Veijo Lassila

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.

Impact strength of denture polymethyl methacrylate reinforced with continuous glass fibers or metal wire

The impact strength of heat-cured acrylic resin test specimens that had been reinforced in various ways was compared in this study. Ten rectangular test specimens were fabricated for each test group. The strengtheners included 1.0-mm-diameter steel wire and continuous E-glass fibers. Both notched and unnotched test specimens were tested in a Charpy-type impact test. In a further analysis the concentration of glass fibers in the test specimens was determined and plotted against the impact strength of the test specimens. The results showed that, compared with the unreinforced specimens, both types of reinforcement increased the impact strength of the test specimens considerably (p < 0.001). There was no clear difference between the mean impact strength value of the test specimens reinforced with metal wire and that of the specimens reinforced with glass fiber. The correlation coefficient between the fiber concentration of the test specimens and their impact strength was 0.818 (p < 0.005). Specimens with fiber concentrations greater than 25 wt% yielded to the higher impact strength more readily than those with metal wire reinforcement did.

Bite force and its correlations in different denture types

Maximal bite force was measured and intraoral condition was examined in 89 patients at the Institute of Dentistry, University of Turku. These patients formed three different denture groups: those with complete dentures, those with full maxillary denture and partial mandibular denture, and those with natural dentition or skeleton-supported partial maxillary denture and partial mandibular denture. There were three age groups: greater than or equal to 70, 60-69, and less than or equal to 59 years old. Maximal bite force was recorded with an appliance at seven different measuring points by placing a biting fork between the antagonistic teeth while at the same time the occlusion was stabilized contralaterally with a plastic tube. Maximal bite force had a correlation with age and sex (P less than 0.01). In partial-denture groups high bite force had a correlation with the breaking of dentures (P less than 0.001 and P less than 0.05, respectively). Satisfied patients had a higher bite force than dissatisfied ones. When there was some disturbance in occlusion, the bite force was smaller, especially in full-denture groups (P less than 0.001). Full-denture wearers also had a good bite force, but the best biting area was located more posteriorly than in patients who still had some natural teeth left in both jaws. Changes in the denture-bearing mucosa in patients with complete dentures and negative height of the mandibular alveolar process decreased the bite force slightly.

Comparison of five interocclusal recording materials.

The initial resistance of interocclusal recording materials to closure changed from 0.5 N to 13.8 N, and a rapid rise in the working time was seen in all elastomers. The resistance exhibited by wax at 60 degrees C was about 7 N. The volumetric contraction of elastomers in polymerization was clinically slight. The dimensional stability of rigid materials, acrylic resin, and zinc oxide pastes was good. Elastomers maintained their reliability for a relatively long time when stored in a tightly sealed plastic bag.

Biometric design of complete dentures related to residual ridge resorption

The aim of this study was to determine whether the location of the incisive papilla, the extent of the alveolar bone remaining on the facial side of the palatal-gingival margin in the canine region, and the height of the palatal vault in the molar region are associated with factors that may affect the volume of the residual ridges in an edentulous maxillae. Results suggest that duration of the edentulousness and skeletal mineral status are important factors in the resorption of the residual ridges in the maxillae. The location of the incisive papilla and the thickness of the ridge on the facial side of the palatal-gingival margin are associated with these two factors.

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.

Bonding of dental porcelain to ceramic-metal alloys

The bond strength of porcelain to the four gold alloys and the three nonprecious alloys was tested with a simplified pull test method, having wide test surfaces. The tests showed that all gold alloys tested were suitable for fusing porcelain. The bond strength in nonprecious alloys was somewhat lower and the location of the fracture lines was more variable. It seems that nonprecious alloys are more sensitive to laboratory procedures. The bond strength and the location of the fracture line had a good correlation in all metal alloys.

Regeneration of D-Galactosamine-Traumatized Rat Liver with Natural Protoberberine Alkaloids from Enantia chlorantha

Experimental liver injury was provoked in test rats with intraperitoneal injections of D-galactosamine. Traumatized rats received further intraperitoneal injections of Hepasor, a protoberberine alkaloid mixture from Enantia chlorantha (Annonaceae). Biochemical assays from blood plasma, serum alanine transferase, serum alkaline phosphatase, serum creatinine, serum hydroxyproline and serum calcium were done and liver and kidney samples for histological processing were taken. The biochemical results obtained indicate a marked influence by Hepasor on the serum alanine transferase activities and serum hydroxyproline values in female rats, but more accidental ones in male rats. A reduction in serum alkaline phosphatase activity and the serum creatinine values was also found, being also dependent on sex. The histological findings in the liver sections of female rats show that 1 week Hepasor therapy greatly furthers the healing process in a D-galactosamine-pretraumatized liver, eliminating megalocytosis, contraction of chromatines and other disorders in the cell architecture. The inhibitory effect of Hepasor on proceeded traumatization caused by D-galactosamine was nearly complete.