Influence of CAD/CAM Abutment Heights on the Biomechanical Behavior of Zirconia Single Crowns

Abstract

The biomechanical behavior of the universal link (titanium base) prosthetic abutment with different heights in implant-supported restorations was evaluated. Forty regular implants (4 × 10 mm) in titanium were used, divided into two groups according to the abutment height (n = 20): 4.5 × 4 mm (short) and 4.5 × 5.5 mm (long). Using CAD/CAM technology, zirconia crowns were milled and cemented onto the prosthetic abutments. Half of the specimens were submitted to the initial maximum fracture load test in a universal testing machine. The long abutments presented fracture load (41.1 ± 6.96 kgf) statistically similar to the short abutments (49.5 ± 7.68 kgf). The other half of the specimens were submitted to mechanical cycling (2,000,000 cycles, 2 Hz with a stainless-steel antagonist with a diameter of 1.6 mm), following ISO 14801:2007. Subsequently, the survival of the specimens was evaluated using the survival analysis function, Kaplan–Meier and Mentel–Cox (log- rank) (p < 0.05). The finite element analysis was performed in similar conditions to those used for the in vitro test through computer-aided engineering software (version 19.2, ANSYS Inc., Houston, TX, USA). The biomechanical behavior of both models was similar regardless of the evaluated structure of the set. It was concluded that both short and long abutment presents promising fatigue behavior and stress distribution for use in long-term implant-supported restorations.

1. Introduction

Dental implants have been used for several decades and the oral rehabilitation of edentulous patients has been improved with their use. However, limitations continue to exist in the area of implant biomechanics and osseointegration maintenance, with regard to a reliable connection between the implant and restoration; this is essential for clinical success [1]. According to different prosthetic systems, the crown can be either directly retained on the implant platform or on the abutment with a prosthetic screw or cement [2]. The use of CAD/CAM technologies allows the manufacture of customized hybrid abutments composed of a metallic link and a ceramic structure, overcoming some aesthetic limitations of conventional metallic abutments due to the peri-implant tissues translucency [3,4].

In general, this metallic link (titanium base) is responsible for a great part of the union in the abutment-implant joint, while the ceramic mesostructure should be cemented to provide an aesthetic substrate for the crown improving the aesthetics of the peri-implant mucosa [5,6]. Another alternative to using the titanium base is milling the ceramic crowns through perforated CAD/CAM blocks, with the cementation of these monolithic crowns directly on the metallic link, characterizing the one-piece design concept [7,8,9]. Recent studies indicate that different mesostructure materials do not interfere with the survival of the restoration [10,11]. However, some considerations are still not clear in the literature regarding the most effective method for fixing these abutment designs, e.g., the type of cement, abutment tapering, surface treatment, and the abutment cementation area [12,13,14]. According to the literature, short axial walls and a high degree of occlusal convergence reduce the mechanical retention of restorative materials [15].

Based on this exposure, previous studies have investigated the influence of abutment height on the mechanical retention of cemented crowns [16]. However, there is a lack of data evaluating the influence of metallic link height on the mechanical behavior of these restorative systems before and after the fatigue cyclic [17]. It is also noteworthy that restorative materials fail under fatigue, especially with lower intensity loads applied for a long period [18,19,20]. In addition, the abutment design may influence the marginal bone loss; however, it is still unclear how this parameter affects the stresses in the peri-implant tissues [18,19].

It is important for clinicians to identify how occlusal force (normal or excessive) may impact implant success rates. Therefore, for a stable dental treatment, it is important to avoid overloading the dental implants and the bone, which can lead to biological and mechanical problems [20]. In summary, dental implants are exposed to inconstant loading during their life span due to chewing, which may lead to mechanical failure under fatigue [20,21]. These failures can occur in the crown, abutment, implant body, or prosthetic screw [21]. The literature reports that the intraoral cavity is a particularly tough environment for survival of biomaterials, due to the strong mechanical and chemical stresses to which these materials are exposed, and due to the bacterial activity [22]. Therefore, a careful selection of the abutment combined with a proper loading protocol is strongly recommended to reduce the effect of forces on the implant system [22]. Although abutment height may have an influence on superstructure retention, light scattering, and restoration translucency and aesthetics, limited data are available investigating its influence on fatigue [23].

