study, success, telescopic crown

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1 Eberhard Frisch Petra Ratka-Kr uger Hans-J urgen Wenz Unsplinted implants and teeth supporting maxillary removable partial dentures retained by telescopic crowns: a retrospective study with >6 years of follow-up Authors affiliations: Eberhard Frisch, Oral Surgery and Periodontology, Northern Hessia Implant Center, Hofgeismar, Germany Petra Ratka-Kr uger, Periodontology, Periodontology Section, Department of Operative Dentistry and Periodontology, University of Freiburg, Freiburg, Germany Hans-J urgen Wenz, Prosthodontics, Clinic of Prosthodontics, Propaedeutics and Dental Materials, Christian-Albrechts University, Kiel, Germany Corresponding author: DMD Eberhard Frisch, MSc Markt 3, Hofgeismar 34369, Germany Tel.: Fax: dres.frisch@t-online.de Key words: double crown, implant, long-term results, maintenance, peri-implantitis, retrospective study, success, telescopic crown Abstract Objectives: Data regarding tooth- and implant-supported maxillary removable partial dentures (TIRPDs) are scarce. The objective of this research was to perform a retrospective evaluation of the clinical long-term outcome of maxillary TIRPDs rigidly retained via telescopic crowns in patients undergoing supportive post-implant therapy (SIT). Material and Methods: The inclusion criteria were met by 26 patients restored with maxillary TIRPDs between 1997 and 2011 in a private practice. Primary crowns (Marburg double crowns, MDCs) on teeth were cemented, whereas those on implants were screw-retained. Using patient records and data from a cross-sectional clinical examination in 2013, the survival rates of the teeth, implants and prostheses, together with the biological and technical complications, were analyzed. Results: After (range: 2 16) years of loading, 23 non-smoking patients with 23 dentures supported by 60 implants and 66 teeth were available for assessment. Nine teeth (survival rate: 86.36%) were lost, whereas 1 implant (survival rate: 98.36%) failed because of periimplantitis. Although 30 implants (50%) in 16 patients (69.57%) showed bleeding on probing (BOP+), no further peri-implantitis was observed. The mean peri-implant probing depth (PPD) was mm. All dentures were functional and required technical maintenance efforts amounting to treatments per patient per year (T/P/Y). Conclusions: Within the limitations of this study, we conclude that TIRPDs retained via MDCs may represent a viable treatment option for patients with residual maxillary teeth. Date: Accepted 18 March 2014 To cite this article: Frisch E, Ratka-Kr uger P, Wenz H-J. Unsplinted implants and teeth supporting maxillary removable partial dentures retained by telescopic crowns: a retrospective study with >6 years of follow-up. Clin. Oral Impl. Res. 00, 2014, 1 7 doi: /clr Implant-supported fixed prostheses or removable partial dentures (IRPDs), mostly using bars or ball attachments for retention on dental implants, have been increasingly used over the past several decades for the rehabilitation of edentulous maxillae. In the literature, data on implant-supported double crowns have been scarce. These restorations were first proposed as conical crowns (Besimo & Kempf 1995) and later as resilient (Heckmann et al. 2004; Krennmair et al. 2007) and non-resilient (Eitner et al. 2008; Krennmair et al. 2012; Frisch et al. 2013b) telescopic crowns. Restorations utilizing this principle have yielded good long-term results (Wenz et al. 2001; Andreiotelli et al. 2010; Frisch et al. 2013a; Frisch et al. 2013b) and have been proposed for the implant-supported rehabilitation of partially dentate patients (Mengel et al. 2002) who may experience an improvement in their oral-health-related quality of life by additional, strategic implant placement (Wolfart et al. 2013). Such IRPDs provide secure function, can be easily repaired and offer good access for oral hygiene measurements (Budtz-J orgensen 1996; Bergman et al. 1997; Wagner & Kern 2000). It has been noted that accessibility of these designs to hygiene instruments is helpful in preventing periodontitis (Wenz & Lehmann 1998). Implant planning based on the use of double crowns offers several advantages, such as the option of utilizing residual teeth, relatively large freedom of anatomical abutment positioning and an opportunity to provide structures resembling fixed prostheses while allowing added soft tissue support and the closure of interproximal spaces for 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd 1

