1

Introduction

In addition to dental implants, minimally invasive fixed dental prostheses retained by inlays (IRFDP) are an attractive option for restoration of single missing molars .

They are a conservative, tissue-preserving alternative to conventional FDP, in particular for abutments with class I or II defects and when, for whatever reason, implant-supported restorations are not indicated. Clinical results have, however, revealed that this kind of restoration suffers from a greater incidence of complications, as a result of debonding and veneer fracture, than do conventional FDP . Thus, knowledge about their long-term performance will help to identify which design or process improvements may be necessary to make them viable definitive restorations. Today, metal-free IRFDP would be the first choice ; IRFDP manufactured from silicate ceramic or glass-fiber-reinforced composites cannot be recommended, however, because of the high likelihood of fracture of these restorations . With the introduction of yttria-stabilized, tetragonal zirconia polycrystal (Y-TZP), a stronger material became available for fabrication of all-ceramic IRFDP, and promising results in laboratory tests soon motivated clinical use of zirconia-based IRFDP to replace single missing molars .

A search of the international literature revealed only two in-vivo studies of zirconia IRFDP in the posterior region, and these studies furnished conflicting results . Whereas one study observed acceptable five-year survival , the other reported a high incidence of complications after observation for 1 year only, raising doubts about the applicability of this type of restoration .

The objectives of this study were, therefore, to determine the long-term (10-year) performance of the restorations in the latter study, to identify possible risk factors for failure of these restorations, and, if possible, to formulate recommendations which might help to prevent complications when IRFDP are used for restoration of missing posterior teeth.

2

Material and methods

More detailed information about the material and methods used in this study is available elsewhere .

Briefly, between July 2005 and June 2006, after use of strict inclusion criteria (good oral hygiene, no signs of periodontal or periapical pathology, tooth mobility not exceeding grade 1) and exclusion criteria (patient age <18, patient legally incompetent, pregnancy or lactation, unacceptable oral hygiene, bruxism, known allergic reaction to the materials used in the study), 27 patients (16 females and 11 males; mean age 42.9 ± 11.7 years, range 24–67 years) participated in this study conducted at the Department of Prosthodontics of Heidelberg University Hospital. All participants gave informed consent, and the study was approved by the local ethics committee (no. MV-326/2004) and conducted in accordance with the Declaration of Helsinki. Between May 2005 and September 2006, participants were provided with a total of 30 IRFDP replacing 15 maxillary first molars, 13 mandibular first molars, and two mandibular second molars ( Fig. 1 ). IRFDP were designed as three-unit end-to-end FDP and, depending on defects of the abutment teeth, combined two dihedral inlays ( n = 13), a dihedral and a trihedral inlay ( n = 1), a partial crown and a dihedral inlay ( n = 6), a complete crown and a dihedral inlay ( n = 8), or a complete crown and a trihedral inlay ( n = 2) as retainers ( Table 1 ).

Inlay-retained fixed dental prosthesis replacing a first molar.

Clinical treatment was performed by three dentists specialized in prosthetic dentistry. After removal of old restorative materials and excavation of caries, abutment teeth were built up with an auto-polymerizing composite resin material (Rebilda SC; VOCO GmbH, Cuxhaven, Germany) in accordance with the manufacturer’s instructions. Inlays were prepared with minimum occlusal reduction of 2 mm. For crown retainers, minimum occlusal reduction was 1.5 mm and axial reduction was 1.0 mm, prepared with a chamfer finish line. A convergence angle of 6° was attempted for all preparations. Impressions were made with polyether material (Impregum; 3 M ESPE, Seefeld, Germany). Laboratory procedures were standardized and conducted by three trained dental technicians. The frameworks were milled (Cerec InLab milling machine; Sirona Dental Company, Bensheim, Germany) from presintered blanks of yttria-stabilized zirconia (IPS e.max ZirCAD; Ivoclar Vivadent GmbH, Ellwangen, Germany) and sintered to their final dimensions at 1500 °C (Nabertherm LHT 02/16; Nabertherm GmbH, Lilienthal, Germany).

After application and firing of the appropriate 0.1 mm thick liner (IPS e.max Ceram ZirLiner; Ivoclar Vivadent GmbH), the zirconia frames were veneered with a pressable fluorapatite glass ceramic (IPS e.max ZirPress; Ivoclar Vivadent GmbH) in accordance with the manufacturer’s recommendations.

Before cementation, the inner surfaces of the frameworks were tribochemically silica-coated (Rocatec system; 3 M ESPE), with protection of the veneering material, and silanized (Monobond S; Ivoclar Vivadent GmbH). To prevent contamination with saliva or sulcus fluid, FDP were cemented under a rubber dam ( n = 7); if fixation of a rubber dam was not possible, cotton rolls and retraction cords were placed. After computerized randomization, the FDP were cemented by use of a dual-polymerizing, self-etch resin cement (either Panavia F; Kuraray Europe GmbH, Frankfurt, Germany; or Multilink Automix; Ivoclar Vivadent GmbH; each n = 15) in accordance with the manufacturers’ recommendations. Cement residues were removed and the participants were instructed in appropriate hygiene around their FDP. Participants were scheduled for regular maintenance visits, and study follow-ups were conducted 2 weeks, 6 months, and 1, 2, and 10 years after cementation. The restorations were evaluated for fractures of the veneer (chipping: veneer fracture primarily located within the veneer; delamination: large parts of the framework exposed; both determined by inspection with the naked eye) or core material, debonding, caries, endodontic and periodontal problems, and abutment tooth fractures; these were recorded, by use of standardized report forms, by a dentist who was not involved in treatment of the patients. In addition, charts were screened and patients were asked to report incidents or intervention related to the IRFDP between the study follow-ups.

