به کارگیری مدل آمیخته خطی چندسطحی در مقایسه آزمایشگاهی انبساط آبی سه نوع کامپوزیت

 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<0.001) and composite (p=0.032) were statistically significant and environment did not have significant effect. Composite   had the lowest hygroscopic expansion between the three underlying composites. Also, random effect of the intercept which showed the effect of unobserved factors on the response, was significant (p<0.001).  Conclusions: Results showed that in addition to the time and composite type, there are other factors that do not exist in the model but affect the hygroscopic expansion. Nevertheless it’s better to choose the best composite that have the lowest hygroscopic expansion (like  ) during the time.