1214 complied with the inclusion criteria and were selected, including 2 randomized controlled trials (RCTs), 1 non-randomized clinical trial and 8 in vitro studies. Quantitative data from Meta-analysis revealed that fit accuracy in the rest region of RPDs fabricated with CLW showed no statistically significant difference with digital techniques (SMD = 0.33, 95%CI (-0.18, 0.83), P = 0.21). Subgroup analysis revealed a significantly better fit accuracy of CLW-fabricated RPDs in the rest region than either additive manufacturing (AM) groups or indirect groups (P = 0.03, P = 0.00), in which wax or resin patterns are milled or printed before conventional casting. While milled RPDs fit significantly better than cast ones in the rest region (P = 0.00). With digital relief and heat treatment, hybrid manufactured (HM) clasps obtained better fit accuracy in the rest region (P < 0.05). In addition, finishing and polishing procedure had no significant influence in the fit accuracy in all groups (P = 0.83).
RPDs fabricated by digital technologies exhibit comparable fit accuracy in rest region with those made by CLW. Digital technologies may be a promising alternative to CLW for the fabrication of RPDs and additional studies are recommended to provide stronger evidence.
In traditional dental practice, removable partial dentures (RPDs) are fabricated by lost-wax casting . 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 . In 1970s, computer-aided design/computer-aided manufacturing (CAD/CAM) was applied into dentistry by Duret and Preston . 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 . When processing polymers and ceramic the industry generally refers to this as SLS whereas for metals the terms used are SLM or DMLS . 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 .
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) . 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 . 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 . 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 . Fit accuracy of digitally fabricated RPD rests have been evaluated and described in several studies [11,12,13, 17,18,19,20,21,22,23,24], 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 , while Soltanzadeh et al. found that compared to 3D-printed groups, the cast RPD group showed better overall fit and accuracy . 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.
This systematic review was registered in the International Prospective Register of Systematic Reviews (PROSPERO: CRD42020201313). A systematic approach was followed according to the Preferred Reporting Items for Systematic Review and Meta-Analyses (PRISMA) Statement  and the Cochrane Handbook . The search strategy was based on the PICOS (Population, Intervention, Comparison, Outcomes, Study) format:
P (Population): Removable Partial Denture
I (Intervention): Digital technologies including AM (3D printing etc.), SM (MI etc.), indirect digital technologies (milling or printing of wax/resin patterns followed by CLW) and HM.
C (Comparator): Conventional lost-wax casting technology
O (Outcome): Fit accuracy in the rest region, which is represented by the gap distances between the rest seats and the intaglio surfaces of the occlusal rests (μm)
S (Study): Clinical studies and in vitro studies
An electronic search was performed in Cochrane Library, PubMed, EMbase, Web of Science and SpringerLink on August 2nd, 2022, including articles published from January 1950 until August, 2022. No publication language restrictions were taken into account (Table 1). We used Medical Subject Headings (MeSH) terms and EMTREE, along with free-words to target the PICOS. In addition to the electronic search, relevant reviews and references lists of included full-text articles were manually checked as well.
Screening and selection criteria
Studies that reported outcome data for both digitally and conventionally fabricated RPDs or clasp samples were included. All related studies with an English abstract were included in this review. For control group, only RPDs fabricated by CLW were included. RPDs fabricated by digital technologies, including AM, SM, HM and indirect digital technologies were included as intervention group.
The main outcome for this review was fit accuracy in the rest region, which was defined as the gap distance in micrometers between the rests and their corresponding rest seats. Randomized controlled trials (RCT), non-randomized clinical studies and in vitro studies were included. Case reports, case series, expert opinions, commentaries, editorials, reviews, and conference abstracts were excluded. A study that was not accessible to read in full or was not available in the databases was also excluded (Table 2).
A reference manager software program (EndNote v.X9.3.1) was used and the duplicates were discarded electronically. The remaining articles derived from the extensive search were screened through title and abstract by two reviewers (JQ, DW) independently. The full-text was checked if title and abstract provided insufficient information with regards to the inclusion criteria. Finally, articles selected from the inclusion and exclusion criteria were further screened in full-text and double-checked by both reviewers (JQ, DW). Any disagreements at the above stages between reviewers were resolved by consulting a third reviewer (LS) and discussion until consensus was reached.
