Comparison of Repeated Chemical and Microwave Disinfection on Dimensional Accuracy of Gypsum Casts

Abstract

OBJECTIVE: The aim of this experimental study is to compare the dimensional accuracy of gypsum casts after repeated disinfection in microwave at 900 Watts, 2450 MHz (5 minutes) and immersion in 0.5% Sodium hypochlorite (10 minutes). Disinfecting casts is recommended to prevent cross infection but may cause dimensional changes. During fabrication of prosthesis, a cast may get contaminated several times so there is a need of repeated disinfection. METHODOLOGY: Sample size was 33 (11 in each group), calculated through WHO software for sample size determination by using standard deviation of 0.16 at 95% confidence interval and 80% power of study. Impressions in irreversible hydrocolloid were recorded of an acrylic cast fabricated for this study. The impressions were poured with die stone and were randomly divided into 3 groups; Group I: Microwave disinfection, Group II: Immersion disinfection in 0.5% Sodium hypochlorite, Group III: Control group. For Groups I and II, each cast was disinfected 7 times with 5 minutes interval between two disinfection cycles, after every cycle anteroposterior and mediolateral measurements were recorded using digital Vernier caliper (accuracy upto 0.01 mm). For group III, casts were rinsed with distilled water, dried in open air within temperature range of 28+/-2OC for 10 mins followed by anteroposterior and mediolateral measurements. This procedure was repeated seven times for each cast. RESULTS: Anteroposterior and Mediolateral differences of dimensional change between and within the Group A, B and C was calculated by One Way ANOVA. Inter/intra examiner reliability was taken into consideration at the time of study. Mean dimensional change in the casts were insignificant through six disinfecting cycles. However, in the seventh cycle, a significant difference (p=0.003) was observed in the anteroposterior dimension (0.03% dimensional change for Group A and 1.26 % for Group B whereas, in mediolateral dimension, dimensional change was 0.35% for Group A and 0.59% for Group B (p=0.004). Dimensional change of >0.5% was considered as the cutoff value for casts to be considered as dimensionally accurate. Casts disinfected through immersion disinfection did not produce dimensionally inaccurate casts in anteroposterior dimension after third cycle and in seventh cycle in mediolateral dimension. However, result is significant only in seventh cycle. Microwave disinfection produced dimensionally accurate casts throughout all cycles. CONCLUSION: Microwave disinfected casts remained dimensionally stable compared to immersion disinfection. KEYWORDS: Disinfection, microwave, immersion, dimensional stability, gypsum casts