1

Introduction

CAD/CAM fabrication with intraoral digitalization utilizes the potential of different materials. Besides ceramics, a number of different resin composites and one resin-infiltrated ceramic are available as CAD/CAM materials. These resin-based systems can be applied without additional treatment (e.g., sintering, crystallization, glazing) and therefore have great potential in cost- and time-effective chairside applications. The materials provide different material compositions and resulting mechanical properties , in vitro behavior , performance on implants , fracture toughness , and machinability . Due to the low expenditure and limited mechanical properties, the materials can be recommended especially for, veneers, onlays, and partly for single crowns. Manufactures promote the materials for their enhanced edge stability and machinability, especially in thin, marginal areas, which represent advantages over ceramics .

The bonding capacity and strength of the materials are dependent on preparation design, marginal fit, and type of cementation , but long-term marginal fit, adaptation, and stability are especially important factors for guaranteeing the clinical success of a restoration. Marginal wear and load transfer to the margins may cause loss of the cement and leave the margins of the restoration without support. To reduce marginal fractures, the margins should not be located under direct loading, and adhesive bonding may be used to improve marginal strength and quality. Minor chipping seems to result in reduced aesthetics and increased marginal staining. Bigger fractures might even cause enhanced tooth sensitivity, plaque accumulation, or microleakage and might result in secondary caries. Especially for brittle materials, cracking, chipping, or fracture at the margins or cusps could initiate a fracture of the entire restoration . Insufficient preparation or fit, due for instance to the radius of the milling tool , may exacerbate these effects. Repeated chewing forces on a material with low elasticity (E-modulus of resin composite: ∼15 GPa, lithium disilicate ceramic: ∼100 GPa) or low edge stability may cause spreading or fracture of the margins and finally debonding. Resin composite restorations may exhibit lower bond strengths than ceramic restorations due to the lower bond strength of the resin composites or due to a higher stress distribution at the central bonding interface .

Edge force tests may allow for the characterization of the stability of a restoration at the margin . Fracture and chipping are supposed to be influenced by the distance from the edge, the thickness of the restoration , the type of intender , and bonding to the underlying (tooth) structure. Watts et al. suggested − based on a linear relationship between force and distance from the edge – that the edge chip test should be performed at a maximum force to be applied at a distance of 0.5 mm . A clinically relevant test method should determine the force required to cause edge failure of the restoration. In this test, a standardized indenter was positioned near the edge of the specimen, and a force was applied with constant velocity. The maximum force required to cause a crack, chip, or fracture of the material was determined. This maximum force was characterized as “edge force”. For investigating the influence of the distance from the edge, tests were performed with varying distances between 0.4 to 1.0 mm.

The null hypotheses of this in vitro study were that edge force does not differ with

• a

  different materials,

• b

  the thickness of the specimens, or

• c

  the distance from the margin.

Individual materials were tested to check for the influence of adhesive bonding on edge force.

2

Materials and methods

Edge force was determined for eight resin composites, one resin-infiltrated ceramic, and one clinically proven lithiumdisilicate ceramic (reference). All materials are intended for processing with computer-aided design and computer-aided manufacturing (CAD/CAM; e.g. Cerec Omnicam, MCXL, Sirona, D). Materials and manufacturers’ details are listed in Table 1 .

CAD/CAM blocks were trimmed and sectioned (Leica SP1600, diamond wheel, water cooling, 600 rpm, n = 10 per material). To investigate the influence of the material’s thickness, specimens (10 mm diameter) were cut into 1-mm- and 2-mm-thick plates. To simulate the situation after milling, resin composites and resin-infiltrated ceramic were used as milled, and the ceramic reference (REF: EmaxCAD, Ivoclar Vivadent) was crystallized according to the manufacturer’s instructions (820 °C–840 °C, 13 min). Surface roughness (Ra, Rz) was determined using a profilometric surface contact measuring device (Perthometer SP6, Feinprüf Perthen; LT = 1.7/0.25, 0.1 mm/s, 2 μm diamond indenter).

