In vitro investigations on retention force behavior of conventional and modern double crown systems Viktor Luft , Peter Pospiech , Axel Schurig and Marc Schmitter Dental Materials, 2021-01-01, Volume 37, Issue 1, Pages 191-200, Copyright © 2020 The Academy of Dental Materials Highlights Conventional and CAD/CAM-fabricated double crowns achieve sufficient retention under artificial aging. The “gold standard” of cast gold alloy provides the most constancy in retention force values over long-term use. CAD/CAM technology generally allows a more predictive setting of retention force and lower retention force losses. Confocal microscopy qualifies for three-dimensional visualizing of wear phenomena. Abstract Objective This study aimed to investigate the effects of long-term use on the retention force and wear behavior of double crown systems. Methods Based on a common double crown design sixty pairs of telescopic crowns were fabricated and divided into six groups, each consisting ten samples: “Gold standard” cast gold alloy primary and secondary crown (GG) and cast non-precious alloy (NN), computer-aided design (CAD)/computer-aided manufacturing (CAM)-milled zirconia primary crown and galvanoformed secondary crown (ZG), CAD/CAM-milled non-precious alloy primary and secondary crown (CC NN), CAD/CAM-milled zirconia primary crown and non-precious alloy secondary crown (CC ZN) and CAD/CAM-milled zirconia primary crown and polyetheretherketone (PEEK) secondary crown (CC ZP). In the constant presence of artificial saliva, all samples were subjected to 10,000 joining-separation cycles at a velocity of 120 mm/min. Wear was analyzed by reflected light microscopy and confocal microscopy before and after artificial aging. Results Retention force losses were observed in each group after long-term use, with significant losses in the groups ZG and CC ZP (p ZG = 0.01, p CC ZP = 0.049). During artificial aging, no significant differences in pull-off force were recorded for groups GG, NN and CC ZN. Regarding wear, merely the Y-TZP primary crowns of the CC ZP group displayed no surface changes. Significance All tested production methods and material combinations seem to be suitable for clinical practice. CAD/CAM technology allows similarly predictable results to be achieved as the gold standard. Confocal microscopy is recommended for surface examinations of double crowns. 1 Introduction Grave finds in Tuscany demonstrated that already in the 17th century people tried to replace missing teeth with natural and artificial materials by using gold bands equipped with human and animal teeth [ ]. For more than 100 years, double crown systems have been constantly in evolution since its first description by Starr in 1886 [ ]. In Germany [ ], Japan and Sweden [ ] double crown retained removable dentures are a well-proven rehabilitation method for reduced residual dentition because of patients’ satisfaction and long-term durability [ ]. Double crown systems consist of a primary crown (inner crown) and a secondary crown (outer crown). While the primary crown functions as a male part and is firmly cemented on the abutment tooth or implant, the secondary crown works as a female part for the retention of the removable denture. The principle is based on the accuracy of the fit of two form-fit crowns. Due to the inserting and removal of the dental restoration, the primary and secondary crown surfaces are exposed to forces that may lead to wear and thus to a loss of function or friction over time [ ]. The surface changes are marked by surface disruption, abrasion, adhesion and tribochemical reaction [ ]. The increasing digitalization of dental technology offers new possibilities for the fabrication of double crowns to avoid problems of the common casting technique [ , ]. In addition to the meanwhile established subtractive milling technique [ ], additive, material-saving manufacturing methods of rapid prototyping are becoming increasingly established, as well as the growing market for three-dimensional printing [ ]. Better fitting accuracy of the milling technique compared to the conventional lost-wax process and rapid prototyping for the production of conventional partial dentures was demonstrated recently [ ]. With further development of CAD/CAM systems in dentistry, the number of tooth-colored, biocompatible and high-performance materials such as ceramics and