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R Rathod, DJ Shen, MX Wang; Relationship between Stromal Ablation Depth and Hyperopic Shift after 6 mm Phototherapeutic Keratectomy using VISX Star Excimer Laser . Invest. Ophthalmol. Vis. Sci. 2002;43(13):159.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To determine the quantitative relationship between the amount of hyperopic shift and the depth of stromal ablation after 6mm central phototherapeutic keratectomy (PTK). Methods: A retrospective study was performed on 17 consecutive eyes in 17 patients that underwent 6mm central PTK in which no masking agent was used (based on clinical indication) with VISX Star excimer laser and by a single surgeon (MW). The depth of corneal stromal ablation and the resultant amount of hyperopic shift when refraction was stabilized were recorded. The data were then correlated and used to derive a general formula to predict the amount of hyperopic shift induced by 6-mm central PTK as a function of the depth of corneal stromal ablation. Results: The mean pre-operative spherical equivalent was -0.87 D (range, -7.5 to +5.5 D), the mean post-operative spherical equivalent was +0.91 D (range, -4.5 to +6.5 D). The mean hyperopic shift after 6 mm central PTK without masking agent was 1.78 D (range, 0.125 to 5.0 D). The average stromal ablation depth (SAD) per diopter change was 42.1 ± 24.1 mm/D. Conclusion: Surgeons should anticipate approximately a 42 mm per diopter shift following 6 mm diameter central PTK. Or inversely, 6 mm PTK will create a shift of 0.024 D per 1 mm of corneal stromal ablation depth. A general formula to predict the hyperopic shift after central 6 mm PTK is: D = SAD/42.
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