J Prosthodont Res. 2024; **(**): ****–**** Journal of Prosthodontic Research Original article Partial glass-ceramic posterior restorations with margins beyond or above the cemento-enamel junction: An observational retrospective clinical study Rijkje A. Bresser a,*, Jelte W. Hofsteenge a, Gerrit J. Buijs b, Carline R. G. van den Breemer a, Mutlu Özcan a,c, Marco S. Cune a,d, Marco M. M. Gresnigt a,e a University of Groningen, University Medical Center Groningen, Center for Dentistry and Oral Hygiene, Department of Restorative Dentistry, Groningen, The Netherlands, b Buijs Tandartsen, Clinic for General Dentistry, Groningen, The Netherlands, c University of Zurich, Center of Dental Medicine, Division of Dental Biomaterials, Clinic for Reconstructive Dentistry, Zurich, Switzerland, d St. Antonius Hospital, Department of Oral Maxillofacial Surgery, Prosthodontics and Special Dental Care, Nieuwegein, The Netherlands, e Martini Hospital, Department of Special Dental Care, Groningen, The Netherlands Abstract Purpose: This observational retrospective clinical study aimed to investigate the survival and success rates of partial indirect lithium disilicate restorations with margins extending above or beyond the cementoenamel junction (CEJ). Methods: The study included patients who underwent partial indirect lithium disilicate restorations with immediate dentin sealing (IDS) between January 2008 and October 2018. All the restorations were placed in a single general dental practice following a standardized protocol. The impact of various predictive variables on the survival rates was assessed. Moreover, modified United States Public Health Service (USPHS) criteria were used to evaluate the survival quality. Results: Totally 1146 partial indirect lithium disilicate restorations in 260 patients were evaluated over an average period of 7.5 years. The cumulative survival and success rates were 97.3% and 95.3%, respectively. Margins extending beyond the cemento-enamel junction did not increase the risk of success or survival failure (P > 0.05). Patients with a high risk of caries, male sex, or non-vital teeth had a significantly higher risk of restoration failure (P < 0.05). Restorations with longer clinical service times exhibited marginally lower clinical quality (P < 0.001). Conclusions: Partial indirect glass-ceramic restorations demonstrated survival and success rates of 97.3% and 95.3%, respectively, over an extended period. However, a higher risk of restoration failure existed in patients with a high caries risk for (pre)molars that had undergone endodontic treatment and in males. In terms of the risk of success or survival failure, comparable results were obtained for the positions of the restoration margin in relation to the cemento-enamel junction. Keywords: Lithium disilicate, Partial indirect restoration, Posterior, Immediate dentin sealing Received 29 August 2023, Accepted 9 April 2024, Available online 27 April 2024 1. Introduction Partial ceramic restoration is an effective method for restoring the function and integrity of severely compromised teeth with minimal invasiveness and limited loss of healthy tooth structures[1]. Glass-ceramic materials such as lithium disilicate can be bonded to the tooth structure using adhesion techniques. These have reported 10-year survival rates ranging from 80.3 to 95.6%[2–4]. The long-term survival failures of ceramic inlays and onlays are primarily caused by restorative fractures and endodontic complications[2]. In addition to long-term failures, partial indirect restorations appear to deteriorate in terms of quality over time[5]. In clinical practice, carious lesions generally extend subgingiDOI: https://doi.org/10.2186/jpr.JPR_D_23_00219 *Corresponding author: Rijkje A. Bresser, University of Groningen, University Medical Center Groningen, Center for Dentistry and Oral Hygiene, Department of Restorative Dentistry, Antonius Deusinglaan 1, 9713 AV Groningen, The Netherlands. E-mail address: r.a.bresser@umcg.nl Copyright: © 2024 Japan Prosthodontic Society. All rights reserved. vally and beyond the cementoenamel junction (CEJ). This poses various challenges. Adhesive materials are hydrophobic and susceptible to contamination by oral fluids. This can adversely affect their bond strength to the tooth structure and ultimately reduce the functional lifespan of the restoration[6,7]. Therefore, although maintaining a WHAT IS ALREADY KNOWN ABOUT THE TOPIC? » Ambiguity remains among the limited number of clinical studies with regard to the impact of the margin location on the success and survival of partial indirect restorations. » Further long-term clinical studies are necessary to evaluate the risk factors and optimize the clinical applicability of glass-ceramic restorations in deep subgingival cavity outlines. WHAT THIS STUDY ADDS? » Partial glass-ceramic restorations demonstrated favorable survival and success rates over 7.5 years of clinical experience. A higher risk of failure existed for patients with high caries risk in non-vital teeth and for males. Glass-ceramic restorations with margins above or beyond the cemento-enamel junction showed comparable results. 2 R. Bresser, et al. / J Prosthodont Res. 2024; **(**): ****–**** dry working field is challenging, it is crucial when the cavity margins extend subgingivally. Bonding to a hydrated dentin substrate is generally considered more difficult than adhering to the enamel[8,9]. To address this issue, the immediate dentin sealing (IDS) technique has been proposed. It involves the immediate sealing of freshly prepared dentin with a layer of adhesive and a flowable composite to enhance the bond strength[10–13]. Ambiguity remains among the limited number of clinical studies on the impact of margin location on the success and survival of partial indirect restorations[14,15]. Further long-term and advanced clinical studies are necessary to evaluate the risk factors and optimize the clinical applicability of glassceramic restorations in deep subgingival cavity outlines. The study by Collares et al. (2016)[14] indicated that if the cavity margin extends beyond the CEJ and reaches into the dentin, the risk of failure may be higher than that for margins confined within the enamel (HR (95% CI); 1.78 (1.31–2.42)). However, it is important to note that contradictory evidence exists on this matter[15]. The primary objective of this study was to assess and compare the clinical survival and success rates of partial lithium disilicate restorations and to identify the relevant risk factors for survival. The study aimed to evaluate the impact of the following predetermined factors: the patient age at placement, margin location, pre-operative endodontic status, location in the mandible or maxilla, (pre)molar status, sex, caries risk, and number of restored surfaces. Therefore, we tested the null hypothesis that the margin location of the restorations does not influence the risk of survival or success rates. The