Two authors (JQ and DW) conducted the data extraction as well as risk of bias assessments independently, and any disagreements were resolved through consensus. The following information was extracted: 1) Author and year of publication; 2) Study design; 3) Groups; 4) Tooth die or model type; 5) Sample type; 6) Method used for evaluating the fit accuracy of the rest); 7) Sample size; 8) Main outcomes; 9) Scanning information; 10) CAD software; 11) Manufacturing machine; 12) Finishing and polishing. We contacted the corresponding authors of individual studies for missing data or additional study information. And those with no respondence after three contact attempts were excluded from meta-analysis and included in the qualitative aspect of this review. For studies that reported the gap distance values before and after polishing of the samples, the data after polishing was selected for the global meta-analysis. And for studies that evaluated vertical and horizontal distances between the RPD rests and rest seats, only the horizontal data (distances between the bottom of rests and rest seats on the occlusal surface of the tooth) was extracted.
The Cochrane Risk of Bias Assessment Tool for Randomized Controlled Trials  were used with the software RevMan version 5.3 (The Cochrane Collaboration, Copenhagen, Denmark) to assess the risk of bias for two included RCTs [17, 23]. And to assess the risk of bias of other included in vitro experiments [11,12,13, 19,20,21,22, 24] and a non-randomized clinical study , we developed a modified version of Methodological Index for Non-Randomized Studies (MINORS) scale based on the original one . A total of 13 items were included in the adapted scale, with an additional item proposed for clinical studies (Table 3). The items are scored 0 (not reported), 1 (reported but inadequate) or 2 (reported and adequate) . Discrepancies of opinion during the assessment were resolved through discussion until a consensus was finally reached between the 2 reviewers (JQ and DW). And finally the overall score was calculated. The ideal global score would be 24 for the in vitro studies and 26 for the clinical studies.
A software program StataMP17.0 was used for data processing and meta-analysis. The number of rests was considered as a statistical unit. Standardized mean difference (SMD) with 95% confidence interval (95% CI) was used to compare digital technologies and CLW fabricated RPDs on fit accuracy in the rest region. The Dersimonian-Laird method was used in the random effects model and the Inverse-variance method was used in the fixed effects model to account for differences between studies. Using Cochrane Q test and I2 test (25–50% slight, 50–75% moderate, and > 75% high heterogeneity), heterogeneity among the pooled studies was tested [28, 29]. The P < 0.05 was considered statistically significant. According to different digital technologies adopted by each experimental group (AM, MI and indirect digital technologies), the included studies were assigned to three subgroups and subgroup analysis was conducted to investigate possible causes of heterogeneity among the results. The final results were presented by forest maps. And to assess robustness of the synthesised results, sensitivity analyses were conducted by excluding the remaining articles into the literature one by one, conducting meta-analysis again and comparing them with the overall results before exclusion. Potential publication bias among studies included in the meta-analysis were assessed and presented by Funnel plots. In order to reduce the risk of bias in our reference list and avoid any risk of auto-citation read, the fi-index tool was used [30, 31]. For the research results that cannot be integrated, a comprehensive description and separate analysis was carried out.
Search and selection
The final electronic search identified 1214 database articles, 337 from Cochrane Library, 389 from PubMed, 214 from EMbase, 114 from Web of Science and 160 from SpringerLink. After removal of duplicates, 956 records remained, from which 905 were excluded through screening on the basis of titles and abstracts. And the remained 51 articles were read in full. Forty publications were further excluded as they did not meet inclusion criteria or lack of available data, leaving 11 as eligible studies for this systematic review. Details of the selection process are presented in Fig. 1.
Among the included 11 studies, one was a double-blind, crossover designed RCT . One was a triple-blinded RCT , and another a non-randomized clinical study . The remaining 8 were in vitro studies [11,12,13, 19,20,21,22, 24]. All of the studies were published in English. The earliest study was published in 2017  and the most recent was in 2022 [17, 21,22,23].