Following the ISO 14801, previous studies compared the effect of diverse implant design, as well as restorative materials, implant dimensions, implant–abutment connection, machining processes, surface finishes, loading, and tightening conditions. Additionally, it is possible to reproduce the in vitro tests with finite element models, aiming to correlate the failure recorded during the experimental tests with the stress result under maximum load [24,25]. Therefore, to elucidate the mechanical performance of CAD/CAM abutments with different heights and their implications on peri-implant tissues, this study aimed to evaluate the influence of CAD/CAM abutments on the maximum fracture load, survival under fatigue, and the stress distribution of implant-supported crowns with in vitro and in silico methods. The null hypotheses were: (1) there would be no difference in the stress concentration with the application of a compressive load between both models; (2) the maximum fracture load and fatigue survival would be similar between both groups.

2. Materials and Methods

2.1. Specimen Preparation

Forty titanium implants (B-fix Profile 4.0 × 10 mm, Titaniumfix, São José dos Campos, SP, Brazil), were randomly divided into 2 groups according to the abutment height (4 and 5.5 mm, n = 20) (Figure 1). For the substrate simulation, polyurethane cylinders (F160, Axson Technologies, Saint-Ouen-I`Aumône, France) with a uniform elastic modulus of 3.6 GPa were created inside a 3/4-inch-diameter polyvinylchloride (PVC) tubular section [26]. According to the literature, polyurethane resin is a valid isotropic substrate to simulate bone tissue, in mechanical studies with dental implants, widely applied in in-vitro reports [5,10,24,26,27,28,29,30].

For the substrate preparation, the polyurethane resin was manipulated with equal measures of a base and catalyst until it reached complete homogenization. The resin was poured into the PVC tubes under 45 lbs of pressure in a vacuum pressurizer (Protecni, Araraquara, SP, Brazil) to avoid incorporation of bubbles. After the resin polymerization, the surfaces were smoothed with SiC sandpaper (#220, #320, #400, and #600) under constant irrigation using an automatic polisher (Ecomet/Automet 250, Buehler, IL, USA) to remove the surface irregularities. A surgical kit (Profile, Titaniumfix, São José dos Campos, SP, Brazil) was used to prepare the perforations in each block. All implants were placed according to the manufacturer’s recommendations, with a torque of 35 N.cm, measured with a manual torque wrench (BTG60CN-S model; Tohnichi, Tokyo, Japan). Each implant was placed maintaining 3 mm of the threads exposed above the resin surface, as per ISO for dental implant fatigue (ISO 14801:2016) [31] (Figure 2).

To standardize the prosthetic restorations, both groups received a maxillary central incisor fully stabilized zirconia crown manufactured with the same anatomical shape. For that, CAD/CAM was used to design and mill the restorations (Prettau® 4 Anterior®, Zirkonzahn Worldwide, Gais, Italy). After the sintering cycle, the try-in of the restorations on the abutments was carried out, observing the sitting. All crowns were inspected and approved and then cemented onto the abutments using resin cement. The cementing process is summarized in Figure 3.

The monolithic restorations were then polished and cleaned. The metallic links (Universal Abutment, Titaniumfix, São José dos Campos, SP, Brazil) were sandblasted with 50 μm aluminum oxide particles (Al₂O₃) at a pressure of 1.5 bar with positioning at 90° using an implant analog to assist in laboratory work. Then, all parts were cleaned in an ultrasonic bath for 5 min with isopropyl alcohol and the surface of the metallic link was dried with an oil-free air jet, followed by a metal primer (Alloy Primer, Kuraray Noritake Dental Inc., Okayama, Japan) for 60 s. The screw access holes were protected by a Teflon tape to prevent leakage of the luting agent.

The application of resin cement (Panavia 2.0, Kuraray Noritake Dental Inc., Okayama, Japan) was conducted with a mixing tip to guarantee a uniform layer. Then, the restoration was placed on the abutment and the excess cement was removed using a brush (Pincel Pelo Marta, Tigre, Joinville, SC, Brazil). After removing all excess cement, light curing was performed using an LED light-curing device (BluePhase, 800 mW/cm², Ivoclar Vivadent, Schaan, Liechtenstein). The cement layer at the margin of the crown was polished using a DHPro Kit Ceram 7 silicone rubber polishing kit (Paranaguá, PR, Brazil) at 5000 rpm. To standardize the prosthetic indexing, all specimens were maintained with the anti-rotational lobe toward the buccal face of the crown.