second null hypothesis stated that other predetermined factors do not significantly influence the survival or success rates. The third and final null hypotheses stated that restorations with longer clinical service times show no difference in the quality of survival over time. 2. Materials and Methods 2.1. Patient population and study design The operative procedures were part of routine dental care. This retrospective study is not considered clinical research with test subjects as defined in the Medical Research Involving Human Subjects Act (WMO) (METc communication 2020/484 and CtC (RR202000588)) and was registered in the National Trial Register (research register number: NL9026). The study identified eligible patients who received partial indirect lithium disilicate restorations with IDS between January 2008 and October 2018. It excluded the cases with active periodontal or pulpal disease. The restorations were performed by five operators in a single general practice in Groningen, Netherlands, following a detailed restorative protocol. Partial ceramic restorations were used to treat (pre)molar teeth with extensive decay, cusp fractures, or failing restorations using minimally invasive preparations. This study was conducted in accordance with The Strengthening the Reporting of Observational Studies (STROBE) guidelines and checklist for observational studies[16]. 2.2. Preparation procedure Prior to the preparation procedure, silicone putty impressions were fabricated (ZETALABOR putty hard and indurent gel (catalyst) Zhermack, Badia Polesine, Italy) in conjunction with a liner from a Speedex Light Body and Speedex Universal Activator (Coltene, Whaledent Inc., Ohio, USA) to create temporary restorations. Rub- berdam isolation (Hygenic Dental Dam; Coltène Whaledent Inc., Ohio, USA) was used throughout the preparation and restoration procedures. Additional clamps on the restored tooth were used only in cases with deep margins or when these were favorable for isolation. The entire procedure was performed using a dental microscope (x10-15, OPMI Pico ZEISS, Jena, Germany). Pre-operative intra-oral radiographs were obtained from all the patients before treatment. Caries and old (composite) restorations were removed meticulously with minimal invasiveness using a red handpiece and several Komet diamonds (FG 001/025, FG 863/018, FG 8379/021, FG 881G/016, FG 6835KR/014, and FG 142C/016) and stainless steel burs (H1SE/023). The intact enamel and dentin were preserved. The cusps were capped only when less than 1 mm of the sound tooth structure of the (pre)molar wall remained after preparation or in the presence of significant cracks. Capped cusps are not standard in (pre)molars that have undergone endodontic treatment although in certain cases, the cusps may have been replaced earlier with direct amalgam or composite restorations. Therefore, the term “cusp replacing restorations” is used to indicate the absence of cusps prior to the operative procedure. Extended buccal veneer preparations were performed only in cases of severe discoloration owing to a previous endodontic treatment or amalgam staining. The location of the margins was not influenced by the position of the occlusal or articulation forces. All the preparation outlines were but-end and prepared using diamond burs or a SONICflex prep ceram handpiece (KaVo, Biberach/Riss, Germany). 2.3. IDS and impression The IDS technique was employed to enhance the adhesive bond strength[10–12]. Immediately after the preparation procedure, the freshly prepared dentin was sealed, and the teeth were placed under a rubber dam to prevent contamination from intra-oral fluids. Various additional measures such as packing Teflon and retracting rubber dam clamps and wedges were utilized to ensure an effective isolation of the teeth. The IDS technique involved the application of a self-etching dentin primer (Clearfil SE Bond; Kuraray, Osaka, Japan) to the dentin. It was then rubbed meticulously for 20 s and air-dried. A thin layer of adhesive bond (Clearfil SE Bond, Kuraray) was applied exclusively to the dentin and photopolymerized for 40 s at > 1000 mW/cm2 (Bluephase powercure lamp, Ivoclar Vivadent, Schaan, Liechtenstein). The output of the LED curing light was measured every week by one of the assistants as one of the standard operating procedures (SOPs) of the practice to assure an effective polymerization degree of the resin adhesive materials. Next, a layer of flowable composite (GrandIO flow, VOCO, Cuxhaven, Germany) was applied to the dentin and undercuts in the preparations and cured for 40 s using photopolymerization. A glycerin gel (K-Y* lubricating jelly; Johnson & Johnson, Sezanne, France) was applied to the preparation and cured for 40 s to eliminate the oxygen-inhibited layer. The IDS layer was inspected under a microscope. The excess adhesive on the enamel was removed using a rubber point (Shofu, Kyoto, Japan). Finally, the rubber dam isolation was removed to obtain impressions. To facilitate impression making, electrosurgery was performed on the subgingival margins of the sulcular epithelium. The gingival height was not modified during the procedure. Retraction cords, specifically Knitterax #0 (Sigma Dental Systems, Handewitt, Germany) and Ultrapak #000 (Ultradent, South Jordan, Utah, USA), were placed circumferentially in the sulcus of the prepared (pre) R. Bresser, et al. / J Prosthodont Res. 2024; **(**): ****–**** molars. Impressions were performed in duplicate using hydrocolloid material (Aqualoid purple extra-strong and orange extra-strong, Gingi-Pak, Camarilla, USA). Temporary restorations were created using the putty impression and chemically cured material Protemp from 3M ESPE (Seefeld, Germany). Then, these were cemented onto the prepared tooth with polycarboxylate cement (Durelon, 3M ESPE, Seefeld, Germany). 2.4. Laboratory procedure All the indirect restorations were made of pressable monolithic lithium disilicate ceramics (e.g., press, Ivoclar Vivadent, Schaan, Liechtenstein) in a single dental laboratory. The lost wax technique was performed in accordance with the manufacturer’s guidelines. Staining (IPS e.max Stains, Ivoclar Vivadent, Schaan, Liechtenstein) was performed to closely replicate the natural tooth color. Subsequently, the glass-ceramic restorations were glazed (IPS e.max Fluoglaze, Ivoclar Vivadent, Schaan, Liechtenstein) and hand-polished (Signum HP diamond polishing, Hereaus Kulzer GmbH, Hanau, Germany). The quality and marginal adaptation of all the restorations were examined rigorously using magnification loupes at 4.2× (Examvision, Rotterdam, The Netherlands) and a dental microscope (OpmiPico, Zeiss, Jena, Germany). The main standard used to assess marginal adaptation was the presence of edges on the margin, as evaluated using a sharp dental probe. If an edge was detected at a margin, it was considered unpermitted. 