Rolf Lappalainen

Transverse strength and fatigue of denture acrylic-glass fiber composite

OBJECTIVES The aims of this experiment were: 1) to test the effect of a high concentration of continuous glass fibers on the transverse strength of test specimens made of heat-cured acrylic resin; and 2) to determine the fatigue weakening of both unreinforced and continuous glass fiber-reinforced specimens of heat-cured acrylic resin shaped into upper complete dentures. METHODS A three-point loading test was used to determine the transverse strength of test specimens (n = 30 per group). The fatigue test was the constant deflection test (n = 10 per group). RESULTS The results revealed that, compared to unreinforced specimens, continuous glass fibers at a concentration of 58 wt% enhanced the transverse strength of the test specimens up to 146% (p < 0.001) and increased the fatigue resistance (p < 0.001) during 5 x 10(5) loading cycles. SIGNIFICANCE This study suggests that by incorporating glass fibers into PMMA denture bases, the strength of the denture can be increased.

Acrylic resin-fiber composite—part I: The effect of fiber concentration on fracture resistance

This study tested the effect on the fracture resistance of acrylic resin test specimens when different amounts of fibers were incorporated in the resin matrix. The fibers used included glass, carbon, and aramid fibers, with 30 test specimens of each concentration of fibers. Transverse sections of the specimens were studied by scanning electron microscope to establish how the fibers behave in the polymerization process. The results indicated that an increase in the amount of fibers enhanced the fracture resistance of the test specimens (p < 0.001). The SEM micrographs of transverse sections of test polymerized specimens revealed void spaces of different sizes inside the fiber roving.

Wetting the repair surface with methyl methacrylate affects the transverse strength of repaired heat-polymerized resin.

This study investigated the transverse strength of repaired test specimens of heat-cured acrylic resin. The repair surfaces of the specimens were wetted with methyl methacrylate for various amounts of time before the autopolymerizing acrylic resin was applied to the joint space. A three-point loading test was used to determine the transverse strength of the test specimens, and the morphologic changes in the methyl methacrylate-wetted repair surface was analyzed by scanning electron microscopy. Visual inspection was used to determine whether the failures were adhesive or cohesive. The results revealed that repaired test specimens were weaker than those unrepaired (p < 0.001). The strength of the test specimens increased as the duration of methyl methacrylate wetting of the repair surfaces increased (p < 0.001). Furthermore, the number of adhesive failures was small if the repair surfaces were adequately wetted with methyl methacrylate. Scanning electron micrographs revealed that after 60- and 180-second wetting periods, the poly(methyl methacrylate) was noted to be dissolved with a smooth surface texture. This study suggests that proper wetting of the repair surface makes an important contribution to the strength of repaired acrylic resin.

Evaluation of damage to removable dentures in two cities in Finland

The number of damaged dentures and type of damage to removable dentures repaired in dental laboratories was investigated using a questionnaire sent to 24 dental laboratories in Finland. Eight variables were examined for each damaged denture. The results showed that the type of dentures most commonly needing repair was the complete upper denture (49%). The most frequent type of damage was breakdown of the acrylic base and loosening of an artificial tooth. The chi-square test established a statistical dependence (p < 0.005) between damaged dentures and their age. Damaged upper partial dentures most frequently had natural teeth or fixed prostheses as antagonist teeth. Damaged lower skeletal dentures and acrylic partial dentures had a complete denture as the antagonist (p < 0.05). Removable dentures made of acrylic resin material seemed to break despite strengtheners, such as clasp wire.

In vitro fatigue fracture of an acrylic resin-based partial denture: An exploratory study

In this study the fatigue life of heat-cured acrylic resin test specimens shaped as maxillary partial dentures was examined. Ten test specimens were prepared from polymethyl methacrylate and were tested by a constant force fatigue test at 150 N immersed in +37 degrees C water. The fatigue-fracture surfaces of the test specimens were compared with a one-bend fracture surface of the control specimen by scanning electron microscopy. The correlation coefficient between the number of loading cycles required to cause fatigue failure in the midline section was calculated as was the concentration of residual methyl methacrylate. Results revealed that the fatigue life of the test specimens varied greatly (483 x 10(3) +/- 371 x 10(3) cycles) and that the correlation between the number of loading cycles and the midline section was poor (r -0.455). The correlation coefficient between the number of loading cycles and the concentration of residual methyl methacrylate was r 0.476 and p > 0.5. The fatigue-fracture surface of the test specimens was smoother in texture on the tension side than on the one-bend fracture surface.