In accordance with ISO 14801:2016 [31], aging simulation was performed using hemispherical loading devices, in which the load was applied to a single point of the palatal area in all specimens, avoiding uneven concentrated forces and the premature deformation of specimens.

2.2. Scanning Electron Microscopy (SEM)

Structural changes induced in the implant’s topographical surfaces were investigated using SEM. Representative implant specimens (B-fix Profile, Titaniumfix, São José dos Campos, SP, Brazil) were analyzed by scanning electron microscopy (SEM, Inspect S50, FEI Company, Brno, Czech Republic). They did not need to be coated with gold to evaluate the composition of the material on the surface, as it is an inherently conductive material.

2.3. Maximum Fracture Load

To evaluate the maximum fracture load, half of the specimens were then subjected to a compressive load (0.5 mm/min) in a universal testing machine (Emic DL-1000, Emic, São José dos Pinhais, PR, Brazil) in accordance with ISO 14801:2016 [31] (Figure 4).

2.4. Survival Fatigue Analysis

To evaluate the long-term effect, the other half of the specimens were positioned with an angle of 30° in the mechanical cycling machine to calculate the fatigue survival until a maximal of 2,000,000 cycles with a 1.6-mm-diameter stainless steel applicator in distilled water at 37 °C, as described in ISO 14801:2016 [31] (Figure 5). Fatigue resistance analysis was performed using the stepwise test [32]. The samples were tested in a mechanical fatigue machine (Biocycle, Biopdi, São Carlos, SP, Brazil), with the same device as the monotonic test, inclined at 30°, with a frequency of 2 Hz [33,34]. Load profiles were analyzed starting at 100 N with the load increasing at each following step, at intervals of 10,000 cycles. The visual inspection under ×7.5 magnification (Stereo Discovery V20, Zeiss, Gottingen, Germany) was used to identify any failure of cohesive fractures at the abutment or implant, or damage to the crowns. Loose crowns or screw fractures were also considered as failed specimens and the test was suspended. The number of cycles and the load at which the specimens fractured during the fatigue test were analyzed by the reliability software version 21 SPSS statistics (IBM, Chicago, IL, USA) using the survival analysis function, Kaplan–Meier and Mentel–Cox (log-rank) (p < 0.05).

2.5. Finite Element Analysis (FEA)

The three-dimensional file from the 4.0 × 10 mm implant, the abutments 4.5 × 1.0 mm (short) and 4.5 × 1.0 mm (long), and the screw were obtained in “. STL” format from the manufacturer (Titaniumfix, São José dos Campos, SP, Brazil) (Figure 6). Polylines were drawn over the mesh surface using Rhinoceros software (version 5.4.2 SR8, McNeel North America, Seattle, WA, USA) [29]. The network surface was created with contiguous and intersected lines using the reverse engineering tool.

Following the in-vitro setup, two different volumetric models containing a cylinder, implant, abutment, and monolithic restoration were created. The 3D models were exported to the computer-aided engineering software (ANSYS 19.2, ANSYS Inc., Houston, TX, USA) to perform the structural analysis [30,31,32,33,34] (Figure 6). Material properties were used based on the literature, and all structures were considered homogeneous, isotropic, linear, and elastic [33,34]. After checking the contact between the three-dimensional models, they were considered bonded and the number of faces tangent between two solids was adjusted with a similar quantity (Table 1).

The meshing process was carried out automatically, for which the software allowed the refinement of the mesh created using tetrahedral elements. For each load (axial and non-axial) a new analysis configuration was used. For all configurations, the cylinder model was fixed on its lower external surface, simulating the support of the model in a plane. The load was defined as a vector in the Z-axis direction with the application of a non-axial load of 450 N at 30°, in accordance with ISO 14801:2016 [31] (Figure 7).

A 3D mesh was generated, subdividing the geometry into a finite number of elements. Tetrahedral element types were considered for all models. After refinement, the total numbers of elements (709,276) and nodes (925,232) for the final setup were determined by a convergence test (Figure 7). After processing, colorimetric maps of Von Mises stress and displacement were obtained for each load at the implants, prosthetic screws, zirconia crowns, and polyurethane blocks.