2.5. Restoration placement and luting procedure The intaglio surface of the lithium disilicate restoration was examined under a microscope to ensure that glaze was absent. If glaze was present, the restoration was removed with caution using a diamond bur with a red handpiece. The temporary restorations (s) and remaining polycarboxylate cement were removed using an ultrasonic tip and/or a hand scaler. Partial indirect restorations were intra-orally fitted and inspected for marginal discrepancies, adaptation, and contact points. The patient received anesthesia (Ultracain D-S Forte; Sanofi GmbH, Frankfurt, Germany), and the teeth were isolated using a rubber dam (Hygenic Dental Dam, Coltène Whaledent, Altstätten, Switzerland). In cases with subgingival margins, additional measures such as Teflon, wedges, retraction clamps, or floss ligatures were used to appropriately isolate the teeth. Subsequently, the neighboring teeth were protected using Teflon tape, and the IDS layer was activated using an intra-oral air-abrasion device (DentoprepTM, RØNVIG A/S, Daugaard, Denmark) with silica-coated aluminum oxide (COJET, 30 μm Al2O3, 3 M ESPE, Neuss, Germany) at a pressure of 2.5 bar and distance of approximately 10 mm for 2–3 s to clean the IDS surface until it appeared dull[17]. The sand particles were rinsed for 30 s, and the operative field was air-dried[7,12]. A new Teflon tape was applied, and the enamel surface was etched with 37% phosphoric acid (Ultra-etch; Ultradent; St Louis, USA) for 20 s. After rinsing with water for 20 s, the surface was air dried. Silane (EPSE-sil; 3M ESPE, Neuss, Germany) was applied to the IDS layer for 60 s and then, air-dried. If dentin was exposed owing to the prerestorative treatment of the tooth surface, a dentin primer (Clearfil SE Bond, Kuraray, Osaka, Japan) was rubbed for 20 s to ensure effective adhesion, and excess air was blown. Finally, the adhesive (Clearfil SE Bond, Kuraray, Osaka, Japan) was applied to the cavity and air-blown but not photopolymerized. Indirect restorations were etched with 9% hydrofluoric acid (Porcelain etch; Ultradent, St. Louis, MO, USA) for 60 s[18]. After rins- 3 ing with water, the samples were cleaned using 37% phosphoric acid (Ultra-etch; Ultradent; St Louis, MO, USA) for 60 s to remove additional impurities from the glass matrix. The restorations were then placed in an ultrasonic bath containing alcohol for 5 min to eliminate the remaining contaminants and etched glass particles. Subsequently, the restorations were air-dried and treated with silane (Monobond Plus, Ivoclar Vivadent, Schaan, Liechtenstein) for 60 s. After silanization, a thin layer of adhesive was applied to the restorations and air-blown but not photopolymerized. A heated composite (55 °C) was applied to the cavity (HFO composite, Micerium S.p.A., Avegno, Italy) to ensure a gentle and uniform distribution. The restorations were seated and photopolymerized for 40 s on all the three sides. Photopolymerization was repeated after applying glycerin gel to eliminate the oxygen inhibition layer. Excess luting cement was removed using a hand scaler, surgical blade (12D), or an EVA handpiece (7LP; 61 LG) (Kavo, Biberach/Riss, Germany). Finally, several polishers were employed (brownie, Shofu, Kyoto, Japan, and Optidisc 12.6 mm, Kerr, Orange, USA) to complete the outline and interface of the restorations. 2.6. Evaluation Intra-oral digital photographs and radiographs were obtained by the operators as part of standard dental care and clinical evaluation of partial indirect restorations. The data were collected and managed using REDCap electronic data capture tools hosted at Buijs Tandartsen, Groningen[19,20]. To eliminate the likelihood of selective reporting bias, all the analyses were performed after all the evaluations were completed. This study used a non-random sampling technique with a convenience sample. The restorations were evaluated based on the survival, success, and quality of survival. Survival failure was defined as secondary decay, debonding, replacement, catastrophic fracture, restorations requiring restorative treatment, or extraction. Successful failures were defined as chipping, minor adjustments to the restoration[21], or the need for endodontic treatment. Extraction owing to severe periodontal breakdown was categorized as an unrelated failure. The predetermined factors investigated for their influence on survival and success were as follows: margin location (above/beyond the CEJ), preoperative endodontic status (vital/nonvital), location (mandible/maxilla), tooth classification (premolar/ molar), sex (sex assigned at birth: male/female), cusp replacement (0/1/2/3/4), caries risk (high/low), and number of surfaces restored (1/2/3/4/5). Intra-oral radiographs were used to determine whether the restoration margins extended beyond the CEJ. A patient was designated as having a high caries risk if (s)he developed at least one new (secondary) carious lesion per year or if a present restoration was replaced owing to secondary caries[22]. Many patients followed stringent oral hygiene protocols as a preventive measure against gingivitis and new caries. The hygiene protocol involved personalized oral hygiene instructions and thorough calculus removal during each examination appointment (every six months). The quality of survival was measured according to the modified United States Public Health Service (USPHS) criteria (Table 1)[23]. Two calibrated investigators (J.W.H. and R.A.B.) who were not involved in the clinical procedures evaluated all the restorations placed between 2008 and 2018 using digital intra-oral photographs and radiographs. To establish the calibration, a third independent and experienced investigator, M.M.M.G., assessed the separate scores of the 20 cases evaluated by the other two investigators. The discrepancies were resolved through discussion until the two investigators were trained effectively and their scores became consistent. R. Bresser, et al. / J Prosthodont Res. 2024; **(**): ****–**** 4 Table 1. Criteria used for clinically evaluating the restorations (adapted version of modified USPHS criteria) Category S* Criteria 1. Photograph–adaptation restoration 0 Restoration contour is continuous with existing anatomical form and margins of the restorations 1 Restoration is slightly under of over contoured 2 Marginal overhang or tooth structure (dentin or enamel) is exposed 3 Restoration is missing, traumatic occlusion or restoration cause pain in tooth or adjacent tissue 0 No visible caries 1 Caries contiguous with the margin of the restoration 0 Excellent continuity at resin—enamel interface; no ledge formation, no discoloration 1 Slight discoloration at resin—enamel interface; ledge at interface 2 Moderate discoloration at resin—enamel interface measuring 1 mm or greater 3 Recurrent decay at margin 0 Smooth and highly shiny, similar to enamel 1 Smooth and satin, highly reflective 2 Rough and shiny, satin, somewhat reflective 3 Rough and dull or satin, not reflective 0 Absent 1 Present 0 Absent 1 Present 