polymers has also increased. In numerous in vitro [ ] as well as in current in vivo studies [ , ] ceramics have proven to be a good alternative to dental metal alloys. Against the background of a society with an increasing demand for aesthetics as well as metal-free treatment options due to increasing material intolerances [ ], PEEK is also coming to the center of attention. The results obtained under laboratory conditions [ , , ] indicate a promising clinical application [ ]. The objective of the study was to compare the classical fabrication techniques particularly cast gold alloy double crowns with newer developed CAD/CAM-fabrication of double crowns and new material combinations ( Fig. 1 ). The main focus of the study was the investigation of the influence of the clinical use (insertion/removing of the denture), manufacturing method and material combination on the retention force and wear behavior. The working hypotheses were therefore that the clinical use phase within the individual test groups (1) and the manufacturing method and the material combination between the individual test groups (2) have no significant influence on the retention forces at the different measuring times of the artificial aging tests. Fig. 1 Test group overview. PC: primary crown, AT: abutment tooth, SC: secondary crown (interior view), TC: tertiary crown. 2 Materials and methods 2.1 Abutment tooth and primary crown design One maxillary premolar abutment tooth ( Fig. 2 ) was designed on the computer (FreeCAD, version 0.17). A total of sixty abutment teeth were fabricated from austenitic stainless steel (1.4404, X2CrNiMi17-12-2) using a universal CNC-lathe machine (TC 400, SPINNER, Sauerlach, Germany). Digital impressions were made tactically (DS 10, Renishaw, Pliezhausen, Germany) and optically (Tizian Smart-Scan, Schütz Dental, Rosbach, Germany) using the “hybrid scanning method” and then merged in CAD software (Tizian Creativ RT, Schütz Dental). Primary crown angles were individually set with 0°, wall thickness 0.7 mm, chamfer margin 1.2 mm and marginal thickness 0.25 mm. Double crown milling was carried out by a 5-axis CNC machine (Coritec 650i, imes-icore, Eiterfeld, Germany). Fig. 2 Steps of the manufacturing process. 2.2 Casting technique: primary and secondary crowns After milling twenty identical primary crowns in wax (Noritake Katana Wax Disc, Kuraray Noritake Dental Inc., Tokyo, Japan) two groups of ten were made and after investing ten were cast in precious alloy (Orplid TK, C. Hafner, Wimsheim, Germany) and ten in cobalt-chromium (CoCr) alloy (Starbond CoS, S&S Scheftner, Mainz, Germany). Primary crowns were adapted to the abutment teeth and ground in a surveyor (S3 Master, Schick, Schemmerhofen, Germany) with carbide cutters providing a taper of 0° (HF364KRNP-060/HF364KRF-060, NTI-Kahla, Kahla, Germany), milling oil and abrasive sand sleeves (Konator-Flex-System 0°, 240 μm/600 μm, DeguDent). Secondary crown fabrication was carried out by using the brush technique to create resin copings (Pi-ku-plast HP 36, bredent, Senden, Germany), which were provided occlusally with a fin in wax for later attachment to the universal testing device. The resin copings were invested and cast in gold (Orplid TK, C. Hafner) and CoCr alloy (Starbond CoS, S&S Scheftner). During the fitting, interfering contacts between the primary and secondary crowns on the inside of secondary crowns were marked and carefully removed with a carbide cutter (141 134, fine criss-cross cut, Horico — Hopf, Ringleb & Co., Berlin, Germany) and polished (Sherapol, Shera, Lemförde, Germany) until non-periodontal harmful friction below 10 N monitored with a standard spring scale was achieved. 2.3 CAD/CAM: primary and secondary crowns Further thirty primary crowns were milled from pre-sintered zirconia (Tizian-Blank Zirkondioxid, Schütz Dental) and ten primary crowns were milled from CoCr alloy (Quattro Disc NEM Soft, Goldquadrat, Hannover, Germany). Yttrium-stabilized tetragonal zirconium oxide (Y-TZP) primary crowns were sintered at a temperature of 1480 °C according to the manufacturer’s recommendations. After the sintering process, the accuracy of fit was checked and adjusted if required with a red ring (30 μm) diamond (FG 8801.314.012, Komet, Lemgo, Germany). The primary crowns of the groups CC ZN and CC ZP were fixed in a surveyor (F1, DeguDent) and ground with parallel