J. Neil Waddell

Measuring Intraoral Pressure: Adaptation of a Dental Appliance Allows Measurement During Function

This article introduces a new way of recording intraoral pressures from a range of locations within the oral cavity. To measure pressure flow dynamics during swallowing, we fitted eight miniature pressure transducers capable of measuring absolute pressures to a chrome-cobalt palatal appliance with a labial bow. Unlike previous devices, our design provides a rigid, custom-fitted platform for the simultaneous recording of pressures at eight locations within the oral cavity during function. We placed an anterior pair of gauges to measure lingual and labial contact against the left central incisor tooth, and two pairs of gauges to measure pressure contributions of the lateral tongue margin and cheeks on the canine and first molar teeth. Finally, lingual pressure on the midline of the palate was measured by two gauges, one at the position of the premolars and one on the posterior boundary of the hard palate. We then recorded intraoral pressures in five adult volunteers seated in an upright position and asked to swallow 10 ml of water. Labial pressures on the canine rose rapidly from a resting level of 10 kPa to 33 kPa, while pressure profiles from the labial aspects of the incisor and first molar teeth followed a negative pattern, peaking at −12 kPa for the incisor and −15 kPa for the molar sensor. Pressure profiles recorded from the palatal aspects of the first molar and the canine appeared to be similar, but the former fell to −13 kPa before rising to 9 kPa, and the canine pressure rapidly increased to 22 kPa before returning to its resting level of 4 kPa. The pressure profile of the palatal aspect of the central incisor was strikingly different; at the start of the swallow, pressure dropped precipitously to −20 kPa, before slowly rising to 10 kPa. It then followed the general pattern of the other two sensors, before peaking again at 10 kPa and then returning to a resting level of 4 kPa. We also showed that there were significant negative pressures in the mouth during function, and that pressure profiles varied markedly between individuals.

Pressed ceramics onto zirconia. Part 1: Comparison of crystalline phases present, adhesion to a zirconia system and flexural strength.

OBJECTIVES To compare the crystalline phases present, quantify the adhesion to zirconia and measure the mechanical properties of four commercially available pressed ceramics suitable for zirconia substructures. MATERIALS AND METHODS This study compares the X-ray diffraction response and the mechanical properties of four different pressed ceramics (Noritake CZR Press, Vita PM9, Wieland PressXzr and IPS e.max ZirPress) to Vita In-Ceram YZ zirconia substrate. The adhesion was determined using the interfacial strain energy release rate fracture mechanics approach; in addition biaxial flexural strength values of each material was determined. RESULTS X-ray diffraction analysis revealed that Noritake CZR Press and Vita PM9 contain leucite whereas IPS e.max ZirPress and Wieland PressXzr are non-leucite amorphous materials. The strain energy release rate results revealed that the pressed ceramics with leucite have better adhesion than non-leucite ceramics to zirconia. Differences were observed between biaxial strength results for the pressed ceramics from bilayer compared with monolayer specimens. CONCLUSIONS Pressed ceramics compatible with zirconia tested in this study were of two types; leucite containing and non-leucite containing essentially glass ceramics. Leucite containing pressable ceramics appears to have better adhesion to zirconia.

Influence of veneering porcelain thickness and cooling rate on residual stresses in zirconia molar crowns

OBJECTIVE The aim of this study was to investigate the influence of increasing veneering porcelain thickness in clinically representative zirconia molar crowns on the residual stresses under fast and slow cooling protocols. METHODS Six veneered zirconia copings (Procera, Nobel Biocare AB, Gothenburg, Sweden) based on a mandibular molar form, were divided into 3 groups with flattened cusp heights that were 1mm, 2mm, or 3mm. Half the samples were fast cooled during final glazing; the other half were slow cooled. Vickers indentation technique was used to determine surface residual stresses. Normality distribution within each sample was done using Kolmogorov-Smirnov & Shapiro-Wilk tests, and one-way ANOVA tests used to test for significance between various cusp heights within each group. Independent t-tests used to evaluate significance between each cusp height group with regards to cooling. RESULTS Compressive stresses were recorded with fast cooling, while tensile stresses with slow cooling. The highest residual compressive stresses were recorded on the fast cooled 1mm cusps which was significantly higher than the 2 and 3mm fast cooled crowns (P<0.05). There was a significant linear trend for residual stress to decrease as veneering porcelain thickness increased in the fast cooled group (P<0.05). No significant differences were found between the various cusp heights during slow cooling (P≥0.05). SIGNIFICANCE Cooling rate and geometric influences in a crown anatomy have substantially different effects on residual stress profiles with increasing veneering porcelain thickness compared to the basic flat plate model.