Nine studies had RPD frameworks as the unit of analysis, while two studies used Akers clasp assemblies [11, 12]. Regarding the fabrication materials, cobalt-chromium (Co-Cr) was most commonly used, while one study made one-piece RPDs from polyetheretherketone (PEEK)  and another one cast clasp samples of CP titanium Grade 3 .
The manufacturing techniques included SLM, SLS, DMLS, MI, indirect digital technologies and HM (Repeated laser sintering (RLS) and MI). 7 studies compared the fit accuracy of CLW fabricated RPDs with selective laser melted ones [13, 17, 18, 20,21,22, 24]. 2 studies evaluated fitness on the rest region of PEEK RPDs fabricated by milling as compared to CLW RPDs [13, 19]. And another two focused on the comparison between hybrid manufactured (RLS and MI) and conventionally cast Akers clasps [11, 12]. As regard to the indirect digital technologies, 3 studies printed resin models for investment and casting [21, 22, 24] while one printed wax patterns for CLW . And in 2 studies wax frameworks were milled and cast [13, 21].
All studies evaluated the horizontal gap between the rests and rest seats, while one addressed the gap distance from both horizontal and vertical dimensions . There’s no standard method for the quantitative measurement of fit accuracy of RPD rests. Among the included studies, 6 used the silicone film method and 2 applied silicone film method combined with digital superimposition to evaluate the fit accuracy [19, 22]. One study scanned the intaglio surface of each RPD framework and superimposed the STL file onto that of its master model  and another 1 made the measurement directly under a light microscopy at × 560 magnification . In addition, clinical observation including visual inspection and pressing test were also conducted by three studies [17,18,19]. Detailed information of individual studies is presented in Tables 4, 5 and 6.
Results of individual studies
Risk of bias in studies
As was shown in Fig. 2a, both RCTs [17, 23] detailed the generation method of the random sequence and carried out allocation and concealment. The outcome indicators were also reported, and appropriate models were used to process the missing data. The reason for the uncertainty of two bias risks was that Chia et al. did not explain whether to implement the blinding method for the test personnel, while Pelletier et al. did not provide sufficient information to explain whether to implement the blinding method for the outcome evaluators. Figure 2b details the analysis of risk of bias results.
As shown in Table 7, among the 9 non-randomized studies accessed by modified MINORS scale, 5 demonstrated low risk of bias, and 3 were classified as medium risk of bias, with only 1 presented high risk of bias . This was mainly caused by a lack of information about the number of measurement sites for each sample, and the outcome data of control group was retrieved from their previous study. All the studies clearly stated their aims and made quantitative evaluation of fit accuracy. An adequate control group was set in both clinical and in vitro studies. One item, viz. prospective calculation of the study size, was of high risk of bias, with none of the included studies reporting on this item (Fig. 3).
Results of syntheses
There are three studies with missing outcome data. We contacted the authors, but only one responded , leaving the two remaining studies to be excluded from the meta-analysis [11, 12]. In the global meta-analysis performed, the SMD was 0.33(95%CI: -0.18, 0.83, P = 0.21) in favor of CLW. But this difference did not show statistical significance (P > 0.05) and high statistical heterogeneity was found (τ2 = 0.99, I2 = 91.19%, H2 = 11.35, Random effects model) (Fig. 4).
The result of the sensitivity analysis is shown in Fig. 5, indicating that research done by Pelletier had the most effect on heterogeneity, and heterogeneity decreased after removing this study [I2 = 85.42%, SMD = 0.06, 95%CI (-0.34, 0.47), P = 0.76] (Fig. 6).
Subgroup analysis was conducted for different types of digital technologies. All studies were compared according to three groups (Table 8). A comparison of fit accuracy in the rest region between AM RPDs and CLW ones involving 9 groups from 8 studies was performed, which showed significant difference between AM and CLW (SMD = 0.83, 95%CI (0.10, 1.56), P = 0.03). Significant heterogeneity between analyses was identified (P = 0.00, I2 = 92.04%) in a random-effects model (Fig. 7).
While in subgroup (MI vs CLW), there was a statistically significant difference with a favorable trend in the MI technique (P = 0.00 < 0.05) (Fig. 8). SMD was -1.35 (95% CI: -1.76 to -0.93) and low heterogeneity was identified (P = 0.20; I2 = 38.73%, fixed effects model) (Fig. 8).
