1

Introduction

Since decades, vital teeth can be safely restored and kept vital using amalgam and more and more resin composites in order to act minimally invasive . During and especially after completed endodontic treatment, the situation considerably changes due to the weakening effect of endodontic access preparation and the often large amount of tissue having been lost by extensive caries excavation . It is proven that the biomechanical stability of posterior teeth after access cavity and even further preparation is dramatically decreased with a high risk of vertical root fracture . Thus, it is logical that clinical studies focusing on endodontically treated teeth (ETT) reveal inferior outcomes compared to vital teeth , although recent studies could not find less tactile sensitivity of ETT compared to vital teeth which makes the “cherry stone” theory questionable, i.e. that the opening reflex for ETT is be delayed, involving heavier load input during routine mastication of hard items .

However, a deeper look into clinical data exhibits that vertical root fractures accounted only for 12% of extractions of ETT, compared to 15% cusp fractures and 40% periodontal problems . Nevertheless, this altogether sums up to 27% fractures as reason for ETT loss . So the appropriate preparation and/or restoration was always a matter of concern in the literature, having been reflected by several studies and reviews with a special focus on postendodontic restorations . Whereas for the restoration of vital teeth it is generally accepted that minimally invasive preparations are supporting clinical long-term success , the influence of preparation invasiveness with ETT is still not fully understood. For eample, the clinical outcome of direct resin composite restorations is discussed contradictorily in the literature of the field ranging from simply catastrophic to acceptable . The same is true for the preclinical estimation of innovative ceramics and hybrid materials, which have been claimed to act as better shock absorbers during mastication which also makes them interesting as ETT restorations . Although prospective, randomized clinical long-term trials are the optimum instrument to investigate clinical behavior of different restorative procedures in vivo, these studies are time-consuming, expensive, and permanently suffering potential patient drop out which is finally corroborating statistical impact . Therefore, in vitro studies are still of concern, especially when thermomechanical loading scenarios are applied being close to the clinically observed circumstances .

Aim of the present study was to evaluate the fracture resistance and behavior of different preparation/restoration concepts of ETT. The null hypothesis was two-fold, that (1) the preparation and (2) the restoration would have no influence on fracture strength of previously thermomechanically fatigued ETT.

2

Methods and materials

264 intact, non-carious, unrestored human lower third molars with similar size and fully developed roots, extracted for therapeutic reasons under informed consent of the patients and upon approval by a local ethics committee (Ref no. 143/09), were stored in an aqueous solution of 0.5% chloramine T at 4 °C for up to 30 days. The teeth were debrided of residual plaque and calculus, and examined to ensure that they were free of defects under a light microscope at ×20 magnification. Beside the control group without any preparation ( n = 8), 256 teeth in 32 groups ( n = 8) received endodontic access cavities and root canal preparation at a working length of −1 mm from the apical foramen using MTwo rotary instruments (VDW, Munich, Germany) up to size .04/#40. Tooth length was established using a C-Pilot file ISO 10 (VDW) that was inserted into the root canal until it could be visualized at the apical foramen. Working length was determined by subtracting 1 mm from this length. Root canals were filled with laterally compacted gutta-percha (VDW) and AH Plus sealer (Dentsply DeTrey, Konstanz, Germany), and immediately sealed (Syntac and SDR/Dentsply).

Specimens were additionally prepared MO or MOD. The cavities were cut using coarse diamond burs under profuse water cooling (80 μm diamond, Komet, Lemgo, Germany), and finished with a 25 μm finishing diamond (one pair of diamonds per four cavities). Inner angles of the cavities were rounded and the margins were not beveled except for direct restorations where margins received a 0.5 mm bevel. Polymerizable materials were light-cured with a Bluephase light-curing unit (Ivoclar Vivadent, Schaan, Principality of Liechtenstein). The intensity of the light was checked periodically with a radiometer (Demetron Research Corp, Danbury, CT, USA) to ensure that 1000 mW/cm ² was always delivered during the experiments.

