Corneal topography of excimer laser photorefractive keratectomy

Anterior and posterior corneal curvature: normal values in healthy Iranian population obtained with the Orbscan II

The objective of study was to determine the normative values of anterior and posterior best fit sphere (A-BFS and P-BFS) measured with Orbscan II Topography System. In this cross-sectional study, patients (age range: 18-40 years) referred to the Khatam Eye Hospital (Mashhad, Iran) were put in an observational cross-sectional study. The A-BFS and P-BFS were measured with the Orbscan II. The differences between genders, between right and left eyes, and age-related changes were evaluated. A total of 977 healthy participants consisted of 614 female and 363 male subjects aged 18-35 years participated. The average A-BFS in our study population was recorded as 43.060 ± 1.541 D (median: 43.00 D, mode: 43.10 D, range: 38.80-55.80 D). The average P-BFS in our study population was recorded as 52.702 ± 2.190 D (median: 52.60 D, mode: 53.10 D range: 46.9-62.20 D). The A-BFS and P-BFS were respectively 42.753 ± 1.629 and 52.327 ± 2.376 D in males and 43.242 ± 1.457 and 52.924 ± 2.041 D in females, which were statistically different between the genders (P < 0.001). However, A-BFS and P-BFS were not statistically different between right and left eyes (P = 0.649 and P = 0.688 respectively). In addition, A-BFS and P-BFS were not correlated with the age (r = 0.038, P = 0.096 and r = -0.142, P = 0.178 respectively). Considering 95 % confidence interval, A-BFS less than 43.13 D and greater than 42.99 D and P-BFS less than 52.80 D and greater than 52.60 D would be considered abnormal. Detailed description and analysis of A-BFS and P-BFS with Orbscan demonstrated that the obtained average value of BFS were higher in male than female and did not change with increasing age.

Relationship between the corneal surface and the anterior segment of the cornea: An Asian perspective

PURPOSE

To determine the values for the anterior best-fit sphere (BFS) and posterior BFS in an Asian population using the Orbscan II (Bausch & Lomb) slit-scanning Placido disk corneal topographer.

SETTING

Hospital-based refractive surgery center.

METHODS

This prospective nonrandomized study comprised of 1 eye of 724 subjects. The eyes were measured with the Orbscan II. Default settings were used to generate relative elevation maps. Anterior BFS, posterior BFS, mean keratometry (K(mean)), spherical equivalent (SE), horizontal white-to-white size (WTW), and anterior chamber depth (ACD) were analyzed.

RESULTS

The mean spherical equivalent (SE) in the 724 eyes was -5.32 diopters (D) +/- 2.88 (SD). The anterior BFS and posterior BFS were normally distributed, with a mean of 43.05 +/- 1.37 D and 52.43 +/- 2.03 D, respectively. The regression equation of anterior BFS and posterior BFS was posterior BFS = 0.443 + 1.208 anterior BFS (r(2) = 0.667, P<.01). The ratio of posterior BFS to anterior BFS was 1.22 +/- 0.03. The anterior BFS was significantly correlated to posterior BFS (P<.01), K(mean) (P<.01), and horizontal WTW (P<.01) but not to SE (P = .28). The posterior BFS was significantly correlated to K(mean) (P<.01) and the horizontal WTW (P<.01) but not to SE (P = .70).

CONCLUSION

There was a close relationship between the various corneal parameters, which is significant in refractive surgery and intraocular lens calculation and design.

Corneal elevation indices in normal and keratoconic eyes

PURPOSE

To determine an index that distinguishes keratoconus and keratoconus-suspect eyes from normal eyes with Orbscan (Bausch & Lomb) corneal elevation maps.

SETTING

Department of Ophthalmology, Tan Tock Seng Hospital, Singapore, and iLaser Centre, Island Hospital, Penang, Malaysia.