Cracking of Porcelain Surfaces Arising from Abrasive Grinding with a Dental Air Turbine

PURPOSE The purpose of this in vitro study was to evaluate porcelain cracking induced by abrasive grinding with a conventional dental air turbine and abrasive diamond burs. MATERIALS AND METHODS Four commercially available porcelains were examined-Wieland ALLUX, Wieland ZIROX, IPS e.max Ceram, and IPS Empress Esthetic Veneering porcelain. Sixty discs of each porcelain type were fabricated according to manufacturer instructions, followed by an auto-glaze cycle. Abrasive grinding using fine, extra-fine, and ultra-fine diamond burs was carried out, using a conventional dental air turbine. The grinding parameters were standardized with regard to the magnitude of the force applied, rotational speed of the diamond bur, and flow rate of the water coolant. A testing apparatus was used to control the magnitude of force applied during the grinding procedure. The ground surfaces were then examined under scanning electron microscope. RESULTS Cracking was seen for all porcelain types when ground with the fine bur. Cracking was not seen for specimens ground with the extra-fine or the ultra-fine bur. CONCLUSION Wet abrasive grinding with a conventional dental air turbine and fine grit diamond burs has the potential to cause cracking in the four porcelain types tested. Similar abrasive grinding with smaller grit size particles does not cause similar observable cracking.

Using a Motion‐Capture System to Record Dynamic Articulation for Application in CAD/CAM Software

PURPOSE One of the current limitations of computer software programs for the virtual articulation of the opposing teeth is the static nature of the intercuspal position. Currently, software programs cannot identify eccentric occlusal contacts during masticatory cyclic movements of the mandible. MATERIALS AND METHODS Chewing trajectories with six degrees of freedom (DOF) were recorded and imposed on a computer model of one subjects maxillary and mandibular teeth. The computer model was generated from a set of high-resolution micro-CT images. To obtain natural chewing trajectories with six DOF, an optoelectronic motion-capturing system (VICON MX) was used. For this purpose, a special mandibular motion-tracking appliance was developed for this subject. RESULTS Mandibular movements while chewing elastic and plastic food samples were recorded and reproduced with the computer model. Examples of mandibular movements at intraoral points are presented for elastic and plastic food samples. The potential of such a kinematic computer model to analyze the dynamic nature of an occlusion was demonstrated by investigating the interaction of the second molars and the direction of the biting force during a chewing cycle. CONCLUSIONS The article described a methodology that measured mandibular movements during mastication for one subject. This produced kinematic input to 3D computer modeling for the production of a virtual dynamic articulation that is suitable for incorporation into dental CAD/CAM software.

Additive Technology: Update on Current Materials and Applications in Dentistry.

Additive manufacturing or 3D printing is becoming an alternative to subtractive manufacturing or milling in the area of computer-aided manufacturing. Research on material for use in additive manufacturing is ongoing, and a wide variety of materials are being used or developed for use in dentistry. Some materials, however, such as cobalt chromium, still lack sufficient research to allow definite conclusions about the suitability of their use in clinical dental practice. Despite this, due to the wide variety of machines that use additive manufacturing, there is much more flexibility in the build material and geometry when building structures compared with subtractive manufacturing. Overall additive manufacturing produces little material waste and is energy efficient when compared to subtractive manufacturing, due to passivity and the additive layering nature of the build process. Such features make the technique suitable to be used with fabricating structures out of hard to handle materials such as cobalt chromium. The main limitations of this technology include the appearance of steps due to layering of material and difficulty in fabricating certain material generally used in dentistry for use in 3D printing such as ceramics. The current pace of technological development, however, promises exciting possibilities.

Effect of autoclave induced low-temperature degradation on the adhesion energy between yttria-stabilized zirconia veneered with porcelain.

OBJECTIVE To investigate the effect of autoclave induced low-temperature degradation on the adhesion energy between yttria-stabilized zirconia veneered with porcelain. METHODS The strain energy release rate using a four-point bending stable fracture test was evaluated for two different porcelains [leucite containing (VM9) and glass (Zirox) porcelain] veneered to zirconia. Prior to veneering the zirconia had been subjected to 0 (control), 1, 5, 10 and 20 autoclave cycles. The specimens were manufactured to a total bi-layer dimension of 30 mm × 8 mm × 3 mm. Subsequent scanning electron microscopy/energy dispersive spectrometry, electron backscatter diffraction and X-ray diffraction analysis were performed to identify the phase transformation and fracture behavior. RESULTS The strain energy release rate for debonding of the VM9 specimens were significantly higher (p<0.05) compared to the Zirox specimens across all test groups. Increasing autoclave cycles lowered the strain energy release rate significantly (p<0.05) from 18.67 J/m(2) (control) to the lowest of 12.79 J/m(2) (cycle 10) for only the VM9 specimens. SEM analyses showed predominant cohesive fracture within the porcelain for all cycle groups. XRD analysis of the substrate prior to veneering confirmed a tetragonal to monoclinic phase transformation with increasing the number of autoclave cycles between 5 and 20. The monoclinic phase reverted back to tetragonal phase after undergoing conventional porcelain firing cycles. EBSD data showed significant changes of the grain size distribution between the control and autoclaved specimen (cycle 20). SIGNIFICANCE Increasing autoclave cycles only significantly decreased the adhesion of the VM9 layered specimens. In addition, a conventional porcelain firing schedule completely reverted the monoclinic phase back to tetragonal.