diamond burs (364-023SF-FGXL/356-026UF-FGXL, NTI-Kahla) clamped in a dental water-cooled turbine. All Y-TZP crowns were finally polished with diamond polishing paste (Legabril Diamond, Cendres+Métaux, Biel, Switzerland; Zi-Polish, bredent). The surface of the CoCr primary crowns was conventionally post-processed with carbide cutters (H364RF.123.023, Komet) in the surveyor (F1, Degudent) and then polished. Similar to the primary crown fabrication, the “hybrid scanning method” was used to create a uniform secondary crown design. An occlusal fin as already described in the casting part was implemented in the CAD software equally. Subsequently, twenty non-precious alloy (Quattro Disc NEM Soft, Goldquadrat) and ten PEEK (BioHPP, bredent) secondary crowns were milled. Interference points could be removed by selective milling of the blanks still mounted in the milling machine. 2.4 Electroforming secondary crowns The electroforming process of the secondary crowns from the ZG group was conducted by C. Hafner (Wimsheim, Germany). A homogeneous silver-varnish layer was applied to the surface of the polished Y-TZP primary crowns using airbrush technique with 0,7 bar. In an automated electroplater (Helioform HF 700, C. Hafner) the desired layer of the gold was set at 0.25 mm thickness. To avoid plastic deformation of the galvanic copings during artificial aging, the electroformed secondary crowns were adhesively attached (AGC Cem “honey”, Wieland Dental, Pforzheim, Germany) to cast CoCr tertiary frameworks (Starbond CoS, S&S Scheftner). 2.5 Experimental set-up and surface examinations All primary crowns were cemented on the abutment teeth by a zinc phosphate cement (Harvard Cement, Harvard Dental, Hoppegarten, Germany) at least 24 h before starting the test cycles. A custom-made abutment tooth holding system, consisting of an M19 block in the middle of a 65 mm hollow cylinder with an 8 mm cylinder bore for abutment teeth insertion, also served as a container for the artificial saliva (Glandosane, Cell Pharm, Hannover, Germany). The continuous force measurement during 10,000 cycles of jointing and separation was carried out by using a universal testing device (Z2,5, Zwick/Roell, Ulm, Germany) at a velocity of 120 mm/min and a fitting force of 50 N. To prevent horizontal tension on the specimens, a self-aligning pad with lasered slotted screw was chosen as a connector between the secondary crown and crosshead bar ( Fig. 3 ). Fig. 3 Experimental set-up and scheme. Retention force results at t 0 (= baseline), t 1 (= 50 cycles), t 2 (= 5000 cycles) and t 3 (= 10,000 cycles) were imported into a statistical program (SPSS 25.0, SPSS Inc., Chicago, USA) and analyzed by a Kolmogorov–Smirnov, Friedman, Wilcoxon, Kruskal–Wallis and Mann–Whitney U-test. The level of significance was set at 5%. Reflected light microscopy at a magnification of 50–500 (Axio Lab. A1, Carl Zeiss, Oberkochen, Germany) on wear changes was carried out on the surface of all primary crowns before and after artificial aging. Profile and areal roughness parameters of one randomly chosen specimen from each group were measured before and after long-term use by confocal microscopy (NanoFocus, Oberhausen, Germany). 3 Results At baseline, the specimens in group ZG showed the highest mean retention force values in the amount of 8.3 N, whereas the lowest retention force values of 1.36 N were found in group ZP ( Fig. 4 ). Simultaneously, group CC ZP displayed significantly lower mean retention force values compared to all other test groups (CC ZP–CC ZN: p = 0.011; CC ZP–NN: p = 0.001; CC ZP–GG: p < 0.001; CC ZP– CC NN: p < 0.001; CC ZP–ZG: p < 0.001). Mann–Whitney-U test revealed comparable mean retention force values between the groups GG, NN and CC ZN (GG–NN: p = 0,247; GG–CC ZN: p = 0,436; NN–CC ZN: p = 0,853). Additionally, the retention force of group GG and CC ZN differed significantly from the groups ZG and CC NN (ZG–GG: p = 0.002; ZG–CC ZN: p < 0.001; CC NN–GG: p = 0.019; CC NN–CC ZN: p = 0.009). Significant differences between the cast and milled CoCr double crowns at baseline were not determinable (NN–CC NN: p = 0.105). Fig. 4 Changes in retention force values of all tested groups. After artificial aging, the loss of mean retention force values varied between 1.58 % (GG) and 36.17 % (NN) in the conventional and between 6.33 % (CC ZN) and 10.82 % (CC NN) in the CAD/CAM