METHODS

In this initial phase of this multicenter nonrandomized retrospective study, 1 eye of 166 normal subjects, 15 keratoconus patients, and 11 keratoconus suspects was examined at 1 clinic. The anterior best-fit sphere (BFS), posterior BFS, anterior elevation, posterior elevation, and maximum and minimum keratometries were analyzed. Two indices with the highest sensitivity and specificity classifying these conditions were identified using receiver operating characteristic curves. These 2 indices were evaluated in a subsequent validation study using 1 eye of 221 normal subjects, 43 keratoconus patients, and 23 keratoconus suspects from another clinic.

RESULTS

The anterior elevation and anterior elevation ratio (anterior elevation ratio = anterior elevation/anterior BFS) best classified the different groups. An anterior elevation ratio of 0.5122 mm or less had 99% sensitivity and 95.2% specificity while a ratio 16.5 mum or less had 80.1% sensitivity and 80.8% specificity in discriminating normal eyes from keratoconus and keratoconus suspects. The results were similar in the validation study. In addition, these anterior elevation and anterior elevation ratio cutoff values had high sensitivity and specificity in identifying keratoconus suspects from normal eyes in the validation study.

CONCLUSION

Anterior corneal elevation parameters are clinically relevant measures for detecting keratoconus and suspected keratoconus eyes.

Predicted corneal visual acuity in keratoconus as determined by ray tracing

PURPOSE

To evaluate the optical quality of the central anterior corneal surface in normal eyes and in eyes with keratoconus, and to investigate the accuracy of the predicted corneal visual acuity (PCVA) index as determined by ray tracing analysis.

METHODS

Twenty keratoconus eyes with contact lens-corrected visual acuity (VA) of 20/20 or better (11 patients, group A) and 20 eyes of 15 normal subjects (group B) were evaluated. After a detailed eye examination including measurement of pupil diameter, keratometry, topography and pachymetry, each subject eye was evaluated using ray tracing analysis with the Technomed C-scan colour ellipsoid topometer, using basic software (Technomed GmbH, Baesweiler, Germany). The PCVA was determined for each patient, and the results were analysed comparatively using two-sample t-test, regression analysis and Pearson correlation analysis.

RESULTS

The average best spectacle-corrected VA was measured as 0.2 +/- 0.2 logMAR (20/32) in group A and -0.1 +/- 0.1 logMAR (20/16) in group B. The average PCVA measurements derived from ray tracing analysis for 3.0 mm, 3.5 mm and 4.0 mm pupil diameters were 0.06 +/- 0.12 logMAR, 0.14 +/- 0.13 logMAR and 0.21 +/- 0.17 logMAR, respectively, in group A, and -0.14 +/- 0.08 logMAR,-0.11 +/- 0.09 logMAR and -0.09 +/- 0.11 logMAR, respectively, in group B. There was good correlation between best corrected VA and PCVA in both groups for all pupil diameters measured (p < 0.007).

CONCLUSION

Predicted corneal visual acuity as determined by ray tracing analysis is useful for estimating best spectacle-corrected VA in normal corneas and the effect of irregular corneal astigmatism on VA in eyes with mild to moderate keratoconus. Further studies are required to evaluate the efficacy of ray tracing in evaluation of aberrations of the optical system of the eye.

Broad Beam vs. Flying Spot Excimer Laser: Refractive and Videokeratographic Outcomes of Two Different Ablation Profiles After Photorefractive Keratectomy

PURPOSE

To assess the refractive outcomes and videokeratographic patterns in photorefractive keratectomy (PRK) of two last-generation excimer lasers: broad beam and flying spot lasers.

METHODS

Forty eyes were treated for a mean myopic correction of -5.59+/-2.17 D with a Ladarvision excimer laser (Autonomous Technology Corp). We compared the refractive outcomes and the videokeratographic patterns with those of 40 eyes treated with the Summit Apex Plus laser (Summit Technology Inc) for a mean myopic correction of -5.60+/-2.24 D.

