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Journal of Prosthetic Dentistry

Fracture resistance of additive manufactured and milled implant-supported interim crowns

      Abstract

      Statement of problem

      Interim dental prostheses can be fabricated by using subtractive or additive manufacturing technologies. However, the fracture resistance of implant-supported interim crowns fabricated by using vat-polymerization additive manufacturing methods remains unclear.

      Purpose

      The purpose of this in vitro study was to evaluate the fracture resistance of anterior and posterior screw-retained implant-supported interim crowns fabricated by using subtractive and vat-polymerization direct light processing (DLP) additive manufacturing procedures.

      Material and methods

      An implant (Zinic Implant RP ∅4.0×10 mm) was placed in a 15×15-mm polymethylmethacrylate block. An implant abutment (ZiaCam, nonrotatory RP) was positioned on each implant. The virtual implant abutment standard tessellation language (STL) file provided by the manufacturer was imported into a software program (exocad v2.2 Valletta) to design 2 anatomic contour crowns, a maxillary right central incisor (anterior group) and a maxillary right premolar (posterior group). Each group was subdivided into 2 subgroups depending on the manufacturing method: milled (milled subgroup) and additive manufacturing (additive manufacturing subgroup). For the milled subgroup, an interim material (Vivodent CAD Multi) and a milling machine were used to fabricate all the specimens (N=40, n=10). For the additive manufacturing subgroup, a polymer interim material (SHERAprint-cb) and a DLP printer (SHERAprint 30) were used to manufacture all the specimens at a 50-μm layer thickness and 45-degree build orientation as per the manufacturer’s instructions. Then, each specimen was cemented to an implant abutment by using composite resin cement (Multilink Hybrid Abutment HO) as per the manufacturer’s instructions. A universal testing machine was used for fracture resistance analysis, and the failure mode was recorded. The Shapiro-Wilk test revealed that data were normally distributed. One-way ANOVA and Tukey multiple comparison were selected (α=.05).

      Results

      One-way ANOVA revealed significant differences among the groups (P<.05). The anterior milled subgroup obtained a significantly higher fracture resistance mean ±standard deviation value of 988.4 ±54.8 N compared with the anterior additive manufacturing subgroup of 636.5 ±277.1 N (P<.001), and the posterior milled subgroup obtained significantly higher mean ±standard deviation of 423.8 ±68 N than the additive manufacturing subgroup of 321.3 ±128.6 N (P=.048). All groups presented crown fracture without abutment fracture.

