Optical coherence tomography evaluation of the corneal cap and stromal bed features after laser in situ keratomileusis for high myopia and astigmatism

Objective: To study the corneal microstructure by optical coherence tomography (OCT) after laser in situ keratomileusis (LASIK) for high myopia with and without astigmatism. Design: Nonrandomized self-controlled comparative trial. Participants: Sixty-three consecutive LASIK eyes with spherical equivalent refraction between -6.0 and -17.0 diopters (D) and astigmatism between 0.0 and -5.0 D were prospectively recruited for examination. Intervention: LASIK was performed with the Chiron Hansatome microkeratome (160-mu m fixed plate) and Summit Apex Plus excimer laser using a 5.5/6.0/6.5-mm multizone pattern. Proper preoperative calculations were performed to ensure stromal beds thicker than 250 mu m. Main Outcome Measures: OCT imaging and measurement of corneal thickness was performed preoperatively. In addition, corneal cap and stromal bed thickness measurements were performed 1 day, 1 month, and 3 months postoperatively. Results: The average central corneal pachymetry was 538.9 +/- 26.2 mu m preoperatively. Mean corneal cap thickness measured 124.8 +/- 18.5 mu m 1-day postoperatively. Mean stromal bed thickness was 295.2 +/- 37.1 mu m on the first postoperative day. Compared with the 1-day postoperative examination, the average stromal bed thickness increased significantly by 5.9 mu m (P = 0.001) and 7.2 mu m (P = 0.001) at the I-month and 3-month postoperative examinations, respectively. Mean difference between actual (118.7 +/- 27.8 mu m) and predicted (104.1 +/- 20.8 mu m) central ablation depths was 14.6 +/- 16.7 mu m (P = 0.0001). A weak but statistically significant positive association was found between preoperative refraction and the difference between expected and real ablation depth values (R = 0.26; P = 0.042). Posterior stromal beds were move than 250-mu m thick in 58 eyes (89.9%) 1 day postoperatively. This safety requirement improved at the 1-month postoperative examination, when the partial regression accounted for slightly thicker stromal beds and only two cases (3.2%) exhibited posterior stromal tissue thinner than 250 mu m. These two cases were seen only for corrections exceeding 12 D (P = 0.04). Conclusions: OCT appears to be a useful tool for the evaluation of both the qualitative and quantitative anatomic,outcome of LASIK. Corrections of higher degrees of ametropia run a higher risk of producing a thinner than expected central cornea. Particularly, corrections greater than 12 D may lead eventually to stromal beds thinner than 250 mu m, despite proper preoperative calculations. Because corneal flaps are usually thinner than expected with the microkeratome used herein, adequate posterior corneal stroma is preserved in most instances. Ophthalmology 2000;107:81-88 (C) 2000 by the American Academy of Ophthalmology.