2 enhanced esthetics and phonetics. Fixed prostheses supported by tooth implant combinations are relatively well documented (Lang et al. 2004; Nickenig et al. 2006, 2008; Pjetursson & Lang 2008). By contrast, few investigations of removable partial dentures supported by tooth implant combinations (TIRPDs) are available, and the evidence they have produced is scanty (Hug et al. 2006; Kaufmann et al. 2009; Bernhart et al. 2012). Only one study of tooth/implant-borne, double-crown-retained dentures in maxillae (Krennmair et al. 2007) was listed in a systematic review by Koller et al. (2011). There is currently a lack of data concerning clinical studies and long-term results, especially those collected in the private practice setting. The Marburg double crown (MDC) (Wenz & Lehmann 1998) uses a clearance fit with no friction or wedging of the primary and secondary crown during insertion and removal. To achieve retention, additional attachments are used (TK-Snap, Si-Tec GmbH, Herdecke, Germany). These attachments consist of an elastic resin body holding a titanium ball, which in the terminal position snaps into a corresponding hollow in the parallel surface of the inner crown (Fig. 1). All metal components are produced from a single cobalt chromium molybdenum alloy, and the framework (including outer crowns) is cast in one piece. Therefore, the complete inner and outer crowns and framework can be produced from base metal alloys rather than the noble alloys typically used for individually milled telescopic and conical crowns (Wenz et al. 2001). Fig. 1. Schematic drawing of the Marburg double-crown system: (a) During insertion of the prosthesis, the resin body of the snap attachment undergoes elastic deformation. (b) In the final position, the titanium ball of the attachment snaps into the corresponding hollow of the inner crown to provide retention. This study was performed to investigate patients with residual maxillary teeth who had been provided with telescopic crownretained TIRPDs in a private dental practice. The purpose of this study was the retrospective, long-term evaluation of the survival/ success rates of implants and dentures, along with their biological/technical complication rates. Material and methods This retrospective clinical study was conducted in a private practice specializing in implants (Northern Hessia Implant Center, Hofgeismar, Germany). A retrospective noninterventional study design was used, based on the analysis of primary patient data that had been extracted from the patients records. We evaluated the clinical outcomes of TIR- PDs rigidly retained via MDCs in maxillae with residual teeth. This study was reviewed and authorized by the Ethics Commission of the Albert-Ludwigs University Freiburg, Germany (application no. 46/ ). The recommendations for strengthening the reporting of observational studies in epidemiology (STROBE) were followed (von Elm et al. 2008). Study population Patients who were provided with maxillary double-crown-retained TIRPDs according to the MDC technique during the period between January 1997 and December 2011 and who were attending a post-implant maintenance program were identified. These patients were approached during the annual maintenance appointments and were asked to participate in the study after having received written information regarding the aims and course of the investigation. Patients who provided written informed consent and met the following inclusion criteria were included: Age 18 years. Having received surgical and prosthetic treatment in the study center. Regular (at least annual) prophylaxis/supportive therapy in the study center. Retention of the denture by at least four double-crown attachments according to the MDC technique. Double crowns exclusively screwed onto the implants and cemented on the teeth. Functional period of the final restoration >2 years. Availability of the complete medical history, including the following potential risk factors: medication (immune suppression and bisphosphonate), diabetes, cardiovascular disease and rheumatoid arthritis, in addition to smoking habits. The following exclusion criteria were applied: Tobacco smoking Use of implant designs other than the Ankylos (Dentsply Friadent, Mannheim, Germany) system. Use of designs other than the MDC technique for telescopic crowns on the teeth and implants. Non-compliance with the post-implant maintenance program (minimum 19/ year). Treatment course The treatment planning aimed at achieving at least two supporting units (implants and/ or natural teeth) on