Topographic analysis of astigmatism induced by scleral shortening in pig eyes

Background: Corneal astigmatism is a severe postoperative problem in foveal translocation surgery. We evaluated the corneal astigmatism induced by scleral shortening in pig eyes in vitro. Methods: We created three sizes of scleral shortening in pig eyes and examined the preoperative and postoperative corneal astigmatism. The three sizes of scleral shortening were; 6 mm x 12 mm, 9 mm x 12 mm, and 6 mm x 16 mm (radial x circumferential). The shortenings were created 11 mm from the limbus with 10 eyes in each group. Videokeratographic measurements were performed using the CAS System 2000, preoperatively and postoperatively, and the astigmatism caused by the scleral shortening was evaluated. Results: The surgically-induced astigmatism was 2.1 +/-1.2 diopters (D) in the 6 mm x 12 mm group, 5.2 +/-1.5 D in the 9 mm x 12 mm group, and 3.7 +/-1.0 D in the 6 mm x 16 mm group. Corneal astigmatism caused by scleral shortening depended on both the radial and circumferential shortening. Pre- and postoperative topographic corneal maps showed an irregular astigmatism pattern (lazy bowtie pattern). Because the central zone of the cornea showed a relatively regular astigmatism, the corneal astigmatism induced by scleral shortening did not affect the predicted corneal acuity. Conclusions: In foveal translocation surgery with scleral shortening, an excessive scleral resection in the radial direction can cause clinically intolerable regular and irregular astigmatism. Minimal scleral shortening that will satisfy the required translocated distance is recommended to reduce the risk/benefit ratio.