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J. S. Pepose, M. A. Qazi, A. M. Mahmoud, J. P. Sanderson, E. Y. Yoon, C. J. Roberts; Comparison of Ocular Response Analyzer Waveforms in LASEK and LASIK. Invest. Ophthalmol. Vis. Sci. 2007;48(13):5354.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the effects of flap creation and/or ablation on biomechanical parameters derived from the applanation and air pressure signals of the Ocular Response Analyzer (ORA, Reichert) following myopic surface (LASEK) and microkeratome-assisted (LASIK) keratorefractive surgery.
Slit-scanning videokeratography, ultrasound pachymetry, and ORA measurements were obtained bilaterally in monocular LASIK (n=15, flap 146 µm, ablation 35 µm, stromal bed 385 µm), bilateral LASIK (n=54, flap 136 µm, ablation 86 µm, stromal bed 331 µm), and bilateral LASEK (n=32, ablation 97 µm, stromal bed 398 µm) eyes. Custom software was used to derive 10 parameters from ORA signals: magnitude of the applanation peaks (peak1 & peak2), onset of the applanation peaks (time1 & time2), width of each peak at its mid-height point (fwhm1 & fwhm2), the peak (pmax) and onset (tpmax) of the air pressure curve, and its slope during the two applanation events (slope1 and slope2). Differences between groups were statistically analyzed.
The most profound changes were seen in the bilateral LASIK group, which had the lowest mean residual stromal bed thickness and correspondingly lower peak1, time1, pmax, and tmax than all other groups (p<.05). While flap thickness did not statistically differ between the monocular and bilateral LASIK eyes, the latter group had a deeper mean ablation, statistically lower waveform amplitudes (peak1, peak2, pmax) and earlier onset of waveform peaks (time2, time2, tpmax). Similarly, time1 and tpmax occurred earlier in bilateral LASIK than ablation-matched LASEK eyes, with the former group requiring lower mean air pulse amplitudes (pmax) to achieve applanation, suggesting that the bilateral LASIK eyes were more easily deformed. Peak2 and time2, but not peak1 nor time1, were reduced in a statistically significant manner in bilateral LASEK eyes compared to residual bed thickness-matched unilateral LASIK eyes, suggesting that anterior stromal ablation may have a greater effect on the rebound properties of the cornea.
The combination of microkeratome flap creation and deeper excimer ablation has the greatest effect upon the ORA applanation and air pulse curves, with reduced amplitudes and earlier onset of peaks suggesting a more easily deformed cornea. There is a complex interaction between the location and number of collagen lamellae altered by photoablation that impacts postoperative corneal viscoelastic and biomechanical properties.
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