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First Author: T.Archer UK
Co Author(s): D. Reinstein Z. Dickeson M. Gobbe
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To demonstrate a standardized method of performing trans-epithelial phototherapeutic keratectomy (TEPTK) for irregularly irregular astigmatism using the epithelium as a natural masking agent with ablation depth calibrated according to epithelial thickness measurements. To review the epithelial thickness irregularity found in such cases to provide a guide of what to expect if a preoperative epithelial thickness measurement is unavailable.
London Vision Clinic, London, UK
Retrospective analysis of 34 TEPTK procedures performed using the MEL80 excimer laser to treat irregularly irregular astigmatism. Stromal surface irregularities are partially compensated for by epithelial thickness remodeling. Therefore the difference between the minimum and maximum epithelial thicknesses can be used a measure of irregularity. Very high-frequency digital ultrasound (VHFDU) scanning was performed preoperatively to produce epithelial thicknesses maps, and Digital Subtraction Pachymetry (DSP) was done to simulate the distribution pattern of remaining epithelium after the application of different TEPTK ablation depths. The first TEPTK ablation was planned to break through the thinnest area of the epithelium. The observed pattern of remaining epithelial was compared to the DSP maps to estimate the actual achieved ablation depth. The ablation depth of subsequent PTK ablations were calculated to reach the desired endpoint based on the DSP maps; until the entire epithelium had been removed or the remaining epithelium was restricted to an isolated area. The epithelial ablation rate was calculated by correlating the first ablation depth entered into the laser with the achieved ablation depth. Postoperative VHFDU scanning was performed. The change in epithelial irregularity was calculated for the 17 procedures that were PTK only, where no further refractive ablation was performed.
The mean preoperative minimum epithelial thickness was 43ḟ4 ṁm (range: 32 to 51 ṁm), and the mean preoperative maximum epithelial thickness was 81ḟ14 ṁm (range: 62 to 122 ṁm). The mean preoperative within-eye epithelial thickness range was 38ḟ15 ṁm (range: 13 to 80 ṁm). The mean first ablation was 54ḟ7 ṁm (range: 41 to 75 ṁm), and the mean total ablation was 67ḟ12 (range: 38 to 91 ṁm). The epithelial ablation rate was found to be 1.06 when using the MEL80. The irregularity corrected was represented by a mean change in within-eye epithelial thickness range of -10ḟ13 ṁm (range: -32 to 8 ṁm).
The use of preoperative epithelial thickness measurements in TEPTK allow ablation depths to be minimized while maximizing the effectiveness of the treatment. TEPTK succeeded in reducing the stromal surface irregularity as demonstrated by the reduction in within-eye epithelial thickness range. If no epithelial thickness measurement is available, the first ablation can be entered in the range 51 to 62 ṁm, which should mean that epithelial breakthrough will be attained where the epithelial is thinnest without exceeding the maximum epithelial thickness. Subsequent ablations can be performed in 10-20 ṁm steps chosen according to the coverage of epithelium remaining and in the context of the distribution of maximum epithelial thickness found in this population. These stepwise ablations can be continued until the epithelium is fully removed or is restricted to an isolated area.
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