The subgroup of Indirect digital technologies vs CLW included 6 groups from 4 studies. Results in a fixed effects model indicated that CLW RPDs obtained a significant better fit accuracy in rest region than RPDs fabricated by indirect digital technologies (SMD = 0.51, 95%CI (0.23, 0.80), P = 0.00). Low heterogeneity between these analyses was identified (P = 0.09, I2 = 47.22%) (Fig. 9).
One study evaluated the fit accuracy of RPD rest in all groups before and after finishing and polishing . To evaluate the potential effect of finishing and polishing procedure to the fit accuracy of RPDs, an additional comparison was made and results were presented in Fig. 10. No significant differences between groups were observed (SMD = 0.09, 95%CI (-0.71, 0.89), P = 0.83 > 0.05) in a fixed effects model.
Hybrid manufacturing was used in two in vitro studies [11, 12] and compared with CLW. Both studies fabricated Aker’s clasp assemblies by the same one-process molding machine. And silicone film method was used to measure the gap distance between the clasp samples and the stainless-steel model. Since part of the outcome data was not provided in the form of Mean ± Standard Deviation and contact was not responded, they were excluded from meta-analysis. The outcome data were extracted from the histograms and box plots provided in the original articles with the assistance of a software program (GetData Graph Digitizer version 220.127.116.11). The results are presented in Table 9.
Nakata et al. reported that compared to cast clasps, the CAM clasps presented significantly greater gap distances (P < 0.05) . With digital relief and heat treatment, Torri et al. fabricated HM clasps with better fit accuracy in the rest region .
A funnel plot was constructed to assess publication bias. As shown in Fig. 11, the funnel plot was visually asymmetric, which indicates that potential publication bias may exist.
Certainty of evidence
As the Galbraith plot shows, most of the points which represent individual studies were within the range of the 95% CI regression line (Fig. 12a) except for two studies: Pelletier et al. 【SLS】and Arnold et al. 【MId】. After excluding these two, all of the remaining studies were within the range of the regression line (Fig. 12b), indicating that these two studies may have some impact on the overall effect.
This manuscript has been checked with the Fi-index tool and obtained a score of 0 for the first author only on the date 28/07/2023 according to SCOPUS® [30, 31]. The fi-index tool aims to ensure the quality of the reference list and limit any auto-citations.
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 . 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  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 . However, Takaichi et al. reported that the fitness of the SLM frameworks and clasps was no better than that of cast ones . Moreover, Pordeus et al. reported a similar fit between CAD-CAM technology and the conventional technique . 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 . 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.  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 .
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®) . 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) . It can be inferred that different AM machines as well as different parameters can affect the fit accuracy of the end product , 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 . 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 [34,35,36,37,38,39]. These studies, because they set no control group [35,36,37] 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 . 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 . 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 . In contrast, three-dimensional surface-matching can be used to assess the fit accuracy of RPDs more comprehensively and effectively than silicone film method . 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 . 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 . Li et al. fabricated 13 one-piece PEEK RPDs and the gap distance was 84.3 ± 23.6 µm in rest region . 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 . 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 . 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 . Most of the RPD samples included in this review were polished [13, 17, 18, 20,21,22,23], 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.
Only 11 studies were included in this meta-analysis after a comprehensive search in the main databases, indicating that the number of original research in relevant fields is still limited. No consensus has been reached for quality assessment of in vitro studies. The modified version of MINORS scale has not been validated in terms of index content as well as scoring, and the results of assessment can only be referenced conservatively. Moreover, since the findings of this review are mainly based on in vitro studies, caution must be exercised when applying the results into clinical practice.
RPDs fabricated by digital technologies exhibit comparable fit accuracy in rest region with those made by CLW.
A universal classification of RPD fabrication workflow is suggested especially for indirect digital methods.
3.Standardizing the measurement method and setting specific values of fit evaluation of RPDs are two important tasks at current research as well as clinical practice.
Availability of data and materials
All data generated and analysed during this study are included in this published article [and its supplementary information files].
