INDUCED ASTIGMATISM AFTER 4 AND 6 MM SCLERAL TUNNEL INCISION - A RANDOMIZED STUDY

Purpose: To study the surgically induced astigmatism after phacoemulsi fication through either a 4 or a 6 mm, scleral tunnel incision by usin g multiple analyses of astigmatism. Methods: 197 eyes from 186 patient s scheduled for phacoemulsification between October 1992 and March 199 4 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 su rgically induced astigmatism was evaluated using at each follow-up: 1) The subtraction method, 2) vector analysis, 3) vector decomposition, 4) Gravy's vertical vector, 5) Naeser's polar values, and 6) the algeb raic method. Results: By subtraction, without regard to axis, the indu ced astigmatism 4 months after surgery was + 0.04 D and + 0.18 D in th e 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, respe ctively. However, cylinder orientation was not found stable until 3 mo nths after surgery, where 94% and 96% of the surgically induced astigm atism (vector decomposition) was against-the-wound in the two groups, respectively. By Gravy'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 mo nths 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 b oth 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 adequatel y described using vector analysis and vector decomposition.