0 No fracture of the restoration 1 Small lines of the restoration 2 Small chippings (1/4 of restoration) 3 Moderate chippings (1/2 of restoration) 4 Severe chippings (3/4 of restoration) 5 Loose of the restoration 0 No wear 1 Wear 0 Restorations contour is continuous with existing anatomical form and margins 1 Restoration is slightly under of over contoured 2 Marginal overhang or tooth structure (dentin or enamel) is exposed 3 Restoration is missing, traumatic occlusion or restoration cause pain in tooth or adjacent tissue 0 No visible caries 1 Caries contiguous with the margin of the restoration 0 Excellent continuity at resin—enamel interface; no ledge formation, no discoloration 1 Slight discoloration at resin—enamel interface; ledge at interface 2 Moderate discoloration at resin—enamel interface measuring 1 mm or greater 2. Photograph-caries 3. Photograph-marginal adaptation 4. Photograph-polishability 5. Photograph-surface staining 6. Photograph-contact points 7. Photograph-fracture of restoration 8. Photograph-wear restoration 9. Radiograph-adaptation restoration 10. Radiograph-caries 11. Radiograph-marginal adaptation 3 Recurrent decay at margin S: score. *Scores of 0, 1, 2, 3, 4, and 5 can also be read as Alpha, Beta, Charlie, Delta, Echo, and Foxtrot. This calibration process was repeated for additional cases until consistency was achieved in the assessments of all the cases. 2.7. Statistical analyses for survival, success and qualitative analysis The results were analyzed using the statistical programs R version 4.2.2 (R Foundation for Statistical Computing, Vienna, Austria) and IBM SPSS statistics 25.0 (IBM Corp. NY, USA). The Kaplan–Meier estimate was used to calculate the overall cumulative survival and success rates. Multilevel Cox regression analysis with a frailty index and univariable subgroups for molars and premolars was conducted to assess the impact of multiple predictor variables on survival and success. Univariable subgroups for molars and premolars were established to account for the variations in the maximum number of cusps that could be replaced by the restoration (molars: n = 4, premolars: n = 2). The frailty index was utilized to account for patient-dependent factors. This was because multiple restorations were frequently (but not always) present in the same patient. Unrelated failures were not included in the statistical analyses. The quality of the restoration (modified USPHS criteria) was analyzed using the chi-squared test. The alpha level was set at 0.05 for all the tests and adjusted accordingly for post-hoc testing. 3. Results A total of 1146 partial lithium disilicate restorations in 260 patients were evaluated over an average follow-up period of 7.5 years. Of these patients, 109 were men (mean age = 52.7 years), and 151 were women (mean age = 51.0 years). Over half of the patients (n = 176) were classified as having a low risk of caries, whereas 84 were classified as having a high risk of caries. The general characteristics of the indirect restorations are presented in Table 2. Table 3 lists the R. Bresser, et al. / J Prosthodont Res. 2024; **(**): ****–**** 5 Table 2. General characteristics of the partial indirect restorations Variable (n) Margin location Pre-operative endodontic status Location Tooth classification Number of surfaces restored Above CEJ Beyond CEJ NA* 569 550 27 Vital Non-vital 970 176 Mandible Maxilla 523 623 Premolar Molar 398 748 One-sided Two-sided Three-sided 17 238 CEJ: cementoenamel junction. *NA: margin outline of restoration not extending approximal. 342 Four-sided Five-sided 318 231 Table 3. Characteristics of partial indirect restorations according to their margin location Margin location Pre-operative endodontic status Caries risk Above CEJ (n) Beyond CEJ (n) NA* (n) Vital 406 537 27 Non-vital 145 31 0 High 201 125 5 Low 350 443 22 1 2 3 4 5 1 2 3 4 5 27 0 67 146 174 164 0 163 196 142 67 CEJ: cementoenamel junction. *NA: margin outline of restoration not extending approximal. Note that two restorations in the NA group slightly involved additional mesial or distal surfaces. Number of surfaces restored Extension surfaces Fig. 1. Cumulative probability for the survival and success rates of partial indirect restorations (n = 1146) specific characteristics of the indirect restorations categorized by their margin location. The cumulative survival and success rates were determined to be 97.3% (95% CI: 95.5–99.0%) and 95.3% (95% CI: 93.4–97.2%), respectively. This is depicted in Figure 1. These results revealed an annual failure rate of 0.36% for survival and 0.63% for success. During the average evaluation period of 7.5 years, 54 restorations failed. Among these failures, 22 were owing to endodontic pathology, 13 were caused by secondary caries, 11 were extracted, three were related to fractures, three resulted in debonding, one was owing to chipping, and one required replacement. Additionally, four (pre) molars were extracted because of severe periodontal breakdown Fig. 2. Illustration of a fracture failure case of a restored maxillary premolar (25) after 10.5 years of function. A: the occlusal view of the fractured premolar. B: the fractured palatinal cusp that is embedded in the indirect restoration. C: the axial view of the fractured partial indirect restoration. D: the occlusal view after placement of the partial indirect restoration and functional examination of the first quadrant. and were classified as unrelated failures. An example of a fracture is shown in Figure 2. The results of the Cox regression analyses for survival and success are presented in Table 4. In the context of survival failure, a high caries risk emerged as a prognostic indicator for failure. It demon- 6 R. Bresser, et al. / J Prosthodont Res. 2024; **(**): ****–**** Table 4. Cox regression analysis on the influence of the variables on the success and survival rate Success Variable HR Age 95% CI Lower Upper Survival P HR 95% CI Lower Upper P 1.01 0.97 1.04 0.75 1.03 0.99 1.08 Margin location Above CEJ vs Beyond CEJ vs NA2 0.35 0.19 0.65 0.10 0.28 0.12 0.67 0.41 Pre-operative endodontic status Vital vs. non-vital 3.15 1.75 5.68 0.10 5.11 2.40 10.88 0.02* Jaw Mandibula vs maxilla 1.42 0.80 2.54 0.52 1.28 0.53 3.08 0.54 Tooth classification Molar vs premolar 1.17 0.65 2.09 1.00 1.42 0.62 3.26 0.91 Sex Male vs Female 0.39 0.20 0.74 0.02* 0.19 0.08 0.50 0.00** Caries risk Low vs high 3.24 1.76 5.98 0.00** 5.90 2.55 13.62 Number of surfaces restores 1 vs 2 vs 3 vs 4 vs 5 1.56 1.18 2.05 0.18 1.92 1.30 2.84 0.12 Operator 1 vs 2 vs 3 vs 4 vs 5 1.17 0.83 1.65 0.37 1.25 0.68 2.28 0.20 0.36 0.17 0.75 0.98 0.25 0.10 0.64 1.00 Type of restoration Partial indirect restoration vs endocrown CEJ: cementoenamel junction, HR: hazard ratio. *P < 0.05, **P < 0.01. 