Purpose: This study compares the change over time of the astigmatism caused by "large" incision extracapsular cataract extraction (ECCE) and three smaller incisions for phakoemulsification. Based on this data, a mathematical model that predicts the course of astigmatism after a superior incision of length 3 to 12 mm has been developed. The relationship of axial length and preoperative astigmatism to induced post-operative astigmatism, the recovery of visual acuity, and the rate of YAG laser capsulotomy after each procedure also are documented.
Methods: Induced astigmatic change was calculated using a simple method of vector analysis. The change in induced astigmatism was calculated for 8 years after ECCE (n = 144), for 3 years after 6 mm superior incisions (6SUP) (n = 93), for 2 years after 3 mm superior incisions (3SUP) (n = 120), and for 18 months after 3 mm temporal incisions (3Temp) (n = 65). Plotted semi-logarithmically, the astigmatic change in each group may be represented mathematically.
Results: Two weeks after ECCE the mean induced cylinder was +3.47 D, which decayed to about -1.25 D after 6 months. Induced cylinder increased gradually to about -1.6 D after 8 years, although this further change was not significantly different than that at 6 months after surgery. For the phako groups, the net induced cylinder on the first post-operative day was: +1.23 D (6SUP), +0.49 D (3Sup), and -0.19 D (3Temp). After 6Sup the wound was astigmatically stable after approximately 3 months, and 3 years after surgery net induced cylinder was -0.66 D. After 3Sup the wound was astigmatically stable after about 6 weeks, and after 18 months net induced cylinder was -0.35 D. No significant change in astigmatism was detected at any time after 3Temp. Maximum visual acuity was reached after a mean of approximately 6 weeks after ECCE, 2 weeks after 6Sup, and between 1 day and 1 week after 3Sup and 3Temp. The rate of YAG laser capsulotomy was higher after ECCE than after any of the phakoemulsification procedures. No relationship of axial length or preoperative astigmatism to astigmatic change was detected.
Conclusions: Incision size and location affect post-operative astigmatism. Induced astigmatism decreases with wound size, and only the 3 mm temporal incision is astigmatically neutral. The time for visual recovery increases with wound size. There appears to be less need for laser capsulotomy after phakoemulsification with capsulorrhexis than after ECCE. Axial length does not affect induced astigmatism after any of the 4 incisions, and preoperative astigmatism does not affect astigmatic change after ECCE and 6Sup.