group. Exact Wilcoxon-Signed-Rank test reported a significant loss of retention load after long-term use in the specimens of the groups with the highest (ZG) and lowest mean retention force values (CC ZP) (ZG: p = 0.010; CC ZP: p = 0.049). At 10,000 cycles, significantly the lowest mean retention values were recorded for group ZP compared to all other test groups (CC ZP–CC ZN: p = 0.003; CC ZP–NN: p = 0.005; CC ZP–GG: p < 0.001; CC ZP–CC NN: p < 0.001; CC ZP–ZG: p < 0.001). As at baseline, the retention force of the groups GG, NN, CC ZN did not differ significantly after 10,000 joining-separation cycles (GG–NN: p = 0.089; GG–CC ZN: p = 0.089; NN–CC ZN: p = 0.853). Further, the double crowns of group GG showed no differences in retention load compared to group ZG (p = 0.105). In contrast, the retention force values of NN and CC NN differed significantly at the end of the test (p = 0.002). In general, smaller deviations in retention force values were observed in the milled groups compared to the conventional groups. Exclusively the GG group showed a lower standard deviation than the majority of the milled groups. Signs of wear were visible on almost every specimen in the contact area between primary and secondary crowns after long-term use. Exclusively, Y-TZP primary crowns of group CC ZP exhibited no wear apart from surface treatment ( Fig. 5 ). In group ZG, only small and flat linear abrasions marks could be detected on the Y-TZP primary crowns. The wear fields on the Y-TZP primary crowns of group CC ZN were limited to isolated areas and displayed linear, thin and superficial abrasion marks. In the group of dental alloys, CAD/CAM-fabricated CoCr and cast gold alloy double crowns exhibited planar wear fields with superficial scratches, while retention areas of the cast non-precious alloy double crowns were concentrated in individual zones with deep abrasion scuffs. Comparing fabrication methods of CoCr alloy, milled double crowns displayed better fittings than the cast group ( Fig. 6 ). Before and after long-term use, the lowest profile roughness and areal roughness values were shown by the CAD/CAM-produced CoCr primary crown surface. The Y-TZP primary crowns of the ZG and CC ZP group showed the highest roughness values. After 10,000 cycles, all R a and S a values increased excluding R a values of group ZG ( Fig. 7 ). Fig. 5 Reflected microscope images (50×)/(500×) and confocal profilometry images. Baseline/10,000 cycles. Fig. 6 Horizontal microsections of conventional fabricated and milled double crowns (50×). a: abutment tooth; b: primary crown; c: secondary crown; d: tertiary framework. Fig. 7 Comparison of profile roughness (R a ) in μm. Baseline/10,000 cycles. 4 Discussion The test parameters were carefully selected since the in vivo complexity of the periodontium cannot be imitated by any in vitro experimental set-up [ ]. The integration of tooth mobility caused by the periodontal apparatus into the experimental set-up has already been tried by several apparatuses and is not subject to ISO standards [ , , , ]. Lateral mobility, extra-axial loads and chewing pressure load change phases could not be reproduced in the test setup of the universal testing device compared to the chewing simulator [ , , ]. The advantage of the universal testing device was the continuous measurement of retention forces without inaccuracies that would be caused by the specimen being repositioned at each retention force measurement in a separate dynamometer after thermocycling and a certain number of cycles in a chewing simulator. The number of 10,000 cycles was described in the wear tests as a sufficiently long fatigue phase to imitate nearly 15 years of wear [ , , , ] since a constancy of the retention forces could be observed after 5000 cycles [ , ]. These aspects were also used to determine the measuring intervals of t 0 –t 3 . The initial wearing phase is characterized by significant fluctuations in retention force, especially within the first 2000 cycles [ , , , , ]. The most pronounced variations are seen in the first 50 cycles during the precise friction adjustment in the laboratory and on the patient [ ]. A crosshead speed of 120 mm/min already represented clinical conditions in several studies [ , , , ]. More important than the speed during removal was the positional stability of the denture under extruding forces caused by sticky