      Conclusions

      Manufacturing procedures and tooth type influenced the fracture resistance of screw-retained implant-supported interim crowns. Milled specimens obtained higher fracture resistance compared with the DLP additive manufacturing groups. The anterior group was higher than the posterior group.
      Clinical Implications
      Anterior implant-supported interim crowns can be manufactured either by using subtractive or additive manufacturing procedures; however, milling procedures produced stronger implant-supported interim crowns than the additively manufactured methods tested.
      Interim restorations are used to restore implants during and after osseointegration, providing esthetics, soft-tissue modeling, and restoring occlusion and function.
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      The effect of the angle of acuteness of additive manufactured models and the direction of printing on the dimensional fidelity: clinical implications.
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      Mechanics of shape distortion of DLP 3D printed structures during UV post-curing.
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      • Wicker R.B.
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      • Fabritius R.
      • Keßler A.
      • Hickel R.
      • Edelhoff D.
      • Stawarczyk B.
      Fracture load of 3D-printed fixed dental prostheses compared with milled and conventionally fabricated ones: the impact of resin material, build direction, post-curing, and artificial aging-an in vitro study.
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      • Unkovskiy A.
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      • Schille C.
      • Geis-Gerstorfer J.
      • Huettig F.
      • Spintzyk S.
      Objects build orientation, positioning, and curing influence dimensional accuracy and flexural properties of stereolithographically printed resin.
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      3D printed versus conventionally cured provisional crown and bridge dental materials.
      Studies have analyzed the mechanical properties, including the fracture resistance of interim protheses fabricated by using milling and AM methods
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      • Stawarczyk B.
      Fracture load of 3D-printed fixed dental prostheses compared with milled and conventionally fabricated ones: the impact of resin material, build direction, post-curing, and artificial aging-an in vitro study.
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      • Park G.S.
      • Kim S.K.
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      Effects of printing parameters on the fit of implant-supported 3D printing resin prosthetics.
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      ; however, studies on the fracture resistance of an AM implant-supported interim crown are lacking.
      The physiologic occlusal forces of natural dentition have been reported to range from 200 N to 900 N, with the maximum occlusal force in the anterior teeth ranging from 50 N to 223 N in women and from 190 N to 244 N in men.
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      and in patients with parafunctional habits such as bruxism, where nocturnal forces may be as much as 790 N, a mean of 53.1% higher than the maximum diurnal forces considered habitual.
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      Therefore, interim implant-supported restorations should be able to withstand such increased forces.
      Screw-retained implant-supported restorations enhance retrievability compared with cement-retained restorations, which facilitates the provision of interim restorations.
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      Perspectives on strength.
      Furthermore, by using intraoral scanning devices, an implant-supported interim crown can be manufactured with a completely digital workflow.
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      The combination implant crown: a cement- and screw-retained restoration.
      The purpose of this in vitro study was to compare the fracture resistance of screw-retained implant-supported interim anterior and posterior crowns manufactured by subtractive and DLP AM methods. The null hypotheses were that no significant difference would be found in fracture resistance between milled and AM screw-retained implant-supported interim crowns and that no significant differences would be found in fracture resistance between anterior and posterior screw-retained implant-supported interim crowns manufactured either by milling or DLP AM methods.

