Determination of optimal pricing and warranty policies

An important problem facing manufacturers in today\u27s competitive market is the determination of the selling price of a product and its warranty period. A longer warranty may serve as a signal of product reliability; however, it may also lead to an increase in cost and hence reduce the profit if the product reliability is low. A burn-in test may be used to improve the reliability of products prior to their shipment.;This research presented integrated models for maximizing the expected profit for products that are subjected to a burn-in test and sold with warranty. The burn-in time, warranty period, and price were chosen as three decision variables in these models. The price and warranty period were treated as marketing variables and a simple multiplicative form was used to model their effect on sales. Solution procedures were developed for several warranty policies. These procedures are applicable for any failure time distribution. Three failure time distributions were further investigated and formulas for optimal solutions were derived. Finally, two sets of data were used to illustrate the application of the models. Two computer programs were developed to solve the models both parametrically and nonparametically

Effect of surface conditioning methods on the bond strength of luting cement to ceramics

Objectives. This study evaluated the effect of three different surface conditioning methods on the bond strength of a Bis-GMA based luting cement to six commercial dental ceramics. Methods. Six disc shaped ceramic specimens (glass ceramics, glass infiltrated alumina, glass infiltrated zirconium dioxide reinforced alumina) were used for each test group yielding a total number of 216 specimens. The specimens in each group were randomly assigned to one of the each following treatment conditions: (1) hydrofluoric acid etching, (2) airborne particle abrasion, (3) tribochemical silica coating. The resin composite luting cement was bonded to the conditioned and silanized ceramics using polyethylene molds. All specimens were tested at dry and thermocycled (6.000, 5-55degreesC, 30 s) conditions. The shear bond strength of luting cement to ceramics was measured in a universal testing machine (2 mm/min). Results. In dry conditions, acid etched glass ceramics exhibited significantly higher results (26.4-29.4 MPa) than those of glass infiltrated alumina ceramics (5.3-18.1 MPa) or zirconium dioxide (8.1 MPa) (ANOVA, P <0.001). Silica coating with silanization increased the bond strength significantly for high-alumina ceramics (8.5-21.8 MPa) and glass infiltrated zirconium dioxide ceramic (17.4 MPa) compared to that of airborne particle abrasion (ANOVA, P <0.001). Thermocycling decreased the bond strengths significantly after all of the conditioning methods tested. Significance. Bond strengths of the luting cement tested on the dental ceramics following surface conditioning methods varied in accordance with the ceramic types. Hydrofluoric acid gel was effective mostly on the ceramics having glassy matrix in their structures. Roughening the ceramic surfaces with air particle abrasion provided higher bond strengths for high-alumina ceramics and the values increased more significantly after silica coating/silanization. 2003 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved

Effect of polishing procedures and hydrothermal aging on wear characteristics and phase transformation of zirconium dioxide

Statement of problem. Yttria-stabilized zirconia used for the fabrication of crowns and fixed prostheses may require intraoral adjustments after placement and cementation. Grinding and polishing methods may result in changes in the surface characteristics of zirconia. Purpose. The purpose of this in vitro study was to assess the effect of polishing procedures on surface roughness, topographical and phase changes of zirconia, and wear of the opposing dentition. Material and methods. Presintered and precut yttria-stabilized zirconia specimens (10×10×1 mm) were divided into 4 groups (control, Intensiv, Shofu, 3M ESPE) depending on the polishing method used to prepare the specimens. All tests were carried out in triplicate. The specimens were polished depending on the polishing regimen, while the control was left untreated. The specimens were thermocycled for 3000 cycles, with a temperature range of 5C to 55C. The surface roughness, elemental, and phase changes caused by polishing before and after thermocycling were assessed with surface profilometry, energy-dispersive spectroscopy, and X-ray diffraction analysis. The wear on antagonist steatite balls was also measured after mastication simulation. Statistical analysis was performed using 1-way ANOVA and the Tukey post hoc test to perform multiple comparison tests (a=.05). Results. The polishing procedures increased surface roughness (Ra) of yttria-stabilized zirconia from 0.52 for the control specimen to 0.73 for Intensiv, 0.70 for Shofu, and 0.70 for 3M ESPE (P<.05), which was reduced by thermocycling to 0.44 (control), 0.58 (Intensiv), and 0.58 (Shofu) (P<.001), while roughness remained unchanged for 3M ESPE specimens (0.75; P=.452). The deposition of aluminum when using Shofu abrasives and nickel in Intensiv was demonstrated. Phase changes were observed on the zirconia surface with formation of the monoclinic phase in all polishing methods. Specimen aging enhanced the surface phase changes and also induced compressive stresses in zirconia polished with Intensiv. The different polishing protocols did not affect the wear to the antagonist (P>.05). Conclusions. Polishing zirconia increased surface roughness and led to surface phase changes, but wear to the antagonist was not affected.peer-reviewe

