A 7.5‐year randomized controlled clinical study comparing cemented and screw‐retained one‐piece zirconia‐based implant‐supported single crowns

Abstract

Objectives

To compare marginal bone levels, biological, and technical outcomes of screw‐retained versus cemented all‐ceramic implant‐supported zirconia‐based single crowns after an observation period of 7.5 years.

Methods

Forty‐four single implants in the esthetic zone in 44 patients (22 females, 22 males) were randomly assigned to two types of restorations: SR (screw‐retained); veneered one‐piece zirconia abutment and CR (cement‐retained); veneered lithium disilicate crown intraorally cemented on a one‐piece zirconia abutment. Patients were recalled annually up to 7.5 years and survival rates, biological, and technical parameters assessed.

Results

A total of 31 patients attended the 7.5‐year follow‐up visit (17 SR group, 14 CR group). The survival rate on the restorative level was 77.5% (74.0% CR, 81.0% SR, p = .6399). Median marginal bone loss (MBL) values yielded −0.073 mm (−0.305; 0.238) in the CR and −0.215 mm (−0.500; 0.555) in the SR group (intergroup p = .6194). Mean bleeding on probing (BoP) values were significantly in favor of group SR with 20 ± 17% compared to 40 ± 22% in group CR (p = .011). The overall biological complication rate amounted to 27.5% (42.1% CR, 14.3% SR, p = .0775), whereas the technical complication rate was 32.5% (42.1% CR, 23.8% SR, p = .314). In total, CR restorations showed significantly more complications (84.2% for CR, 38.1% for SR, p = .0041).

Conclusion

One‐piece zirconia‐based single crowns on two‐piece dental implants exhibited a high rate of technical and biological complications at 7.5 years of follow‐up. Cemented restorations revealed significant higher rates of bleeding on probing and total complications compared to screw‐retained restorations.

1. INTRODUCTION

Restoring a single tooth gap with an implant‐supported single crown in partially edentulous patients is considered a well‐documented treatment modality (Jung et al., 2012; Pjetursson, Valente, et al., 2018; Rabel et al., 2018). Metal‐ceramic crowns and metal abutments have been used for many years as a standard restoration material (Pjetursson, Zarauz, et al., 2018; Sailer et al., 2009). Later, ceramic abutments in combination with all‐ceramic crowns were introduced to overcome possible esthetic deficits and limitations of metal abutments in cases with thin peri‐implant mucosa in the anterior zone (Sailer et al., 2009). Despite esthetic advantages, ceramic abutments present a higher incidence of technical and biological complications compared to metal abutments, at least on the short‐/mid‐term follow‐ups (Pjetursson, Zarauz, et al., 2018), whereas long‐term data are scarce.

Moreover, the type of retention should be further clarified, considering that there is still a lack of consistent long‐term data showing the possible differences between screw‐retained and cemented implant restorations with respect to technical and biological complications (Pjetursson, Zarauz, et al., 2018; Sailer et al., 2012).

Therefore, the aim of this randomized controlled clinical study was to compare marginal bone levels as well as biological and technical outcomes of screw‐retained and cemented all‐ceramic implant‐supported zirconia‐based single crowns after an observation period of 7.5 years.

2. MATERIALS AND METHODS

The present randomized controlled clinical trial was performed at the Clinic of Reconstructive Dentistry, University of Zurich, Switzerland. It was approved by the local ethics committee (Kantonale Ethik‐Kommission Zürich; KEK‐ZH‐Nr. 2010‐0041) and registered in the German Clinical Trials Register (DRKS; Nr. DRKS00006221) following the CONSORT guidelines. All the patients/participants provided written informed consent and were treated following recognized clinical standards in accordance with the Declaration of Helsinki.