Removable partial dentures
Conventional lost-wax technique
Modified methodological index for non-randomized studies
Randomized controlled trials
Standardized mean difference
Computer-aided design/computer-aided manufacturing
Computer Numerical Controlled
Selective laser melting
Selective laser sintering
Direct metal laser-sintering
Fused deposition modeling
Selective electron beam melting
Preferred Reporting Items for Systematic Review and Meta-Analyses
Population, Intervention, Comparison, Outcome and Study
Medical Subject Headings
Methodological Index for Non-Randomized Studies
Repeated laser sintering
Selective laser melting from stone model
Lost-wax technique from resin model
Indirect rapid prototyping
Direct rapid prototyping
Metal 3D printing
Resin printing and subsequent casting
Conventional casting of milled sacrificial patterns
Conventional casting of printed sacrificial patterns
Conventional lost-wax technique
Laverty DP, Thomas MBM, Clark P, Addy LD. The use of 3D metal printing (Direct Metal Laser Sintering) in removable prosthodontics. Dent Update. 2016;43(9):826–828, 831–822, 834–825.
Miyazaki T, Hotta Y, Kunii J, Kuriyama S, Tamaki Y. A review of dental CAD/CAM: current status and future perspectives from 20 years of experience. Dent Mater J. 2009;28(1):44–56.
Duret F, Preston JD. CAD/CAM imaging in dentistry. Curr Opin Dent. 1991;1(2):150–4.
Braian M, Jönsson D, Kevci M, Wennerberg A. Geometrical accuracy of metallic objects produced with additive or subtractive manufacturing: a comparative in vitro study. Dent Mater. 2018;34(7):978–93.
van Noort R. The future of dental devices is digital. Dent Mater. 2012;28(1):3–12.
Balhaddad AA, Garcia IM, Mokeem L, Alsahafi R, Majeed-Saidan A, Albagami HH, Khan AS, Ahmad S, Collares FM, Della Bona A, et al. Three-dimensional (3D) printing in dental practice: applications, areas of interest, and level of evidence. Clin Oral Investig. 2023;27(6):2465–81.
Della Bona A, Cantelli V, Britto VT, Collares KF, Stansbury JW. 3D printing restorative materials using a stereolithographic technique: a systematic review. Dent Mater. 2021;37(2):336–50.
Masood SH. 10.04 - Advances in fused deposition modeling. In: Comprehensive materials processing. edn. Edited by Hashmi S, Batalha GF, Van Tyne CJ, Yilbas B. Oxford: Elsevier; 2014. p. 69–91.
Ren X, Peng H, Li J, Liu H, Huang L, Yi X. Selective Electron Beam Melting (SEBM) of Pure Tungsten: Metallurgical Defects, Microstructure, Texture and Mechanical Properties. Materials (Basel). 2022;15(3):1172.
Torabi K, Farjood E, Hamedani S. Rapid prototyping technologies and their applications in prosthodontics, a review of literature. J Dent (Shiraz). 2015;16(1):1–9.
Nakata T, Shimpo H, Ohkubo C. Clasp fabrication using one-process molding by repeated laser sintering and high-speed milling. J Prosthodont Res. 2017;61(3):276–82.
Torii M, Nakata T, Takahashi K, Kawamura N, Shimpo H, Ohkubo C. Fitness and retentive force of cobalt-chromium alloy clasps fabricated with repeated laser sintering and milling. J Prosthodont Res. 2018;62(3):342–6.
Arnold C, Hey J, Schweyen R, Setz JM. Accuracy of CAD-CAM-fabricated removable partial dentures. J Prosthet Dent. 2018;119(4):586–92.
Boitelle P, Mawussi B, Tapie L, Fromentin O. A systematic review of CAD/CAM fit restoration evaluations. J Oral Rehabil. 2014;41(11):853–74.
Carr AB, Brown DT. McCracken's Removable Partial Prosthodontics. 13th ed. Saint Louis: Mosby; 2015.
Stern MA, Brudvik JS, Frank RP. Clinical evaluation of removable partial denture rest seat adaptation. J Prosthet Dent. 1985;53(5):658–62.
Chia VAP, See Toh YL, Quek HC, Pokharkar Y, Yap AU, Yu N. Comparative clinical evaluation of removable partial denture frameworks fabricated traditionally or with selective laser melting: a randomized controlled trial. J Prosthet Dent. 2022;S0022-3913(22)00011–7.