The fit of tapered posts in root canals luted with zinc phosphate cement:A histological study

Objectives. Stress transmission to the root through passive fitting dental posts is partly influenced by the thickness of the cement layer between the post and the prepared root canal surface as well as the fit of the post in the root canal. The objective of this study was to compare the cement gap between the post surfaces and the root canals using five prefabricated, tapered, unthreaded titanium posts of different manufacturers, without and with cement. Methods. Following the endodontic treatment with hand instruments of 100 intact anterior teeth, post spaces were prepared using opening drills of the corresponding size of post. Fifty posts were cemented with zinc phosphate cement into the roots for each system while another 50 posts were inserted into the canal without using the cement. After histological sectioning, the cement gap was measured at six sites for three times at the coronal, middle and apical regions between the root canal wall and the post surface under a light microscope before and after cementation. Results. Before cementation, the highest overall cement gap was observed with the Dr Mooser post system (R) (46 mu m) and the lowest with the Velva post system (R) and Cylindro-Conical system (R) (30 mu m). Significantly less (P <0.05) mean cement gap was observed with respect to the Erlangen post system (R) (41 mu m), the Dr Mooser post system (R) (48 mu m), the MP Pirec post system (R) (34 mu m) and Velva post system (R) (33 pm) when compared with the Cylindro-Conical system (R) (62 tm). The Cylindro-Conical system (R) (79, 61 gm) and MP Pirec post system (R) (25, 24 tm) demonstrated no significant difference (P > 0.001) compared with Velva-Post (R) (38, 20 mu m) at the coronal and middle part, respectively (Mann-Whitney U-test, Boneferroni correction). Significant differences (P <0.001) were observed between the cement gap at the coronal and apical part for the Cylindro-Conical system (R) (79, 46 mu m), Dr Mooser post system (R) (45, 56 mu m) and MP Pirec post system (R) (25, 52 mu m). After cementation, the highest cement gap at the coronal part was obtained with the Cylindro-Conical system (R) (79 +/- 21 mu m) and the lowest with the MP Pirec post system (R) (25 +/- 9 mu m). However, at the apical end, the MP Pirec post system (R) (52 +/- 89 gm) and Dr Mooser post system (R) (56 +/- 16 gm) revealed the highest gap. Significance. Form-congruence between the preparation drill and the post systems exhibited differences. The most consistent cement gap either at the coronal, middle or apical parts of the root canals was obtained with the Erlangen post system (R). (c) 2005 Academy of Dental Materials. Published by Elsevier Ltd. All. rights reserve

Influence of various surface-conditioning methods on the bond strength of metal brackets to ceramic surfaces

