Changes in the Hardness of Resilient Denture Liners Embedded in Maxillary Complete Dentures and Their Association With Patient Characteristics

1. Participants This study was approved by the Human Ethics Committee of Nihon University School of Dentistry, Matsudo (EC 13-006). Thirty wearers of complete maxillary dentures (12 men; mean age, 71.8 ± 9.2 years and 18 women; mean age, 70.2 ± 10.7 years) who visited the Department of Removable Prosthodontics at our institute were recruited for this study. The volunteers were enrolled only after they provided written informed consent. Complete maxillary denture wearers who had been treated with a tissue conditioner, RDLs, and denture adhesives were excluded. Before starting the clinical trial, the participants were asked about the following environmental factors in a questionnaire: smoking, drinking, denture wearing during sleeping, and denture cleanser usage.

After specimen making, all participants were instructed to wear their dentures as per their normal routine. 2. Specimen Preparation A cylinder shapes six places with a 4-mm diameter and 2-mm depth , that was drilled into the inner surface of the denture base. Then, the RDL materials were mixed by each product, packed into the cavity, and polymerized at room temperature . The six commercial RDLs used in this study:Evatouch Super (EVA; Neo Dental Chemical products, Washington, USA), GC RELINE (GCR; GC Dental Products Corp, Tokyo, Japan), Mucopren soft (MCP; Kettenbach GmbH & Co KG, California, USA) Soften (SFT; KAMEMIZU CHEM, Osaka, Japan), FD Soft (FDS; KAMEMIZU CHEM, Osaka, Japan), and Bio Liner (BIO; Nissin Dental Products, Kyoto, Japan).

Hardness was measured using a Vesmeter® (WaveCyber Corp, Saitama, Japan). When the probe, which included a built-in position sensor connected to a personal computer, was placed in a perpendicular orientation to an RDL, the indenter of the probe was depressed onto the RDL at a constant speed through electromagnetic power. Simultaneously, the path of the indenter was constantly traced by the position sensor. The computer processed electrical signals from the measuring device and calculated the Shore D hardness, which was the primary outcome in the present study. The same investigator measured the hardness at each appointment. The original hardness and the hardness at 1 month after oral exposure were used for analysis. Each specimen was measured five times, and the highest and lowest values were eliminated from the calculation of the mean representative value for each specimen. The specimens embedded in the dentures were measured on a hot plate to maintain the temperature at 37°C or as close to the oral temperature as possible. 3. Statistical Analyses Before other statistical analyses, the normality of the data was tested using the Kolmogorov - Smirnov test, and then parametric statistical methods were applied.Two-way repeated measures analysis of variance ( ANOVA ) with the material type (EVA, MCP, GCR, FDS, SFT, and BIO) and time period (baseline and one month after) as factors was used to assess changes in the hardness of the six RDLs over time. The Tukey-Kramer test was used as a post hoc test after the two-way repeated measures ANOVA .The effects of categorical variables such as sex, smoking, drinking, denture wearing during sleeping, and denture cleanser usage on the hardness of the RDLs were analyzed using a two-way ANOVA without a post hoc test; we were not interested in comparing each RDL but rather in analyzing the effects of categorical variables on the RDLs. The effects of continuous variables, including age, salivary flow rate, and occlusal force, were analyzed using Pearson's correlation coefficients. All statistical analyses were performed using IBM® SPSS® Statistics 21 (IBM, Armonk, NY, USA). A p-value of <0.05 was considered statistically significant.