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

Mariya Khalid                              BDS, FCPS
Mohammad Ali Chughtai           BDS, FCPS, MHPE, FFDRCSI
Sohrab Shaheed                         BDS, FCPS, FFDRCSI
Syed Nasir Shah                         BDS, FCPS


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
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
HOW TO CITE: Khalid M, Chughtai MA, Shaheed S, Shah SN. Comparison of repeated chemical and microwave disinfection
on dimensional accuracy of gypsum casts. J Pak Dent Assoc 2021;30(4):235-242.
Received: 13 January 2021, Accepted: 01 August 2021

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