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

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Abstract
To investigate the influence of incision depth and site on wound strength and postoperative astigmatism. Virchow Memorial Hospital Eye Clinic, Berlin, Germany. In this prospective, randomized study, 180 patients with a 7.0 mm tunnel incision depth of 300 and 500 microns; 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. 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 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 microns led to induced astigmatism of 0.94 +/- 0.50 D; a depth of 300 microns 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). Incisions 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.