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Basic and Clinical Research

Effect of CAD/CAM Abutment Height and Cement Type on the Retention of Zirconia Crowns

Silva, Camila E. P. DDS*; Soares, Simone DDS, MS, PhD; Machado, Camila M. DDS, MS; Bergamo, Edmara T. P. DDS, MS§; Coelho, Paulo G. DDS, MS, PhD; Witek, Lukasz MS, PhD; Ramalho, Ilana S. DDS, MS#; Jalkh, Ernesto B. B. DDS, MS; Bonfante, Estevam A. DDS, MS, PhD

Author Information
doi: 10.1097/ID.0000000000000811
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Abstract

Implant dentistry is a well-established treatment option that has been demonstrating high success rates in long-term follow-up studies.1,2 When implant surgical protocols are strictly followed, metallic device osseointegration is expected to be uneventful. Despite the high success rates, biological and technical complications are frequently described for implant-supported reconstructions.3,4 Although still debated, it seems that the rate of such complications in internally connected implants are lower compared with external connections.5 In a systematic review that analyzed more than 1000 patients, only 61.3% of them were free of any complication after 5 years of function.3 Abutment or occlusal screw loosening was the most common technical complication (12.7%), followed by crown loss of retention (5.5%), veneer fracture (4.5%), and screw and abutment fracture (0.35%). Consequently, current research in implantology has focused on the improvement of prostheses' longevity and retrievability.6

Most implant rehabilitations comprise one- or 2-piece restorations where a crown can be cemented or screwed on the abutment. Substantial literature findings are categorical concerning the advantages and disadvantages of both prosthesis retention systems.3,7–13 Although screw-retained restorations are known for their retrievability, cement-retained advantages include enhanced esthetic, easy handling, reduced laboratory technique sensitivity, ability to correct minor casting misfits between the superstructure and abutment, prevention of microorganism colonization of the inner part of the implant, and possibility of implant alignment correction. Nonetheless, loss of retention is the weakness of such system along with periodontal concerns relative to remnants of cement in the gingival sulcus.13

Considering the increased use of the digital workflow in restorative dentistry, the use of abutments with geometry stored in computer-aided design/ computer-aided manufacturing (CAD/CAM) systems for rapid restoration fabrication, also known as Ti-Base abutments, has gained interest. The rationale behind its indication lies in the possibility of conventional or digital implant transfer for the design of monolithic or bilayered restorations of any given material that will be cemented chairside and then screwed to the implant. Improved control of bonding procedures can be achieved especially during excess cement removal. This feature is advantageous because extraneous cement in the periimplant tissue may lead to marginal bone loss. As a disadvantage, the screw-access hole will still be visible, which may impair excellent esthetic results.11,14

Cementation protocols for Ti-Base abutments have been scarcely explored.15 There are several factors that may affect the retention of the final prostheses such as Ti-Base height and texturization, cement type, superstructure fit, and surface treatment.12,16,17 When the prostheses material is zirconia, additional challenges are encountered for adequate bonding. Because of its high crystalline content and absence of a glass phase,18 zirconia demands different surface treatments, compared with other glass-matrix ceramics, that should be carefully undertaken because loss of retention is an important issue affecting clinical success rates of zirconia prostheses.19,20 The use of temporary cements has been indicated as a safe alternative for implant-supported metal-ceramic crowns compared with conventional and resin-based cements.4 Also, the use of self-adhesive resin cements, indicated to bond all-ceramic crowns to titanium abutments, has been suggested as a straightforward alternative because the application of adhesive systems is not required. Self-adhesive cements comprise a single-step cement that contain a resin matrix packed with phosphoric groups that simultaneously react with the ceramic surfaces and have demonstrated satisfactory bond strength to zirconia.21 However, the lack of an adhesive layer has been claimed to hamper bond strength,22 suggesting that strategies, including the use of 10-methacryloyloxydecyl dihydrogen phosphate (MDP)-based resin cements, may improve the bond between Ti-Base abutments and zirconia crowns.23–26

The understanding of the relative retentiveness of Ti-Base abutments to zirconia crowns using different cements should improve clinician's ability to choose a particular cementation protocol. Accordingly, this study sought to evaluate the effect of different Ti-Base abutment heights and cement type on the pull-out retention of zirconia-based restorations. The postulated null hypotheses were as follows: (1) that Ti-Base abutment height would not influence pull-out retention values and (2) cement type would not influence pull-out retention values.