      Material and methods

      Forty internal hexagonal connection implants (Zinic Implant RP ∅4.0×10 mm; Ziacom) were placed in custom 15×15-mm polymethylmethacrylate blocks with the implant shoulders located at the same level of the acrylic resin material to simulate the clinical bone level placement. An implant abutment (ZiaCam, nonrotatory RP; Ziacom) of 5 mm in height and with a 0.5-mm shoulder was positioned on each implant and tightened to 30 Ncm with a torque wrench (Tork70; Ziacom) as per the manufacturer’s recommendations.
      The virtual standard implant abutment (ZiaCam, nonrotatory RP; Ziacom) standard tessellation language (STL) file was provided by the manufacturer and imported into a CAD dental software program (exocad v2.2 Valletta; exocad GmbH) to design 2 different anatomic contour crowns, a maxillary right central incisor (anterior group) crown and a maxillary right premolar (posterior group) crown (Fig. 1).
      Figure thumbnail gr1
      Figure 1Anatomic contour crown virtual design. A, Anterior group. B, Posterior group.
      In the anterior group, the virtual crown design included the ideal dimensions of a maxillary central incisor with a cervicoincisal dimension of 11 mm and a mesiodistal width of 9 mm. The screw-access hole was positioned in the center of the lingual surface without extending to the incisal edge of the crown. In the posterior group, the maxillary premolar crown had a cervico-occlusal dimension of 9 mm and a mesiodistal width of 7 mm. The screw-access hole was positioned in the center of the occlusal surface of the crown. In both groups, the virtual design of the crowns included a uniform die space of 50 μm, and the screw access holes had a standard diameter of 2.33 mm.
      When the virtual crown designs were completed, 2 STL files were exported (STLA and STLP files). Each group was further subdivided into 2 subgroups depending on the manufacturing method used to fabricate the specimens: milled (milled subgroup) and AM (AM subgroup) technologies (Table 1).
      Table 1Manufacturing procedures completed for specimens
      Group; SubgroupMaterialManufacturing MethodAnatomic Contour Design
      Anterior groupMilled subgroupPMMA-based interim dental material (Vivodent CAD Multi; Ivoclar Vivadent AG)5-axis milling machine (PrograMill CAM V4; Ivoclar Vivadent AG)Maxillary right central incisor crown
      AM subgroupPhotopolymer interim dental resin (SHERAprint-cb; Shera)Vat-polymerization DLP 3D printer (SHERAprint30; Shera)Maxillary right premolar crown
      Posterior groupMilled subgroupPMMA-based interim dental material (Vivodent CAD Multi; Ivoclar Vivadent AG)5-axis milling machine (PrograMill CAM V4; Ivoclar Vivadent AG)Maxillary right central incisor crown
      AM subgroupPhotopolymer interim dental resin (SHERAprint-cb; Shera)Vat-polymerization DLP 3D printer (SHERAprint30; Shera)Maxillary right premolar crown
      AM, additive manufacturing; DLP, direct light processing.
      For the milled subgroup, the STLA and STLP files were imported into the milling machine software program (PrograMill CAM V4; Ivoclar Vivadent AG) to define the milling strategy, as well as the positions and supports of the specimens on the interim dental material block. A total of 10 PMMA-based interim (Vivodent CAD Multi; Ivoclar Vivadent AG) specimens for each group (anterior and posterior groups) were manufactured by using a 5-axis milling machine (PrograMill CAM V4; Ivoclar Vivadent AG). Subsequently, the specimens were trimmed from their supports and polished with low-speed polishing burs and disks (Kit ref. 4409; Komet) marketed for finishing interim dental restorations (Fig. 2).
      Figure thumbnail gr2
      Figure 2A, Milled specimen of anterior group. B, Additive manufactured specimen of anterior group. C, Milled specimen of posterior group. B, Additive manufactured specimen of posterior group.
