Abstract
Objectives:
The present thesis was conducted to characterize a novel dental CAD/CAM restorative material, based on two interpenetrating networks of ceramic and polymer. The novelty of the material is defined by its structure. Some natural biomaterials such as spongy bone and dentin exhibit interconnected dual phase structures enhancing the mechanical properties under loading. The goal during development of the novel material (PICN; polymer-infiltrated-ceramic-network) was to emulate the structure of natural materials and thereby resulting in superior characteristics compared with traditional dental restorative materials.
The major objective of this study was the assessment of the suitability and characteristics of the novel PICN material intended for dental restorations by means of comparative in-vitro studies.
The specific objectives were to determine and identify correlations between flexural strength, strain at failure, elastic modulus and hardness versus ceramic network densities of a range of PICN materials. Furthermore to determine the damage tolerance of these materials, by comparing the strength degradation, following various sharp and blunt contact indentation loading situations. The response of the PICN materials is compared with a range of dental CAD/CAM ceramic materials and the results are discussed in terms of the factors contributing to the toughness of these different materials. In addition the damage tolerance and strength degradation after typical clinician and technician adjustments of PICNs and comparative materials was simulated in-vitro by utilizing different bur grinding procedures.
Methods:
First, a literature review was undertaken to identify the structural and mechanical behavior of existing dental CAD/CAM materials as well as materials based on interpenetrating networks. Dental and non-dental interpenetrating phase composites as well as biomaterials were included.
Secondly, experimental in-vitro studies were carried out for a range of PICNs and comparative dental materials:
a) Four experimental porous feldspar ceramic network densities ranging from 59 % to 72 % of theoretical density, resin infiltrated PICN as well as pure polymer and dense feldspar ceramic cross-sections were subjected to Vickers indentations for hardness evaluation. The flexural strength and elastic modulus were measured using three-point-bending. The fracture response of PICNs was determined for cracks induced by Vickers indentation. Scanning electron microscopy (SEM) was employed to observe the indented areas and material structures.
b) A comparative study with an number of existing CAD/CAM materials (Mark II, PICN test material 1 and 2, In-Ceram Alumina, VM 9, In-Ceram YZ; all VITA Zahnfabrik, Bad Saeckingen, Germany) and (IPS e.max CAD, Ivoclar Vivadent, Schaan, Liechtenstein) was conducted. Bending bars were cut and lapped. The initial flexural strength was determined in three-point-bending. To evaluate the damage tolerance, Vickers indentations were placed on the bending bars with varying loads (1.96 - 98.07 N). The indented bending bars were subsequently loaded to fracture in three-point-bending. In addition, the fracture toughness and R-curve behavior was determined by the indentation strength (IS) and single-edge-vee-notch beam (SEVNB) technique.
c) The same materials as listed above (b) were used to evaluate in-vitro the damage tolerance to blunt indenters using two spheres. Indentations with tungsten carbide spherical indenters (1.25 mm and 0.5 mm radius) were placed on bending bars with varying loads (1.96 - 1000 N). The indented bending bars were subsequently loaded to fracture in three-point-bending. The contact induced damage was analyzed by light microscopy and SEM. The spherical contact response was measured on high-gloss polished cross-sections.
d) Seven materials were analyzed comprising six commercially available dental restorative materials (Mark II, ENAMIC, In-Ceram Alumina, VM 9, In-Ceram YZ, IPS e.max CAD) and one experimental material (PICN 2). Bending bars were fabricated according to manufacturer’s instructions and subjected to standardized grinding with three different diamond grit burs (coarse, 151 μm; medium, 107 μm and extra fine 25 μm) and two grinding directions (transversal and longitudinal). The ground specimens were subsequently loaded to fracture in three-point-bending and analyzed by SEM. Additionally the elastic modulus and Poisson’s ratio were determined by the resonant frequency method.
