TY - JOUR T1 - Application of multilevel linear mixed model for experimental comparison of hygroscopic expansion in three composites TT - به کارگیری مدل آمیخته خطی چندسطحی در مقایسه آزمایشگاهی انبساط آبی سه نوع کامپوزیت JF - RJMS JO - RJMS VL - 20 IS - 110 UR - http://rjms.iums.ac.ir/article-1-2674-en.html Y1 - 2013 SP - 1 EP - 7 KW - Multilevel linear mixed model KW - Hygroscopic expansion KW - Composite. N2 -  Background: Tooth damage is one of the most common reasons that patients visit the dentists. Nowadays composites are increasingly used in dentistry because they have the colour of tooth and are more compatible with tooth tissue. Hygroscopic expansion is one of the most reasons that reduce the efficacy of the composite. Main goal of this article is to compare the hygroscopic expansion of the three common composites. Methods: In this longitudinal study, laboratory data from three common composites (         ,   ) that were kept in two different environments (distilled water and natural mouth saliva) for three months were used. Their cylinder length was measured 18 times after preparation of 540 samples and their hygroscopic expansion was comprised using multilevel linear mixed model by SAS 9.1.3 software. Results: In this study both, time (p M3 ER -