With the increase in adult orthodontic treatment comes the need to find a reliable method for bonding orthodontic brackets onto metal or ceramic crowns and fixed partial dentures. In this study, shear bond strength and surface roughness tests were used to examine the effect of 4 different surface conditioning methods: fine diamond bur, sandblasting, 5% hydrofluoric acid, and silica coating for bonding metal brackets to ceramic surfaces of feldspathic porcelain. Sandblasting and hydrofluoric acid were further tested after silane application. A total of 120 ceramic disc samples were produced, and 50 were used for surface roughness measurements. The glazed ceramic surfaces were used as controls. Metal brackets were bonded to the ceramic substrates with a self-curing composite. The samples were stored in 0.9% NaCl solution for 24 hours and then thermocycled (5000 times, 5degreesC to 55degreesC, 30 seconds). Shear bond tests were performed with a universal testing device, and the results were statistically analyzed. Chemical surface conditioning with either hydrofluoric acid (4.3 mum) or silicatization (4.4 mum) resulted in significantly lower surface roughness than mechanical conditioning (9.3 mum, diamond bur; 9.7 mum, sandblasting) (P <.001). The surface roughness values reflect the mean peak-and-valley distances. The bond strengths of the brackets bonded to the ceramic surfaces treated by hydrofluoric acid with and without silane (12.2 and 14.7 MPa, respectively), silicatization (14.9 MPa), and sandblasting with silane (15.8 MPa) were significantly higher (P <.001) than those treated by mechanical roughening with fine diamond burs (1.6 MPa) or sandblasting (2.8 MPa). The highest bond strength values were obtained with sandblasting and silicatization with silane or hydrofluoric acid without silane; these fulfilled the required threshold. The use of silane after hydrofluoric acid etching did not increase the bond strength. Diamond roughening and sandblasting showed the highest surface roughness; they can damage the ceramic surface. Acid etching gave acceptable results for clinical use, but the health risks should be considered. The silicatization technique has the potential to replace the other methods; yet cohesive failures were observed in the ceramic during removal of the brackets

Factors associated with substance use among preclinical medical students in Turkey: a cross-sectional study

Introduction: Medical students experience high levels of stress due to their rigorous training, which can negatively affect their mental health. This study aimed to investigate substance use habits of medical students at Istanbul University-Cerrahpaşa and the association on their mental health and demographic factors. Methods: This cross-sectional survey study was conducted in March-April 2022 among preclinical medical students (years 1-3 of a 6-year program). A confidential, anonymous online survey consisting of four sections on sociodemographic and educational characteristics, nicotine use and dependence [Fagerström Test for Nicotine Dependence (FTND)], alcohol use [Alcohol Use Disorders Identification Test (AUDIT)], mental health status [12-item General Health Questionnaire (GHQ-12)], was distributed to 1131 students via WhatsApp and Telegram text messages. Mann-Whitney U and Kruskal Wallis tests compared variables’ distribution in the questionnaire categories. Spearman's correlation assessed associations between scales. Significance was p &lt; 0.05. Results: The study included 190 medical students. A total of 26.3% of the participants were smokers, with 8.4% showing moderate to high levels of nicotine dependence. An estimated 45.8% and 8.4% reported low-risk consumption and risky usage of alcohol, respectively. There were statistically significant associations between substance use and demographic factors such as sex, GPA, and religious belief. The study found a statistically significant correlation between FTND scores and GHQ-12 scores, and, between FTND scores and AUDIT scores. Conclusion: The findings of this study will inform the development of interventions to improve the mental health and academic performance of medical students at Istanbul University-Cerrahpaşa. Furthermore, it will raise awareness about the importance of addressing substance use among medical students in Turkey

Evaluation of spatial and functional roughness parameters on air-abraded zirconia as a function of particle type and deposition pressure