0.23 0.00** strated a 5.9-fold increase in the associated risk (HR = 5.90, 95% CI [2.55, 13.62], P < 0.001). The analysis further revealed that males had a 5.26 times higher risk of failure (HR = 0.19, 95% CI [0.08, 0.50]), P < 0.001). Additionally, non-vital teeth were associated with a 5.11 higher risk of failure (HR =5.11, 95% CI [2.34, 10.88], P = 0.02). Margin location in dentin as a single risk factor for restoration failure was not statistically significant (P = 0.41). Similarly, a high risk of caries was observed to increase the risk of failure of indirect partial restorations by 3.24 times (HR = 3.24, 95% CI [1.76, 5.98], P < 0.001). The Cox regression analyses also revealed an increased risk of failure of 2.56 times for males (HR = 0.39, 95% CI [0.20, 0.74], P = 0.02). Consistent with the survival failures, the margin location in dentin did not increase the risk of success failures (HR = 0.35, 95% CI [0.19, 0.65], P = 0.10). The number of surfaces included in partial indirect restorations did not affect the risk of success or survival. The quality of survival was assessed statistically using the chisquared test. The 54 failures were excluded from the qualitative analysis. This resulted in 1092 restorations being scored based on the clinical and radiographic USPHS criteria, with an average evaluation time of 6 years. Although minimal degradation was observed in the modified USPHS criteria scores, the clinical quality of the restorations deteriorated over time. The results revealed that restorations with a clinical service period exceeding six years exhibited marginally lower marginal adaptation than those with a shorter service period (χ2(2) = 154.95, P < 0.001). Furthermore, restorations with a longer clinical service period showed marginally diminished polished surfaces (χ2(2) = 96.66, P < 0.001) and an increased likelihood of surface staining (χ2(1) = 47.31, P < 0.001). The incidences of wear (χ2(1) = 1.78, P = 0.182) and fractures (χ2(2) = 1.82, P = 0.40) were not higher in the restorations with a longer clinical service period or in non-vital teeth (χ2(2) = 0.34, P = 0.845). The radiographic USPHS evaluation did not yield additional observations. Figure 3 shows multiple partial indirect restorations in the clinical and radiographic settings. These provide insights into their long-term qualitative condition. 4. Discussion The primary objective of this study was to assess and compare the clinical survival and success rates of partial lithium disilicate restorations, and to identify the relevant risk factors for survival. Fig. 3. Illustration of multiple partial indirect restorations and their qualitative condition. A: intra-oral clinical situation of three partial indirect restorations in the first quadrant after six years of function. B: intra-oral clinical situation of three partial indirect restorations in the fourth quadrant after 14 years of function. C: radiographic image of the partial indirect restorations in the first and fourth quadrant. Note a subtle degradation between the clinical images (A-B) in terms of marginal discoloration and reduced polished surface of the restorations after 14 years of function. The first hypothesis could not be rejected because the placement of the margin in either dentin or enamel did not significantly influence the risk of restoration failure in terms of success or survival. There are several potential explanations for the observations of this study, particularly with regard to the rigorous adhesive protocol employed. Rubber dam isolation was implemented consistently in all the cases to minimize the likelihood of contamination. This may have contributed to the enhanced adhesive bond strength to both dentin and enamel[24,25]. The use of an IDS may provide additional advantages[26]. Extensive in vitro studies support the use of IDS because this technique improves the adhesive bond strength to dentin[10–13]. We are aware of the inadequacy of clinical evidence regarding this technique. This raises the question of whether the absence of the IDS technique would have decreased the survival and success rates of partially indirect restorations in the present study. This was an important and informative follow-up study. Additionally, factors such as the stringent oral hygiene protocol, utilization of a microscope, and use of a preheated resin composite as a luting agent during procedures could also be contributing operator and technical factors to the observed data[27–30]. This observational retrospective clini- R. Bresser, et al. / J Prosthodont Res. 2024; **(**): ****–**** 7 cal study involved the evaluation of 1146 restorations with a mean follow-up period of 7.5 years. The cumulative survival rate for these restorations was 97.3% (95% CI: 95.5–99.0%), whereas the success rate was 95.3% (95% CI: 93.4–97.2%). These observations correspond to annual survival and success rates of 0.36% and 0.63%, respectively. The results are consistent with those reported previously for indirect partial ceramic restorations[2,3,31]. These data verify the viability of partial indirect lithium disilicate restorations as a suitable treatment option for severely damaged premolars and molars. higher risk of restoration failure in cases involving fractures. However, it is important to highlight that only three of the 54 failures were attributed to fractures[36]. With regard to dental caries (a common cause of failure in the present study), it was peculiar to observe a higher risk of failure among males. Typically, females are considered to have a higher risk of developing dental caries because of factors such as lower saliva secretion flow rates and lower pH levels[37]. The underlying cause of the elevated failure rates among males remains unclear. Several predetermined factors had a significant impact on both success and survival. This resulted in the rejection of the second null hypothesis. Approximately 25% of all the failures are attributed to the development of secondary caries, which is a relatively common cause. The results of this retrospective clinical study demonstrate that a high caries risk has an adverse influence on the survival and success rates of partial indirect restorations. In the case of a high caries risk, the probability of failure increases by 5.9 and 3.24 times, respectively. Previous studies have also revealed that a high risk of caries is a significant risk factor for direct composite restorations[30,32]. It is essential to note that caries risk can vary across patients and fluctuate over time. The third null hypothesis was also rejected because it was observed that restorations exhibited a marginal albeit statistically significant reduction in their quality of survival after six years of functioning, according to certain modified USPHS criteria. Certain deterioration in restoration quality was evident in terms of marginal adaptation, a marginally diminished polished surface, and increased surface staining. These observations are consistent with those of previous studies that examined the performance of partial indirect restorations[5]. However, although the quality of survival reduced moderately, another important