The Long-term Course of the Surgically-induced Astigmatism after a Scleral Tunnel Incision

OBJECTIVE: To study the 1-year course of the surgically-induced astigmatism after phacoemulsification through a 6 to 6.5 mm scleral tunnel incision by using six different analytic techniques. STUDY DESIGN: Prospective study with follow-up at 1 day, 1 week, 1 month, 4 months and 1 year after surgery. SETTING: Department of Ophthalmology, Aarhus University Hospital. PATIENTS: A consecutive series of 110 patients between 22-81 years of age (mean age 60) scheduled for phacoemulsification between October 1991 and May 1992. MAIN OUTCOME MEASURES: The surgically induced astigmatism was evaluated using 6 different methods: by (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, respectively. RESULTS: By subtraction, without regard to axis, the induced astigmatism was found to reach baseline level 1 month after surgery. By vector analysis, the mean induced astigmatism was 1.33 D,1.31 D, 0.75 D, 0.72 D and 0.75 D 1 day, 1 week, 1 month, 4 months and 1 year after surgery, respectively. By vector decomposition, 27%, 25%, 55%, 85% and 86% of the surgically-induced astigmatism was found to be against-the-rule at the same postoperative visits, respectively. By Cravy's method, the mean induced cylinder was +0.75, +0.89, +0.08, -0.44D and -0.50D, respectively. Similar values were found with Naeser's method and with the algebraic method. CONCLUSIONS: We conclude the surgically induced astigmatism to be stable 4 months after phacoemulsification with a 6 to 6.5 mm scleral tunnel incision. Regarding the six analytic techniques we recommend the vector analysis, which is capable of reporting both the magnitude and the axis of the induced astigmatism within the theory of Gaussian optics. To illustrate the with-the-rule and against-the-rule component in a simple manner, we suggest the vector decomposition to be used as an adjunct to the vector analysis.