Title : Planned vs. objectively measured postoperative ablation depth and the amount of postoperative epithelial remodeling after Alcon's "Streamlight" laser vision surgery
Purpose: To find the difference between the planned ablation depth (given by the laser manufacturer) and the measured reduction in corneal stroma thickness in the ablation area.
Setting: Memira Eyecenter, Tromsø
Methods: Prospective study of 40 eyes of 40 patients undergoing treatment for low myopia and astigmatism with mean spherical equivalent of -2.75±1.1 D, using transepithelial (tPRK) "StreamLight" ablation, with the Alcon EX-500 laser. Twenty-nine corresponding points (one point in the center and one point at every 0.5 mm in the 7-mm ablation zone along two cylinder meridians, were measured. The stromal and epithelial thickness map taken with the OptoVue-Avanti OCT, as well as the laser ablation map were used. The difference in thickness between preoperative and 6-month postoperative measurements produced changes in the stroma and epithelium thickness, respectively.
Results: Mean planned and achieved central stromal ablation depths were 47.89 µm and 58.76 µm, respectively, corresponding to a mean exceeded depth of 10.87 µm (p= 0.132)) = 22.7%. The mean pre- and postoperative central epithelial thickness was 53.97 and 62.11 µm, respectively, which corresponds to a mean postoperative increase in thickness of 8.14 µm (p=? 0.001) (15.0%). The mean epithelial thickness increase added to the mean exceeded stromal ablation depth resulted in 2.73 µm (5.1%) more corneal tissue reduction relative to the planned ablation depth.
Conclusion: Measured mean central reduction in stromal thickness after StreamLight tPRK was 22.7% greater than the manufacturer's stated ablation depth. However, it seems that this excessive reduction was planned by the laser manufacturer to compensate for the postoperative increase in epithelial thickness, which brought the mean difference between the planned and the achieved central corneal tissue reduction down to only 5.1%. The 22.7% higher than nominal stromal ablation depth should be still taken into consideration when treating cases with low calculated postoperative residual stromal thickness.