Induction of astigmatism by straight transverse corneal incisions, 45 degrees long, at different clear zones in human cadaver eyes.

Background: Two of the major factors affecting the amount of astigmatism correction are the length of the transverse incision and its distance from the center of the cornea. Many nomograms used in clinical practice have been created by varying the length or clear zone diameter of the incisions. A simplification of this situation has been suggested by Thornton, who has theorized that straight transverse incisions, subtending 45 degrees of arc, have equal astigmatic corrective effect at different clear zones. Our study tested Thornton's theory in human donor eyes.

Methods: Ten eyes were tested at four clear zones: 5.0, 6.0, 7.0, and 8.0 mm. Paired straight transverse incisions, subtending an arc of 45 degrees (2.1 to 3.3 mm long), were centered on the 90-degree meridian. Preoperative keratometric readings at the 180- and 90-degree meridians were compared to the postoperative readings; the difference was the total astigmatism induced. We also calculated the coupling ratio.

Results: Student's t-tests comparing clear zones 6.0 and 7.0 mm revealed a statistical difference (p = .0085) in total astigmatic induction, greater for the 6.0-millimeter zone. The coupling ratio decreased as the clear zone diameter increased, presumably as a result of diminished flattening effect along the incised meridian. One-way analysis of variance indicated that the groups were different (p = .0001), and that the theory noted above was incorrect.

Conclusions: The effect of transverse incisions subtending the same angular length, drops off dramatically with clear zones larger than 6.0 mm, contrary to the theory of Thornton. This effect may be due to reduction in coupling as the clear zone diameter increases, suggesting that the greatest efficacy is achieved for transverse incisions placed between 5.0- and 6.0-millimeter zones.