conclusion from the data was evident. The survival rates were unaffected by the extent of the surfaces involved in the partial indirect restoration. This contradicts the trend observed in direct composite restorations. Therein, the involvement of multiple surfaces significantly decreases the long-term survival rates, with AFR up to 6% and an increased risk of failure[29,30]. Hence, these observations substantiate the conjecture that partial indirect restorations could be superior to direct composite restorations when treating extensive cavities and significant loss of tooth structure. The risk of survival failure for partial indirect restorations is 5.11 times higher if (pre)molars have undergone endodontic treatment prior to restorative treatment. This observation is consistent with previous research[33]. Of the 54 failures, 27 were attributed to endodontic complications that developed after restorative treatment, such as periapical lesions or pulpitis. It is important to note that certain endodontic failures may not have been caused by partial indirect restorations in terms of factors such as leakage, decay, or marginal overhangs. Typically, these endodontic complications occur in teeth treated with partial indirect restorations in close proximity to pulp tissue. To further reduce the loss of healthy tooth structure, the undercuts were filled with flowable material on top of the IDS to enhance preservation. Circumferential crowns require larger removal of the tooth structure, are more damaging to the pulp, and may cause more endodontic problems than partial tooth preparations[34]. Endodontic treatment significantly reduces the probability of survival of both indirect and direct restorations[29,33]. Root canal treatment challenges the structural integrity of the tooth and requires further removal of healthy tooth tissue. This renders the tooth less resistant to mechanical stress and more susceptible to failure. Therefore, it is not reasonable that structurally compromised teeth are generally treated with (partial) indirect restorations. In the future, it may be effective to categorize these endodontic survival failures in terms of endodontic complications within the root canals rather than considering these as failures of indirect restorations. Recent trends have classified failures as either survival or success failures based on their severity[21]. Therefore, endodontic treatment of the tooth may need to be considered before proceeding with restorative treatment. Furthermore, the risk of restoration failure was more than five times higher and statistically more significant among males than among females for both survival and success. There are two potential explanations for this observation. First, it is likely that males exhibit significantly inferior oral hygiene habits than females[35], which could increase the likelihood of restoration failure. However, it should be noted that among patients with a high caries risk, 53% were male and 47% were female. This indicated a relatively equal distribution within the population. Alternatively, it could be speculated that males have higher bite forces than females, which involves a It should be noted that the current study did not assess the periodontal health status of the patients, which is important for oral health and comfort[38]. Therefore, it remains unclear whether the location of the restorative margin beyond the CEJ in the present study had any adverse effects on the periodontal tissue or marginal alveolar bone. Previous studies have shown that restorative margins of conventional circumferential crowns located beyond the CEJ can cause increased sulcular and/or papillary bleeding, particularly when approaching the crestal bone level and invading the biological width[39–44]. In addition, larger marginal discrepancies were associated with higher periodontal inflammation and bone loss[45–47]. Nevertheless, the transition to adhesive ceramic restorations including partial restorations may have a positive effect on periodontal health, as indicated by recent studies[48–50]. For example, Eichelsbacher et al. (2009)[49] observed no adverse periodontal effects after adhesively luting crown-root fractures within the biological width. Similarly, Opperman et al. (2016)[50] reported no significant differences in periodontal health between crown-lengthening procedures and the deep-margin elevation (DME) technique for cavities with restoration margins close to the alveolar bone level. However, it is important to note that these studies had only a two-year follow-up period. Further research is required to determine the long-term effects of (partial) ceramic restorations on periodontal health. A clinical alternative for mitigating deep margins with adhesive materials is DME, as described by Opperman et al. (2016)[50]. The DME technique involves elevating the subgingival margin to a supragingival level using a direct class II composite restoration, wherein the matrix system provides additional isolation. Several clinical studies have reported potential results regarding clinical survival and periodontal health outcomes of this technique[5,49–52]. The DME technique simplifies the process of rubber dam isolation and impression-taking. It may be a viable treatment option for complex R. Bresser, et al. / J Prosthodont Res. 2024; **(**): ****–**** 8 cases where isolation is challenging. In this study, a non-random sampling technique with a convenience sample was utilized. It is important to discuss its potential influence on the results. To draw reliable conclusions that could be widely applicable to the population, a large number of restorations needed to be included. It was determined through an a priori sample size calculation. The calculation was based on the data of van den Breemer et al. (2019)[7]. It revealed a minimal number of failures. To gather such a large sample, we opted for a convenience sample approach. We attempted to evaluate the maximum number of restorations performed in private practice since 2008. Although this method was not random and could have introduced bias, we mitigated this concern by including over 40% of all the restorations completed since 2008 in our study. Given that, this extensive inclusion significantly reduced bias. Thus, the use of the convenience sample in this study is not likely to have adversely impacted the validity of the results. To conclude, partial indirect glass-ceramic restorations demonstrated favorable survival and success rates of 97.3% and 95.3%, respectively, over an extended period. However, a higher risk of restoration failure exists in patients with a high caries risk for (pre) molars that have undergone endodontic treatment and in males. In terms of the risk of success or survival failure, the position of the restoration margin and the cementoenamel junction were comparable. Although the degradation was minimal, the clinical quality of restorations reduced significantly over time. The