food supply [ ]. Okhawa et al. and Rößler et al. found out in clinical studies that the speed of removal is over 1000 mm/min [ , ]. Rößler detected a constancy of the pull-off forces from 30 mm/min and a decrease in retention force above 200 mm/min. The selected pull-off speed represented a sufficient approximation to clinical maximum speeds based on normal masticatory behavior. As a representative component of the tribological system [ ], saliva-lubricated experiments were conducted to imitate physiological conditions. Bayer et al. considered the use of artificial saliva as an intermediate substance in pull-off tests to be an indispensable factor [ ]. Glandosane (Cell Pharm), serving as a saliva substitute in numerous studies [ , , , , , , ], was presented superior to the use of sodium chloride 0.9 % and tylose-sorbitol solution for pull-off tests [ ]. From literature research, the total retention force of a prosthesis results from the addition of the individual retention force values of the double crowns, whereby a retention force range between 1 and 10 N per single telescope crown was described to ensure sufficient retention of double crown attached prostheses [ , , ]. This value range was maintained by the mean retention force values of the tested groups before and after long-term use. In general, it must be borne in mind that according to Bayer et al. prostheses lose their functionality as soon as the total retention force falls below 2.5 N [ ] and the retention values of a restoration measured in vitro exceed the values obtained in vivo [ ]. Lost-wax technique with a precious metal alloy and a non-precious metal alloy and electroplating technique were representatives of the conventional production methods. The occurrence of retention force losses of cast double crowns made of precious and non-precious alloys after artificial aging was confirmed by numerous investigations [ , , , ]. Dillschneider et al. reported a comparable mean retention force loss of the cast gold group by approximately 10 % from 5.43 N to 4.92 N., whereas the cast CoCr group recorded a loss of approximately 40 % from 5.82 N to 3.59 N. Past investigations have already presented conflicting results regarding the retention force behavior using the electroforming technique for double crown fabrication [ , , ]. Due to the high standard deviation of the initial and final measurement results and the significant retention loss, the greater predictability of the retention force values, analogous to the findings of Arnold et al. [ ], cannot be confirmed for the material combination Y-TZP primary crowns and electroformed gold secondary crowns as in previous studies [ , , , , ]. It has to be considered, in this group no adjustment of friction before the test phase took place. Additionally, different application techniques (brush or spray technique) and conductive silver varnish thicknesses significantly influence the retention force behavior of double crown restorations during the fabrication of electroformed gold copings [ ]. Concerning the test results, the gold standard should be preferred when choosing a conventional production technique due to its low retention force losses and high predictability. Clinically, this initially leads to higher material costs, but in the long-term lower repair and maintenance costs, fewer dental appointments and higher patient satisfaction can be obtained accordingly. As seen in former investigations, CAD/CAM fabrication can be considered as an alternative to the conventional casting technique when dealing with CoCr alloys regarding the fracture behavior [ , ], the accuracy of fit [ ] and especially the higher retention force of double crowns [ ]. In the current study, CAD/CAM-milled CoCr-telescopes demonstrated higher retention force values with minimal, likewise insignificant retention force losses. The considerably lower standard deviation of the initial and terminal retention force in this group suggested more predictable results compared to the conventional casting technique. The material combinations Y-TZP/CoCr and Y-TZP/PEEK represented less expensive, aesthetic and completely CAD/CAM-fabricated alternative groups compared to the electroplating group. Turp et al. equally showed slight losses of retention force for the material combination of Y-TZP/CoCr in a previous study, but the non-precious metal alloy secondary