      For the AM subgroup, the STLA and STLP files were imported into the printer software program (Autodesk Netfabb Standard 2019.1; Autodesk) to slice the virtual designs and determine the printing parameters. A total of 10 virtual crown designs for each group (anterior and posterior groups) were positioned on the build platform by using the printer software program. A layer thickness of 50 μm and a 45-degree print orientation were determined. To standardize the manufacturing procedure, the printing parameters were adjusted as per the manufacturer’s instructions, and all the specimens were fabricated at the same time. A polymer interim dental material (SHERAprint-cb; Shera) and a vat-polymerization DLP printer (SHERAprint 30; Shera) were used to manufacture the specimens of the AM subgroup. After printing, all the specimens were washed in an ultrasonic bath (Biosonic UC300; Coltène) of a 98% isopropyl alcohol solution (Shera Ultra-P; Shera), followed by a second bath in a clean isopropyl alcohol 98% solution. Subsequently, the specimens were placed inside a UV polymerizing unit (Shera Flash-Light Plus; Shera) for 25 minutes at 220 W for final polymerization as per the manufacturer’s instructions. Then, the support material was trimmed, and the specimens were polished with the same protocols as for the milled subgroup (Fig. 2B, 2D).
      A total of 40 screw-retained implant-supported interim crowns were fabricated (n=10). Each specimen was randomly assigned (by using a shuffled deck of cards) to a standard implant abutment-implant system prepared at the start of the experiment. Each specimen was evaluated for good adjustment to the implant abutment by using a profile projector with ×4 magnification (Xenoplan; Schneider). Successively, each specimen was cemented to the corresponding implant abutment (ZiaCam, nonrotatory RP; Ziacom) with an autopolymerizing composite resin cement (Multilink Hybrid Abutment HO; Ivoclar Vivadent AG) as per the manufacturer’s instructions. Previously, the implant abutment had been airborne-particle abraded at 0.2 MPa air pressure (Basic Quatro IS Renfert Neumatic Sandblaster; Renfert) with 50-μm aluminum oxide particles for 10 seconds at a 10-cm distance and cleaned with a steam jet cleaner (Steam Cleaner Model i702C Ea; Zhan Henry Schein). After the cementation procedures, the specimens were stored at 37 °C for 24 hours.
      To simulate aging, the specimens were subjected to a thermocycling process (Thermocycler THE-1100; D-Sat) by following the International Organization for Standardization (ISO) 10477 standard: 52 5000 cycles (5 °C to 55 °C, 5 seconds for transfer and 30 seconds for dwell time) were run automatically.
      International Organization for Standardization
      ISO 10477:2018. Dentistry. Polymer-based crown and veneering materials.
      All specimens completed the thermocycling procedures without observable cracks or debonding from the implant abutment.
      A universal testing machine (BT1-FR2.5TS.D14; Zwick Roell) was used for the fracture resistance analysis. Force was applied with a 3-mm-diameter round tip at a specific point and load angle for each sample group: on the lingual face at 30 degrees from the implant axis in the anterior group and perpendicular to the implant axis in the posterior group (Fig. 3). A 0.2-mm sheet of tinfoil was placed between the specimens and the punch.
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      • Raab P.
      • Hahnel S.
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      • Preis V.
      In-vitro performance of CAD/CAM-fabricated implant-supported temporary crowns.
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      • Albrecht T.
      • Kirsten A.
      • Kappert H.F.
      • Fischer H.
      Fracture load of different crown systems on zirconia implant abutments.
      The load was applied at a speed of 1 mm/min until fracture occurred. The fracture loads were automatically registered in newtons (N) by the software program (TestXpert 2.1; Zwick Roell). After testing, the specimens were photographed for fracture mode and location analysis with classifications as follows: crown fracture without abutment fracture, crown fracture with abutment fracture, fracture of both crown and abutment, and screw fracture.
      Figure thumbnail gr3
      Figure 3A, Anterior specimen under fracture load testing by using universal testing machine with load applied test application of force at angle of 30 degrees to implant axis. B, Posterior specimen with load force applied to central fossa of occlusal surface at angle of 90 degrees to implant axis.
      A Shapiro-Wilk test revealed that the data were normally distributed. One-way ANOVA and Tukey multiple comparison tests were selected to analyze the data (α=.05) by using a statistical software program (Matlab R2019b; MathWorks). An analysis of power for the 4 groups (n=10) ANOVA (F-statistics) was performed (G∗power v.3.1; Universität Düsseldorf) for a normalized size of the effect of 0.5 (Cohen effect size), a power of 0.8, and 0.1 significance level.