Materials and Methods

Sample Preparation

Crowns with identical anatomy were designed in a computer-aided design (CAD) software (Ceramill Mind; Amann Girrbach, Curitiba, Brazil) to fit 2 different Ti-Base abutment heights (Emfils, Itu, Brazil) as follows (n = 10/group): (1) 4-mm-height Ti-Base abutment (Tall) and (2) 2.5-mm-height Ti-Base abutment (Short) (Fig. 1, A and B). An internal space of 50 μm for cementation was planned, starting 0.5 mm from the margin. Abutments presented internal conical connection because of their implant counterpart geometry (n = 20, Novo Colosso; Emfils). At the buccal and lingual cusps of the crowns, projections beyond the occlusal surface were designed with a round perforation of 2 mm to provide the attachment of stainless steel wires for pull-out testing (Fig. 1, C and E). Such projections were designed at the cusps to allow occlusal access for abutment screw torquing.

Fig. 1
Fig. 1:
Schematic images with dimensions for the A) tall (4 mm height) and (B) short (2.5 mm height) Ti-Base abutments. (C) Proximal view of the STL file with projections that were designed with a round perforation of 2 mm (D) to provide the attachment of stainless steel wires for pull-out testing. (E) Occlusal view showing the projections located at the cusp regions to allow occlusal access for abutment screw torqueing after cementation.

Ti-Base abutments and their corresponding implants were embedded in acrylic resin (Jet; Clássico Artigos Odontológicos, São Paulo, Brazil). For standardized long axis alignment, Ti-Base abutments digitally torqued to the implants were fixed in a surveyor (Delineador B2; Bio-Art, São Carlos, Brazil) and acrylic resin poured in 25-mm diameter polyvynil chloride (PVC) tubes with their long axis aligned to the tube's long axis, simulating the bone level of the implant for the tests. This procedure certified that the implant and Ti-Base were on the same axis for pull-out. The acrylic base held the samples in the same position once fixed to the universal testing machine (Kratos Equipamentos, São Paulo, Brazil). To reduce the distortion of the acrylic resin, pouring was accomplished in 2 steps, where the first step poured most of the material still allowing the implant to be free of contact. After polymerization, the second step consisted in pouring sufficient resin to cover the implant up to the collar surface.

Four cement types were selected: (1) temporary cement (provisional) (Temp-Bond; Kerr, Orange, CA); (2) glass-ionomer cement (Meron) (Voco; Meron, Cuxhaven, Germany); (3) self-adhesive resin cement (U200) (RelyX U200; 3M Oral Care, St. Paul, MN), and (4) resin cement (Ultimate) (RelyX Ultimate, 3M) with universal adhesive for treatment of titanium and zirconia substrates (Scotchbond Universal, 3M). Before cementation, Ti-Base abutments were torqued as per manufacturer's recommendations (32 N/cm). To standardize temporary cement base and catalyst portions, a digital scale was used. After cement mixing and crown insertion onto the abutments, a weight of 5 kg was used for standardization for 10 minutes until cement cure.

Pull-Out Retention Testing

A stainless steel wire of approximately 1.8 mm was inserted through both projections of the occlusal surface of each crown and fixed in a device positioned directly below the load cell. Pull-out testing was performed in a universal testing machine (Kratos Equipamentos, São Paulo, Brazil) at a cross-head speed of 1 mm/min until crown displacement and load drop. The force was recorded for statistical analysis.

Statistical Analysis

Preliminary data analyses showed indistinguishable variances in the study of the dependent variable (Levene test, all P > 0.25). Data were statistically evaluated through 2-way analysis of variance with fixed factors of Ti-Base height (short and tall) and cement (Provisonal, Meron, U200 and Ultimate) after post hoc comparisons by the Tukey test. Data are presented as a function of mean values with the corresponding 95% confidence interval. All analyses were accomplished using SPSS (IBM SPSS 23; IBM Corp., Armonk, NY).

