Postoperative astigmatism and relative strength of tunnel incisions: A prospective clinical trial

Purpose: To investigate the influence of incision depth and site on wound strength and postoperative astigmatism. Setting: Virchow Memorial Hospital Eye Clinic, Berlin, Germany. Methods: In this prospective, randomized study, 180 patients with a 7.0 mm tunnel incision were examined. They were divided into the following subgroups: primary incision depth of 300 and 500 mu m; limbal incision and scleral incision; temporal scleral incision and scleral incision at the 12 o'clock position; temporal limbal incision and limbal incision at the 12 o'clock position. Postoperative astigmatism was measured by keratometry and videokeratoscopy 1 day, 1 and 4 weeks, and 8 months postoperatively. Wound strength was measured with an ophthalmodynamometer on the first postoperative day and after 1 week at the site with the least mechanical stability adjacent and posterior to the primary incision. Results: The temporal incision, which was performed 1.0 mm behind the surgical limbus, led to induced astigmatism of 0.65 diopters (D) +/- 0.23 (SD) after 8 months. When the incision was at the 12 o'clock position, the induced astigmatism was 0.97 +/- 0.41 D. Induced astigmatism was highest following a limbal incision in the 12 o'clock position (1.33 +/- 0.63 D). This effect was less pronounced with a temporal incision. Incision depth did not significantly influence induced astigmatism. An incision depth of 500 mu m led to induced astigmatism of 0.94 +/- 0.50 D; a depth of 300 mu m led to induced astigmatism of 0.78 +/- 0.64 D. After 1 week, wound strength was highest with temporal scleral incisions (38.6 +/- 2.1 kPa by ophthalmodynamometer) and lowest with limbal incisions in the 12 o'clock position (30.8 +/- 7.7 kPa). Conclusions: Incision site significantly influenced mechanical wound strength and induced astigmatism; incision depth influenced neither. In general, incisions in the 12 o'clock position induced more astigmatism than temporal incisions.