The different restorative procedures are displayed in Table 1 . Direct adhesive procedures: cavities (MO, MOD, partial crown preparations, Figs. 1 and 2 ) were surrounded with a metal matrix band, bonded with Syntac, and restored with Tetric EvoCeram Bulk Fill in oblique layers of 2–4 mm thickness. The increments were separately light-cured for 40 s each with the light source in contact with the coronal edge of the matrix band. After removal of the matrix band, the restorations were light-cured from their buccal and lingual aspects for an additional 20 s on each side. Prior to the finishing process, visible overhangs were removed using a posterior scaler (A8 S204S, Hu-Friedy, Leimen, Germany). Proximal margins were finished with flexible disks (SofLex Pop-on, 3M ESPE, St. Paul, USA).

Table 1

Experimental set up of the study.

Abbreviations: F: filling, DPC: direct partial crown, I: inlay, PC: partial crown, RC: resin composite, IP: IPS Empess, EM: e.max CAD, CD: Celtra Duo, LU: Lava Ultimate, EN: Enamic, CG: Cast gold, AM: Amalgam, MO: additional MO preparation, and MOD: additional MOD preparation.

Illustration of the experimental setup with (a) Control tooth. (b) Endodontic access cavity. (c) Situation directly after root canal filling. (d) Inlay preparation for ceramic/hybrid materials. (e) Inlay dimensions and rounded angles. (f) Cast gold partial crown preparation with step and bevel.

Recordings from the restorative procedures. (a) Partial crown preparation with postendodontic seal (SDR) during further adhesive pretreatment. (b) e.max CAD inlay after finishing, polishing, and individualization. (c) Celtra Duo partial crown during adhesive luting in vitro. (d) The inlay from (b) directly after adhesive luting.

Direct amalgam procedures: cavities (MO, MOD, partial crown preparations with flattened cusps) were surrounded with a metal matrix band, and amalgam (Dispealloy, Dentsply, Konstanz, Germany) was applied with a syringe in consecutive layers. In partial crown situations, one parapulpal pin was applied on each replaced cusp (Edenta, Au, Switzerland). Amalgam restorations were polished after 24 h.

Indirect adhesive procedures (I: Inlay vs. PC) were IPS Empress CAD I/PC and e.max CAD I/PC (Ivoclar Vivadent), Celtra Duo I/PC (Dentsply), Lava Ultimate I/PC (3 M Espe), and Enamic I/PC (Vita Zahnfabrik, Bad Säckingen, Germany), all adhesively luted using Syntac/Variolink (Ivoclar Vivadent) after pretreatment of the intaglio surface as recommended by the manufacturers (HF etching or airborne particle abrasion). The chosen indirect conventional procedure was cast gold I/PC (Degunorm, Degudent, Hanau, Germany), cemented with Ketac Cem (3M Espe, Seefeld, Germany). The experimental set up is displayed in Table 1 , compositions of involved materials are shown in Table 2 . CAD/CAM restorations were made in a Cerec 3-D device (Sirona, Bensheim, Germany), cast gold was manufactured on stone cast dies according to the lost-wax technique. Inlay preparations for indirect adhesive restorations were butt-joint, preparations for cast gold received a 1 mm bevel. PC preparations for indirect adhesive procedures were cut according to previously published guidelines for CAD/CAM restorations ( Fig. 1 ), PC preparations for cast gold were conventional with step and bevel ( Fig. 2 ).

