Processing shrinkage of heat-curing acrylic resin reinforced with high-performance polyethylene fibre

Dimensional change in complete dentures fabricated by injection molding and microwave processing

STATEMENT OF PROBLEM

Acrylic resin complete dentures undergo dimensional changes during polymerization. Techniques with injection molding and polymerization and microwave polymerization are reported to reduce these changes and thereby improve clinical fit. These dimensional changes need to be quantified.

PURPOSE

The purpose of this study was to compare differences in dimensional changes of simulated maxillary complete dentures during polymerization and storage in water after injection molding and conventional polymerization, or microwave polymerization against a control of conventionally packed and polymerized simulated maxillary complete dentures.

MATERIAL AND METHODS

Forty identical maxillary denture bases were prepared in dental wax with anatomic teeth. They were invested and the wax eliminated from the molds. Ten specimens each were randomly assigned to 1 of 4 groups. Group 1 was compression molded and conventionally polymerized; group 2 was injection molded and conventionally polymerized (Success); group 3 was injection molded and microwave polymerized (Acron MC); and group 4 was injection molded and microwave polymerized (Microbase). Intermolar width and changes in vertical dimension of occlusion, were determined after polymerization and after storage in water for 28 days. Measurements in triplicate were made between points scribed on the second molar teeth with a traveling microscope (accurate to 0.005 mm). Vertical dimension of occlusion was measured between points scribed on the upper and lower members of an articulator by use of an internal micrometer (accurate to 0.05 mm). Data were analyzed by use of a 1-way analysis of variance with Tukey post-hoc contrasts (P <.05).

RESULTS

Polymerization contractions (intermolar widths) for each group were: group 1, -0.24%; group 2, -0.27%; group 3, -0.35%; and group 4, -0.37%. The Microbase specimens had greater shrinkage than conventionally polymerized specimens, but there were no significant differences between the groups. All injection methods had less postpolymerization increase in vertical dimension of occlusion (0.63 to 0.41 mm) than the conventional Trevalon control (0.74 mm), but only group 4 was significantly different (P<.004). After storage in water for 28 days, all specimens increased in vertical dimension of occlusion (0.10% to 0.16%) from polymerization techniques, but there were no significant differences between groups.

CONCLUSION

Within the limitations of this study, injection molding resulted in a slightly less increase of vertical dimension of occlusion than conventional polymerization techniques, the difference being significant for Microbase compared with the conventional Trevalon control.

Fabrication of complete denture bases reinforced with polyethylene woven fabric

Incorporation of 5 layers of woven, high-modulus polyethylene fiber into acrylic resin denture bases produces substantial improvements in mechanical properties and dimensional changes. A modified split-pack technique has been developed using conventional dental-laboratory compression molding to accommodate multiple layers of woven fabric in complete denture bases. A recess formed in the resin by a spacer allows the reinforcement to be embedded in the denture base without exposing the fibers. Embedded fibers do not compromise the esthetics of complete dentures.

Effect of processing method on the dimensional accuracy and water sorption of acrylic resin dentures

STATEMENT OF PROBLEM

Acrylic resin complete dentures exhibit certain unavoidable dimensional changes. Processing shrinkage and expansion due to water uptake are 2 important aspects influencing dimensional accuracy.

PURPOSE

This study investigated linear dimensional changes and water sorption of dentures processed by dry and wet heat with different rates of cooling.

METHODS

Fine crosses marked on tinfoil inserts were placed at the incisive papilla and tuberosity regions of edentulous maxillary casts and incorporated into the dentures during polymerization by 3 processing techniques. A traveling microscope was used to measure the distances between the reference points to determine dimensional changes. Water uptake and content were determined by the mass changes of the dentures with an electronic balance. Data of linear dimensional change and water sorption were analyzed by multivariate analysis of variance and analysis of variance, respectively. Bonferroni simultaneous confidence intervals (95%) were applied for multiple comparison.

RESULTS

Dry heat-processed and water bath-processed acrylic resin dentures did not exhibit significant differences in shrinkage (0.42% to 0.58%) at water saturation. Amounts of water sorption of dentures processed by dry and wet heat (0.50 and 0.48 mass%, respectively) were not significantly different, and their associated expansion did not entirely compensate for the processing shrinkage. The initial water content of dry heat-processed dentures (1.77 mass%) was unexpectedly slightly higher than that of wet heat-processed dentures (1.68 mass%). The rate at which the dentures cooled did not affect their initial water content and subsequent water uptake.

CONCLUSIONS

Water uptake of dry and wet heat-processed acrylic resin dentures after deflasking was in both cases low, and the dentures did not reveal significant differences in shrinkage at water saturation. Air oven-processed and water bath-processed acrylic resin dentures show similar dimensional shrinkage at water saturation.