each side of the maxilla. Edentulous sides were usually provided with 2 (and not more than 3) implants. Surgical treatment was performed under local anesthesia and followed the manufacturer s protocol. All clinical procedures were performed by the same experienced clinician (EF). Antibiotics were given 1 h before and continued for 1 week after the surgery (amoxicillin /day). Wound assessment was performed after 7 days (suture removal) and 28 days. Second-stage surgery was performed after a healing period of 3 months. Prosthodontic treatment (Figs 2 5) was conducted according to the Marburg doublecrown (MDC) technique, with a clearance fit (Wenz & Lehmann 1998) and additional attachments (TK-Snap, Si-tec GmbH, Herdecke, Germany). The secondary structures of all TIRPDs were cast in one piece and included no solder or welding joints. The primary telescopes were either screwed to the implants or cemented to the residual teeth. Fig. 2. Internal view of the Marburg double crown with an additional retention element TK-Snap. 2 Clin. Oral Impl. Res. 0, 2014 / John Wiley & Sons A/S. 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3 Fig. 3. Maxilla with two residual teeth and three additional implants prepared for restoration. Fig. 4. Primary telescopes cemented to the teeth and screwed to the implants. Fig. 5. Intra-oral view of the tooth- and implant-supported MDC-RPD. Following delivery of the TIRPDs, oral hygiene instructions were given to all patients. Subsequently, the patients were scheduled for a supportive post-implant hygiene therapy program (SIT) with trimonthly visits. These sessions included an evaluation of peri-implant tissue status using the Quigley-Hein plaque index (QHI) (Quigley & Hein 1962) and measurement of periimplant probing depths (PPDs) using a millimeter-scaled periodontal probe (PCP 15; Hu-Friedy, Chicago, IL, USA) at four locations per implant (mesio-buccal, disto-buccal, mesiooral, disto-oral). Any bleeding upon probing (BOP; 30 s following the probing) was noted, and radiographs for implants with a positive BOP and a PPD 5 mm were performed using the long-cone parallel technique. All followup sessions included reinforcement of patient motivation and repeated instruction regarding at-home plaque control. Finally, all implants and teeth were professionally cleaned using a polishing paste and a rubber cup (FSI Slimline, De Trey GmbH, Konstanz, Germany). During the complete study period, prosthesis design, material selection, surgical/technical procedures and SIT remained unchanged. Data collection Between January 1, 2013 and December 1, 2013, the patients in our study were evaluated according to the following parameters using patient records: age and gender, medical history, smoking habits, anatomical position of the implants (according to the Federation Dentaire Internationale [FDI] scheme), number of implants, loss of implants, time of denture placement, opposing dentition and the period of observation. Moreover, during the last SIT appointment, the subjects were clinically examined by an EF, who evaluated the following biological and technical complications of the teeth/ implants and removable dentures: screw loosening, material fractures, loss of retention, changing retention elements, relinings, secondary caries (teeth) or peri-implantitis (implants). A periodontal examination (including PPD and BOP) was performed for all implants. To confirm the diagnosis of peri-implant disease in implants with a positive BOP and PPD 5 mm, radiographs were taken to measure the extent of peri-implant bone loss with respect to the baseline radiograph (prosthetic delivery). To assess the periimplant bone level, intra-oral radiographs were taken using the parallel technique. Diagnostic criteria and statistical analysis Survival was defined as the continued presence of the implant or prosthetic reconstruction in the mouth, independent of biological and/or technical complications (Rinke et al. 2011; Frisch et al. 2013a). Any technical complication related to the overdenture or the implant abutment (e.g., abutment screw loosening, fracture of the abutment, fracture of the denture base or denture teeth, loss of retention or defects in the attachments) was recorded. Incidence rates for technical complications were calculated based on the number of treatments occurring per patient per year (T/P/Y). Every recorded BOP incident was defined as peri-implant mucositis (Fransson et al. 2008). No true endpoints have been identified to diagnose peri-implantitis (Lee 2011; Klinge et al. 