Thermo-mechanical loading of all specimens – also control teeth – was then performed in an artificial oral environment (CS4 professional line, SD Mechatronik, Feldkirchen, Germany). One specimen was arranged in one simulator chamber and occluded against a steatite (a multi-component semi-porous crystalline ceramic material) antagonist (6 mm in diameter, Fig. 3 a ) obliquely hitting cusps of restored teeth for 1,200,000 cycles at 100 N at a frequency of 0.5 Hz. The specimens were previously subjected to 300,000 thermal cycles between +5 °C and +55 °C (THE 1100, SD Mechatronik, Feldkirchen, Germany). The mechanical action and the water temperature within the chewing chambers were checked periodically to ensure a reliable thermo-mechanical loading (TML) effect. After completion of the thermomechanical fatigue regimen, specimens were loaded in a universal testing machine (Zwicki, Zwick, Ulm, Germany) until fracture. Failed specimens were subjected to photographic documentation ( Figs. 3 and 4 ).

Recordings from the fracture strength evaluation in the universal testing machine. (a) Cast gold partial crown during loading. (b) Crashing of the roots because for cast gold partial crowns no restoration fracture occurred. (c) Occlusal view after root fracture in a cast gold partial crown: a distinct plastic deformation of the gold is visible (arrows).

Recordings from the fracture strength evaluation in the universal testing machine. (a) Lava Ultimate inlay during loading exhibiting a cusp fracture. (b) Failure mode in direct resin composite restoration: the resin-composite interface as well as the cusp tip show distinct fracture lines.

For statistical appraisal of recorded results, the Kolmogorov–Smirnov test proved values to be consistent with a normal distribution curve, so parametric statistical analyses were carried out (one-way ANOVA and post hoc Tukey–Kramer test), considering the preparation and restoration techniques as variable. The significance level was set as 5% (SPSS 15.0, SPSS Inc., Chicago, IL, USA).

2

Methods and materials

264 intact, non-carious, unrestored human lower third molars with similar size and fully developed roots, extracted for therapeutic reasons under informed consent of the patients and upon approval by a local ethics committee (Ref no. 143/09), were stored in an aqueous solution of 0.5% chloramine T at 4 °C for up to 30 days. The teeth were debrided of residual plaque and calculus, and examined to ensure that they were free of defects under a light microscope at ×20 magnification. Beside the control group without any preparation ( n = 8), 256 teeth in 32 groups ( n = 8) received endodontic access cavities and root canal preparation at a working length of −1 mm from the apical foramen using MTwo rotary instruments (VDW, Munich, Germany) up to size .04/#40. Tooth length was established using a C-Pilot file ISO 10 (VDW) that was inserted into the root canal until it could be visualized at the apical foramen. Working length was determined by subtracting 1 mm from this length. Root canals were filled with laterally compacted gutta-percha (VDW) and AH Plus sealer (Dentsply DeTrey, Konstanz, Germany), and immediately sealed (Syntac and SDR/Dentsply).

Specimens were additionally prepared MO or MOD. The cavities were cut using coarse diamond burs under profuse water cooling (80 μm diamond, Komet, Lemgo, Germany), and finished with a 25 μm finishing diamond (one pair of diamonds per four cavities). Inner angles of the cavities were rounded and the margins were not beveled except for direct restorations where margins received a 0.5 mm bevel. Polymerizable materials were light-cured with a Bluephase light-curing unit (Ivoclar Vivadent, Schaan, Principality of Liechtenstein). The intensity of the light was checked periodically with a radiometer (Demetron Research Corp, Danbury, CT, USA) to ensure that 1000 mW/cm ² was always delivered during the experiments.

The different restorative procedures are displayed in Table 1 . Direct adhesive procedures: cavities (MO, MOD, partial crown preparations, Figs. 1 and 2 ) were surrounded with a metal matrix band, bonded with Syntac, and restored with Tetric EvoCeram Bulk Fill in oblique layers of 2–4 mm thickness. The increments were separately light-cured for 40 s each with the light source in contact with the coronal edge of the matrix band. After removal of the matrix band, the restorations were light-cured from their buccal and lingual aspects for an additional 20 s on each side. Prior to the finishing process, visible overhangs were removed using a posterior scaler (A8 S204S, Hu-Friedy, Leimen, Germany). Proximal margins were finished with flexible disks (SofLex Pop-on, 3M ESPE, St. Paul, USA).

Table 1

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