Overall, 44 patients (22 females, 22 males), presenting a single tooth gap in the aesthetic area, were recruited, including 14 smokers (but not heavy smokers; >10 cigarettes) and four subjects with a history of (treated) periodontitis. The reason for tooth loss was mainly due to deep caries, root fractures, and endodontic complications. Details on the surgical and prosthetic procedures were reported earlier (Kraus et al., 2022; Thoma et al., 2016). In brief, two‐piece dental implants (OsseoSpeed, Astra Tech Implant System, Dentsply Sirona Implants, Mölndal, Sweden) were placed in single‐tooth gaps in the esthetic zone including maxillary and mandibular anterior teeth regions as well as first and second premolar regions. Patients were then randomly assigned to the screw‐retained (SR) or cemented (CR) group. Customized CAD/CAM zirconia abutments (Atlantis, Dentsply Sirona Implants, Mölndal, Sweden) were fabricated for all cases. In the SR group, abutments were directly veneered and directly fixed onto the implants. In the CR group, abutments were screwed onto implants, and then veneered lithium disilicate crowns (e.max, e.max Ceram, Ivoclar Vivadent, Schaan, Liechtenstein) were intraorally cemented (Panavia 21, Kuraray Medical, Kuraray Europe GmbH) according to manufacturer's instructions. Cement remnants were carefully removed. Patients received ongoing supportive care with annual check‐ups and dental hygiene appointments at the University clinic.

At 6‐month (FU‐6M), 1 (FU‐1Y), 3 (FU‐3Y), 5 (FU‐5Y), and 7.5 year (FU‐7.5Y) follow‐up visits, the following parameters were evaluated and compared to baseline (crown insertion): (1) marginal bone level; measured from standardized digital radiographs with an open‐source software (Image J; National Institute of Health), after calibration using the pitch distance between implant threads (to the nearest of 0.1 mm); (2) biological parameters at implant sites and control teeth such as plaque control record (PCR) (O'Leary et al., 1972); probing depth (PD) (Ramfjord, 1959); bleeding on probing (BoP) (Ainamo & Bay, 1975), six sites per implant/tooth; thickness of the buccal mucosa (1 mm below the margin, using an endodontic file) and width of keratinized mucosa (KM), 1 site per implant/tooth; and (3) technical parameters such as fractures of framework or veneering ceramic, occlusal roughness, loss of retention or over−/undercontouring, according to USPHS (United States Public Health Service) criteria (Cvar & Ryge, 2005): alpha (A) if there were no problems, bravo (B) for minor complications, charlie (C) for major complications, and delta (D) if the restoration had to be replaced due to the complication.

Peri‐implant mucositis was defined as the presence of profuse bleeding (line or drop) and/or suppuration on probing with or without an increase in probing depths compared to baseline, and an absence of bone loss beyond crestal bone‐level changes resulting from the initial remodeling. Whereas peri‐implantitis was defined as the presence of profuse bleeding (line or drop) and/or suppuration on probing, an increase in probing depths compared to baseline, and progressive bone loss in relation to the radiographic bone‐level assessment at FU‐1Y (≥2 mm) (Berglundh et al., 2018; Renvert et al., 2018).

For the marginal bone‐level values, biological and technical findings, the acquired data were reported as means, standard deviations, minimum, maximum, and quartiles, and analyzed descriptively.

2.1. Statistical analysis

Data were computed in Excel (Microsoft Corporation, Redmond, USA) and the statistical analysis was performed with SAS 9.4 (SAS Institute Inc., Cary, NC, USA). All comparisons of the two group medians were performed with the Wilcoxon‐Mann–Whitney test and the level of significance was set at 5%. Changes of a parameter within a group were analyzed with the Wilcoxon signed rank test. Categorical data were summarized with counts and percentages. Comparisons were based on the Chi‐square test with exact derivations of the p‐values. Survival analyses were performed with the Kaplan–Meier survival approach.

3. RESULTS

Of the 44 originally recruited patients, 31 attended the 7.5‐year follow‐up visit (17 SR group, 14 CR group). Of the remaining 13 patients, one passed away (CR group), three were not available for the visit (dropouts, SR group only), while nine patients experienced failures: two implant losses (CR group only, p = .219), seven abutment fractures (four SR and three CR group, p = 1.0).