Ye HQ, Ning J, Li M, Niu L, Yang J, Sun YC, Zhou YS. Preliminary clinical application of removable partial denture frameworks fabricated using computer-aided design and rapid prototyping techniques. Int J Prosthodont. 2017;30(4):348–53.
Ye HQ, Li XX, Wang GB, Kang J, Liu YS, Sun YC, Zhou YS. A novel computer-aided design/computer-assisted manufacture method for one-piece removable partial denture and evaluation of fit. Int J Prosthodont. 2018;31(2):149–51.
Bajunaid SO, Altwaim B, Alhassan M, Alammari R. The fit accuracy of removable partial denture metal frameworks using conventional and 3D printed techniques: an in vitro study. J Contemp Dent Pract. 2019;20(4):476–81.
Muehlemann E, Özcan M. Accuracy of removable partial denture frameworks fabricated using conventional and digital technologies. Eur J Prosthodont Restor Dent. 2022;30(2):76–86.
Oh KC, Yun BS, Kim JH. Accuracy of metal 3D printed frameworks for removable partial dentures evaluated by digital superimposition. Dent Mater. 2022;38(2):309–17.
Pelletier S, Pelletier A, Al Dika G. Adaptation of removable partial denture rest seats in prostheses made with selective laser sintering or casting techniques: a randomized clinical trial. J Prosthet Dent. 2022;S0022-3913(22)00327–4.
Soltanzadeh P, Suprono MS, Kattadiyil MT, Goodacre C, Gregorius W. An in vitro investigation of accuracy and fit of conventional and CAD/CAM removable partial denture frameworks. J Prosthodontics-Implant Esthetic Reconstruct Dent. 2019;28(5):547–55.
Page MJ, McKenzie JE, Bossuyt PM, Boutron I, Hoffmann TC, Mulrow CD, Shamseer L, Tetzlaff JM, Akl EA, Brennan SE, et al. The PRISMA 2020 statement: an updated guideline for reporting systematic reviews. BMJ. 2021;372: n71.
Higgins JP, Altman DG, Gøtzsche PC, Jüni P, Moher D, Oxman AD, Savovic J, Schulz KF, Weeks L, Sterne JA. The Cochrane Collaboration’s tool for assessing risk of bias in randomised trials. BMJ. 2011;343: d5928.
Mai HY, Mai HN, Kim HJ, Lee J, Lee DH. Accuracy of removable partial denture metal frameworks fabricated by computer-aided design/ computer-aided manufacturing method: a systematic review and meta-analysis. J Evid Based Dent Pract. 2022;22(3): 101681.
Higgins JP, Thompson SG. Quantifying heterogeneity in a meta-analysis. Stat Med. 2002;21(11):1539–58.
Pordeus MD, Santiago Junior JF, Venante HS, Bringel da Costa RM, Chappuis Chocano AP, Porto VC. Computer-aided technology for fabricating removable partial denture frameworks: a systematic review and meta-analysis. J Prosthet Dent. 2022;128(3):331–40.
Fiorillo L. Fi-Index: a new method to evaluate authors hirsch-index reliability. Publ Res Q. 2022;38(3):465–74.
Fiorillo L, Cicciù M. The use of Fi-Index tool to assess per-manuscript self-citations. Publ Res Q. 2022;38(4):684–92.
Takaichi A, Fueki K, Murakami N, Ueno T, Inamochi Y, Wada J, Arai Y, Wakabayashi N. A systematic review of digital removable partial dentures. Part II: CAD/CAM framework, artificial teeth, and denture base. J Prosthodont Res. 2022;66(1):53–67.
Tan FB, Song JL, Wang C, Fan YB, Dai HW. Titanium clasp fabricated by selective laser melting, CNC milling, and conventional casting: a comparative in vitro study. J Prosthodont Res. 2019;63(1):58–65.
Maruo R, Shimpo H, Kimoto K, Hayakawa T, Miura H, Ohkubo C. Fitness accuracy and retentive forces of milled titanium clasp. Dent Mater J. 2022;41(3):414–20.