3. Results

3.1. Surface Analysis (SEM)

Figure 8 presents the scanning electron microscopy (SEM) images of representative specimens from the experimental groups. No topographical differences were found on the surfaces of the materials, and the implants showed irregular surfaces from the processing method visible at magnifications of 20,000×, 50,000×, and 100,000×.

3.2. Maximum Fracture Load

The results of maximum fracture load are described in Table 2 and Table 3. The statistical analysis was carried out with one-way ANOVA with a significance level of 5% (Table 2). There was no statistically significant difference between the long universal ink and short universal link abutments, regarding maximum fracture load (Table 2).

3.3. Survival Fatigue Analysis

The stepwise test was also performed to evaluate the long-term effect. Regardless of the group, all specimens showed fracture of the screw. Therefore, the means and confidence intervals for fracture resistance and cycles to fracture were obtained using the Kaplan–Meier and Mantel–Cox tests (log-rank, 95%) and summarized in Table 4. Study groups (short and long) were statistically similar in the number of cycles required for fracture (p = 0.078); however, in terms of load, the long abutment group was statistically superior to the short abutment group (p = 0.014).

The groups’ survival graphs are presented in Figure 9. Table 5 shows the probability of survival of the experimental groups in each of the load steps and several cycles.

According to the load or number of cycles, the probability of survival was different among both groups. However, there was no difference until 350 N between both of them.

3.4. Finite Element Analysis (FEA)

After processing the results it was possible to observe the stress distribution in the implant, prosthetic abutment with screw, and restorative material. From colorimetric stress maps, the biomechanical behavior was evidenced as a function of the load application. To facilitate the qualitative analysis and the comparison between models, the same scale (MPa) was used. In this way, warmer colors indicate areas with a higher stress concentration, while colder colors indicate lower values. The maximum displacement of the set shows a high trend of movement at the incisal region of the prosthetic crown (Figure 10), with a maximum peak of 1.34 mm on both short and long abutments.

The maximum principal stress was analyzed in both groups (Figure 11), with a high stress concentration at the contact between bone and implant, and the contact of the prosthetic abutment with the prosthetic connection of the implant. There were no qualitative differences between the behaviors of both evaluated models.

The stress in the prosthetic screw suggests that in both models, this structure is prone to failure (Figure 12). Additionally, similar behavior of the prosthetic screw can be observed from the visual plot in both groups in terms of stress distribution.

4. Discussion

The present study evaluated the survival probability of the CAD/CAM metallic link (universal link) with different heights in the single-unit implant-supported anterior crown. There was no difference between both evaluated groups in terms of maximum fracture load and stress distribution; however, during certain steps of the stepwise fatigue, both groups behaved differently. Therefore, it was possible to partially reject the proposed hypotheses. The assembly consisting of the implant, prosthetic abutment, and monolithic zirconia single crown showed the same failure mode when subjected to fatigue cycling: fracture of the prosthetic screw.

There is a wide variety of designs and features of prosthetic abutments available for dental application and consequently, several variations in biomechanical behavior [34,35,36,37,38]. The abutments for CAD/CAM evaluated by Silva et al. (2018) [34] ranged between long and short, respectively, from 4 to 2.5 mm (37.5%), while, in the present study, the variation was between 5.5 and 4 mm (27.7%). To evaluate the effectiveness of luting the prosthetic restoration onto the abutment, they submitted the specimens to the pull-out test and found no statistically significant differences according to the abutment height. Likewise, in the present study, there was no adhesive failure of the monolithic restoration on the abutments at the two different heights.

The study by Lee et al. (2018) [39] related the effects of prosthetic abutment height on marginal bone loss and concluded that longer abutments were more favorable, highlighting that the abutment height should not exceed 4 mm, at risk of loosening the prosthetic screw. The evaluated abutments considered both the adhesive area and the collar height as abutment height. Contrasting the reported methodology, the present study standardized the collar height at 1 mm and only varied the adhesive area height, starting from the premise that a greater adhesive area will theoretically favor the bending moment, increasing the risk of screw loosening [35,40].

Likewise, Spinato et al. (2018) [41] considered different abutment heights and their influence on marginal bone loss. Similar to Lee et al. (2018) [39], they considered the total length as abutment height and concluded that higher collar heights were more favorable from a biological point of view. According to them, the optimization of results can be achieved with implants whose prosthetic connection is characterized by the platform switch concept, similar to the present implant design that contemplated this characteristic.

