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.
This study aimed to investigate the effects of long-term use on the retention force and wear behavior of double crown systems.
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.
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.
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.
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.
Materials and methods
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).
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.
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.
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).