Purpose: This study evaluated the spatial and functional roughness parameters on air-abraded zirconia as a function of particle type and deposition pressure. Materials and Methods: Polished zirconia blocks (Cercon, Degussa/Dentsply) (N=30) with dimensions of 5 × 4 × 4 mm3 were air abraded according to 2 factors: a) particle type – 30-μm silica-coated alumina (CoJet) or alumina particles (45 μm); b) deposition pressure (1.5, 2.5 and 4.5 bar). Roughness parameters (Sdr, Vi, Sci and Svi) were measured in an optical profilometer (Wyko NT 1100) at the center of the air-abraded area (301.3 × 229.2 μm). Two measurements were made for each parameter from each surface. The means of each group were analyzed by 2-way ANOVA followed by Tukey’s adjustment test and Student’s t-test (alpha = 0.05). Results: Both the particle type (p 0.05) in these parameters for either particle type. Conclusion: Considering roughness parameters for micromechanical retention and parameters for adsorption mechanisms of adhesion, zirconia surfaces presented better morphological features when air abraded with silica-coated alumina than alumina particles at pressures higher than 1.5 bar. Particle deposition at 2.5 bar may be preferable to 4.5 bar pressure for avoiding possible deposition-related damage on zirconia, as there were no significant differences for the functional parameters

Gastric intramucosal pH is stable during titration of positive end-expiratory pressure to improve oxygenation in acute respiratory distress syndrome

BACKGROUND: Optimal positive end-expiratory pressure (PEEP) is an important component of adequate mechanical ventilation in acute lung injury and acute respiratory distress syndrome (ARDS). In the present study we tested the effect on gastric intramucosal pH of incremental increases in PEEP level (i.e. PEEP titration) to improve oxygenation in ARDS. Seventeen consecutive patients with ARDS, as defined by consensus criteria, were included in this clinical, prospective study. All patients were haemodynamically stable, and were not receiving vasopressors. From an initial level of 5 cmH(2)O, PEEP was titrated at 2 cmH(2)O increments until the partial arterial oxygen tension was 300 mmHg or greater, peak airway pressure was 45 cmH(2)O or greater, or mean arterial blood pressure decreased by 20% or more of the baseline value. Optimal PEEP was defined as the level of PEEP that achieved the best oxygenation. The maximum PEEP was the highest PEEP level reached during titration in each patient. RESULTS: Gastric mucosal pH was measured using gastric tonometry at all levels of PEEP. The thermodilution technique was used for measurement of cardiac index. Gastric mucosal pH was similar at baseline and at optimal PEEP levels, but it was slightly reduced at maximum PEEP. Cardiac index and oxygen delivery remained stable at all PEEP levels. CONCLUSION: Incremental titration of PEEP based on improvement in oxygenation does not decrease gastric intramucosal perfusion when cardiac output is preserved in patients with ARDS

A Scoping review on the polymerization of resin-matrix cements used in restorative dentistry

In dentistry, clinicians mainly use dual-cured or light-cured resin-matrix cements to achieve a proper polymerization of the organic matrix leading to enhanced physical properties of the cement. However, several parameters can affect the polymerization of resin-matrix cements. The main aim of the present study was to perform a scoping review on the degree of conversion (DC) of the organic matrix, the polymerization, and the light transmittance of different resin-matrix cements used in dentistry. A search was performed on PubMed using a combination of the following key terms: degree of conversion, resin cements, light transmittance, polymerization, light curing, and thickness. Articles in the English language published up to November 2022 were selected. The selected studies’ results demonstrated that restorative structures with a thickness higher than 1.5 mm decrease the light irradiance towards the resin-matrix cement. A decrease in light transmission provides a low energy absorption through the resin cement leading to a low DC percentage. On the other hand, the highest DC percentages, ranging between 55 and 75%, have been reported for dual-cured resin-matrix cements, although the polymerization mode and exposure time also influence the DC of monomers. Thus, the polymerization of resin-matrix cements can be optimized taking into account different parameters of light-curing, such as adequate light distance, irradiance, exposure time, equipment, and wavelength. Then, optimum physical properties are achieved that provide a long-term clinical performance of the cemented restorative materials

Güçlendirilmiş dental seramiklerin vickers sertlikleri ve yük altında kırılma davranışları