Therefore, the heights of 1- and 3-mm prosthetic abutments previously evaluated [37,41] exclusively considered the variation of the total height, depending on the level of implant placement in the bone with a variation of the collar heights, and did not correlate mechanical failures with the different height characteristics of the prosthetic abutments [39] or different parameters in the implant-supported restoration [40,41,42,43,44]. Contrastingly, Bordin et al. (2019) [32] performed the analysis from a mechanical point of view using finite element analysis. They found 40% more stresses when a 2 mm higher abutment was simulated; therefore, with a greater risk of failure. The universal link abutment used in the present study has a screw, which, also according to Bordin et al. (2019) [32] does not reduce the survival of the assembly; however, in terms of failure, these tend to present the screw loosening first before another fracture occurs, while solid abutments do not present the loosening, and failure occurs due to abutment fracture.

The failure in the screw also justifies the results in the maximum fracture load, since short and long abutments did not present statistically significant differences. When considering progressive load in the stepwise fatigue test, there was no difference in the number of cycles between both groups. However, for the fatigue failure load, the long abutment was statistically superior to the short one with high values of cyclic load. Despite the results and the superiority of the long abutment, the difference is visible only after 400N and the applied load was much higher than that recommended by the ISO 14801 standard and compatible with clinical extrapolations [39].

While Spitznagel et al. (2021) [45] analyzed the conformations of CAD/CAM monolithic restorations, the specimens were subjected to a mechanical fatigue test with an occlusal load of 198 N in 1,200,000 cycles at a frequency of 1.6 Hz, corresponding to the 5-year behavior. The mechanical fatigue tests aim to simulate the masticatory function in vitro, and the number of cycles has a significant impact on the force applied until fracture [40]. After aging, they subjected all specimens to maximum fracture load, and all specimens showed fracture origin in the screw access channel, while in the present study, failures occurred due to screw fracture with the non-axial application of the load [45].

Although Tribst et al. (2019) [5] concluded that in hybrid abutments there is a higher stress concentration in the cervical areas, with the possible occurrence of fractures for posterior crowns, Pitta et al. (2019) [46] determined that the implant restoration composed of a ceramic crown with a metallic link with a ceramic structure reduces the stress concentration in the restoration cervical area. Therefore, it is possible to assume that the biomechanical characteristic of the assembly composed of the universal link provided a favorable condition for stress dissipation without difference between both abutment designs.

The digital workflow in rehabilitation using implant-supported prostheses allows restoration designs with proper seating accuracy combined with reproducibility precision assisted by artificial intelligence manufacturing technology. This approach allows the use of monolithic restorative materials such as zirconia, as well as different “metal-free” materials, with unique elastic modulus and diverse mechanical response during compressive loading [47,48]. Monolithic zirconia was the material of choice only for the single crown due to its high strength, which allowed the assessment of the abutment behavior without cohesive failure of the restorative material. Therefore, in the present one-piece restoration design, there was no need to create a framework to support the crown.

In addition to the characteristic of the metallic links allowing adaptation of ceramic restorations on the abutment with insignificant gap and without any micro-movement [43], the universal link abutment allows the selection of a collar height from 0.7 to 4 mm. To not generate bias in the analysis of the behavior of the adhesive height of the abutment, in the present study, both groups were standardized with the same collar height of 1 mm, which is the most frequent height for this kind of component in implant dentistry [5]. However, further studies should be carried out to elucidate the effect of collar height as another parametric variation in the mechanical behavior of CAD/CAM crowns.

Due to the open-source characteristic of the performed workflow, there were no technical limitations between the implant manufacturer and the zirconia material [7]. The choice of the prosthetic restoration design in the present study for a maxillary central incisor occurred due to the less favorable biomechanical condition of the prosthetic rehabilitation of this tooth, considering that the long axis of the implant does not coincide with the axis of the load application, as well as a greater chance of contacts in excursive movements [49,50].

Despite the unfavorable mechanical conditions determined by ISO 14801:2016 [31] the application of non-axial axis load on an implant containing marginal bone loss simulates the worst clinical condition, increasing the probability of implant fracture [50,51,52], which is why the characterization of the implant by SEM was performed. However, even under these conditions and with irregular surfaces, the implant showed no fractures and its composition described as commercial grade IV titanium met the mechanical requirements, as shown in the present investigation [53].