crowns were cast conventionally in the test set-up of that time [ ]. Recent investigations have already proven the suitability of PEEK as a secondary crown material [ , ]. Merk et al. and Schubert et al. highlighted predictable results for the material combination Y-TZP/PEEK, especially for CAD/CAM processing. This finding correlates with the low and constant standard deviation during the artificial aging of the present study. Nevertheless, the double crowns of group CC ZP were characterized by a significant loss of mean retention force. In total, the CAD/CAM-fabricated double crowns convinced with predictable results based on the low deviation of retention forces within the individual test groups, whereby the group CC ZN showed the least loss of retention force and the greatest similarity in retention behavior to the gold standard. In contrast to the milled CoCr telescopes, the use of the material combination Y-TZP/PEEK owing to the minimal retention force of the individual double crowns is only conditionally recommendable when treating patients with a low number of residual abutment teeth. When regarding double crown systems, the advantage of the milling groups is the possibility of digital storage and modification of the data on the secondary crown parameters [ ]. Additionally to re-milling and replacing of secondary crowns of dentures, retention force can be set individually by variation of the gap size [ ]. Conventionally, increased friction can be achieved by friction varnish or electroforming methods in the case of insufficient retention [ ]. Fine adjustment of friction in casting double crowns presents the dental technician with great challenges in every patient case when fabricating unique double crowns [ , ]. Similarly to electroforming [ , ] due to a more standardized and reliable production procedure [ ] even less-experienced dental technicians might be able to fabricate well-working double crown-retained dentures using CAD/CAM-technology compared to casting technique. Additionally, the reduction of costs through time and manpower speaks for the production of digitally milled dentures compared to conventional analog laboratory production [ ]. Friction loss caused by wear is another typical problem in the double crown technique [ ]. Except for group CC ZP, signs of wear on the primary crowns were observed after the pull-off test in all test groups. In general, less surface wear was observed in the groups of heterologous material combinations on Y-TZP primary crowns as shown before [ ]. The higher Vickers hardness of the CoCr alloy of 350 HV (Quattro Disc NEM Soft, Goldquadrat) [ ] and the electroforming gold of 160–170 HV (Helioform H, C. Hafner) [ ] seems to be responsible for the presence of wear on the Y-TZP crowns with approximately 1200 HV (Tizian Blank Zirkondioxid, Schütz Dental) [ ] compared with low values of 30 HV of PEEK (BioHPP, bredent) [ ]. Since cold welding at room temperature mainly occurs only in homologous material combinations of dental alloys [ ], such surface deformations were more apparent in group GG, NN and CC NN. Regarding the different fabrication methods, a more accurate fit resulted in a lower retention force loss and fewer wear phenomena in the groups of CoCr alloy. CAD/CAM manufacturing and the use of a solid primary crown material such as Y-TZP can be postulated as advantageous due to the minimal surface changes in terms of longevity and the possibility of re-milling secondary crowns in case of friction loss. The determination of the surface roughness parameters by confocal microscopy was contactless in comparison to the commonly used tactically perthometer [ ] and delivered three-dimensional surface information additionally. In former investigations, fringe projection was an optical measuring method for three-dimensional surface analysis [ , ], but was reported as disadvantageous due to the reflection artifacts that occurred [ ]. Using confocal microscopy, measurement value deviations due to reflective scattered light were prevented by using a multi-pinhole filter. Although the R a values of Y-TZP exceeded the specific surface roughness values of representative dental materials by Dabrowa et al. (zirconia: 0.02 μm, CoCr: 0.35 μm, gold: 0.44 μm/0.51 μm) [ ], comparable values were obtained as in surface roughness investigations of zirconia processed by different polishing systems [ ]. Furthermore, it could be shown that grinding with a diamond bur under water-cooling (CC ZP: 0.443 μm) achieves approximately the same profile roughness as mere polishing of the primary crown surface with the goat hair brush after the sintering process (ZG: 0.454 μm). Dental materials with a surface roughness of more than 0.2 μm show a higher accumulation of plaque than smoother surfaces [ ], which locally provides corrosive processes and inflammation [ ]. In the recent study, the groups with a higher corrosion tendency as dental alloys showed lower surface roughness than the materials with a generally lower corrosion tendency as zirconia. Although confocal microscopy has proven to be an effective method for three-dimensional visualization of wear phenomena and determination of the surface roughness parameters, no quantitative statement can be postulated about the extent of roughness increase. For this purpose, more specimens per group should be evaluated. Second, the whole surface of the double crowns should be examined, since a prognosis of the occurrence of wear areas is not possible before testing. The retention of parallel telescopes is based on the sliding and static friction phenomena between the contacting areas of the primary and secondary crowns [ ]. An interference fit between the contact points exists, if the minimum clearance, meaning the minimum dimension of the outer crown and the maximum dimension of the inner crown, the waviness and the roughness depth of the corresponding double crown surfaces are approximately the same. The interference fit decreases with repeated separation and joining due to wear of the waviness [ ]. Former in vitro studies revealed, that conventional in lost-wax technique fabricated double crowns exhibit solely linear contacts between primary and secondary crowns [ , ]. In contrast, the retention mechanism in manufacturing processes with a high accuracy of fit such as electroforming is mainly caused by hydraulic effects, while friction and adhesion forces are of subordinate importance [ ]. Nonetheless, more extensive contact zones were present on the inside of the secondary crowns in electroforming as in the classic casting technique [ ]. In summary, the differences in retention force between the tested double crown systems resulted from the addition of the various gap dimensions, roughness values, material combinations and wear tendency since identical experimental conditions and specimen geometries were applied. To achieve a high long-term retention force in double crown fabrication, a precise and uniform fit, a low surface roughness and a wear-resistant material combination are mandatory. 5 Conclusion Within the limits of this study, it can be stated that the tested manufacturing methods and material combinations achieved sufficient retention forces, although there were considerable differences regarding the magnitude and predictability of the retention force between the individual test groups. In general, more predictable retention forces were found in the milled groups, however, these were in the range of the gold standard retention force deviation. The digital workflow seems to be suitable in clinical practice and represents a serious alternative to conventional manufacturing methods. However, the suitability of CAD/CAM-fabricated double crowns should also be proven in long-term clinical studies. Ethical approval This article does not contain any studies with human participants or animals performed by any of the authors. Acknowledgments The study was supported by material provided by C. Hafner (Wimsheim, Germany) and bredent (Senden, Germany). CAD/CAM-fabrication of double crowns was conducted by the dental laboratory Matthias Gürtler (Schwarzheide, Germany). 1 Both authors contributed equally to this paper. References 1. Minozzi S., Panetta D., De Sanctis M., Giuffra V.: A dental prosthesis from the early modern age in Tuscany (Italy). Clin Implant Dent Relat Res 2017; 19: pp. 365-371. 2. Starr W.R.: Removable bridge-work — porcelain cap-crowns. Dent Cosmos 1886; 28: pp. 17-19. 3. Hofmann M.: The telescopic total prosthesis (cover denture). Dent Labor (Munch) 1968; 16: pp. 589-593. 4. Böttger H.: Das Teleskopsystem in der zahnärztlichen Prothetik.1960.J.A. BarthLeipzig 5. Körber K.H.: Das rationelle Teleskopsystem. 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