The reason for implant failure was due, in one case, to peri‐implantitis (explantation 3 years and 4 months after baseline), while in the other case, it was due to loss of osseointegration (9 months after baseline), without any previous sign of peri‐implantitis. The overall survival rate at the restorative level was 77.5% (74.0% for CR and 81.0% for SR, p = .6399) (Figure 1). At the implant level, survival rates were 95.0% (overall), 88.8% (CR group), and 100% (SR group, p = .1459).

FIGURE 1.

Kaplan–Meier cumulative survival rates for cement‐retained (CR) and screw‐retained (SR) zirconia‐based implant crowns.

At FU‐7.5Y, the implant sites showed a median PD of 3.3 mm (Q1: 2.8; Q3: 4.0) in the CR and 3.0 mm (2.6; 4.0) in the SR group (p = .435). Changes in PD during the follow‐up time were not significantly different between the two groups (p = .244). The width of keratinized mucosa remained stable over time (intergroup comparison: p = .866). At FU‐7.5Y, plaque control records (PCR) at the implant sites were 17 ± 18% for the CR group and 10 ± 15% for the SR group (intergroup comparison p = .291). For bleeding on probing (BoP) mean values were 40 ± 22% for the CR and 20 ± 17% for the SR group, showing a statistically significant difference in favor of group SR (p = .011). Implants in the CR group showed, during the 7.5‐year follow‐up period, a significant increase in BoP (+33 ± 20%, p = .0001), while implants in the SR group had relatively stable BoP values (+6% ± 24%, p = .109). The difference between the groups was statistically significant (p = .003).

Peri‐implant mucositis was diagnosed in eight cases (20%): two in the SR group (9.5%) and six in the CR group (31.6%). Peri‐implantitis occurred at one single implant site (2.5% of cases) in the SR group (4.8%) only. There were no significant differences in the prevalence of both diseases between the two groups (Peri‐implantitis: p = 1.0, Peri‐mucositis: p = .1202).

The radiographic analysis at FU‐7.5Y (Figures 2 and 3) yielded median MBL values of −0.073 mm (−0.305; 0.238) in the CR and −0.215 mm (−0.500; 0.555) in the SR group (intergroup comparison p = .6194). The changes over time (Baseline – FU‐7.5Y) were not significantly different between the groups (p = .9789). Figure 4 reports the cumulative frequencies of implants showing different bone‐level changes over time (every 0.5 mm) with no significant differences between the groups.

FIGURE 2.

All‐ceramic implant screw‐retained restoration SR at baseline (a) and at 7.5‐year follow‐up (b). Site 21.

FIGURE 3.

All‐ ceramic implant cemented restoration CR at baseline (a) and at 7.5‐year follow‐up (b). Site 24.

FIGURE 4.

Cumulative frequencies of implants showing bone‐level changes over time (every 0.5 mm, from BL to FU‐7.5), with no significant differences between the groups (cement‐retained (CR) and screw‐retained (SR) implant crowns).

Table 1 displays the observed complications during the 7.5‐year observation period. The overall biological complication rate was 27.5% (42.1% CR group, 14.3% SR group, p = .0775), whereas the technical complication rate reached 32.5% (42.1% CR group, 23.8% SR group, p = .314) (Table 2). Overall, cemented restorations exhibited a statistically significant higher rate of complications during the observation period (Total complications: 84.2% CR group, 38.1% SR group, p = .0041).

TABLE 1.

Total complications as number of restorations (and in percent) during the 7.5 years of follow‐up.

Complications	Total amount (n = 40)	Screw‐retained restorations (n = 21)	Cement‐retained restorations (n = 19)	p‐value
Technical complications
Abutment fracture	7 (17.5%)	4 (19.1%)	3 (15.8%)	1.000
Screw loosening	2 (5.0%)	0 (0.0%)	2 (10.5%)	.219
Minor chipping	4 (10.0%)	1 (4.8%)	3 (15.8%)	.331
Biological complications
Implant loss	2 (5.0%)	0 (0.0%)	2 (10.5%)	.219
Peri‐implant mucositis	8 (20.0%)	2 (9.5%)	6 (31.6%)	.1202
Peri‐implantitis	1 (2.5%)	1 (4.8%)	0 (0.0%)	1.000
Total technical complications	13 (32.5%)	5 (23.8%)	8 (42.1%)	.314
Total biological complications	11 (27.5%)	3 (14.3%)	8 (42.1%)	.0775
Total complications	24 (60.0%)	8 (38.1%)	16 (84.2%)	.0041
Total restorations with complications	23 a (57.5%)	8 (38.1%)	15 a (78.9%)	.0123

^a Patient 37 (CR) had two complications (screw loosening/abutment fracture, 2 years later).

