Decentration measurements using Placido corneal tangential curvature topography and Scheimpflug tomography pachymetry difference maps after small-incision lenticule extraction

Purpose To assess the agreement of decentration measurements between tangential curvature topography and tomography pachymetry difference maps after a small-incision lenticule extraction (SMILE) procedure. Setting Yonsei University College of Medicine and Eyereum Eye Clinic, Seoul, South Korea. Design Retrospective case series. Methods Decentrations and pupillary offset (angle κ) were analyzed and compared using two different methods. A difference map of the tangential curvature was generated with a Placido corneal topographer (Keratron Scout) and a pachymetry difference map was generated with a Scheimpflug tomographer (Pentacam) for each eye, using preoperative and 3-month postoperative scans. Decentrations were also compared using a vector difference plot for the two maps. Results The study comprised 155 eyes (155 patients). The magnitude of decentration was 0.27 mm ± 0.17 (SD) (range 0.02 to 0.88 mm) with the tangential curvature difference maps and 0.26 ± 0.20 mm (range 0.00 to 0.91 mm) with the pachymetry difference maps, showing no significant difference in the arithmetic values of decentration (P = .124). However, the vector difference in pupillary offset between preoperative and postoperative measurements from the pachymetry difference map significantly correlated with the magnitude of vector differences of decentration between the two maps (P = .007). The vector difference plot also showed a wide distribution spread to all 4 quadrants, demonstrating a lack of agreement. Conclusions Although there was no significant difference between the scalar magnitude of decentration with the tangential topography difference maps and tomography pachymetry difference maps, the vector differences did not agree and increasing error was present with increasing differences in pupillary offset in the pachymetry difference maps.