Lee JW, Park JM, Park EJ, Heo SJ, Koak JY, Kim SK. Accuracy of a digital removable partial denture fabricated by casting a rapid prototyped pattern: a clinical study. J Prosthet Dent. 2017;118(4):468–74.
Ni D, Dong Y, Peng JP, Xu Y, Yang MX, Dai YJ. Effect of different support angles on the fitness of removable partial denture framework fabricated using selective laser melting technique. Zhonghua Kou Qiang Yi Xue Za Zhi. 2020;55(3):165–70.
Li XX, Liu YS, Sun YC, Chen H, Ye HQ, Zhou YS. Evaluation of one-piece polyetheretherketone removable partial denture fabricated by computer-aided design and computer-aided manufacturing. Beijing Da Xue Xue Bao Yi Xue Ban. 2019;51(2):335–9.
Zhang M, Gan N, Qian H, Jiao T. Retentive force and fitness accuracy of cobalt-chrome alloy clasps for removable partial denture fabricated with SLM technique. J Prosthodont Res. 2022;66(3):459–65.
Negm EE, Aboutaleb FA, Alam-Eldein AM. Virtual evaluation of the accuracy of fit and trueness in maxillary Poly(etheretherketone) removable partial denture frameworks fabricated by direct and indirect CAD/CAM techniques. J Prosthodontics-Implant Esthetic Reconstruct Dent. 2019;28(7):804–10.
Takahashi K, Torii M, Nakata T, Kawamura N, Shimpo H, Ohkubo C. Fitness accuracy and retentive forces of additive manufactured titanium clasp. J Prosthodont Res. 2020;64(4):468–77.
Alabdullah SA, Hannam AG, Wyatt CC, McCullagh APG, Aleksejuniene J, Mostafa NZ. Comparison of digital and conventional methods of fit evaluation of partial removable dental prosthesis frameworks fabricated by selective laser melting. J Prosthet Dent. 2022;127(3):478.e471-478.e410.
Chen H, Li H, Zhao YJ, Zhang XY, Wang Y, Lyu PJ. Adaptation of removable partial denture frameworks fabricated by selective laser melting. J Prosthet Dent. 2019;122(3):316–24.
Yin X, Zhou H, Yan F, Wu X, Wu G, Pang D. Effects of 3 kinds of processing techniques on the fitness of metal clasp. Zhong Nan Da Xue Xue Bao Yi Xue Ban. 2021;46(10):1122–8.
Xie W, Zheng M, Wang J, Li X. The effect of build orientation on the microstructure and properties of selective laser melting Ti-6Al-4V for removable partial denture clasps. J Prosthet Dent. 2020;123(1):163–72.
Hsu YT. A technique for assessing the fit of a removable partial denture framework on the patient and on the definitive cast. J Prosthet Dent. 2016;116(4):630–1.
Gan N, Ruan YY, Sun J, Xiong YY, Jiao T. Comparison of adaptation between the major connectors fabricated from intraoral digital impressions and extraoral digital impressions. Sci Rep. 2018;8(1):529.
Yoon JM, Wang ZX, Chan CK, Sun YC, Liu YS, Ye HQ, Zhou YS. Evaluation of methods for fitness of removable partial denture. Beijing Da Xue Xue Bao Yi Xue Ban. 2021;53(2):406–12.
Dunham D, Brudvik JS, Morris WJ, Plummer KD, Cameron SM. A clinical investigation of the fit of removable partial dental prosthesis clasp assemblies. J Prosthet Dent. 2006;95(4):323–6.
The study was financially supported by North Sichuan Medical College Scientific Research and Development Projects [grant numbers CBY22-QDA11] and National Natural Science Foundation of China [grant numbers 81970958].
Ethics approval and consent to participate
Consent for publication
The authors declare no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
About this article
Cite this article
Qiu, J., Liu, W., Wu, D. et al. Fit accuracy in the rest region of RPDs fabricated by digital technologies and conventional lost-wax casting: a systematic review and meta-analysis. BMC Oral Health 23, 667 (2023). https://doi.org/10.1186/s12903-023-03348-6
- Removable partial denture
- Digital technology
- Conventional lost-wax casting
- Fit accuracy