2012, Mombelli et al. 2012). Therefore, the following surrogate endpoints were used: positive BOP, PPD 5 mm and a maximum bone loss of 3.5 mm (Rinke et al. 2011; Frisch et al. 2013a). Because of the small sample size, no meaningful statistical analysis of potential factors influencing the treatment outcome was possible. Therefore, only descriptive statistics (no statistical test or confidence interval) were applied. Results Patients In total, 26 patients met the inclusion criteria. Because one patient moved out of the area and two patients changed their dental provider, the dropout rate was 11.54%. Therefore, 23 non-smoking patients aged (range: ) years were available for the study. All patients attended 1 4 SIT appointments/year. Of these patients, 15 were female (65.22%) and 8 were male (34.78%). The medical histories revealed cardiovascular disease in 11 (47.83%) patients. No patient suffered from diabetes. The mean follow-up was years, and the median follow-up was 5.03 (range: ) years. Table 1 summarizes the pertinent patient data. Implant treatment In total, 61 implants with a Morse taper connection (Ankylos, Dentsply Friadent, Mannheim, Germany) were provided with unsplinted telescopic crowns according to the MDC technique. The mean implant length was (range: ) mm. Table 2 summarizes the distribution of implants and teeth using the FDI numbering system. One implant was lost within the observation period after 5 years of intra-oral service as a result of peri-implantitis (implant survival rate: 98.36%). Tooth treatment In total, 66 teeth were provided with telescopic crowns according to the MDC technique. Within the observation period, we observed the loss of nine teeth (13.64%) in seven patients (30.43%) after a mean of 5.58 (range: ) years. Hence, the cumulative survival rate of telescope-fitted teeth was 86.36% after a mean of 6.12 years. Reasons for tooth loss were caries (seven teeth), failed endodontic treatment (two teeth) and one crown fracture. No cases of tooth intrusion or periodontal disease were observed. Tables John Wiley & Sons A/S. Published by John Wiley & Sons Ltd 3 Clin. Oral Impl. Res. 0, 2014 / 1 7

4 Table 1. Pertinent data for the patients and dentures (n = 23) Mean age (SD) years (range: ) Sex Female 15 (65.22%) Male 8 (34.78%) Systemic conditions Diabetes mellitus 0 Coronary heart disease 11 (47.83%) Mean (median) follow-up (5.03) years (range: ) Mean implant length (range: ) mm Implants (n = 61) Ankylos 61 (100%) Opposing dentition Total denture 0 Removable Partial 10 (43.48%) Denture Fixed 13 (56.52%) Number of implant-borne abutments per denture (mean) 2.61 (range: 1 5) Number of tooth-borne abutments per denture (mean) 2.65 (range: 1 7) Number of TK snaps per denture* 4 (as a rule) * Additional retaining elements of the Marburg double crowns. Table 2. Distribution of the maxillary implants (n = 61) and teeth (n = 66) Tooth position (FDI) Implants (n) Teeth (n) Federation Dentaire Internationale (FDI). Table 3. Outcomes of the teeth (n = 66) n % Survival Loss Loss resulting from Caries Periodontitis 0 0 Intrusion 0 0 Endodontic complications Fracture Table 4. Outcomes of the implants (n = 60) and TIRPDs (n = 23) after a mean follow-up of 6.12 years Mean probing depth (range: 2 6) mm Bleeding on probing 30 (50%) (peri-implant mucositis) mod. QHI Median 0 Grade 0 43 (71.67%) Grade 1 17 (28.33%) Grade 2 0 Implants lost 1 (1.64%) Peri-implantitis 0* Implant survival rate 60 (98.36%) Implant success rate* 60 (98.36%) TIRPD survival rate 23 (100%) mod. QHI, mean plaque index. *The one implant with peri-implantitis was lost. and 4 summarize the results obtained for the clinical parameters related to the teeth and implants. Dentures Each of the 23 included patients had received a removable telescopic denture supported by a combination of teeth (n = 66) and implants (n = 61). All TIRPDs were fabricated by onepiece casting of the secondary structure and were retained via MDCs. All dentures were found to be functional at the time of investigation, and none had been renewed; therefore, the survival probability for the TIRPDs in our study was 100% after a mean observation period of 6.12 years. The opposing dentition consisted of teeth or a fixed restoration in 13 cases (56.52%), and 10 patients were restored with a removable partial denture (43.48%); no patient had full dentures (Table 1). Technical maintenance requirements In addition to treatments necessitated by tooth loss, the patients required 18 visits to address technical requirements, for a total incidence of treatments