Purpose: The aim of this study was to assess the random error of the thinnest point and midperipheral pachymetry with a combined Placido–scanning-slit system (Orbscan II) in the intermediate-term follow-up of excimer laser keratorefractive surgery for treating myopia.
Methods: Sixty-five patients who had undergone aspheric surface ablation for treating myopia with a minimum follow-up of 6 months, and showed no biomicroscopically detectable corneal haze, were subjected to 5 consecutive topographic examinations. All eyes underwent a complete ophthalmic examination before the surgery. The within-subject SD (Sw), repeatability (2.77 × Sw), coefficient of repeatability (2.77 × Sw/mean), and intraclass correlation coefficients of pachymetry at the thinnest point and in each 4 quadrants at a 6-mm diameter were calculated.
Results: The patients had a mean age of 31.9 ± 6.2 years, and the average follow-up was for 7.5 ± 2.6 months. The mean ablation depth for myopia was 63.6 ± 23.6 μm. The average pachymetry for the thinnest point and midperipheral superior, inferior, nasal, and temporal locations was 456.9, 602.0, 595.5, 609.7, and 566.1 μm, respectively. The repeatability (and coefficient of repeatability) for the thinnest point and midperipheral superior, inferior, nasal, and temporal locations was 26.5 (5.8%), 37.9 (6.3%), 31.0 (5.2%), 30.5 (5.0%), and 35.4 μm (6.2%), respectively. The intraclass correlation coefficients were 0.96, 0.88, 0.89, 0.97, and 0.91 for each location, respectively.
Conclusions: This study provides the repeatability of the thinnest and midperipheral pachymetry using the combined Placido–scanning-slit system in transparent corneas after laser keratorefractive surgery for treatment of myopia. The test–retest reliability provided here will help differentiate real corneal thickness change from measurement noise. For this, only pachymetric changes >6% are likely to be real and therefore useful when evaluating postoperative keratectasia suspects.