The finite element analysis did not show differences in the biomechanical behavior of the short abutment from the long ones due to their very similar physical characteristics. Despite having a greater length, the cement layer and rigid ceramic crown may have made the assembly behave as a single body [49]. Therefore, the total length of the assembly formed by the implant, prosthetic abutment, and crown remained similar between the groups. In addition, the interposition of a cement layer between the prosthetic abutment and the crown may have favored the biomechanical behavior of the assembly by creating a damping space, which reduces the stress concentration on the implant body [53,54]. In this sense, the FEA was carried out under the same conditions as the laboratory tests for a better understanding of the stress concentrations and displacements of the structures, and with the same objective, polyurethane blocks were used in the theoretical models.

Although the literature presents lower loads in the finite element analysis, varying in the central incisor region between 100 N [54,55,56] and 200 N [57], the present study applied a load of 450 N based on a previous fatigue test [58], as well as in the fatigue failure load of the stepwise method, to reproduce the critical conditions observed in vitro. In addition, the stress concentration regions in the present study were similar to the results found in the literature [54,55,56,57].

The prosthetic screw was the most susceptible structure to failure, agreeing with previous reports [32,34]. Although the stress peak of 379.3 MPa in the neck of the screw was not present in the entire cervical area of the screw, it can induce damage and possible screw loosening. However, it can be easily resolved in implant-screwed prostheses just by replacing the screw, in further follow-ups. In the case of the fatigue test performed, there was no interruption of the test after screw loosening, which led to the displacement of the set and subsequent fracture of the prosthetic screw [58,59].

As a clinical implication, it can be demonstrated that the height of the abutment does not seem to weaken the mechanical behavior of implant-supported single crowns. Therefore, aesthetic parameters and interocclusal space should be driven by the abutment selection; instead, biomechanics when similar abutments and different heights are considered. It is important to mention the limitations of this study with simplified substrate, uniform loading method, absence of temperature variation, and sliding contact with the antagonist. Additionally, the results were obtained based on the restrict analysis of the adhesive height as the only variable on the biomechanical behavior of the universal link abutment. Therefore, further studies are necessary to evaluate other variables to make the correct prosthetic selection, aiming for greater longevity of the rehabilitation of CAD/CAM implant-supported prostheses.

5. Conclusions

Based on the results of this in vitro and in silico study, it can be concluded that:

• Both abutment heights presented similar and adequate fracture strength;

• Long universal link abutment showed greater fatigue strength during the survival test, however, without difference in the number of cycles;

• The stress concentration results corroborate the mechanical failure in the prosthetic screw during fatigue for implant-supported anterior crowns.

Author Contributions

Conceptualization: J.D.M.d.M., L.S.G., J.P.M.T., A.L.S.B., and G.d.R.S.L.; methodology, J.D.M.d.M., L.S.G., T.M.B.C., G.d.R.S.L., J.P.M.T., and M.A.B.; formal analysis, J.D.M.d.M., T.M.B.C., N.d.C.R., and G.d.R.S.L.; investigation, J.D.M.d.M., L.S.G., T.M.B.C., N.d.C.R., and G.d.R.S.L.; resources, J.D.M.d.M., G.d.R.S.L., and M.A.B.; data curation, J.D.M.d.M., N.d.C.R., and L.S.G.; writing—J.D.M.d.M., L.S.G., D.A.Q., and G.d.R.S.L.; writing—review and editing, J.D.M.d.M., L.S.G., D.A.Q., N.d.C.R., J.P.M.T., A.L.S.B., and G.d.R.S.L.; supervision, M.A.B., N.d.C.R., J.P.M.T., T.J.A.P.J., A.L.S.B., and G.d.R.S.L.; project administration, M.A.B., N.d.C.R., J.P.M.T., T.J.A.P.J., A.L.S.B., and G.d.R.S.L.; funding acquisition, J.D.M.d.M. and M.A.B. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are available upon request.

Acknowledgments

The authors appreciate the support of Titaniumfix (®) (Titaniumfix, CNPJ: 01.786.547/0001-27, São José dos Campos, SP, Brazil); for donating the implants, abutments and screws and STL files used in this research.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. Schematic sequence of the composition of the two experimental groups.