TABLE 2.

USPHS criteria (number of restorations) during the 7.5 years of follow‐up, including all crowns (38 patients): 17 cement‐retained restorations (CR) and 21 screw‐retained restorations (SR).

	Alpha (A)	Bravo (B)	Charlie (C)	Delta (D)
Fracture of framework
SR	81.0% (17)	–	–	18.2% (4)
CR	82.4% (14)	–	–	17.6% (3)
Fracture of veneering ceramic
SR	95.2% (20)	4.8% (1)	0.0% (0)	0.0% (0)
CR	82.4% (14)	17.6% (3)	0.0% (0)	0.0% (0)
Occlusal roughness
SR	66.7% (14)	33.3% (7)	0.0% (0)	0.0% (0)
CR	23.5% (4)	76.5% (13)	0.0% (0)	0.0% (0)
Loss of retention
SR	81.0% (17)	Not applicable	0.0% (0)	19.0% (4)
CR	70.6% (12)	0.0% (0)	11.8% (2)	17.6% (3)
Contour of restorations
SR	85.7% (18)	14.3% (3)	0.0% (0)	0.0% (0)
CR	94.1% (16)	5.9% (1)	0.0% (0)	0.0% (0)

4. DISCUSSION

The presented 7.5‐year follow‐up data on biological and technical outcomes of screw‐retained and cemented all‐ceramic implant‐supported single crowns on zirconia abutments showed: (1) a high rate of major technical and biological complications, resulting in (2) low survival rates of the crowns; (3) a significant increase in BoP in the cemented group (and not in the SR group).

The rate of major complications in the present study, 22.5% over a 7.5‐year time period, is to be considered high. Technical complications leading to the loss of the restoration were exclusively caused by abutment fractures and occurred in 17.5% of the cases (19.1% SR; 15.8% CR). In comparison, the estimated annual abutment fracture rate in a recent systematic review was 0.37%, resulting after 7.5 years in a considerably lower percent (2.8%) (Pjetursson, Zarauz, et al., 2018). A further reason for failures was implant loss (two patients, 5.0% overall, only CR). This can be considered to be in line with data from a systematic review reporting an implant failure rate of 0.97 per 100 implant years (after 5 years) and of 0.38 per 100 implant years (after 10 years) (Rabel et al., 2018).

All these complications resulted in a relatively low survival rate at the restorative level (77.5%) at 7.5 years. Cemented restorations seemed to show a lower percentage (74.0%) compared to screw‐retained ones (81.0%). According to recent literature, for all‐ceramic implant supported single crowns, a much higher survival rate of 94.4% could be expected after an observation period of 10 years (Rabel et al., 2018). The reason for this notable difference in survival rates could be related to the connection design used in the present study. However, another clinical study with the same implant system and abutment type, did not report any abutment fracture over a 5‐year period (Cooper et al., 2021). The differences in technical failures (abutment fracture) to present study might be explained by the location of the restorations. Cooper et al. included maxillary anterior and first premolar regions in their study, where as in the present study maxillary and mandibular anterior and first and second premolar regions were included. Another similar randomized clinical trial including 34 zirconia one‐piece abutments for restoring single tooth gaps in anterior or first and second premolar sites revealed comparable survival rates of single crowns after 5 years (82.4%). The authors highlighted that restorations with one‐piece zirconia abutments directly engaging the implant's internal connection (two‐piece implants with non‐matching implant‐abutment junction) were susceptible to technical complications, including abutment fractures (Lamperti et al., 2022). It is important to emphasize that the implants reported in the latter publication were from a different system to those used in the present investigation. Therefore, the described complications appear to be more related to the connection design and location of the restorations in general, rather than being specific to a particular implant system.

