The effect of dehydration on the dimensional stability of acrylic partial denture bases

Abstract

Removable acrylic dentures are often used to restore both function and aesthetics of edentulous patients. This treatment option is commonly advocated because acrylic partial dentures are simple to manufacture, finish and adjust during clinical fitting. The main drawback of an acrylic denture, however, is that it is porous in nature. Consequently, water, saliva or any other oral fluids can be absorbed during intraoral use and storage. This severely compromises the fit, comfort and stability of the denture base in the patient‟s mouth. Acrylic dentures that are not stored in water during periods of disuse can also deform and become brittle. Deformation of acrylic denture base stored post-manufacture, and prior to intraoral fitting, may also occur in the absence of hydration. The atmosphere in which the acrylic partial denture is stored may contribute to the level of hydration, which can further cause dimensional instability of the denture bases. The focus of this study was to investigate the extent of dimensional instability of acrylic partial denture bases by temperature and humidity.

A positivist paradigm was followed and an experimental research design within a quantitative framework was used. There were three sample groups namely; the hydrated 9-hour cure (Control); dehydrated 9-hour cure (Group 1); and dehydrated 1½-hour cure (Group 2). Each sample group had ten specimens (n=30). In determining the extent of the dimensional instability of the acrylic partial denture bases by temperature and humidity, the acrylic partial denture bases for the dehydrated 9-hour cure and dehydrated 1½-hour cure were stored in a custom-made incubator for 28 days with the temperature and humidity levels regulated at 21ºC and 40%, respectively. The 9-hour cured acrylic partial denture bases were stored in a water bath at 37ºC for 28 days. The dimensional deformation of the acrylic partial denture bases were measured immediately after fabrication (day 1) and subsequently on days 2, 7, 14, 21 and 28.

The silicone wafer method was used to measure the degree of deformation of the acrylic partial denture bases. The process entailed pouring a silicone (Mold Max® 30) mixture onto the gypsum models and fitting the acrylic partial denture bases using finger pressure. Following the setting of the silicone, the acrylic Page | vi

partial denture bases were carefully removed from the gypsum model. The thickness of the silicone layer was measured using a micrometer (Mitutoyo® S293, America Corporation, Illinois, United States of America) at four reference areas. These reference areas were the incisive papilla; the area of the first molars (16 and 26); and the deepest part of the midline posteriorly. Validity was achieved following the South African National Standard for Denture base polymers (SANS 861). Reliability was maintained using a temperature control unit and a calibrated thermo-hygrometer. The Kruskal-Wallis non-parametric test and Dunn‟s Multiple Comparison test (SPSS® Version 24) were used to analyse the degree of deformation in the areas mentioned above (p=0.05). The slope test was further used to compare the deformation trends of each sample group (Graph Pad Prism® Version 5).

The Kruskal-Wallis test and Dunn‟s Multiple Comparison test revealed no significant differences for the three groups (p>0.05). The slope test revealed no significant differences in the incisal area (p>0.42). There were significant differences in the posterior area of the acrylic denture bases, particularly in the first molar (26) area (p<0.04). Overall, the prominent features of this study showed that the dehydrated 1½-hour cured acrylic partial denture bases had the highest deformation. The hydrated 9-hour cured acrylic partial denture bases, by contrast, had the lowest deformation. Notably, this study conclusively showed that curing acrylic partial dentures for nine hours increases the dimensional fit, which is further enhanced if kept hydrated on storage. Future research to consider the dry-stored parameters of acrylic partial denture bases is recommended, as this could provide useful guidelines to both clinicians and patients in reducing deformation on storage, thereby improving denture care practices.