The International Journal of Prosthodontics, 04/1994

A new method of adherend surface treatment was evaluated to determine the effect of adhesion between dentin and adhesive resins. The pretreatment method consisted of the application of 40% phosphoric acid solution to dentin to remove the smear layer and decalcify the superficial dentin layer, followed by 10% sodium hypochlorite to dissolve and remove the exposed dentinal collagen and provide a fresh dentin apatite surface to which adhesive resin is applied. This method features a direct adhesion between the adhesive resin and dentin, with no resin-reinforced layer of dentinal collagen and adhesive resin. The adhesive tensile strength after storage in water at 37 degrees C for 24 hours measured 6.7 MPa with the phosphoric acid treatment alone, while the test method produced an initial adhesive strength as high as 9.7 MPa. After 10,000 thermal cycles (4 degrees C to 60 degrees C), the adhesive strength was 6.2 MPa, which was approximately 1.5 times greater than the value obtained with the method using the phosphoric acid solution alone.

Reinforcement of acrylic denture base resin with high-performance polyethylene fiber in woven form produced a substantial improvement of stiffness and impact strength, as well as reducing the sensitivity of the material to notches that mimic anatomic features. The mechanical properties of the new systems were not affected by water immersion. Reinforced complete maxillary and mandibular dentures having good appearance and satisfactory fiber/resin integration were achieved using conventional laboratory techniques. These prostheses are undergoing clinical trials with encouraging results.

This study investigated the effects of various immersion times and concentrations of sodium hypochlorite on irreversible hydrocolloid impressions and resultant gypsum casts. Irreversible hydrocolloid impressions of a test die were immersed for 1, 5, or 10 minutes in water (control), 5.25%, 0.525%, and 0.0525% sodium hypochlorite and then cast in a Type III stone and a Type V stone. Each stone specimen was evaluated for detail reproduction, dimensional change, surface roughness, and surface hardness. The results indicated that impressions may be immersed in sodium hypochlorite for any of the experimental times and concentrations without negative effects on Type V stone casts. However, immersion of impressions in 5.25% sodium hypochlorite causes some surface deterioration on Type III stone casts.

Two commercially available, screw-retained attachments and an experimental attachment were modeled and analyzed by computerized finite element method. Mechanical test specimens were then developed. Cast rectangular bars were fabricated with the attachments or were joined by soldering at midspan as a control. These bars were subjected to three-point loading. The soldered connector was stronger than the three attachment designs, and the experimental attachment was stronger than the two commercial products. There was general agreement between the mechanical and finite element testing. The use of computer-aided technology to design dental devices with improved mechanical properties was demonstrated by this study.

The fracture strengths of four types of all-ceramic premolar crowns (conventional Vita-Dur, In-Ceram, Dicor, and IPS-Empress) were compared after a preload cycling in aqueous atmosphere. Preload cycling significantly decreased the strength of Vita-Dur crowns. Fracture strength of Vita-Dur crowns was improved when they were luted with either polyalkenoate or adhesive resin cement. The average (standard deviation) of load to fracture of ceramic crowns luted using zinc phosphate cement and after applied preload cycling in distilled water were: Vita Dur, 770 (237) N; Vita In-Ceram, 1060 (341) N; Dicor, 840 (226) N; and IPS-Empress: 891 (185) N. The In-Ceram crowns fractured in two modes: complete fractures at 1276 (207) N; and fractures with the core remaining intact at 808 (292) N.

This study examined the desirable properties of a soft denture liner. The physical behavior of palatal mucosa of young adults was quantified by analyzing creep behavior using Voigt's four-element model. Instantaneous elastic recovery was significantly smaller than instantaneous elastic displacement, and delayed elastic recovery was significantly larger than delayed elastic displacement (P .05). Viscous recovery was small but detectable. To ascertain whether the characteristics of the mucosa can be replaced with a soft material, a new soft lining material was developed and both behaviors, especially after load release, were compared with each other. It was indicated that by properly controlling the amounts of the crosslinking agent and inorganic filler of the soft material, its properties may be made to approximate those of the mucosa.

The fit of spark-eroded titanium and cast gold alloy copings was compared by assessment of retrieved cement film analogues. This technique was chosen for its simplicity, nondestructive nature, and for the information it provided on overall fit. The present study showed that the overall fit of titanium copings was comparable to that of gold copings. In marginal areas, the space between die and coping was found to be larger for spark-eroded than cast copings.

The shades of several resin composite materials are keyed to the Vita Lumin shade guide. This study was designed to determine whether different composites of the same designated Vita shade were actually similar in color. A small-area colorimeter was used to compare the L*a*b* color parameters of three hybrid resin composites - Pertac-Hybrid, Prisma AP.H, and XRV Herculite. Four Vita shades (A2, B3, C2, and D3) of each material were evaluated. Color differences (delta E values) between composites of the same designated shade ranged from 2.07 for the A2 shades of Herculite and AP.H to 7.64 for the D3 shades of Herculite and Pertac. Color differences in this range are perceptible to many observers under ideal viewing conditions.

A raised ridge of Novus resilient denture liner was removed from samples using one of three burs, three stones, or a coarse sandpaper disc. Samples were polished using pumice alone, tin oxide alone, or both agents. A flat, unfinished sample served as a control. Average surface roughness of control samples was 0.96 microns. Average surface roughness measurements for reduced samples ranged from 7.21 microns (sandpaper disc) to 3.34 microns (Prolastic wheel). Without polishing, sandpaper discs and serrated burs produced rougher surfaces than stones or the nonserrated bur. The sandpaper disc reduced the liner the quickest. There were no significant differences between treatments or controls after the use of pumice alone or after the use of a combination of pumice and tin oxide. An acceptable finishing protocol for this poly(fluoroalkoxyphosphazene) resilient liner appears to differ from that previously established for a silicone resilient denture liner.

This study investigated the mandibular height reduction in relationship to aging and the length of edentulism period. Data were collected from 120 edentulous male subjects (complete denture wearers) who were divided into two resorptive age groups. The edentulism period of the subjects was classified as 1 to 5, 5 to 10, and over 10 years. Ridge resorption was estimated on the panoramic radiograph by estimating the original height of the alveolar process as being three times the distance from the inferior border of the mandible to the inferior edge of the mental foramen. According to statistical analysis, the amount of mandibular height reduction increased in the older age group. The mandibular resorption rate was greatest in the earlier stages of edentulism and slowed with longevity of edentulism.

One light-polymerized, three heat-polymerized, and three autopolymerized denture base polymers were exposed to coffee, tea, water at 50 degrees C ± 1 degree C, as well as artificial sunlight and water, and evaluated for color stability. The color characteristics were determined using a computer-controlled spectrophotometer. Water sorption and solubility were assessed, and the materials were also chemically characterized. Coffee and tea stained the denture base materials superficially. Brushing with toothpaste and moderate grinding reduced discoloration to an acceptable level. All materials were relatively color stable when immersed in water at 50 degrees C ± 1 degree C. The materials behaved differently when exposed to artificial sunlight and water.