Induced astigmatism after 4 and 6 mm scleral tunnel incision

. Purpose : To study the surgically induced astigmatism after phacoemulsification through either a 4 or a 6 mm scleral tunnel incision by using multiple analyses of astigmatism. Methods : 197 eyes from 186 patients scheduled for phacoemulsification between October 1992 and March 1994 were randomly assigned two different‐sized incisions with follow‐ups at 1 day, 1 week, 2 weeks, 1 month and 4 months after surgery. The surgically induced astigmatism was evaluated using at each follow‐up: 1) The subtraction method, 2) vector analysis, 3) vector decomposition, 4) Cravy's vertical vector, 5) Naeser's polar values, and 6) the algebraic method. Results : By subtraction, without regard to axis, the induced astigmatism 4 months after surgery was + 0.04 D and + 0.18 D in the 4 mm and the 6 mm incision group, respectively. By vector analysis, the numerical value of the induced cylinder was stable one month after surgery at 0.61 D and 0.77 D in the 4 mm and in the 6 mm group, respectively. However, cylinder orientation was not found stable until 4 months after surgery, where 94% and 96% of the surgically induced astigmatism (vector decomposition) was against‐the‐wound in the two groups, respectively. By Cravy's method, the mean induced astigmatism changed from − 0.08 D to − 0.32 D and from − 0.42 D to − 0.60 D between 1 and 4 months in the 4 mm and the 6 mm group, respectively. Similar values were found with Naeser's method and with the algebraic method. Conclusion : We conclude the mean cylinder of the surgically induced astigmatism (vector analysis) to be stable 1 month after phacoemulsification with both the 4 mm and 6 mm scleral tunnel incision. However, the direction of the induced axis (vector decomposition) was still drifting between 1 and 4 months in both groups. These astigmatic changes were adequately described using vector analysis and vector decomposition.