Peri‐implant mucositis was diagnosed in 20% of the implants, while peri‐implantitis was diagnosed in a single case (SR group). No significant differences between the groups were detected. However, focusing on BoP values, significant intergroup differences were observed: mean BoP values were significantly higher in group CR (40 ± 22%) compared to group SR (20 ± 17%). Moreover, implants in the cemented group demonstrated an increase in BoP over time (+33 ± 20%), while implants in the SR group revealed stable BoP values (+6% ± 24%). In contrary, the clinical study of cement retained crowns on same implant system and abutment type by Cooper et al. (2021) reported lower bleeding on probing scores, but similar marginal bone loss over time. Also some other studies have not been demonstrating a substantial biological advantage of screwed‐retained restorations (Jemt, 2009; Kotsakis et al., 2016) and a recent systematic review did not show a significant difference in the risk of peri‐implant mucositis between cement‐ and screw‐retained restorations (Reis et al., 2023). However, the topic is still a matter of debate, as in the literature, cement excess has been described as a possible cause of peri‐implant diseases (Linkevicius, Puisys, et al., 2013; Pesce et al., 2015; Renvert & Polyzois, 2015; Staubli et al., 2017) and it seems that dental radiographs are not a reliable method of evaluating excess cement (Linkevicius, Vindasiute, et al., 2013). Additionally, emergence profile design (concave), long distance of crown‐abutment margin to mucosal margin and area of inspection (oral and interdental) increase the risk of cement remnants (Sancho‐Puchades et al., 2017). These factors were not evaluated in the present or above‐mentioned studies, which might make the comparison difficult.

The present study provides long‐term outcomes of all ceramic, zirconia‐based implant crowns in the esthetic zone. The primary limitation of this study is the inclusion of premolar sites, which may have influenced the high number of abutment fractures as discussed. Other authors have also reported failures of zirconia abutments placed in posterior sites, suggesting that one‐piece zirconia abutments should only be used in the anterior region (Ferrari et al., 2016). The number of enrolled patients (44) and the 6.8% dropout rate after 7.5 years could be considered further limitations of the present investigation. Finally, the necessity of publishing the 7.5‐year follow‐up data, after a recent 5‐year follow‐up, invites critical discussion. In the first years, subjects in this study encountered multiple complications following the use of all‐ceramic one‐piece abutments. A period of stability was observed after the third year, yet by the 7.5‐year milestone, a significant increase in both technical and biological complications was evident, including an abutment fracture. This trend over time is evident in Figure 2, and emphasize the limitations associated with the use of one‐piece zirconia abutments and underscoring the essential need for clinicians to be aware of these potential issues.

5. CONCLUSIONS

All‐ceramic, zirconia‐based single crowns on two‐piece dental implants exhibited in the present study set‐up a high rate of technical and biological complications after 7.5 years of follow‐up. Cemented restorations showed significantly higher rates of total complications as well as a higher degree of positive peri‐implant inflammatory parameters at 7.5 years and over time compared to screw‐retained restorations.

AUTHOR CONTRIBUTIONS

Riccardo D. Kraus: Data curation; formal analysis; validation; visualization; writing – review and editing; writing – original draft. Jenni Hjerppe: Data curation; formal analysis; validation; visualization; writing – original draft; writing – review and editing. Nadja Naenni: Conceptualization; methodology; validation. Marc Balmer: Conceptualization; methodology; validation. Ronald E. Jung: Conceptualization; methodology; validation. Daniel S. Thoma: Conceptualization; data curation; formal analysis; methodology; validation; writing – review and editing.

CONFLICT OF INTEREST STATEMENT

The authors report no conflict of interest.

Supporting information

Data S1:

Kraus, R. D. , Hjerppe, J. , Naenni, N. , Balmer, M. , Jung, R. E. , & Thoma, D. S. (2024). A 7.5‐year randomized controlled clinical study comparing cemented and screw‐retained one‐piece zirconia‐based implant‐supported single crowns. Clinical Oral Implants Research, 35, 1669–1675. 10.1111/clr.14346

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from the corresponding author upon reasonable request.