Effect of electrode characteristics on electromyographic activity of the masseter muscle.

OBJECTIVES: The study investigated effects of electrode material, inter-electrode distance (IED), and conductive gel on electromyographic (EMG) activity recorded from the masseter muscle. MATERIALS AND METHODS: EMG was recorded unilaterally, as ten volunteers performed standardized oral tasks. Ag/AgCl and Ag coated with Au were the gel-based; Ag alloy coated with graphene, pure Ag coated with graphene and silver nanowire embedded electrodes were the gel-free materials tested. Ag/AgCl electrodes were tested at three different IEDs (i.e. 15 mm, 20 mm, 25 mm). An electrode relative performance index (ERPI) was defined and calculated for each of the standardized oral tasks that the volunteers performed. ERPI values obtained for the different oral tasks with different electrode materials and IEDs were compared using two-way repeated-measures ANOVA. RESULTS: ERPI values were not significantly influenced by IED. However, for the electrode materials statistically significant differences were found in ERPI values for all oral tasks. Of the gel-free electrode materials tested, pure silver electrodes coated with graphene had the highest ERPI values followed by Ag alloy electrodes coated with graphene and silver nanowire embedded electrodes. CONCLUSIONS: Within the limitations of the study, IED between 15 and 25 mm has a negligible effect on masseter muscle EMG. Graphene coated and silver nanowire embedded electrodes show promise as gel-free alternatives.

The influence of undercut depths on the accuracy of casts poured from irreversible hydrocolloid impression materials.

OBJECTIVES: To evaluate the elastic recovery of a typical irreversible hydrocolloid impression material in terms of the amount of undercut identified and additional thickness of spacer provided before custom tray fabrication. METHODS: This study surveyed the undercut depths of the teeth surfaces and lingual sulci of one hundred partially dentate mandibular dental casts which were subsequently scanned and the data exported as STL files. A typical cast from each undercut category (0.5mm, 1mm, 1.5mm, 2mm, 2.5mm, 3mm, 4mm, 5mm) was 3D printed. Three custom trays were constructed with spacer thicknesses representing 20%, 30% and 50% compression against the wall of the custom trays on removal, impressions recorded, and casts fabricated in dental stone. Measurements for elastic recovery of irreversible hydrocolloid were done using an internal caliper and a digital caliper. All measurements were done by the principal investigator. Non-parametric analyses were used for comparisons of the undercut values between the 3D printed and poured casts to determine the amount of elastic recovery of the irreversible hydrocolloid impression material. RESULTS: There was no statistically significant difference in measurements between the original 3D printed casts and the poured casts, irrespective of percentage compression (p > 0.05). CONCLUSIONS: In removable partial denture construction, casts should be blocked out parallel to the path of insertion and then an additional 3mm of spacer applied as a standard technique before custom tray fabrication to ensure elastic recovery of the irreversible hydrocolloid impression. CLINICAL IMPLICATIONS: In severe undercut situations, accurate impressions can be taken using irreversible hydrocolloid by blocking out all undercut surfaces on the teeth and lingual sulci before adapting additional three-millimetre wax spacer on casts when constructing custom trays thereby ensuring accurate and reliable impressions for removable prosthesis construction.

Changes in surface characteristics of moderately roughened grade iv titanium disc following a standardised implantoplasty technique: an in-vitro study

Aim: To analyze the changes in surface characteristics of moderately roughened grade IV titanium discs following a standardized implantoplasty protocol. Materials and Method: Nine moderately roughened titanium discs (ø9.0 mm; 2-mm thickness) with a comparable surface to commercially available oral implants (Southern Implants (Pty) Ltd, Irene, South Africa) were used. One disc was used as a control sample while the remaining 8 discs were modified using a standardized technique. Each disc was divided into 4 sections of which each quadrant was instrumented in the same manner. Regular grit and superfine grit diamond burs were used for 10s each, followed by brown and green silicone burs for 15s respectively. The surface characteristics of all samples were analysed using scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM), and energy dispersive x-ray spectroscopy (EDS). Results: SEM analysis of the surfaces showed that as the instrumentation progressed from diamond burs to silicone burs, surface irregularities began to diminish. However, using silicone burs tended to increase the amount of surface debris and the roughness still remained significantly higher than the control sample. EDS identified some foreign elements originating from the silicone burs that were used. Conclusion: The proposed standardized implantoplasty protocol did not decrease the roughness of the surface below that of the control disc. The clinical implications of foreign elements remaining on the titanium surfaces need further investigation.

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.

Mechanical properties of dental tissues in dolphins (Cetacea: Delphinoidea and Inioidea).

