Central ablation depth and postoperative refraction in excimer laser myopic correction measured with ultrasound, Scheimpflug, and optical coherence pachymetry.

PURPOSE: To compare measurements of ultrasound, Scheimpflug, and optical coherence pachymetric techniques to describe ablated depth after myopic astigmatic corneal laser refractive surgery and achieved refractive correction.

METHODS: Ninety-six myopic astigmatism treatments using LASIK or LASEK in 58 patients with 3-month followup were retrospectively analyzed. In all cases, standard examinations, pre-/postoperative corneal topography, ocular aberrometry, and pachymetry were performed. SCHWIND Custom Ablation Manager (CAM) software and the ESIRIS laser were used for planning treatments and performing ablations. Outcomes were evaluated in terms of predictability, safety, and wavefront aberration. Pachymetry was taken before treatment (ultrasound [DGH Pachette 2], Scheimpflug [Oculus Pentacam HR], and optical coherence pachymetry [OCP] [Heidelberg-Engineering OCP]), after lifting the flap (Pachette 2, OCP), immediately after finishing ablation (Pachette 2, OCP), and at 3-month follow-up (Pachette 2, Pentacam HR).

RESULTS: At 3 months, 87 (91%) of eyes achieved 20/20 UCVA, and 89 (93%) of eyes were within +/-0.50 diopters (D). Postoperative mean spherical equivalent refraction was -0.15 +/- 0.30 D. Best spectacle-corrected visual acuity improved in 30 (31%) of eyes. Differential pachymetry correlated to intended central ablation depth for all techniques: r2 = 0.60, P < .0001, slope 0.81 for ultrasound; r2 = 0.75, P < .0001, slope 0.97 for Scheimpflug; and r2 = 0.76, P < .0001, slope 1.03 for OCR Relative differential pachymetry correlated only marginally to achieved refractive correction for ultrasound and OCP.

CONCLUSIONS: Differential pachymetry is a metric useful for describing intended central ablation depth but not for achieved refractive correction. The rotating Scheimpflug technique offers the best estimation (closest slope to 1) and OCP offers the best correlation (closest r2 to 1) for describing intended central ablation depth achieved. The three techniques give different measurements for ablation depth, with OCP being substantially different from ultrasound and Scheimpflug. Only borderline correlations were obtained for achieved refractive correction with ultrasound and OCP.