Abstract: Objective To assess the time course of Q value after myopic laser-assisted in situ keratomileusis (LASIK) and preliminarily evaluate the determinants of the difference of Q value between before and after LASIK. 
Methods We performed a retrospective, longitudinal investigation on patients undergoing wavefront optimized LASIK therapy for emmetropization. A total of 418 eyes from 222 cases were examined preoperatively, and partly followed up at one week (172 eyes), one month (134 eyes) and three months (51 eyes) after surgery. The horizontal, vertical and total Q values of cornea were calculated from eccentricity measured at the central 6-mm corneal zones respectively. Potential determinants of the change of Q value were analyzed using multiple linear regressions. 
Results The mean Q value was -0.17±0.13 preoperatively, and 0.99±0.70, 0.97±0.66, and 0.86±0.41 one week, one and three months postoperatively, respectively. One way analysis of variance (ANOVA) demonstrated significant differences between measurements made before surgery and at all postoperative times (at one week, one and three months; all P<0.0001, Bonferroni post hoc), but no significant differences were found among postoperative groups. Significant differences of Q values between horizontal and vertical meridians were found before surgery and at all postoperative times (all P<0.0001). Multiple regression analysis revealed that change of Q value significantly correlated with manifest refraction spherical equivalent (r=0.116, P<0.0001) and axial length (r=0.264, P<0.0001). 
Conclusions Over the study period, the primary changes in Q value occur within 1 week after surgery, and then become slightly decreased and nearly stable. Manifest refraction spherical equivalent and axial length play a significant role in the change of postoperative Q value.

Key words: corneal asphericity, Q value, myopia, laser-assisted in situ keratomileusis