restorations with a longer clinical service duration exhibited an observably lower marginal adaptation, diminished polished surfaces, and an increased likelihood of surface staining. Nonetheless, further research is required to evaluate its potential long-term effects on periodontal health. Acknowledgements We wish to thank Ilja Nolte, PhD, of the Department of Epidemiology UMCG, for writing the R script with frailty index. We also thank the team of dental technicians in the laboratory Kwalident, Beilen, the Netherlands, for fabricating the partial indirect glass-ceramic restorations. Conflict of interest statement This study did not receive any grants from funding agencies in the public, commercial, or not-for-profit sectors. The authors declare no conflicts of interest. References [1] Dietschi D, Magne P, Holz J. Recent trends in esthetic restorations for posterior teeth. Quintessence Int. 1994;25:659–77. PMID:9161244 [2] Morimoto S, Rebello de Sampaio FBW, Braga MM, Sesma N, Özcan M. Survival rate of resin and ceramic inlays, onlays, and overlays: A systematic review and meta-analysis. J Dent Res. 2016;95:985–94. https://doi. org/10.1177/0022034516652848, PMID:27287305 [3] Malament KA, Margvelashvili-Malament M, Natto ZS, Thompson V, Rekow D, Att W. 10.9-year survival of pressed acid etched monolithic e.max lithium disilicate glass-ceramic partial coverage restorations: performance and outcomes as a function of tooth position, age, sex, and the type of partial coverage restoration (inlay or onlay). J Prosthet Dent. 2021;126:523–32. https://doi.org/10.1016/j.prosdent.2020.07.015, PMID:33012530 [4] Strasding M, Sebestyén-Hüvös E, Studer S, Lehner C, Jung RE, Sailer I. Long-term outcomes of all-ceramic inlays and onlays after a mean observation time of 11 years. Quintessence Int. 2020;51:566–76. https://doi. org/10.3290/j.qi.a44631, PMID:32500864 [5] Bresser RA, Gerdolle D, van den Heijkant IA, Sluiter-Pouwels LMA, Cune MS, Gresnigt MMM. Up to 12 years clinical evaluation of 197 partial indirect restorations with deep margin elevation in the posterior region. J Dent. 2019;91:103227. https://doi.org/10.1016/j.jdent.2019.103227, PMID:31697971 [6] Guess PC, Strub JR, Steinhart N, Wolkewitz M, Stappert CFJ. All-ceramic partial coverage restorations—midterm results of a 5-year prospective clinical splitmouth study. J Dent. 2009;37:627–37. https://doi.org/10.1016/j. jdent.2009.04.006, PMID:19477570 [7] Van den Breemer CRG, Buijs GJ, Cune MS, Özcan M, Kerdijk W, Van der Made S, et al. Prospective clinical evaluation of 765 partial glass-ceramic posterior restorations luted using photo-polymerized resin composite in conjunction with immediate dentin sealing. Clin Oral Investig. 2021;25:1463–73. https://doi.org/10.1007/s00784-020-03454-7, PMID:32785851 [8] Nair P, Hickel R, Ilie N. Adverse effects of salivary contamination for adhesives in restorative dentistry. A literature review. Am J Dent. 2017;30:156–64. PMID:29178762 [9] Nair P, Ilie N. The long-term consequence of salivary contamination at various stages of adhesive application and clinically feasible remedies to decontaminate. Clin Oral Investig. 2020;24:4413–26. https://doi.org/10.1007/ s00784-020-03307-3, PMID:32519235 [10] Magne P, So WS, Cascione D. Immediate dentin sealing supports delayed restoration placement. J Prosthet Dent. 2007;98:166–74. https://doi. org/10.1016/S0022-3913(07)60052-3, PMID:17854617 [11] van den Breemer CRG, Özcan M, Cune MS, Ayres APA, Van Meerbeek B, Gresnigt MMM. Effect of immediate dentin sealing and surface conditioning on the microtensile bond strength of resin-based composite to dentin. Oper Dent. 2019;44:E289–98. https://doi.org/10.2341/18-052-L, PMID:31084533 [12] Gresnigt MMM, Cune MS, Schuitemaker J, van der Made SAM, Meisberger EW, Magne P, et al. Performance of ceramic laminate veneers with immediate dentine sealing: An 11 year prospective clinical trial. Dent Mater. 2019;35:1042–52. https://doi.org/10.1016/j.dental.2019.04.008, PMID:31084936 [13] van den Breemer CR, Özcan M, Pols MR, Postema AR, Cune MS, Gresnigt MM. Adhesion of resin cement to dentin: effects of adhesive promoters, immediate dentin sealing strategies, and surface conditioning. Int J Esthet Dent. 2019;14:52–63. PMID:30714054 [14] Collares K, Corrêa MB, Laske M, Kramer E, Reiss B, Moraes RR, et al. A practicebased research network on the survival of ceramic inlay/onlay restorations. Dent Mater. 2016;32:687–94. https://doi.org/10.1016/j.dental.2016.02.006, PMID:26975695 [15] Frankenberger R, Petschelt A, Krämer N. Leucite-reinforced glass ceramic inlays and onlays after six years: clinical behavior. Oper Dent. 2000;25:459– 65. PMID:11203857 [16] von Elm E, Altman DG, Egger M, Pocock SJ, Gøtzsche PC, Vandenbroucke JP; STROBE Initiative. The Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) Statement: Guidelines for reporting observational studies. Int J Surg. 2014;12:1495–9. https://doi.org/10.1016/j. ijsu.2014.07.013, PMID:25046131 [17] Gresnigt MMM, Sugii MM, Johanns KBFW, van der Made SAM. Comparison of conventional ceramic laminate veneers, partial laminate veneers and direct composite resin restorations in fracture strength after aging. J Mech Behav Biomed Mater. 2021;114:104172. https://doi.org/10.1016/j. jmbbm.2020.104172, PMID:33172798 [18] Almiro M, Marinho B, Delgado AHS, Rua J, Monteiro P, Santos IC, et al. Increasing acid concentration, time and using a two-part silane potentiates bond strength of lithium disilicate–reinforced glass ceramic to resin composite: An exploratory laboratory study. Materials (Basel). 2022;15:2045. https://doi.org/10.3390/ma15062045, PMID:35329495 [19] Harris PA, Taylor R, Minor BL, Elliott V, Fernandez M, O’Neal L, et al.; REDCap Consortium. The REDCap consortium: building an international community of software platform partners. J Biomed Inform. 2019;95:103208. https://doi. org/10.1016/j.jbi.2019.103208, PMID:31078660 [20] Harris PA, Taylor R, Thielke R, Payne J, Gonzalez N, Conde JG. Research electronic data capture (REDCap)—A metadata-driven methodology and workflow process for providing translational research informatics support. J Biomed Inform. 2009;42:377–81. https://doi.org/10.1016/j.jbi.2008.08.010, PMID:18929686 R. Bresser, et al. / J Prosthodont Res. 2024; **(**): ****–**** [21] Anusavice KJ. Standardizing failure, success, and survival decisions in clinical studies of ceramic and metal–ceramic fixed dental prostheses. Dent Mater. 