      Results

      For the anterior group, the milled subgroup had a fracture resistance mean ±standard deviation of 988.4 ±54.8 N and the AM subgroup had 636.5 ±277.1 N. For the posterior group, the milled subgroup obtained a fracture resistance mean ±standard deviation of 423.8 ±68.0 N and the AM subgroup obtained 321.3 ±128.6 N (Table 2).
      Table 2Fracture resistance values (N) for studied groups
      Tooth TypeSubgroupMeanSDDifferenceCI (95%)P
      Anterior groupMilled988.454.8351.9164.2-539.6<.001
      Significant difference (P<.05) using 1-way ANOVA and Tukey multiple comparison tests.
      AM636.5277.1
      Posterior groupMilled423.868.0102.55.8-199.2.048
      Significant difference (P<.05) using 1-way ANOVA and Tukey multiple comparison tests.
      AM321.3
      ProcessingTooth typeMeanSDDifferenceCI (95%)P
      AMAnterior group636.5277.1315.2112.2-518.2<.001
      Significant difference (P<.05) using 1-way ANOVA and Tukey multiple comparison tests.
      Posterior group321.3128.6
      MilledAnterior group988.454.8564.6506.6-622.6<.001
      Significant difference (P<.05) using 1-way ANOVA and Tukey multiple comparison tests.
      Posterior group423.868.0
      AM, additive manufacturing; CI, confidence interval; SD, standard deviation.
      Significant difference (P<.05) using 1-way ANOVA and Tukey multiple comparison tests.
      One-way ANOVA test revealed significant mean value differences for fracture resistance among the groups (P<.05). The Tukey multiple comparison test showed that the anterior milled subgroup had a significantly higher fracture resistance mean value compared with the anterior AM subgroup (P<.001) and that the posterior milled subgroup had a significantly higher mean value compared with the AM subgroup (P=.048). The Tukey multiple comparison test showed that the anterior AM subgroup presented a significantly higher fracture resistance mean value than the posterior AM subgroup (P<.001) and that the anterior milled subgroup obtained a significantly higher mean value than the posterior milled subgroup (P<.001) (Fig. 4).
      Figure thumbnail gr4
      Figure 4Box plot of fracture loads computed among groups tested. AM, Additive manufacturing.
      In both the anterior and posterior groups, all specimens had a crown fracture without abutment fracture. Furthermore, in all specimens, the screw or implant abutment were not fractured. The failure mode in the anterior group consisted of multiple fractures, splitting the crown into fragments (Fig. 5A). However, in the posterior group, the predominant failure pattern was a single longitudinal fracture from the central fossa, splitting the crown into 2 pieces (Fig. 5B).
      Figure thumbnail gr5
      Figure 5Failure mode that included fracture of interim crown with integrity of screw and implant abutment. A, Anterior specimen. B, Posterior specimen.