per patient per year (T/P/Y). No case of abutment loosening occurred. Screw loosening (primary crowns were screwed onto the abutments) was found seven times, which resulted in an incidence of 0.05 T/P/Y. Within the observation period, one TK-Snap retaining element of a Marburg double crown had to be replaced (0.007 T/P/Y). Furthermore, eight treatments were necessary to address prosthetic requirements (i.e., relining or veneer fracture) (0.057 T/P/Y). Table 5 presents an overview of the technical complications and prosthetic maintenance requirements. Peri-implant mucositis/peri-implantitis The mean plaque index (mod. QHI) was (median 0), with 43 implants (71.67%) exhibiting no plaque and 17 implants (28.33%) rated as 1. The mean peri-implant probing depth was (2.25 6) mm, with a median value of 3.75 mm. Bleeding on probing (peri-implant mucositis) was observed in 30 implants (50%) and 16 patients (69.57%). BOP+ and PPD 5 mm were exhibited by two implants (3.33%), but no implant presented a mean bone loss 3.5 mm; therefore, we determined no diagnosis of peri-implantitis according to the selected criteria. Discussion The goal of this study was to present longterm data on the clinical outcomes of maxillary MDC-retained TIRPDs in patients attending a SIT program in a private practice. The study revealed a limited incidence of biological and technical complications over a >6-year period of intra-oral service. In interpreting our results, it should be considered that only a small number of patients could be presented and that only 23 of 26 patients in whom dentures were placed could be followed for the complete observation period. Furthermore, a control group with fixed or exclusively implant-supported restorations matched to the study group could not be presented. The provision of all Table 5. Prosthetic maintenance requirements of maxillary dentures (n = 23) retained on 66 teeth and 61 implants by Marburg double crowns (mean follow-up: 6.12 years) Total Abutment loosening Screw loosening TK-Snap* renewal RCMT Denture repair Number of treatments (n) Number of treatments (%) Treatments/P/Y *Additional retaining elements of the Marburg double crowns; re-cementation (primary telescope to the tooth); per patient per year. 4 Clin. Oral Impl. Res. 0, 2014 / John Wiley & Sons A/S. Published by John Wiley & Sons Ltd

5 treatments by a single provider represents another limitation. Because of the retrospective nature of the study, the observation period ranged from a minimum of 2 years up to 16 years. The investigated implant system (Ankylos â ) offers a platform switch. Recent literature indicates that this characteristic may reduce peri-implant bone loss (Al-Nsour et al. 2012). Furthermore, the possible influences of a platform switch, different implant designs or abutment necks on biological width formation and on pocket depths has not been clarified. These factors should be considered in the interpretation of the present results. Despite these limitations, the present study is one of the first investigations focused on double-crown-retained TIRPDs in maxillae with residual teeth. This investigation may contribute to further evaluation of the long-term performance of TIRPDs because of the comparably long observational period of >6 years and the fact that non-resilient telescopic crowns were assessed. In previous studies, different types of double crowns, often using resilient crowns, were investigated. The present clinical data were generated under the typical conditions of a private practice, whereas previous clinical studies of telescopic crown-retained overdentures were predominantly conducted in university settings. The tooth survival rate of 85.25% in our study fell into the lower range of similar studies of exclusively tooth-supported double-crown restorations (Wagner & Kern 2000; Verma et al. 2013). The 6-year observation period of the present study appears particularly relevant with regard to tooth loss because these events occurred after a mean of years. Previous studies typically did not cover this observation period, which may emphasize the relevance of clinical long-term studies. Implant survival rates of % have been described for removable implant-supported dentures in edentulous mandibles featuring bars or ball attachments for retention (Bergendal & Engquist 1998; Gotfredsen & Holm 2000; Oetterli et al. 2001; Naert et al. 2004). The limited published data regarding double-crown-retained IODs in edentulous jaws (Koller et al. 2011; Krennmair et al. 2012) reveal success rates of 97% 