Figure 2. (a) Polyurethane cylinder with a diameter of ¾ inch; (b) drilling the block with the spear cutter Ø 2.0 mm; (c) use of the Ø 3.4 mm helical cutter; (d) implant placement with 3 mm exposure; (e) implant with universal link abutment 4.5 × 1.0 mm in position.

Figure 3. (a) Universal link abutment positioned and covered with Teflon tape; (b) application of the resin cement in the crown’s intaglio surface; (c) application of the cement on the metallic link surface; (d) seating the crown on the abutment with leakage of the cementing agent; (e) removal of excess cement.

Figure 4. (a) Specimen fixed on a base with 30° of angulation about the ground (ISO 14801:2016); (b) and (c) Specimen being submitted to a compressive load of 0.5 mm/min in a universal testing machine.

Figure 5. Illustration of the base at 30° to the ground during stepwise fatigue.

Figure 6. STL files obtained from the manufacturer (Titaniumfix, São José dos Campos, SP, Brazil); (a) front view of the screw; (b) short universal link; (c) long universal link; and (d) implant.

Figure 7. Assembly of sets consisting of implant, abutment, screw, and monolithic crown. Three-dimensional assembly of the short abutment (a), and long abutment (b); section view of the assembly of the short model (c); and section view of the assembly of the long model (d). Initial mesh division (e), and load application axis (f).

Figure 8. Micrographs of representative samples of the implants at (a) 20,000×, (b) 50,000×, and (c) 100,000× magnification.

Figure 9. Survival graph of groups as a function of time (cycles) (a) and function load (N) (b).

Figure 10. The maximum displacement of the assembly with the short abutment (a), and the long abutment (b).

Figure 11. (A) Stress concentration in the assembly with short (a), and long (b) abutments; and (B) stress concentration in the implant with a short abutment (a), and long abutment (b).

Figure 12. Stress concentration in the screw with short abutment (a), and long abutment (b).

Table 1

Material properties used in the present numerical analysis.

Material	Composition	Young Modulus (GPa)	Poisson Ratio
Polyurethane [25]	Poly[oxy(methyl-1,2-ethanediyl)], .alpha.-hydro-.omega.-hydroxy-, polymer with 1,1′-methylenebis [4-isocyanatobenzene]4,4′-Diphenylmethane diisocyanateBenzene, 1,1′-methylenebis [4-isocyanato-, homopolymer]4-methyl-1,3-dioxolan-2-one	3.6	0.30
Resin Cement [30]	10-MDP, hydrophobic aromatic and aliphatic photoinitiator, dibenzoylperoxide dimethacrylate, hydrophilic dimethacrylate, silanized silica	7.5	0.25
Titanium [31]	Titanium alloy ASTM F136	110	0.30
Y-TZP [32]	8 mol% Y-TZP: ZrO2 principal component, Y2O3 < 12%, Al2O3 < 1%, SiO2 < 0.02%, Fe2O3 < 0.02%	220	0.30

Table 2

One-way ANOVA for the fracture load between the long universal link group and the short universal link group.

Conversion Source	SQ	gl	QM	F	p-value	F-critical
Groups	63.68	1	63.68	1.86	0.18	4.41

Table 3

Mean ± standard deviation of the data obtained by fracture resistance for the long universal link group and the short universal link group.

Group	Mean Value (kgf)	SD	CI 95%
Long	39.4 a	±6.96	(46.36–32.44)
Short	43.1 a	±7.68	(50.78–35.42)

Legend: CI indicates confidence interval. Different letters indicate statistically significant differences between columns.

Table 4

Mean and standard deviation during fatigue according to the cycles to failure and load to failure for both groups.

Group	Mean (Cycles)	SD	CI-Minimum	CI-Maximum
Long	64,610.4	4791.4	55,219.12	74,001.67
Short	60,760.0	1264.8	58,280.83	63,239.16
Group	Mean (Load)	SD	CI-Minimum	CI-Maximum
Long	450.0	16.6	417.3	482.6
Short	400.0	0.0	400.0	400.0

Table 5

Probability of survival of experimental groups according to applied load and number of steps (cycles).

Total Cycles/Load (N)
Group	5 k/100	15 k/200	25 k/250	35 k/300	45 k/350	55 k/400	65 k/450	75 k/500	85 k/550
Long	100%	100%	100%	100%	90% (9%)	60% (15%)	50% (15%)	30% (14%)	0
Short	100%	100%	100%	100%	100%	87.5% (11%)	0	0	0

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