      Discussion

      Based on the results obtained in the present investigation, both null hypotheses were rejected, as significant differences in fracture resistance were found between the milled and AM specimens and for both manufacturing methods. Significant differences in fracture resistance were also found between the anterior and posterior groups.
      In the present study, the manufacturing procedures tested influenced the fracture resistance of screw-retained implant-supported interim crowns. The subtractive technique demonstrated significantly higher mean values for fracture resistance than the vat-polymerization AM method tested. The authors are unaware of a previous study evaluating the fracture resistance of AM implant-supported interim crowns; therefore, comparisons with previous data were not feasible.
      Previous studies have demonstrated the influence of build orientation on the mechanical properties of an AM device.
      • Alharbi N.
      • Osman R.
      • Wismeijer D.
      Effects of build direction on the mechanical properties of 3D-printed complete coverage interim dental restorations.
      Vertically printed specimens with the layers oriented perpendicular to the load direction have shown improved mechanical properties compared with those of horizontally printed specimens with the layers oriented parallel to the load direction. In the present study, all the specimens were manufactured at a 45-degree build orientation as per the manufacturer’s recommendations. Further studies are needed to evaluate the mechanical properties, including fracture resistance, different printing parameters, and postprocessing procedures.
      The anterior milled specimens had a fracture resistance 351.9 N higher than that of the anterior AM crowns, and the posterior group had lower fracture resistance differences between both manufacturing methods than for the anterior group. Similarly, the posterior milled specimens showed a fracture load 102.5 N higher than that of the posterior AM crowns. Establishing whether such fracture resistance differences are clinically significant is complex. Nevertheless, except for the AM posterior group, mean fracture load values were in the physiological clinical range.
      • Kiliaridis S.
      • Kjellberg H.
      • Wenneberg B.
      • Engström C.
      The relationship between maximal bite force, bite force endurance, and facial morphology during growth. A cross-sectional study.
      • Mao J.
      • Osborn J.W.
      Direction of a bite force determines the pattern of activity in jaw-closing muscles.
      • Dean J.S.
      • Throckmorton G.S.
      • Ellis 3rd, E.
      • Sinn D.P.
      A preliminary study of maximum voluntary bite force and jaw muscle efficiency in pre-orthognathic surgery patients.
      • Bakke M.
      • Holm B.
      • Jensen B.L.
      • Michler L.
      • Möller E.
      Unilateral, isometric bite force in 8-68-year-old women and men related to occlusal factors.
      • Varga S.
      • Spalj S.
      • Lapter Varga M.
      • Anic Milosevic S.
      • Mestrovic S.
      • Slaj M.
      Maximum voluntary molar bite force in subjects with normal occlusion.
      • Ferrario V.F.
      • Sforza C.
      • Zanotti G.
      • Tartaglia G.M.
      Maximal bite forces in healthy young adults as predicted by surface electromyography.
      • Braun S.
      • Bantleon H.P.
      • Hnat W.P.
      • Freudenthaler J.W.
      • Marcotte M.R.
      • Johnson B.E.
      A study of bite force, part 1: relationship to various physical characteristics.
      • Van Eijden T.M.
      Three-dimensional analyses of human bite-force magnitude and moment.
      • Trulsson M.
      • Gunne H.S.
      Food-holding and -biting behavior in human subjects lacking periodontal receptors.
      • Fontijn-Tekamp F.A.
      • Slagter A.P.
      • Van Der Bilt A.
      • Van ’T Hof M.A.
      • Witter D.J.
      • Kalk W.
      • et al.
      Biting and chewing in overdentures, full dentures, and natural dentitions.
      Therefore, the AM implant-supported interim crowns for the anterior region might be expected to withstand physiological forces, but AM crowns in the posterior region could pose a higher risk of fracture. These results can be partly explained by the area and direction in which force was applied. In the present study, a different direction of the loading force was applied to replicate the clinical setting. The distance between the loading area and the underlying metal of the implant abutment might differ among the groups, and the use of tinfoil might have created variations on the loading area among the specimens. However, studies that evaluated the influence of the direction of force and the thickness of the material on the fracture resistance of interim implant-supported fixed dental prostheses are lacking. More studies evaluating this variable would therefore be advisable.
      Differences in the fracture resistance of screw- and cement-retained implant-supported crowns have been reported.
      • Kim W.D.
      • Jacobson Z.
      • Nathanson D.
      In vitro stress analyses of dental implants supporting screw-retained and cement- retained prostheses.
      • Pietrabissa R.
      • Gionso L.
      • Quaglini V.
      • Di Martino E.
      • Simion M.
      An in vitro study on compensation of mismatch of screw versus cement-retained implant supported fixed prostheses.
      • Kelly J.R.
      Perspectives on strength.
      • McGlumphy E.A.
      • Papazoglou E.
      • Riley R.L.
      The combination implant crown: a cement- and screw-retained restoration.