To determine the load to failure/crack (maximum loading force), specimens were fixed and loaded with a Vickers diamond indenter (1 mm/min, Zwick 1446) at a distance of 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 mm from the edge using a micrometer adjustment. Force detection was set to a 10% drop of the actual force. To guarantee parallel impression of one ridge, the Vickers intender was positioned with the second ridge in direction to the edge of the specimen. Specimens were separated into four sections in order to perform the measurements. Edge force results at a distance of 0.5 mm were used .

To test the influence of an underlying tooth structure, 1-mm discs were bonded to a 1-mm-thick resin composite disc. In view of the above results and to reduce expenditure, only LU, BC, and VE were used for this test. Mechanical properties such as the modulus of elasticity are considered important factors for simulating an underlying (tooth) structure and the performance of an edge force test . Therefore, LU discs (modulus of elasticity: ∼13 GPa) were used which provided a modulus of elasticity in the range of human dentin . Before bonding, both resin composites were sandblasted (Al ₂ O ₃ , 50 μm, 2 bar) and resin-infiltrated ceramic was etched (5% HF, 60 s). Following the clinical recommendation, adhesive bonding was performed with silane and a dual-curing universal self-adhesive resin (Ceramic Bond, Bifix SE [both Voco], 20 s with Elipar Trilight, 3 M). Edge force was tested as described above, with the tested material on top.

Failures were characterized as (A) crack, (B) minor chipping, (C) severe chipping, and (D) fracture of the plate. Exemplary scanning electron microscopy (SEM Quanta, Philips, NL; SE detector, 10 KeV, low vacuum, WD ∼40 mm, magnification up to 500x) was performed to show the details of the fracture pattern.

Calculations and statistical analyses were performed using SPSS 23.0 for Windows (SPSS Inc., Chicago, IL, USA). Normal distribution was controlled with the Kolmogorov-Smirnov test. Means and standard deviations were calculated and statistical analysis was performed with one-way analysis of variance (ANOVA) and the Bonferroni post hoc test. The level of significance (α) was set to 0.05.

2

Materials and methods

Edge force was determined for eight resin composites, one resin-infiltrated ceramic, and one clinically proven lithiumdisilicate ceramic (reference). All materials are intended for processing with computer-aided design and computer-aided manufacturing (CAD/CAM; e.g. Cerec Omnicam, MCXL, Sirona, D). Materials and manufacturers’ details are listed in Table 1 .

Table 1

Material and material properties (literature and manufactures’ information, Bis-EMA: ethoxylated bisphenol A-glycol dimethacrylate; Bis-GMA: bisphenol A-glycidyl methacrylate; Bis-MEPP: 2,2-Bis(4-methacryloxypolyethoxyphenyl)propane; TEGDMA: triethylene glycol dimethacrylate; UDMA: urethane dimethacrylate, MA: methacrylate).

Code	Material, Manufacturer	Comment	Flexural strength (MPa)	Modulus of elasticity (GPa)	Filler content (wt%)
CS	Cerasmart, GC, B	Resin-based composite (Bis-MEPP, UDMA, MA)	231	12.1 ± 0.8	64
BC	BRILLIANT Crios, Coltene AG, CH	Resin-based composite	198 ± 14	10.3 ± 0.5	70
VE	Vita Enamic, Vita Zahnfabrik, D	Resin-infiltrated ceramic (ceramic/polymer dual-network)	150–160	30	86
ET	Estelite Tokuyama Dental, J	Resin-based composite	225	13.8	70
KA	Katana Avencia, Kuraray Noritake dental Inc., J	Resin- based composite (UDMA, TEGDMA)	189.8	12.4	62
KJ	KZR-CAD Jamakin, Jamakin Co., J	Resin-based composite	235	10.4	65
LU	LAVA Ultimate, 3 M, USA	Resin-based composite (approximately 80% by weight nanoceramic particles bound in the resin matrix, Bis-GMA, UDMA, Bis-EMA, TEGDMA)	204 ± 19	12.77 ± 0.99	80
SB	SHOFU Block HC, SHOFU, US	Resin-based composite (UDMA, TEGDMA)	191	9.5 [30]	61
VG	VOCO Grandio blocs, VOCO, D	Resin-based composite	330	18	86
REF	Reference E.max CAD, Ivoclar Vivadent, FL	Lithiumdisilicate (LiSi₂), crystallized	360 ± 60	95 ± 5	100

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