[Vector analysis of surgically-induced astigmatism in cataract operation with 4 tunnel incision techniques]

Evaluation of astigmatism induced by cataract surgery requires the calculation of surgically induced astigmatism using vector analysis. This method was performed for four tunnel-incision techniques and compared with the results of computer-assisted videokeratoscopy.

Phacoemulsification with IOL implantation was performed in 59 eyes using four different incisions (4- or 5- to 6-mm scleral tunnel and 4- or 5.5-mm clear cornea). Surgically induced astigmatism was calculated by vector subtraction using the formulas of Retzlaff. In addition, corneal topography was performed and astigmatism was calculated within three concentric corneal zones (3 mm, 3-5 mm, 5-7 mm).

Surgically induced astigmatism after a 4-mm scleral tunnel incision was 0.55 D (pre-op 0.58 D, post-op 0.72 D), the 5-6-mm incision induced 0.89 D (pre-op 0.97 D and post-op 1.02 D). The 4-mm clear cornea incision induced 2.00 D (from 0.89 D to 1.56 D), the 5.5-mm corneal incision, performed in the steepest meridian to reduce preexisting astigmatism, induced 3.57 D; pre-op 3.38 D, post-op 2.09 D. Corneal topography revealed a slight increase in astigmatism within all corneal zones (4-mm scleral tunnel: 0.26 D, 5-6-mm scleral tunnel: 0.36 D and 4-mm clear cornea incision: 0.22 D). After a 5.5-mm clear cornea incision, however, astigmatism of the central 3-mm zone was reduced by 1 D, while astigmatism of the 5-7 mm zone increased by 0.22 D.

The 4- and 5- to 6-mm scleral tunnel as well as the 4-mm clear cornea incision were shown to be nearly astigmatism-neutral. The 5.5-mm clear cornea incision reduced astigmatism of the central cornea by about 35%, but induced irregular astigmatism in the periphery.