Comparative evaluation of refractive surgery candidates with Placido topography, Orbscan II, Pentacam, and wavefront analysis.

PURPOSE: To study the role of the Pentacam (Oculus), Orbscan II (Bausch & Lomb), and WaveScan (Visx) in evaluating topographic features identified as risk factors for ectasia after laser in situ keratomileusis to identify parameters that may be important in interpreting elevation topography and wavefront data when screening refractive surgery candidates. SETTING: Private practice, New York, New York, USA. METHODS: One hundred forty-five eyes of 75 consecutive patients were evaluated for refractive surgery by ultrasound pachymetry (Humphrey Atlas), videokeratography, WaveScan, Orbscan II, and Pentacam. Eyes were classified as normal or suspect based on the Rabinowitz criteria for keratoconus suspect on Placido disk-based videokeratography. Forty-six parameters were evaluated in a comparison of topographically normal eyes and eyes that met the criteria for keratoconus suspect. RESULTS: The suspect group had thinner pachymetry, multiple distinguishing characteristics on the anterior and posterior corneal surfaces by elevation topography, and larger amounts of coma by wavefront analysis. Multivariable regression analysis identified the following as the strongest predictors of a suspect Placido topography: Pentacam, thinner pachymetry and larger differences between the highest and lowest points on the posterior elevation; Orbscan II, higher anterior maximum elevation, horizontal location of the thinnest point on the pachymetry map, and larger differences between the highest and lowest points on the posterior elevation. CONCLUSION: Several parameters provided by the Pentacam, Orbscan II, WaveScan, and pachymetry were statistically correlated with keratoconus suspect, defined by higher asymmetry and steeper curvature on Placido topography.

Intraocular lens calculations after refractive surgery.

PURPOSE: To evaluate the effect of refractive surgery on intraocular lens (IOL) power calculation, compare methods of IOL power calculation after refractive surgery, evaluate the effect of pre-refractive surgery refractive error on IOL deviation, review the literature on determining IOL power after refractive surgery, and introduce a formula for IOL calculation for use after refractive surgery for myopia. SETTING: Laser & Corneal Surgery Associates and Center for Ocular Tear Film Disorders, New York, New York, USA. METHODS: This retrospective noncomparative case series comprised 21 patients who had uneventful cataract extraction and IOL implantation after previous uneventful myopic refractive surgery. Six methods of IOL calculation were used: clinical history (IOL(HisK)), clinical history at the spectacle plane (IOL(HisKs)), vertex (IOL(vertex)), back-calculated (IOL(BC)), calculation based on average keratometry (IOL(avgK)), and calculation based on flattest keratometry (IOL(flatK)). Each method result was compared to an "exact" IOL (IOL(exact)) that would have resulted in emmetropia and then compared to the pre-refractive surgery manifest refraction using linear regression. The paired t test was used to determine statistical significance. RESULTS: The IOL(HisKs) was the most accurate method for IOL calculations, with a mean deviation from emmetropia of -0.56 diopter +/-1.59 (D), followed by the IOL(BC) (+1.06 +/- 1.51 D), IOL(vertex) (+1.51 +/- 1.95 D), IOL(flatK) (-1.72 +/- 2.19 D), IOL(HisK) (-1.76 +/- 1.76 D), and IOL(avgK) (-2.32 +/- 2.36 D). There was no statistical difference between IOL(HisKs) and IOL(exact) in myopic eyes. The power of IOL(flatK) would be inaccurate by -(0.47x+0.85), where x is the pre-refractive surgery myopic SE (SEQ(m)). Thus, without adjusting IOL(flatK), most patients would be left hyperopic. However, when IOL(flatK) is adjusted with this formula, it would not be statistically different from IOL(exact). CONCLUSIONS: For IOL power selection in previously myopic patients, a predictive formula to calculate IOL power based only on the pre-refractive surgery SEQ(m) and current flattest keratometry readings was not statistically different from IOL(exact). The IOL(HisKs), which was also not statistically different from IOL(exact), requires pre-refractive surgery keratometry readings that are often not available to the cataract surgeon.