Ultra-high-modulus polyethylene ribbon as reinforcement for denture polymethyl methacrylate: a short communication

OBJECTIVE

The aim of this study was to determine the effect of polyethylene fiber ribbon on the transverse strength of denture base polymer.

METHODS

One or two layers of ultra-high-modulus or ultra-high-molecular weight polyethylene (UHMP) ribbon were placed into rhombic test specimens of denture base polymer. The specimens were tested with three-point-loading, and the ribbons were positioned on the tension side of the specimen. Fractured test specimens were examined with SEM.

RESULTS

The transverse strength of the specimens increased slightly from the incorporation of UHMP fiber ribbons (unreinforced polymer: 91 MPa; with one ribbon layer: 118 MPa; with two ribbon layers: 125 MPa). Adherence of the UHMP fibers to the denture base polymer was poor.

SIGNIFICANCE

Improvement in transverse strength of the specimens was modest although the fibers were placed into an ideal position of the specimens. This lack of improvement was due to poor adhesion of the UHMP fibers to the polymer and inadequate impregnation of the fibers with denture base resin.

The effect of untreated UHMWPE beads on some properties of acrylic resin denture base material

OBJECTIVES

To evaluate the effect of surface untreated ultra high molecular weight polyethylene (UHMWPE) beads on a range of properties of the polymerised acrylic resin denture base material.

METHODS

UHMWPE in the form of surface untreated beads was added to acrylic resin denture base material in various percentages (2.5%, 5%, 7.5% and 10%) to form a composite material. The influence on doughing and manipulation times, transverse and impact strength and hardness characteristics were examined.

RESULTS

The results indicated that the manipulation and setting times gradually increased as the percentage of UHMWPE beads in the acrylic resin powder increased. By increasing the percentage of incorporated UHMWPE beads, there was a decrease in modulus of rupture and impact fracture resistance. There was also an effect on the hardness with an increasing percentage of beads lowering the hardness value, although this was not consistent.

CONCLUSION

It was concluded that the use of untreated UHMWPE beads is of no significant benefit to the properties of acrylic resin.

A review of fiber-reinforced denture base resins

PURPOSE

One method of reinforcing denture base material is to use fiber composite reinforcement. Different types of fibers, such as glass (GF), carbon/graphite, aramid, and ultrahigh-modulus polyethylene (UHMP) fibers have been tested for this purpose.

MATERIALS AND METHODS

This article reviews the studies conducted on the fiber-reinforced denture base resin systems.

RESULTS

The literature has reported that the fiber concentration and its adhesion to polymer matrix influences the transverse strength of the fiber composite. The highest transverse strength value (265 MPa) with polymethyl methacrylate (PMMA) was obtained by incorporating 58 wt% GF into the resin. UHMP fibers incorporated into PMMA resin yielded the highest impact strength value (134 kJm-2) of the fiber-PMMA composites.

CONCLUSIONS

Despite the improved mechanical properties of fiber-reinforced denture materials, further research is required to show the clinical usefulness of the fiber reinforcement.

Temperature and dimensional changes in the two-stage processing technique for complete dentures

OBJECTIVES

This study concerned the temperature and linear dimensional change of heat-cured acrylic resin in the two-stage processing technique for complete dentures.

METHODS

Thermocouples were incorporated in the acrylic resin for recording temperatures. Measurements between reference marks were made by a high-resolution digital measuring microscope.

RESULTS

No increase in temperature associated with the exothermic nature of the polymerization reaction was recorded. The temperature of the resin followed the waterbath temperature closely. The temperatures recorded at various regions were in phase with each other. The total linear shrinkage of the base after two processing cycles was less than 1% and compares favourably with studies on the single-stage processing technique.

CONCLUSIONS

Temperature differential is excluded as a reason for the warpage of dentures. The dimensional changes of the denture base resulting from the two-stage processing technique cannot be considered to be of any clinical significance.

Acrylic resin reinforced with chopped high performance polyethylene fiber--properties and denture construction

A newly developed incremental mixing technique has been used to incorporate over 30 vol % chopped high performance polyethylene fiber into acrylic denture base resin. The reinforcement produced a substantial improvement in several clinically important properties, namely: 1) stiffness and impact strength were higher; 2) the mechanical properties were insensitive to notches that mimic anatomical features; and 3) samples damaged during bending and impact did not break up into separate fragments. Reinforced complete maxillary and mandibular dentures of good esthetic appearance were successfully manufactured and polished with conventional laboratory techniques.