Objective: The objectives of this study were to determine the Vicker`s hardness of reinforced dental ceramics and determine the modes of fractures under load. Methods: Four ceramic core groups (n=7/group) from leucite (Evopress,Wegold&amp;De), low leucite (Finesse, Ceramco), glass-infiltrated (Inceram Alumina,Vita) and lithium disilicate materials (E.max press, Ivoclar) were fabricated according to each manufacturers’ instructions (thickness: 3 mm, diameter: 5 mm). Their individual veneering ceramics were vibrated, condensed in a stainless steel mold (diameter: 5 mm, height: 5 mm) and fired on the core materials. The specimens were stored in distilled water at 37°C for 24 hours prior to indentation tests and embedded in polyesther moulds. Vickers hardness values (DUH±SD) were measured (cross-head speed:7,2 gf/s, load:200 gf) and statistically analysed (ANOVA). A load of 400 N was applied on the surfaces of specimens with a diamond indentor (diameter:1 mm) at the macro hardness test machine for crack formation. The crack modes for each group were observed under the scanning electrone microscope. Results: The Vickers hardness values for low leucite veneering ceramic were significantly (P&lt;0.05) higher (236±17), followed by the leucite (129±51), glass-infiltrated (117±38), and lithium disilicate (85±34) veneering ceramic materials in decreasing order. Mainly radial or cone cracks were observed after the application of load. Conclusion: The increase in the hardness of the material led to more crack formation and resulted in longer cracks. No crack formation extending to the core materials were observed in neither of the ceramic groups under these experimental conditions. ÖZET Amaç: Bu çalışmanın amacı güçlendirilmiş dental seramiklerin Vickers sertlik değerlerinin ve yük altındaki kırılma şekillerinin belirlenmesidir. Gereç ve Yöntem: Lösit (Evopress,Wegold&amp;De), düşük lösit (Finesse, Ceramco), cam infiltrasyonlu aluminöz seramik (Inceram Alumina,Vita) ve lityum disilikat (E.max press, Ivoclar) bazlı dört farklı seramik alt yap_ materyali (n=7/grup) her bir üretici firmanın önerileri doğrultusunda hazırlandı (3 mm kalınlıkta; 5 mm çapta). Her bir alt yapı seramiğine özgü kaplama seramikleri; paslanmaz çelik bir metal kalıpta (5mm çap 5mm yükseklikte) vibrasyonla kondanse edildi ve alt yapı seramiklerinin üzerine pişirildi. Örnekler batırma testlerinden önce 37°C’ de 24 saat distile suda bekletildikten sonra polyester kalıplara gömüldü. Vickers sertlik değerleri (DUH±SD) ölçüldü (çene hızı:7,2 gf/s, yük:200 gf) ve veriler istatistiksel olarak analiz edildi (ANOVA). Çatlak oluşumu için örneklerin üst yüzeylerine makro sertlik test cihazında batıcı elmas uç ile (1 mm çaplı) 400 N yük uygulandı. Alınan taramalı elektron mikroskop görüntüleri ile her bir gruba ilişkin çatlak şekilleri gözlemlendi. Bulgular: Gruplar arasında ortalama Vickers sertlik değerleri düşük lösit grubu için anlamlı olarak (P&lt;0.05) en yüksek bulunur iken (236±17), bunu lösit (129±51), cam infiltrasyonlu aluminöz seramik (117±38), ve lityum disilikat (85±34) kaplama seramik materyalleri azalan sırayla izledi. Yük uygulaması sonrasında genellikle ışınsal ya da koni şekilli çatlakların oluştuğu gözlendi. Sonuç: Seramik materyalin sertliğinin artması daha fazla ve daha uzun çatlak oluşumuna yol açtı. Bu çalışmadaki deneysel koşullar altında kaplama seramik gruplarının hiçbirinde alt yapı seramiklerine ulaşan çatlak oluşumu gözlenmedi