Mechanical properties of protective spectacles fitted with corrective lenses.

The majority of commercially available corrective spectacles used by workers do not provide effective eye protection against mechanical hazards in the workplace. One of the risks commonly occurring during work is hitting the head on some protruding elements, such as components of machines, buildings or tree branches in a forest. Because of the considerable weight of the human head and the speed of movement during impact, this type of accident may be very serious. This article presents a method of testing the mechanical strength of corrective lenses, simulating the results of an impact of the head on elements of workplaces. The results of tests of commercially available materials used for the construction of corrective and protective spectacles are also presented and discussed.

Impact resistance of common spectacle and safety lenses to airgun and rimfire projectiles.

PURPOSE: To evaluate the penetration resistance of common spectacle and safety lenses to high-velocity projectiles and to examine the current recommendations regarding the use of such lenses. METHODS: The penetration threshold of glass, high-index, and polycarbonate spectacle and safety lenses was determined by firing BBs, pellets, and 0.22-caliber projectiles. The mass, diameter, velocity, and energy of each projectile were measured, and the center thickness, mass, and refractive power of each lens were recorded. The penetration threshold for each lens type was calculated, and a comparison of impact resistance was made. RESULTS: The maximum velocity of the BB was measured at 221 m/s; pellets at 210 m/s; CB.22 rimfire projectiles at 204 m/s; and 0.22 rimfire projectiles at 290 m/s. The BB penetration thresholds were as follows: glass lenses = 84.0 m/s (2.4 J) and high-index lenses = 107.7 m/s (4.09 J). Polycarbonate spectacle and safety lenses were not penetrated by BBs regardless of their velocity; however, these lenses were penetrated by CB.22 rimfire projectiles (89.2 J). Review of the penetrated lenses showed that center thickness and refractive power was comparable between the various groups of projectiles. CONCLUSIONS: Polycarbonate offers a vastly superior degree of penetration resistance compared with other commonly used lens materials. The current recommendations regarding the use of polycarbonate in prescription and protective lenses, as endorsed by the American Academy of Ophthalmology and the American Academy of Pediatrics, must be reevaluated.

[Study on clinical standard consistency of the luting cement].

Various properties of dental luting cement are influenced by powder/liquid ratio. The standard consistency of luting cement is determined in Japanese industrial standard and American dental association's specifications. However, it is not considered that a constant consistency is best for luting cement of all kinds. This study were performed on suitable consistency of individual cement with regard to bond strength and disintegration. Most suitable consistency was almost the same as standard consistency by the specification in the case of Zinc phosphate cement (GC's Elite cement 100). But, in the case of Polycarboxylate cement (Shofu's HY-Bond carbo cement) and Glass alkynoate cement (GC's Fuji ionomer Type I Liv), maximum bond strength can be obtained with a more powder/liquid ratio than standard consistency by the specification. Especially, Glass alkynoate cement shows this tendency strongly. Therefore, it must be manipulated quickly after mixing.

A laboratory and clinical evaluation of three dental luting cements.

The loss of material from specimens of three luting cements was measured after continuous erosion cycling in the laboratory. The glass ionomer luting cement showed significantly less material loss than the zinc polycarboxylate and zinc phosphate luting cements. Two hundred and fifty restorations cemented with one of the three materials were studied clinically for marginal integrity and retention over 3.5 years. The data were tested using survival analysis. Zinc phosphate cement gave the best clinical performance. Possible explanations for the poor correlation between the findings in the laboratory and clinical study are discussed.

CEMENTO DE POLICARBOXILATO COPMO BASE CAVITARIA

Como resultado de los casos clínicos realizados en la presente tesis; podemos deducir; que las bases cavitarias practicadas con cemento de policarboxilato "DURELON" en cavidades superficiales como prfundas, no presentan ningún tipo de reacción pulpar reversible ni reacción irreversible, Por su tiempo de fraguado se puede realizar la obturación definitiva en una sóla sesión, si no se tiene dudas sobre el diagnóstico, Posee un alto grado de adhesión química alos tejidos duros del diente, Puede ser usado como único material de base cavitaria, aislante térmico, protector pulpar y buen sellador marginal, Sirve para la obturación definitiva en piezas deciduas por su baja solubilidad

Adhesives in orthopaedic surgery. A review of the literature and in vitro bonding strengths of bone-bonding agents.

Adhesives may offer a number of advantages over conventional metal osteosynthesis in the treatment of fractures: ease and speed of fixation are improved, they have the anatomy to anatomically coapt small fragments, and hardware removal is not necessary. A review of the literature was performed to evaluate experiments with collagen and other biologic adhesives, epoxy resins, polyurethane foam, cyanoacrylates, zinc polycarboxylate, polymethylmethacrylates, and fibrin adhesives pertaining to osteosynthesis. Cyanoacrylates, polymethylmethacrylates, and fibrin adhesives are currently being investigated. These agents are tested to evaluate their bone-bonding strength. Cyanoacrylates of biologically tolerated chain length did not develop useful bonding in cancellous bone; they bond well to cortical bone only with meticulous surface preparation. Fibrin adhesives developed only minimally useful bonding. Polymethylmethacrylate developed surgically useful bonding in osteosynthesis where conventional fixation is impossible.

An evaluation of mechanical and adhesive properties of polycarboxylate and glass ionomer cements.