Results

Results from 2-way analysis of variance are shown in Figure 2. Standard 4-mm-height Ti-Base abutments (Tall) demonstrated similar retention to 2.5-mm-height abutments (Short) when data are collapsed over cement type (P > 0.74) (Fig. 2, A). On the other hand, resin-based cements (U200 and Ultimate) presented significantly higher pull-out values relative to provisional and glass-ionomer cement groups (P < 0.01), without significant difference between U200 and Ultimate and between provisional and glass ionomer groups (data collapsed over Ti-Base height) (P > 0.34) (Fig. 2, B).

Fig. 2
Fig. 2:
Pull-out retention values as a function of mean and 95% confidence interval. (A) Ti-Base abutment height has not influenced pull-out retention values (P > 0.74). (B) Resin-based cements evidenced significant higher retentiveness than provisional and Meron (P < 0.01). (C) Tall and short abutments presented similar force to dislodgment for all within cement comparisons (P > 0.42), except U200 (P = 0.032); and, resin-based tall and short abutments demonstrated higher retentiveness than their provisional and Meron counterparts (P < 0.001). Different symbols indicate significant difference between groups (P < 0.05).

Evaluation of pull-out data as a function of both factors, Ti-Base height and cement, demonstrated similar force to dislodgment between tall and short abutments for all within cement comparisons (P > 0.42), except for U200 (P = 0.032) where a significantly higher pull-out value was observed for the short abutment. In addition, tall abutments cemented with Ultimate evidenced higher pull-out values compared with U200 (P = 0.043), and both were significantly more retentive than tall provisional and Meron (P < 0.001). No significant difference was observed between U200 and Ultimate cements used in short abutments (P = 0.758), and both presented statistically higher pull-out values than provisional and glass-ionomer groups (P = 0.001).

Discussion

One of the critical factors for implant-supported restorations' success is the connection integrity between the prosthetic superstructure and implant.27 Such integrity is provided as a function of implant–prostheses retention using screw and/or cement. The advantages and disadvantages of restoring dental implants with a screw- or cement-retained superstructure are well documented,3,7–12 but less frequently documented is the use of hybrid restorations that are both cemented and screwed. Nonetheless, besides the evident biomechanical advantages of cement-retained prostheses, most clinicians still opt for screw-retained because of the retrievability inherent to such assemblies.10 It is worth mentioning that the concern in supported prostheses cementation lies on ranking various cements to establish a guide for clinicians because retrievability and longevity in cement-retained prosthesis with provisional cements has been previously reported.28,29 Also, not only the adhesive bond strength is important to cementation longevity but also mechanical factors, such as frictional resistance.12,16 Then, final prostheses' retention depends on cement type, abutment height and texture, fit of the superstructure, and material composition of the prosthesis.12,16,17

Ti-Base abutments are a somewhat novel concept that can be indicated not only for conventionally cemented rehabilitations but also as a link between implant and a monolithic customized crown or a high-strength customized ceramic abutment joint to a cement-retained crown (screwed three-piece restorations). The main advantages of such concept, as previously explained, comprise the absence of ceramic material inside the implant connection, reproduction of a tailored emergence profile, and last but not the least, the opportunity of performing the bonding procedure before crown placement. Such sequence of procedures supports a final bond area polishing that decreases soft-tissue reaction regarding remnants of cement.11,14 Nonetheless, literature findings are scarce in addressing the influence of Ti-Base height and cement type regarding abutment-to-crown retention.15

The abutment geometry, mainly taper degree and height, has been previously shown to affect the retentiveness of implant-supported rehabilitations.17,30 In the present study, taper degree was standardized by the manufacturer, and the force to dislodgment for 4.0 mm (tall) and 2.5 mm (short) Ti-Base abutments was compared. Both abutments' cone heights demonstrated similar retentive force (data collapsed over cement). Even when data were evaluated as a function of both factors, similar force to dislodgment between tall and short abutments for all within cement comparisons was evidenced, except for U200. Consequently, the first postulated null hypothesis that Ti-Base abutment height would not influence pull-out retention values was accepted. This result is in contrast to previous literature findings that have shown that the higher the abutment's axial walls or the height to width ratio the better the effect on the uniaxial testing values because of higher contact area.17,30 In one investigation, twofold more retentiveness has been demonstrated by abutments possessing 1.5-mm higher axial walls relative to standard abutments.17

