We analyzed J0 and J45 of astigmatism over 3.5 years and found no significant difference between the IVB and laser groups (T = −0.50, df = 46, P = 0.61 for J0; T = −0.18, df = 46, P = 0.86 for J45). We therefore modeled only the magnitude of astigmatism (CYL). At the final visit, CYL for the laser group was on average 0.46D ± 0.45D (0.00D to 2.00D), whereas CYL for the IVB group was 0.36D ± 0.55D (0.00D to +2.50D). There was no significantly different CYL in two groups (t = −0.67, df = 40.4, P = 0.50) during the study. At the final visit, 16/26 (61.5%) in the laser group and 9/22 (41%) in the IVB group had significant astigmatism (CYL ≥ 1D), but these differences did not reach statistical significance (OR = 2.3; 95% CI, 0.7–7.4; P = 0.15).
Longitudinal cylinder changes were fit by the equation: CYL(Age) = 0.2D × Age (YR) + 0.3D. There was a significant increase in cylinder magnitude with age (T = 5.49, df = 167, P < 0.001), but there were no significant differences between the two treatment groups (T = −0.17, df = 46, P = 0.86).