Grinding damage assessment for CAD-CAM restorative materials.

OBJECTIVES: To assess surface/subsurface damage after grinding with diamond discs on five CAD-CAM restorative materials and to estimate potential losses in strength based on crack size measurements of the generated damage. METHODS: The materials tested were: Lithium disilicate (LIT) glass-ceramic (e.max CAD), leucite glass-ceramic (LEU) (Empress CAD), feldspar ceramic (VM2) (Vita Mark II), feldspar ceramic-resin infiltrated (EN) (Enamic) and a composite reinforced with nano ceramics (LU) (Lava Ultimate). Specimens were cut from CAD-CAM blocs and pair-wise mirror polished for the bonded interface technique. Top surfaces were ground with diamond discs of respectively 75, 54 and 18µm. Chip damage was measured on the bonded interface using SEM. Fracture mechanics relationships were used to estimate fracture stresses based on average and maximum chip depths assuming these to represent strength limiting flaws subjected to tension and to calculate potential losses in strength compared to manufacturer's data. RESULTS: Grinding with a 75µm diamond disc induced on a bonded interface critical chips averaging 100µm with a potential strength loss estimated between 33% and 54% for all three glass-ceramics (LIT, LEU, VM2). The softer materials EN and LU were little damage susceptible with chips averaging respectively 26µm and 17µm with no loss in strength. Grinding with 18µm diamond discs was still quite detrimental for LIT with average chip sizes of 43µm and a potential strength loss of 42%. SIGNIFICANCE: It is essential to understand that when grinding glass-ceramics or feldspar ceramics with diamond discs surface and subsurface damage are induced which have the potential of lowering the strength of the ceramic. Careful polishing steps should be carried out after grinding especially when dealing with glass-ceramics.

The role of the fluid phase in the viscous response of bovine periodontal ligament.

The mechanical response of the periodontal ligament (PDL) is complex. This tissue responds as a hyperelastic solid when pulled in tension while demonstrating a viscous behavior under compression. This intricacy is reflected in the tissue's morphology, which comprises fibers, glycosaminoglycans, a jagged interface with the surrounding porous bone and an extensive vascular network. In the present study we offer an analysis of the viscous behavior and the interplay between the fibrous matrix and its fluid phase. Cylindrical specimens comprising layers of dentine, PDL and bone were extracted from bovine first molars and affixed to a tensile-compressive loading machine. The viscous properties of the tissue were analyzed (1) by subjecting the specimens to sinusoidal displacements at various frequencies and (2) by cycling the specimens in 'fully saturated' and in 'partially dry' conditions. Both modes assisted in determining the contribution of the fluid phase to the mechanical response. It was concluded that: (1) PDL showed pseudo-plastic viscous features for cyclic compressive loading, (2) these viscous features essentially resulted from interactions between the porous matrix and unbound fluid content of the tissue. Removing the liquid from the PDL largely eliminates its damping effect in compression.

Mechanical response of periodontal ligament: effects of specimen geometry, preconditioning cycles and time lapse.

This study was conducted as part of research line addressing the mechanical response of periodontal ligament (PDL) to tensile-compressive sinusoidal loading. The aim of the present project was to determine the effect of three potential sources of variability: (1) specimen geometry, (2) tissue preconditioning and (3) tissue structural degradation over time. For the three conditions, selected mechanical parameters were evaluated and compared. (1) Standard flat specimens (obtained by sequentially slicing portions of bone, PDL and dentin using a precision band saw) and new cylindrical specimens (extracted with a diamond-coated trephine drill) were obtained from bovine mandibular first molars and subjected to a sinusoidal load profile. (2) Specimens were loaded with up to 2000 cycles. (3) Specimens were immersed in saline and tested after 0, 30 and 60 min. From the data generated, the following was concluded: (1) specimen geometry and preparation technique do not influence the mechanical response of the PDL; (2) the mechanical response stabilizes after approximately 1000 cycles; and (3) no major structural degradation occurs when PDL is immersed in saline for a time lapse up to 60 min.

Fixed prosthodontics centric relation registration technique using resin copings.

The registration of an accurate centric relation is one of the most critical steps in a prosthodontic rehabilitation, yet it sometimes is an uncertain procedure. A technique using resin copings, each supporting an occlusal contact, is presented. By careful, selective addition of resin, the centric relation recording can be controlled. The technique requires a pattern of abutments to provide stability, and is, therefore, primarily applicable to fixed prosthodontics. Nonetheless, it is versatile enough to accommodate a variety of clinical situations.

Mechanical and structural characteristics of commercially pure grade 2 Ti welds and solder joints.

This study aimed at determining whether data previously gathered for a laser welds and IR brazings using a Au-Pd alloy were applicable to titanium joints. As to its resistance under fatigue loading, Au-Pd alloy had shown a poor response to pre-ceramic laser welding and post-ceramic brazing. The present study was designed to assess the mechanical resistance, the microstructure and the elemental diffusion of laser welded, electric arch welded and brazed joints using commercially pure titanium as substrate metal. Mechanical resistance was determined by determining the joints' ultimate tensile strength and their resistance to fatigue loading. Elemental diffusion to and from the joints was assessed using microprobe tracings. Optical micrographs of the joints were also obtained and evaluated. Under monotonic tensile stress, three groups emerged: (1) the GTAW and the native (i.e. as received) substrate, (2) the annealed substrate and the laser welds and (3) the brazed joints. Under fatigue stress, the order was: first the native and annealed substrate, second the brazings and laser welds, third the GTAW joints. No Au-filler brazing withstood the applied fatigue loading. The micrographs showed various patterns, an absence of HAZ cracking and several occurrences of Widmanstätten structures. Elemental diffusion to and from the Ti substrate was substantial in the Ti filler brazings and virtually nil in the Au-based brazings. Under fatigue stress application, the titanium-based brazings as well as the laser- and electric arc welds performed equally well if not better than a previously tested AuPd alloy. There was a definite increase in grain size with increased heat application. However, no feature of the microstructures observed or the elemental analysis could be correlated with the specimen's resistance to fatigue stress application.

In vivo wear of three types of veneering materials using implant-supported restorations: a method evaluation.

In a previous study, we determined the precision and the positional duplicability of a system that used implant connectors to transfer restorations between the oral cavity and the measuring device. The objectives of the present study were to determine whether this testing procedure was suitable for clinical use, and to apply the procedure to the longitudinal assessment of the wear of three types of veneering materials. Ten patients received a total of 12 restorations. The restorations were made either of ceramic (positive control), poly(methylmethacrylate) resin (negative control) or of composite resin. The restorations were profiled at baseline and at 3, 6, 9, 12, 18 and 24 months. For profiling, the restorations were secured to the x-y table of the measuring device using the octagonal connector of the ITI implant system. Numerical analyzes were performed using a commercial array-oriented software package. The ceramic and the composite wore at roughly 12-14 microm yr(-1) (height) 10-12 microm3 microm2 yr(-1) (volume). The resin wore at c. 50 micorm yr(-1) (height) and c. 45 microm3 microm2 yr(-1) (volume). The error of the procedure was estimated at +/- 13%. It was concluded that the procedure was applicable for clinical studies, and that the composite did not differ from the ceramic as to its wear rate.