(1) Mammalian teeth play a major role in food acquisition and processing. While most mammals are heterodont and masticate their food, dolphins are homodont with simplified tooth morphology and negligible mastication. Understanding mechanical properties of dental tissues in dolphins is fundamental to elucidate the functional morphology and biomechanics of their feeding apparatus. This paper aims to study the hardness and elastic modulus of enamel and dentine in dolphins. (2) Teeth of 10 extant species (Inioidea and Delphinoidea) were longitudinally sectioned, polished and mounted in a UMIS nanoindenter. Indentations were performed from dentine to outer enamel. Hardness and elastic modulus were calculated using the Oliver-Pharr method. (3) Mean values of hardness and elastic modulus were similar on buccal and lingual surfaces. While dentine hardness was statistically similar among species, enamel hardness varied from 3.86GPa (±0.4) in Steno bredanensis (rough-toothed dolphin) to 2.36GPa (±0.38) in Pontoporia blainvillei (franciscana). For most species, there was a gradational increase in hardness values from inner to outer enamel. Enamel and dentine elastic modulus values clearly differed among species. In enamel, it ranged from 69.32GPa (±4.08) in the rough-toothed dolphin to 13.51GPa (±2.80) in Stenella coeruleoalba (striped dolphin). For most species, elastic modulus values were highest at inner and outer enamel. (4) Differences in mechanical properties between species, and within the enamel of each species, suggest functional implications and influence of ultrastructural arrangement and chemical composition.

A natural functionally graded biocomposite coating--human enamel.

Human enamel has been found to be a coating with excellent mechanical performance, and has undergone extensive investigation and discussion. However, most of the reported studies consider the enamel as a homogeneous anisotropic biocomposite. The current study illustrated the graded properties of the biocomposite from its functional load-bearing direction. Within the thickness of the enamel, from the outer surface towards the enamel-dentin junction (EDJ), the elastic modulus (E(x)) and hardness (H(x)) of enamel exist in an exponential relationship with normalized thickness (x) as E(x)=111.64x(0.18) (R(2)=0.94) and H(x)=4.41x(0.16) (R(2)=0.87) GPa, respectively. Moreover, the creep ability of enamel increases towards the EDJ. The graded properties of the biocomposite can be explained by both microstructural and compositional changes along the thickness of the material towards the EDJ. Finite element analysis indicates that the graded properties of enamel have important roles in reducing the enamel-dentin interface stresses and maintaining the integrity of the multilayer tooth structure. The results provide a new angle to understand the excellent mechanical behaviour of the multilayer tooth structure and may inspire the development of new functionally graded materials and coating structures.

Development of a dental model hardener.

The paper presents a novel method of strengthening dental gypsum materials by infiltrating with a low-viscosity autopolymerising monomer. Gypsum specimens were infiltrated with diluted butyl-cyanoacrylate monomer at different concentrations. Nanoindentation was employed to measure their hardness and elastic modulus. A standard single-edge-notched beam (SENB) method was used to measure fracture toughness. Biaxial strength was measured using a universal test machine with a piston-on-ring jig. Scanning Electron Microscopy (SEM) and affiliated energy dispersive X-ray spectroscopy (EDX) were used to observe the fracture topography and the depth of infiltration. Infiltration and autopolymerisation of the cyanoacrylate improved the biaxial strength and fracture toughness of the gypsum by around 39% and around 30%, respectively. The hardness and elastic modulus for the treated specimens were also higher, albeit slightly. The proposed method provides a convenient way to strengthen and repair dental gypsum models in situ.

Full-arch milled titanium implant bridge: technical report.

The manufacturing of full-arch fixed implant-supported bridges with the use of the traditional lost wax technique remains a technical challenge. Distortion of the alloy during casting and subsequent heating cycles during porcelain build-up causes numerous problems. Fracturing of porcelain on large restorations is difficult and costly to restore. The fitting problems can be eliminated by utilising CAD/CAM technology in the manufacturing of long-span or full-arch titanium bridges. Repair of damaged porcelain can be simplified with the use of discrete, individually-removable crowns on the bridge.

A micro-mechanical evaluation of the effects of die hardener on die stone.

The purpose of this study was to investigate the properties of a die hardener penetrated layer and evaluate its protective effects on the surface of die stone. A commercial die hardener (PDQ die hardener, Whipmix corp., USA) was tested on a die stone (GC Fujirock EP die stone, GC Europe, Belgium) and a dental plaster (Dental Stone, United States Gypsum Company, USA). Nanoindentation and micro-scratch tests were performed on both coated and uncoated specimens. The scratch damage was observed by SEM and the penetration depth of die hardener was detected by the affiliated EDX. Upon drying, the die hardener penetrated into the die stone to a depth of 3-5 microm, and deposited a thin film on the surface of die stone. Although the die hardener penetrated layer did not show improved mechanical properties, the die hardener film on the surface did protect the specimens from abrasion damage.