Statistical evidence for the relation between citrate and carbonate in human cortical bone

SummaryBone specimens from the anterior iliac crest were collected post-mortem from 128 subjects whose death had been primarily caused by acute coronary disease or accidents. The cortical bone was separated and used for the following determinations: citrate by an enzymatic method, carbon dioxide by microdiffusion, chloride and phosphorus by spectrophotometry, fluoride by the ion-selective electrode technique, and Ca, Mg, Zn by atomic absorption spectrophotometry. The citrate concentration (±SD) was 10.9±3.1 mg/g (dry weight) and was not age-dependent. A statistically significant negative correlation was obtained between citrate and carbon dioxide, however, which was confirmed by regression analysis. The only statistically significant differences in carbon dioxide and fluoride concentrations between three citrate groups were obtained between the lowest group (3.2–8.7 mg/g) and the highest (13.1–16.8 mg/g). In conclusion an inverse relationship may suggest either an effect of citrate on the binding of carbonate to apatite structures, or that the carbonate concentration is dependent to a slight degree on citrate metabolism.

Fluoride content related to the elemental composition, mineral density and strength of bone in healthy and chronically diseased persons

The fluoride (F) content of bone was determined and compared to the elemental composition, mineral density and compressive strength in bone specimens from the crista iliac of 88 subjects who had died suddenly, and from 50 subjects who had died as the result of chronic immobilizing diseases. The elemental composition of bone was determined using an atomic absorption spectrophotometer, mineral density using gamma-ray attenuation, compressive strength using a strain transducer, and F content using an ion-selective electrode. The increase of F content with age was not statistically significantly different between the groups. The Ca content of bone decreased more with age in post-menopausal women who had suffered from chronic diseases than in women who had died suddenly. The same was true for the negative correlation coefficient between Ca and F. A similar trend was found for mineral density. This does not support the idea that the F concentration obtained produces any effect that would clearly prevent bone loss in post-menopausal women when compared to those who had died suddenly. In both groups F content was correlated with Zn and Mn; in the sudden-death group Zn concentration explained 12.9% of the F concentration. The relation between Zn and F may be important in the formation and structure of biological apatite.

The Concentrations of Pb, Cu, Co and Ni in Extracted Permanent Teeth Related to Donors' Age and Elements in the Soil

Extracted permanent molars, premolars or incisors were collected from 89 subjects aged 8–67 years living in a rural area of eastern Finland where large scale geochemical analyses of the soil have been carried out. A sample of 89 teeth was analyzed for Pb, Cu, Co and Ni by atomic absorption spectro-photometry. The concentration of Pb increased with age in the dentin but not in the enamel. The concentrations of Cu, Co and Ni remained unaltered with age. The mean concentrations (μg/g) of Cu in the dentin were 9.1, 8.6 and 7.2 when the mean concentrations (μg/g) in the soil were over 150, 50–150 and under 50, respectively. The concentration of Cu in the enamel was not dependent on the content in the soil. The mean concentrations (μg/g) of Co and Ni were higher in the enamel (26.3 and 43.8) than in the dentin (13.8 and 31.4).

Cadmium content of human cancellous bone

The cadmium content of human cancellous bone was related to age, sex, bone loss, physical properties, and elemental composition. Bone specimens from the anterior iliac crest were collected from 889 cadavers with a normal mineral status, and from 50 cadavers which had bone loss from chronic diseases and immobilization. The element concentrations were analyzed using atomic absorption spectrophotometry. Bone fluoride levels ere determined with the ion specific electrode, the mineral density with the gamma ray attenuation method, and the compressive strength with a strain transducer. The data were analyzed using multiple linear regression analysis. The mean cadmium content of 0.22 +/- 9.16 micrograms/g dry weight (+/- SD) in the samples did not change with age and its content was slightly greater in males than in females. Furthermore, no statistically significant relation was found in cadmium content to bone loss changes or to the calcium content of bone. The cadmium content had a high statistically significant positive correlation with the strontium and nickel content.

A 55 Fe excited X-ray fluorescence method for the analysis of Ca content in enamel

A 55Fe excited X-ray fluorescence method has been developed for the quantitative nondestructive analysis of Ca content in surface layers of teeth. To eliminate the effects due to curved enamel surface and the localisation of the tooth a special measuring geometry was constructed. The reproducibility of the method remained under 2%. The effect of acid treatment on the Ca content in enamel was demonstrated.