Results:
The results of the conducted studies a)-d) outlined above are presented as follows:
a) Depending on the density of the porous ceramic network the flexural strength of PICNs ranged from 131 to 160 MPa, the hardness values ranged between 1.05 and 2.10 GPa and the elastic modulus ranged between 16.4 and 28.1 GPa. SEM observations of the indentation induced cracks indicate that the polymer network causes greater crack deflection than the dense ceramic material. The results were compared with simple analytical expressions for property variation of two phase composite materials.
b) With increasing pointed indentation loads the fracture strength of all materials tested decreased. The material with the highest fracture resistance to indentation induced damage, was the PICN test material 1 with an indentation load - flexural strength curve slope of 0.21. In-Ceram YZ exhibited the highest damage susceptibility with a slope of 0.4. The fracture toughness varied with measurement technique and material in the range of 0.82 (VM 9) to 4.94 (In-Ceram YZ) MPa√m for the SEVNB method and 0.96 (VM 9) to 4.97 (In-Ceram YZ) MPa√m for the IS method respectively.
c) The initial strengths for the individual materials were found to reduce above specific indentation loads, which were a function of the indenter radius. Employing a 0.5 mm radius sphere resulted in the following strength degrading loads and order of materials: VM 9 (98 N) < Mark II ∼ PICN 1 (147 N) < In-Ceram Alumina ∼ IPS e.max CAD (300 N) < PICN 2 ∼ In-Ceram YZ (500 N). For the materials indented with the 1.25 mm sphere, higher loads were required for the onset of strength degradation: VM 9 (190 N) < Mark II (300 N) < PICN 1 (400 N) < IPS e.max CAD (500 N) < In-Ceram Alumina (700 N) < PICN 2 (1000 N) < In-Ceram YZ (above 1000 N). Two different damage modes were observed by scanning electron and light microscopy: brittle cone cracking and plastic deformation. The PICN materials exhibited elastic-plastic behavior with creep. In contrast YTZP showed entirely elastic behavior upon loading with both spheres.
d) Except for the YTZP bending bars, the initial materials strength of all tested materials decreased significantly with the tested diamond burs upon adjustments in both transversal and longitudinal grinding directions. The resistance of the ground materials to strength reduction follow the order from highest to least damage tolerant material: PICN 2 > ENAMIC > Mark II > VM 9 > In-Ceram Alumina > IPS e.max CAD. The loss in strength of all examined materials after longitudinal grinding is generally less compared to transversal grinding. The lowest loss in strength occurred for VM 9 (7.79 %) and ENAMIC (9.18 %) upon longitudinal grinding direction with extra fine and medium diamond grit bur respectively.
Conclusions:
Based on the results of the present thesis the following conclusions were drawn:
1. Toughening of dental materials, can be engineered by incorporation of a lower elastic modulus second phase into porous ceramic precursors to improve the damage tolerance of restorations against introduced flaws.
2. The ratio between porous ceramic and polymer content influences the mechanical properties, especially flexural strength, elastic modulus, hardness and strain at failure of the novel PICN (polymer-infiltrated-ceramic-network) material. The Halpin-Tsai relationship appears to reliably predict the PICN materials elastic modulus values.
3. All studied materials with the exception of In-Ceram YZ and VM 9 showed some R-curve behavior. The interpenetrating network materials PICN 1, PICN 2 and In-Ceram Alumina as well as Mark II appear to exhibit the most significant R-curve behavior. PICN 1, PICN 2 and In-Ceram Alumina are porous ceramic microstructures infiltrated by polymer or glass respectively.
4. The tested PICN materials are found to be more damage tolerant than commonly used dental ceramics available on the market, and imply therewith suitability as dental restorative material.
5. The experimental PICN materials exhibit the greatest loading displacements among the tested ceramics, with plastic deformation and creep upon spherical contact. This outcome implies that in these less brittle materials occlusal contacts are distributed over a greater contact area and as such decreases the contact pressure and resulting stress intensity on surrounding defects during mastication.
6. The damage tolerance of restorative materials upon adjustments depends on specific mechanical properties and the adjustment procedure. The outcomes of the simulated grinding protocols can be adopted clinically in terms of the selection of appropriate materials, burs and adjustment parameters.
7. The interpenetrating and polymer containing composites ENAMIC and PICN exhibit the most damage tolerant behavior upon typical clinical bur grinding procedures.