100% for implants and % for overdentures. The implant survival rate of 98.36% and the TIR- PD survival rate of 100% after 6 years found in our study were comparable to those of previous studies, that is, to the rates observed for solely double-crown-retained IODs and other implant-supported retention systems. In the literature, available data for toothand implant-supported RPDs have been rare, and most of the observation periods covered by these data are relatively short. Hug et al. (2006) monitored eight patients with eight maxillary dentures supported by tooth implant combinations over 2 years, mostly using ball attachments, but including 5 conical crowns. Krennmair et al. (2007) reported implant and tooth survival rates of 100% each and a 5% incidence of screw loosening in 22 patients with 60 maxillary implants followed for a mean of 3.2 years. Nickenig et al. (2008) did not observe any differences in complication rates between implant-supported fixed prostheses and telescopic removable dentures in 224 patients. Kaufmann et al. (2009) inserted removable dentures in 65 partially dentate jaws, mostly using root caps and ball attachments for anchorage. They included only seven toothsupported and eight implant-supported telescopes in their study and indicated observation periods 3 years for 31 implants. No data were specifically given for the telescopes. Bernhart et al. (2012) analyzed 16 patients with maxillary (n = 14) and mandibular (n = 2) restorations supported by 40 implants and 44 teeth over 2 years and reported 100% survival and success rates for both teeth and implants, with one case of screw loosening and two veneer fractures. Schwarz et al. (2012) reported on doublecrown-retained dentures after a mean observation period of 3.4 years. Thirty-six TIRPDs (24 maxillary) supported by 80 implants and 102 teeth and 30 IODs supported by 129 implants were investigated. The survival rate of the TIRPDs was 100%. The risk of tooth-supported telescopic dentures undergoing loss of function was considerably reduced by instituting a systematic recall program with professional maintenance (W ostmann et al. 2007). Our results support this finding for telescopic crown-retained TIRPDs, as the denture survival rates were found to be considerably higher than those reported for tooth-supported double-crown dentures and were similar to those offered by purely implant-supported double-crown dentures (Koller et al. 2011; Frisch et al. 2013a). Presumably, the high-implant survival rate and the low prevalence of peri-implant disease (mucositis and peri-implantitis) can be explained, at least partially, by excellent access to the abutments for oral hygiene and a high degree of patient compliance in our SIT program (with its comparatively extensive schedule of trimonthly visits). Prosthetic maintenance requirement values are an important factor in evaluating restorative treatment concepts. Published data on technical complications of IRPDs have been scarce; the maintenance requirements vary between 0.27 and 4.03 treatments per patient per year (T/P/Y) (Rentsch-Kollar et al. 2010; Weinl ander et al. 2010; Mackie et al. 2011; Krennmair et al. 2012). In a prospective study assessing four interforaminal implants in the mandible and covering 3 years of follow-up, the requirement for prosthetic maintenance was 0.41 treatments per patient per year (T/ P/Y) with bar structures, compared with 0.45 T/P/Y for telescopic restorations (Krennmair et al. 2012). Another 5-year study of edentulous mandibles revealed 0.37 T/P/Y for milled bars supported by four implants, 1.2 T/P/Y for egg-shaped bars supported by four implants and 1.04 T/P/Y for ovoid transversal bars supported by two implants (Weinl ander et al. 2010). Maintenance requirements varying from 1.09 to 4.03 T/P/Y were observed in an 8-year comparative study of various implant systems used with ball attachments on two interforaminal implants (Mackie et al. 2011). A 10-year study of cases predominantly involving two interforaminal implants resulted in 0.25 T/P/Y for bars and 0.37 T/P/Y for ball attachments (Rentsch- Kollar et al. 2010). Our finding of T/P/ Y confirms the relatively low-maintenance requirements for the implant-supported telescopic designs that were used (i.e., dentures fabricated by 1-piece casting of the secondary structure and retained by MDCs). Of these visits, 44% were exclusively devoted to the dentures themselves (i.e., relining or resin fractures). In another 11%, it was necessary to re-cement the tooth-retained telescopes, whereas only 45% of