Previous studies have demonstrated that provisional cements can satisfactorily work for conventional cement-retained implant-supported crowns.28,29 Consequently, the second postulated null hypothesis of the current study consisted on the assumption that cement type would not influence pull-out retention values, even for Ti-Base abutments that possess a screw access. Nonetheless, the results demonstrated that resin-based cements presented significantly higher force to dislodgment relative to provisional and glass-ionomer groups, without significant difference between both resin cements and also between provisional and glass-ionomer cements (data collapsed over Ti-Base height). The same behavior was evidenced when data were evaluated as a function of both factors, with resin-based cements demonstrating similar force to dislodgment between tall and short abutments and both significantly more retentive than tall and short provisional and Meron cements. This rank order corroborates with previous studies that evaluated pull-out retention values for metal abutments with different cements.12,31

The absence of significant difference between resin-based cements lies behind their similar composition (methyl methacrylate monomers).21 The lack of an adhesive layer between zirconia superstructure and resin-based cements has been previously shown to hamper bond strength,22 however, the retention is proposed to be improved with the use of chemical surface treatments such as the addition of phosphate groups in the cement system.24–26 Both cements tested in the current study presented phosphate groups; self-adhesive U200 cement presents such monomers in its composition, and Ultimate, even as a conventional cement, is recommended to be used with an adhesive system that also presents MDP in its composition. On the other hand, the notably lower pull-out retention values for glass-ionomer and provisional cements are based on the absence or weak bond to zirconia because of lack of any bonding agent, as previously reported.32

Although time-consuming, the pull-out test is an efficient alternative to determine the retentive force of abutment joint to zirconia restorations. Such a test is influenced by several factors such as the degree of fit, height of the planned restoration, taper angle, and luting agent.33 Consequently, the general coincidence with literature findings allows us to conclude that the rank ordering of the cements in the current study is valid. Actually, there is no determination based on what constitutes a minimum pull-out value that provides long retentiveness to an implant-supported rehabilitation so that the patient would not have to return for prostheses recementation at an unexpected time. Then, the current and previous studies provide a scenario of the bonding behavior of various cements and abutment designs that may be used as a guide until clinical studies are conducted.34

Conclusion

Although Ti-Base abutment heights have not influenced zirconia superstructures' retentiveness, resin-based cements significantly evidenced higher retention than glass ionomer and temporary cements.

Disclosure

The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the manuscript.

Roles/Contributions by Authors

Camila E. P. Silva: contributed to conception, design, and acquisition of data, participated in drafting the manuscript, and proofreading the version being submitted. Simone Soares: contributed to conception, design, and interpretation of data, participated in revising the manuscript critically for important intellectual content, and proofreading the version being submitted. Camila M. Machado: contributed to study design and acquisition of data, participated in drafting the manuscript, and proofreading the version being submitted. Edmara T. P. Bergamo: contributed to conception and interpretation of data, participated in revising the manuscript critically for important intellectual content, responsible for the graphics and proofreading the version being submitted, and played a key role in statistics discussions. Paulo G. Coelho: contributed to conception, design, and interpretation of data, participated in revising the manuscript critically for important intellectual content, and proofreading the version being submitted. Lukasz Witek: contributed to conception, and interpretation of data, participated in revising the manuscript critically for important intellectual content, proofreading the version being submitted, and played a key role in statistics analysis. Ilana S. Ramalho: contributed to study conception, data interpretation and statistical analysis, participated in drafting the manuscript, responsible for the graphics, and proofreading the version being submitted. Ernesto B. B. Jalkh: contributed to study conception, data interpretation and statistical analysis, participated in drafting the manuscript, and proofreading the version being submitted. Estevam A. Bonfante: contributed to conception, design, interpretation of data, revising it critically, and proofreading the version being submitted.

Acknowledgments

To Grant # 2012/19078 to 7 São Paulo Research Foundation (FAPESP)—Young Investigators Awards, FAPESP EMU 2016/18818 to 8 and To Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Grant # 309475/2014–7.

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Keywords:

abutment design; dental cements; zirconia; dental implants; pull-out test

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