      In addition, different parameters have been identified as an influencing factor on the fracture resistance of dental crowns, for example, the mechanical properties of the selected restorative material, design and characteristics of the implant abutment interface, fracture load parameters, cement thickness, and cement type selected.
      • Zimmermann M.
      • Ender A.
      • Egli G.
      • Özcan M.
      • Mehl A.
      Fracture load of CAD/CAM-fabricated and 3D-printed composite crowns as a function of material thickness.
      ,
      • Rosentritt M.
      • Raab P.
      • Hahnel S.
      • Stöckle M.
      • Preis V.
      In-vitro performance of CAD/CAM-fabricated implant-supported temporary crowns.
      ,
      • Krejci I.
      • Reich T.
      • Lutz F.
      • Albertoni M.
      An in vitro test procedure for evaluating dental restoration systems. A computer-controlled mastication simulator.
      • Guess P.C.
      • Schultheis S.
      • Wolkewitz M.
      • Zhang Y.
      • Strub J.R.
      Influence of preparation design and ceramic thicknesses on fracture resistance and failure modes of premolar partial coverage restorations.
      • De Kok P.
      • Kleverlaan C.J.
      • De Jager N.
      • Kuijs R.
      • Feilzer A.J.
      Mechanical performance of implant-supported posterior crowns.
      • Keul C.
      • Müller-Hahl M.
      • Eichberger M.
      • Liebermann A.
      • Roos M.
      • Edelhoff D.
      • et al.
      Impact of different adhesives on work of adhesion between CAD/CAM polymers and resin composite cements.
      • Bähr N.
      • Keul C.
      • Edelhoff D.
      • Eichberger M.
      • Roos M.
      • Gernet W.
      • et al.
      Effect of different adhesives combined with two resin composite cements on shear bond strength to polymeric CAD/CAM materials.
      As the majority of those studies were performed on definitive restorative materials,
      • Rosentritt M.
      • Raab P.
      • Hahnel S.
      • Stöckle M.
      • Preis V.
      In-vitro performance of CAD/CAM-fabricated implant-supported temporary crowns.
      ,
      • Kim W.D.
      • Jacobson Z.
      • Nathanson D.
      In vitro stress analyses of dental implants supporting screw-retained and cement- retained prostheses.
      • Pietrabissa R.
      • Gionso L.
      • Quaglini V.
      • Di Martino E.
      • Simion M.
      An in vitro study on compensation of mismatch of screw versus cement-retained implant supported fixed prostheses.
      • Kelly J.R.
      Perspectives on strength.
      • McGlumphy E.A.
      • Papazoglou E.
      • Riley R.L.
      The combination implant crown: a cement- and screw-retained restoration.
      ,
      • Albrecht T.
      • Kirsten A.
      • Kappert H.F.
      • Fischer H.
      Fracture load of different crown systems on zirconia implant abutments.
      • Krejci I.
      • Reich T.
      • Lutz F.
      • Albertoni M.
      An in vitro test procedure for evaluating dental restoration systems. A computer-controlled mastication simulator.
      • Guess P.C.
      • Schultheis S.
      • Wolkewitz M.
      • Zhang Y.
      • Strub J.R.
      Influence of preparation design and ceramic thicknesses on fracture resistance and failure modes of premolar partial coverage restorations.
      • De Kok P.
      • Kleverlaan C.J.
      • De Jager N.
      • Kuijs R.
      • Feilzer A.J.
      Mechanical performance of implant-supported posterior crowns.
      • Keul C.
      • Müller-Hahl M.
      • Eichberger M.
      • Liebermann A.
      • Roos M.
      • Edelhoff D.
      • et al.
      Impact of different adhesives on work of adhesion between CAD/CAM polymers and resin composite cements.
      • Bähr N.
      • Keul C.
      • Edelhoff D.
      • Eichberger M.
      • Roos M.
      • Gernet W.
      • et al.
      Effect of different adhesives combined with two resin composite cements on shear bond strength to polymeric CAD/CAM materials.
      extrapolation of those results to both definitive and interim implant-supported restorations should be performed carefully. In the present study, only interim dental materials were tested, and the screw-access hole was designed with a standardized diameter. Moreover, the cement space provided in the digital restoration designs and the bonding technique was similar in the groups.
      Limitations of the present investigation included the in vitro design, which might not reflect clinical conditions, the limited number of interim dental materials and manufacturing procedures tested, the restricted printing parameters and postprocessing procedures evaluated in the AM group, and the constrained implant restoration design. Further in vitro and clinical trials are recommended to broaden the analysis of the mechanical properties and include biocompatibility, color stability, and the repairability of implant-supported interim restorations manufactured by using AM procedures.

      Conclusions

      Based on the findings of this in vitro investigation, the following conclusions were drawn:
      • 1.
        Manufacturing procedures and tooth type influenced the fracture resistance of screw-retained implant-supported interim crowns on internal-hexagonal connection implants.
      • 2.
        Screw-retained implant-supported interim crowns manufactured with subtractive methods presented higher fracture resistance than those manufactured with vat-polymerized DLP AM methods.
      • 3.
        The anterior group showed higher fracture resistance than the posterior specimens.

      Acknowledgments

      The authors thank Mr Lorenzo del Río for help and support with the design and manufacturing of the specimens.

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