Data were evaluated by use of statistical software (SPSS 24; IBM Corp., New York, USA) with the FDP as the statistical unit. Statistical evaluation included Kaplan–Meier survival and success (complication-free survival) and Cox regression analysis, with α = 0.05.

2

Material and methods

More detailed information about the material and methods used in this study is available elsewhere .

Briefly, between July 2005 and June 2006, after use of strict inclusion criteria (good oral hygiene, no signs of periodontal or periapical pathology, tooth mobility not exceeding grade 1) and exclusion criteria (patient age <18, patient legally incompetent, pregnancy or lactation, unacceptable oral hygiene, bruxism, known allergic reaction to the materials used in the study), 27 patients (16 females and 11 males; mean age 42.9 ± 11.7 years, range 24–67 years) participated in this study conducted at the Department of Prosthodontics of Heidelberg University Hospital. All participants gave informed consent, and the study was approved by the local ethics committee (no. MV-326/2004) and conducted in accordance with the Declaration of Helsinki. Between May 2005 and September 2006, participants were provided with a total of 30 IRFDP replacing 15 maxillary first molars, 13 mandibular first molars, and two mandibular second molars ( Fig. 1 ). IRFDP were designed as three-unit end-to-end FDP and, depending on defects of the abutment teeth, combined two dihedral inlays ( n = 13), a dihedral and a trihedral inlay ( n = 1), a partial crown and a dihedral inlay ( n = 6), a complete crown and a dihedral inlay ( n = 8), or a complete crown and a trihedral inlay ( n = 2) as retainers ( Table 1 ).

Inlay-retained fixed dental prosthesis replacing a first molar.

Clinical treatment was performed by three dentists specialized in prosthetic dentistry. After removal of old restorative materials and excavation of caries, abutment teeth were built up with an auto-polymerizing composite resin material (Rebilda SC; VOCO GmbH, Cuxhaven, Germany) in accordance with the manufacturer’s instructions. Inlays were prepared with minimum occlusal reduction of 2 mm. For crown retainers, minimum occlusal reduction was 1.5 mm and axial reduction was 1.0 mm, prepared with a chamfer finish line. A convergence angle of 6° was attempted for all preparations. Impressions were made with polyether material (Impregum; 3 M ESPE, Seefeld, Germany). Laboratory procedures were standardized and conducted by three trained dental technicians. The frameworks were milled (Cerec InLab milling machine; Sirona Dental Company, Bensheim, Germany) from presintered blanks of yttria-stabilized zirconia (IPS e.max ZirCAD; Ivoclar Vivadent GmbH, Ellwangen, Germany) and sintered to their final dimensions at 1500 °C (Nabertherm LHT 02/16; Nabertherm GmbH, Lilienthal, Germany).

After application and firing of the appropriate 0.1 mm thick liner (IPS e.max Ceram ZirLiner; Ivoclar Vivadent GmbH), the zirconia frames were veneered with a pressable fluorapatite glass ceramic (IPS e.max ZirPress; Ivoclar Vivadent GmbH) in accordance with the manufacturer’s recommendations.

Before cementation, the inner surfaces of the frameworks were tribochemically silica-coated (Rocatec system; 3 M ESPE), with protection of the veneering material, and silanized (Monobond S; Ivoclar Vivadent GmbH). To prevent contamination with saliva or sulcus fluid, FDP were cemented under a rubber dam ( n = 7); if fixation of a rubber dam was not possible, cotton rolls and retraction cords were placed. After computerized randomization, the FDP were cemented by use of a dual-polymerizing, self-etch resin cement (either Panavia F; Kuraray Europe GmbH, Frankfurt, Germany; or Multilink Automix; Ivoclar Vivadent GmbH; each n = 15) in accordance with the manufacturers’ recommendations. Cement residues were removed and the participants were instructed in appropriate hygiene around their FDP. Participants were scheduled for regular maintenance visits, and study follow-ups were conducted 2 weeks, 6 months, and 1, 2, and 10 years after cementation. The restorations were evaluated for fractures of the veneer (chipping: veneer fracture primarily located within the veneer; delamination: large parts of the framework exposed; both determined by inspection with the naked eye) or core material, debonding, caries, endodontic and periodontal problems, and abutment tooth fractures; these were recorded, by use of standardized report forms, by a dentist who was not involved in treatment of the patients. In addition, charts were screened and patients were asked to report incidents or intervention related to the IRFDP between the study follow-ups.

Data were evaluated by use of statistical software (SPSS 24; IBM Corp., New York, USA) with the FDP as the statistical unit. Statistical evaluation included Kaplan–Meier survival and success (complication-free survival) and Cox regression analysis, with α = 0.05.