Clinical tooth preparations and associated measuring methods: A systematic review

STATEMENT OF PROBLEM The geometries of tooth preparations are important features that aid in the retention and resistance of cemented complete crowns. The clinically relevant values and the methods used to measure these are not clear. PURPOSE The purpose of this systematic review was to retrieve, organize, and critically appraise studies measuring clinical tooth preparation parameters, specifically the methodology used to measure the preparation geometry. MATERIAL AND METHODS A database search was performed in Scopus, PubMed, and ScienceDirect with an additional hand search on December 5, 2013. The articles were screened for inclusion and exclusion criteria and information regarding the total occlusal convergence (TOC) angle, margin design, and associated measuring methods were extracted. The values and associated measuring methods were tabulated. RESULTS A total of 1006 publications were initially retrieved. After removing duplicates and filtering by using exclusion and inclusion criteria, 983 articles were excluded. Twenty-three articles reported clinical tooth preparation values. Twenty articles reported the TOC, 4 articles reported margin designs, 4 articles reported margin angles, and 3 articles reported the abutment height of preparations. A variety of methods were used to measure these parameters. CONCLUSIONS TOC values seem to be the most important preparation parameter. Recommended TOC values have increased over the past 4 decades from an unachievable 2- to 5-degree taper to a more realistic 10 to 22 degrees. Recommended values are more likely to be achieved under experimental conditions if crown preparations are performed outside of the mouth. We recommend that a standardized measurement method based on the cross sections of crown preparations and standardized reporting be developed for future studies analyzing preparation geometry.

The influence of opaque application methods on the bond strength and final shade of PFM restorations

OBJECTIVES The aim of this study is to compare the influence of the opaque application method on the adhesion and final shade of porcelain-fused-to-metal restorations. METHODS Four samples (n=12) were cast in a NiCrMo alloy and opaqued using three application methods: aerosol-spray (one application), aerosol-spray (two applications), paste and powder-liquid prior to veneering with dentine porcelain to a thickness of 1.0mm to mimic the clinical thickness of porcelain-fused-to-metal crowns. The porcelain to metal adhesion was determined by measuring the interfacial fracture toughness or strain energy release rate with a four-point bending configuration developed by Charalambides et al.,(13) at a loading rate of 0.05mm min(-1) in kerosene. The resultant porcelain shade was measured with a spectrophotometer under daylight. Values of L*, a* and b* were used to calculate the total color difference ΔE with different equations CIELAB, CMC(1:1), CIE94, and CIEDE2000. The resulting values were also compared with a reference unbonded sample made from the same dentine porcelain used to veneer the porcelain-fused-to-metal specimens. RESULTS The application technique of the opaque did not influence the adhesion between the porcelain and metal. All the application methods demonstrated brittle cracking behaviour through the opaque layer. No visible color differences were found between the applications methods in the final shade of the porcelain. SIGNIFICANCE The application method in terms of bond strength and final porcelain shade is not a factor in choosing which method to use in the dental laboratory.

Scanning electron microscopy observations of failures of implant overdenture bars: a case series report.

BACKGROUND Soldered or cast bars are used as a standard of care in attachment systems supporting maxillary and mandibular implant overdentures. When failures of these bars occur, currently there is a lack of evidence in relation to their specific etiology, location, or nature. PURPOSE To investigate the failure process of a case series of six failed soldered bars, four intact soldered bars, and one intact cast milled bar, which had been supporting implant overdentures. MATERIALS AND METHODS A total of 11 different overdenture bars were removed from patients with different configuration of opposing arches. A failed bar (FB) group (n = 6) had failed soldered overdenture bars, which were recovered from patients following up to 2 years of wear before requiring prosthodontic maintenance and repair. An intact bar (IB) group (n = 5) had both soldered bars and a single cast milled bar, which had been worn by patients for 2 to 5 years prior to receiving other aspects of prosthodontic maintenance. All bars were examined using scanning electron microscopy to establish the possible mode of failure (FB) or to identify evidence of potential failure in the future (IB). RESULTS Evidence of a progressive failure mode of corrosion fatigue and creep were observed on all the FB and IB usually around the solder areas and nonoxidizing gold cylinder. Fatigue and creep were also observed in all the IB. Where the level of corrosion was substantial, there was no evidence of wear from the matrices of the attachment system. Evidence of an instantaneous failure mode, ductile and brittle overload, was observed on the fracture surfaces of all the FB, within the solder and the nonoxidizing gold cylinders, at the solder/cylinder interface. CONCLUSION Corrosion, followed by corrosion fatigue, appears to be a key factor in the onset of the failure process for overdenture bars in this case series of both maxillary and mandibular overdentures. Limited sample size and lack of standardization identify trends only but prevent broad interpretation of the findings.