A comparison of polymeric denture base materials

Since its introduction in 1937, poly(methyl methacrylate) (PMMA) has become the most commonly used material for denture bases. This is largely due to its favourable although not ideal characteristics. One of the main problems associated with PMMA is the polymerisation shrinkage exhibited by the material. Injection moulding systems have been developed to compensate for this by continuously injecting PMMA resin at pressure throughout a carefully controlled polymerisation procedure. This study aimed to compare acrylic specimens processed by injection moulding and conventional pressure packing in relation to dimensional accuracy. Subsequent experiments evaluated adherence of Candida albicans to denture base materials. A cobalt chrome control denture base was used to fabricate stone moulds in which 20 injection moulded and 20 conventional pressure packed PMMA resin denture bases were produced. These denture bases incorporated 6 reference points between which sequential measurements were taken using digital callipers. Base plate adaptation was additionally measured by weighing a vinyl polysiloxane film to reproduce any discrepancy between the denture base and master model. Linear dimensional measurements revealed that changes in dimension did not occur evenly over the entire denture base for either sample group. Injection moulded samples exhibited statistically significant differences when compared to control in two of the six measured linear dimensions. Conventional pressure packed materials exhibited a statistically significant difference in one of the measured linear dimensions compared to control. Statistically significant linear dimensional differences were determined between injection moulded and conventional pressure packed materials in three measured dimensions. For injection moulded materials, the location of the injection moulding inlet may have influenced the dimensional accuracy For the weighed vinyl polysiloxane data, a greater weight of material was recovered from conventional pressure packed material samples than injection moulded samples. These data demonstrate that injection moulded denture bases have superior internal surface adaptation compared to conventional pressure packed acrylic resin. Squares of denture base material were produced by injection moulding and conventional pressure packing techniques. Self cured PMMA resin was additionally included in candidal adherence and surface morphology analysis. Profilometer testing determined self-cure resin surfaces had more irregular surface characteristics than surfaces of conventional or injection moulded samples. Conventionally processed samples exhibited the smoothest material surface. However, conventional and injection moulded sample groups were similar. Scanning electron microscopy of the three material sample groups was performed to determine surface morphology and patterns of candidal adherence and subsequent biofilm formation. SEM examination revealed variations in surface morphology and following 1 hour, Candida albicans cells were observed to adhere and aggregate within the various surface irregularities of all three materials. Examination after 24 hours demonstrated the complex intertwining hyphae evident on all the material samples, irrespective of initial candidal adherence patterns. No significant differences were observed between attachment of the 9 C. albicans clinical strains when tested independently against each sample group. However, comparison of the mean attachment of all strains to the 3 sample groups revealed a statistically significant difference in attachment capacity between conventional and self cured sample groups. Self cured PMMA resin samples exhibited significantly less candidal attachment than conventionally processed samples, indicating that material surface factors may play a greater role in promoting or preventing candidal adhesion that the organism per se. As the denture bearing mucosa is compressible and the achieved palatal seal largely dependent on the prepared post dam, small dimensional changes demonstrated in this study may be of limited clinical relevance to the success or failure of the material as a denture base. C. albicans were found to adhere to all three types of PMMA resin and if left undisturbed, Candida cells proliferated to form a biofilm upon all resin materials. Therefore, the observed differences in attachment are likely to be of limited clinical importance in the prevention of candidal infection without consideration to denture and oral hygiene