2012;28:102–11. https://doi.org/10.1016/j.dental.2011.09.012, PMID:22192254 [22] Mettes TG, van der Sanden WJM, van Eeten-Kruiskamp L, Mulder J, Wensing M, Grol RPTM, et al. Routine oral examination: clinical vignettes, a promising tool for continuing professional development? J Dent. 2010;38:377–86. https://doi.org/10.1016/j.jdent.2010.01.004, PMID:20080144 [23] Ryge G. Clinical criteria. Int Dent J. 1980;30:347–58. PMID:6935165 [24] Falacho RI, Melo EA, Marques JA, Ramos JC, Guerra F, Blatz MB. Clinical in‐situ evaluation of the effect of rubber dam isolation on bond strength to enamel. J Esthet Restor Dent. 2023;35:48–55. https://doi.org/10.1111/ jerd.12979, PMID:36325593 [25] Miao C, Yang X, Wong MCM, Zou J, Zhou X, Li C, et al. Rubber dam isolation for restorative treatment in dental patients. Cochrane Libr. 2021;2021:CD009858. https://doi.org/10.1002/14651858.CD009858.pub3, PMID:33998662 [26] Hardan L, Devoto W, Bourgi R, Cuevas-Suárez CE, Lukomska-Szymanska M, Fernández-Barrera MÁ, et al. Immediate dentin sealing for adhesive cementation of indirect restorations: A systematic review and meta-analysis. Gels. 2022;8:175. https://doi.org/10.3390/gels8030175, PMID:35323288 [27] Browet S, Gerdolle D. Precision and security in restorative dentistry: the synergy of isolation and magnification. Int J Esthet Dent. 2017;12:172–85. PMID:28653049 [28] Eichenberger M, Biner N, Amato M, Lussi A, Perrin P. Effect of magnification on the precision of tooth preparation in dentistry. Oper Dent. 2018;43:501–7. https://doi.org/10.2341/17-169-C, PMID:29513642 [29] Laske M, Opdam NJM, Bronkhorst EM, Braspenning JCC, Huysmans MCDNJM. Longevity of direct restorations in Dutch dental practices. Descriptive study out of a practice based research network. J Dent. 2016;46:12–7. https:// doi.org/10.1016/j.jdent.2016.01.002, PMID:26790901 [30] Laske M, Opdam NJM, Bronkhorst EM, Braspenning JCC, Huysmans MCDNJM. Risk factors for dental restoration survival: A practice-based study. J Dent Res. 2019;98:414–22. https://doi.org/10.1177/0022034519827566, PMID:30786222 [31] Fan J, Xu Y, Si L, Li X, Fu B, Hannig M. Long-term clinical performance of composite resin or ceramic inlays, onlays, and overlays: A systematic review and meta-analysis. Oper Dent. 2021;46:25–44. https://doi.org/10.2341/19107-LIT, PMID:33882133 [32] Opdam NJM, van de Sande FH, Bronkhorst E, Cenci MS, Bottenberg P, Pallesen U, et al. Longevity of posterior composite restorations: a systematic review and meta-analysis. J Dent Res. 2014;93:943–9. https://doi. org/10.1177/0022034514544217, PMID:25048250 [33] Dioguardi M, Alovisi M, Troiano G, Caponio CVA, Baldi A, Rocca GT, et al. Clinical outcome of bonded partial indirect posterior restorations on vital and non-vital teeth: a systematic review and meta-analysis. Clin Oral Investig. 2021;25:6597–621. https://doi.org/10.1007/s00784-021-04187-x, PMID:34628547 [34] Edelhoff D, Sorensen JA. Tooth structure removal associated with various preparation designs for anterior teeth. J Prosthet Dent. 2002;87:503–9. https://doi.org/10.1067/mpr.2002.124094, PMID:12070513 [35] Abe M, Mitani A, Hoshi K, Yanagimoto S. Large gender gap in oral hygiene behavior and its impact on gingival health in late adolescence. Int J Environ Res Public Health. 2020;17:4394. https://doi.org/10.3390/ijerph17124394, PMID:32570913 [36] Varga S, Spalj S, Lapter Varga M, Anic Milosevic S, Mestrovic S, Slaj M. Maximum voluntary molar bite force in subjects with normal occlusion. Eur J Orthod. 2011;33:427–33. https://doi.org/10.1093/ejo/cjq097, PMID:21062965 9 [37] Li-Hui W, Chuan-Quan L, Long Y, Ru-Liu L, Long-Hui C, Wei-Wen C. Gender differences in the saliva of young healthy subjects before and after citric acid stimulation. Clin Chim Acta. 2016;460:142–5. https://doi.org/10.1016/j. cca.2016.06.040, PMID:27374299 [38] Padbury A Jr, Eber R, Wang HL. Interactions between the gingiva and the margin of restorations. J Clin Periodontol. 2003;30:379–85. https://doi. org/10.1034/j.1600-051X.2003.01277.x, PMID:12716328 [39] Reeves WG. Restorative margin placement and periodontal health. J Prosthet Dent. 1991;66:733–6. https://doi.org/10.1016/0022-3913(91)90405-L, PMID:1805020 [40] Günay H, Seeger A, Tschernitschek H, Geurtsen W. Placement of the preparation line and periodontal health--a prospective 2-year clinical study. Int J Periodontics Restorative Dent. 2000;20:171–81. PMID:11203559 [41] Goldberg PV, Higginbottom FL, Wilson TG Jr. Periodontal considerations in restorative and implant therapy. Periodontol 2000. 2001;25:100–9. https:// doi.org/10.1034/j.1600-0757.2001.22250108.x, PMID:11155185 [42] Schätzle M, Lang NP, Ånerud Å, Boysen H, Bürgin W, Löe H. The influence of margins of restorations on the periodontal tissues over 26 years. J Clin Periodontol. 2001;28:57–64. https://doi.org/10.1111/j.1600-051X.2001.280109.x, PMID:11142668 [43] Reitemeier B, Hänsel K, Walter MH, Kastner C, Toutenburg H. Effect of posterior crown margin placement on gingival health. J Prosthet Dent. 2002;87:167–72. https://doi.org/10.1067/mpr.2002.121585, PMID:11854673 [44] Ercoli C, Caton JG. Dental prostheses and tooth‐related factors. J Clin Periodontol. 2018;45(suppl 20):S207–18. https://doi.org/10.1111/jcpe.12950, PMID:29926482 [45] Felton DA, Kanoy BE, Bayne SC, Wirthman GP. Effect of in vivo crown margin discrepancies on periodontal health. J Prosthet Dent. 1991;65:357–64. https://doi.org/10.1016/0022-3913(91)90225-L, PMID:2056454 [46] Sorensen SE, Larsen B, Jörgensen KD. Gingival and alveolar bone reaction to marginal fit of subgingival crown margins. Eur J Oral Sci. 1986;94:109–14. https://doi.org/10.1111/j.1600-0722.1986.tb01373.x, PMID:3518035 [47] Ercoli C, Tarnow D, Poggio CE, Tsigarida A, Ferrari M, Caton JG, et al. The relationships between tooth‐supported fixed dental prostheses and restorations and the periodontium. J Prosthodont. 2021;30:305–17. https://doi. org/10.1111/jopr.13292, PMID:33210761 [48] Borges GA, Kyrillos M, Moreira M, Cal L. The Area of adhesive continuity. Quintessence Dent Technol. 2013;36:9–26. [49] Eichelsbacher F, Denner W, Klaiber B, Schlagenhauf U. Periodontal status of teeth with crown–root fractures: results two years after adhesive fragment reattachment. J Clin Periodontol. 2009;36:905–11. https://doi.org/10.1111/ j.1600-051X.2009.01458.x, PMID:19682174 [50] Oppermann R, Gomes S, Cavagni J, Cayana E, Conceição E. Response to proximal restorations placed either subgingivally or following crown lengthening in patients with no history of periodontal disease. Int J Periodontics Restorative Dent. 2016;36:117–24. https://doi.org/10.11607/ prd.2015, PMID:26697548 [51] Muscholl C, Zamorska N, Schoilew K, Sekundo C, Meller C, Büsch C, et al. Retrospective clinical evaluation of subgingival composite resin restorations with deep-margin elevation. J Adhes Dent. 2022;24:335–44. https:// doi.org/10.3290/j.jad.b3240665, PMID:35983705 [52] Bertoldi C, Monari E, Cortellini P, Generali L, Lucchi A, Spinato S, et al. Clinical and histological reaction of periodontal tissues to subgingival resin composite restorations. Clin Oral Investig. 2020;24:1001–11. https:// doi.org/10.1007/s00784-019-02998-7, PMID:31286261 This is an open-access article distributed under the terms of Creative Commons Attribution-NonCommercial License 4.0 (CC BYNC 4.0), which allows users to distribute and copy the material in any format as long as credit is given to the Japan Prosthodontic Society. It should be noted however, that the material cannot be used for commercial purposes.