Mechanical and adhesive properties of polycarboxylate and glass ionomer cements were studied. Water had an adverse effect on the compressive and tensile strengths of both materials. Both cements showed good bonding to enamel and dentine. Bonds between polycarboxylate cement and sterling silver were strong enough to cause cohesive failure of the cement rather than adhesive failure, when the specimens were tested. Surface cleaning and conditioning with 5% sodium hypochlorite solution showed an improvement in the bonds of the cements to dentine and they were significantly stronger than the bonds formed after the use of 50% citric acid solution (ASPA conditioner.)

[Retention of three different luting cements (author's transl)].

Employing zinc phosphate, polycarboxylate and glass ionomer cements, effect of cement layer thickness on retention of crowns at their axial wall was investigated. Stainless steel preparations and occlusally perforated crowns having taper angles of 2.9 and 5.7 degrees were cemented, regulating the thickness at the axial wall from 5 to 60 mu. These dies were stored in a 100% relative humidity atmosphere at room temperature or in distilled water. After 48 hr, retention was measured, applying the load vertically to occlusal preparation surface through a plunger (Fig. 1). Due to the inclusion of unreacted powder particles in the set structure of all three cements, evidence of a critical value was always shown in the cement thickness-retention relationship (Figs. 2, 3 and 4). When the cement thickness was reduced below the critical value, the retention increased remarkably and was not affected by the environment. Above the critical value, however, the retention of these cements was relatively small and affected considerably by the environment. By water immersion, the increase in the retention was found for the two polyacrylate, but not for the zinc phosphate. Correlation between retention and mechanical strengths of the three cements in the respective environments was indefinite.

The quantitative measurement of the opacity of aesthetic dental filling materials.

A method is described for measuring the opacity of anesthetic dental filling materials using a goniophotometer. Procedures are given for converting measured opacity values to those referred to a 70% reflectance background (C0.7 values) as required by existing specifications. The conversion is involved, and, since most available white surfaces have reflectance of about 80%, it is recommended that in future specifications opacity be recorded as C0.8 values. Opacity values for 28 materials are reported. It was found that the opacity of dental cement decreases for several days following preparation and that powder/liquid ratio has little effect, while pigmentation has a marked effect on opacity.

[Compressive strength and deformation of dental cements]. / Kompresjonsstyrke og deformering hos dentale sementer.

Several dental cements have been tested for compressive strength, modulus of elasticity and plastic strain at fracture. Sylindrical specimens (4 x 6 mm) were compressed to fracture with two different crosshead speeds (2 mm/min, 0.1 mm/min) at two temperatures (23 +/- 1 degrees C and 37 +/- 1 degrees C). The results showed that the zinc phosphate cement had relatively high strength and a very small elastic and plastic strain during compression. The luting type of glass-ionomer cements showed properties near to that of the zinc phosphate cement. The other cements had either lower strength or higher elastic and plastic deformation than the zinc phosphate cement. Several cements showed a marked reduction of the mechanical properties when tested at a low crosshead speed or at 37 degrees C.

Stress/strain behavior of some dental luting cements.

The compressive strength, modulus of elasticity and the plastic strain at fracture have been studied for several dental luting cements. Stress/strain diagrams of cylindrical specimens using two different crosshead speeds (2 mm/min and 0.1 mm/min) at 23 degrees and 37 degrees C showed that large differences existed between various luting cements. A zinc phosphate cement exihibited high strength, high modulus of elasticity and a small plastic strain at fracture. A resin cement also had high strength, but elastic and plastic strains were high. A polycarboxylate and an EBA-cement both showed low values of strength and modulus of elasticity combined with a high degree of plastic deformation at fracture. Testing with low strain rate at 37 degrees C accentuated the differences between these two materials and the zinc phosphate cement.

The extent of slits at the interfaces between luting cements and enamel, dentin and alloy.

Four different cements were used to assess the presence of slits at the cement/tooth or the cement/alloy interfaces using a tooth-crown model. The model consisted of ground sections of teeth and plane plates of silver/palladium alloy. The plates were fixed with bolts between two brass plates and with three different dimensions of the cement film between tooth and alloy, i.e. 50 micrometer, 100 micrometer and 200 micrometer. The tooth-alloy specimens were sectioned and the adaption of cements was studied with an indirect technique (replica) in a scanning electron microscope. The extent of slits was expressed as the length of all slits relative to the total length of the interface in each specimen. The results showed that the zinc phosphate cement and polycarboxylate cement exhibited a slight to moderate tendency to formation of slits at the interfaces. The EBA cement had a small extent of slits adjacent to thin cement films, but more slits were observed with increasing film thickness. The composite resin cement had a marked tendency to slit formation independent of the cement film thickness.

Adhesive bonding of dental luting cements; influence of surface treatment.

Tensile bond strength of four different luting cements to smooth dentin surfaces was measured. A chisel edged, stainless steel ring was cemented to the butt end of a dentin cylinder. The dentin was polished to a plane and smooth surface before cementation. The cements were also applied to dentin surfaces that were treated with a pumice slurry, etched with different acid solutions, or covered with different liners. The results showed that the polycarboxylate cement had a tensile bond strength to smooth, untreated dentin of approximately 4 MN/m2. The zinc phosphate and EBA cements had a bond strength of 0,6 MN/m2 and the composite resin cement had no measurable bond to untreated dentin. All dentin treatments showed in general a decreasing effect on the bond strength of zinc phosphate, polycarboxylate and EBA cements, whereas that of composite resin cement showed a slight increase.