visits were related to the technology connecting the implants to the IODs. Fixed dentures supported by tooth implant combinations and featuring non-rigid connections were previously found to intrude in approximately 5% of the natural teeth involved (Lang et al. 2004). The present findings for removable double-crown-retained restorations revealed no intrusion of teeth. The Consensus of the 7th European Workshop on Periodontology describes periimplant mucositis as a host response to the presence of bacterial biofilms and as a phase preliminary to peri-implantitis (Lang & Berglundh 2011). With a prevalence of 48% for mucositis and no cases of peri-implantitis, the data from the present study reveal 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd 5 Clin. Oral Impl. Res. 0, 2014 / 1 7

6 lower rates for peri-implant disease than those reported by Lindhe & Meyle (2008), who found mucositis and peri-implantitis, respectively, in 50% and 22 43% of the implant sites studied. Recent studies have yielded lower peri-implantitis rates of 18%, 11.2% and 9.1%, respectively, among individuals attending professional SIT programs (Rinke et al. 2011; Costa et al. 2012; Frisch et al. 2013a). The results of our study appear to support these findings. It should be mentioned that soft tissue quality (keratinzed mucosa width) and soft tissue level (mucosal recession) were not evaluated in our study. Therefore, not all peri-implant tissue changes could be included and the dimension of attachment loss might be underestimated. According to the literature, different types of double crowns have successfully been used for teeth and implants, but data comparing different double-crown systems are scarce. All of these studies reveal the same advantages with regard to accessibility for oral hygiene measurements. Therefore, we hypothesize that comparable data concerning biological complications may be obtained for each type. However, our results may not be applicable to other double-crown systems, because they include differences from the MDC system. IRPDs based on the MDC technique are cost-effective because noble alloys can currently be avoided. These restorations are well suited for double-crownretained overdentures on both implants and teeth and therefore may be recommended for clinical use. Implications for dental practice 1. The reported treatment concept of rigidly retained TIRPDs may be well suited for patients with few remaining maxillary teeth. Based on the rates of implant and denture survival, in addition to biological and technical complications, this technique is comparable to the alternative retention concepts and may be utilized clinically. 2. Patients with maxillary TIRPD restorations and compliance with a SIT program demonstrated a low risk for peri-implantitis. The present data may lead clinicians to reconsider the importance of implant accessibility for hygiene instruments. Easy access should be prioritized, especially in elderly patients. 3. The reported design of the dentures (onepiece casting of the secondary structure and retention by MDCs) showed no increased risk for technical complications, even over extended periods. Conclusion In summary, our study revealed comparably high success rates and low rates of biological and technical complications for implants and dentures over a >6-year period for maxillary TIRPDs in patients attending supportive post-implant therapy (SIT) in a private practice. Overdentures supported by teeth and implants and rigidly retained by MDCs appear to be well suited for rehabilitating maxillae with residual teeth. Further prospective studies with more patients are necessary to validate the present results and to compare this treatment concept with alternative concepts of retaining RPDs and with alternative types of double crowns. Disclaimer The authors report no conflicts of interest or third-party funding of the study. References Al-Nsour, M.M., Chan, H.L. & Wang, H.L. (2012) Effect of the platform-switching technique on preservation of peri-implant marginal bone: a systematic review. International Journal of Oral and Maxillofacial Implants 27: Andreiotelli, M., Att, W. & Strub, J.R. (2010) Prosthodontic complications with implant overdentures: a systematic literature review. International Journal of Prosthodontics 23: Bergendal, T. & Engquist, B. (1998) Implant-supported overdentures: a longitudinal prospective study. International Journal of Oral and Maxillofacial Implants 13: Bergman, B., Ericson, A. & Molin, M. 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