RESULTS

During the 6-month follow-up, we found no significant difference in uncorrected and spectacle-corrected visual acuity, haze grade, regularity index, or decentration. The eyes treated by Ladarvision showed a lower refractive error at 1 month (P = .04), a lower incidence of central islands at 1 (P = .003) and 3 months (P = .04), a wider mean effective ablation area at 1 (P = .0004), 3 (P = .0000009), and 6 months (P = .0000000004), a less steep ablation edge at 1 (P = .0026), 3 (P = .015), and 6 months (P = .011).

CONCLUSIONS

The small beam excimer laser provides better videokeratographic outcomes, which lead to quicker refractive stability and better visual performance.

200 Hz flying-spot technology of the LaserSight LSX excimer laser in the treatment of myopic astigmatism: six and 12 month outcomes of laser in situ keratomileusis and photorefractive keratectomy

PURPOSE

To evaluate safety, efficacy, predictability, and stability in the treatment of myopic astigmatism with laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK) using the 200 Hz flying-spot technology of the LaserSight LSX excimer laser.

SETTING

SynsLaser Clinic, Tromsø, Norway.

METHODS

This retrospective study included 110 eyes treated with LASIK and 87 eyes treated with PRK that were available for evaluation at 6 and 12 months, respectively. The mean preoperative spherical equivalent (SE) was -5.35 diopters (D) +/- 2.50 (SD) (range -1.13 to -11.88 D) in the LASIK eyes and -4.72 +/- 2.82 D (range -1.00 to -15.50 D) in the PRK eyes. The treated cylinder was 4.00 D in both groups. Eleven (8.5%) LASIK eyes and 8 (7.4%) PRK eyes had secondary surgical procedures before 6 and 12 months, respectively, and were excluded when the 6 and 12 month outcomes were analyzed.

RESULTS

None of the eyes lost 2 or more lines of best spectacle-corrected visual acuity. Seventy-seven percent of the LASIK eyes and 78% of the PRK eyes achieved an uncorrected visual acuity of 20/20 or better; 98% in both groups achieved 20/40 or better. The SE was within +/-0.5 D of the desired refraction in 83% of the LASIK eyes and 77% of the PRK eyes; it was within +/-1.0 D in 97% and 98%, respectively. The cylinder correction had a mean magnitude of error of 0.04 +/- 0.31 D (range -0.96 to +0.85 D) in the LASIK eyes and 0.02 +/- 0.37 D (range -1.44 to +0.72 D) in the PRK eyes. Refractive stability was achieved at 1 month and beyond in the LASIK eyes and at 3 months and beyond in the PRK eyes.

CONCLUSION

The outcomes of this study are comparable to those achieved with lasers that use small-beam technology with a lower frequency, as well as with other types of delivery systems. They suggest that the 200 Hz technology used in the LaserSight LSX excimer laser is safe, effective, and predictable and that with LASIK and PRK the results are stable when treating low to moderate myopia and astigmatism up to 4.0 D.

The accuracy and precision performance of four videokeratoscopes in measuring test surfaces

In this study we evaluated the accuracy and precision of three placido-disk videokeratoscopes (the Keratron, Medmont and TMS) and one videokeratoscope that uses the raster-stereogrammetry technique (PAR-CTS) in elevation topography with six test surfaces. The test surfaces were a sphere, an asphere, a multicurve, and three bicurve surfaces. Each instrument performed well on certain test surfaces, but none of the instruments excelled on all of the surfaces. The results showed high accuracy of the Keratron and Medmont instruments in measuring the sphere, asphere, and multicurve surfaces, but not the bicurve surfaces. The precision of the Keratron and Medmont instruments were high. The TMS and PAR-CTS instruments showed poorer accuracy than the Keratron and Medmont instruments for the multicurve test surface but showed better performance for the bicurve surfaces. The PAR-CTS had the poorest performance in precision of the four instruments. The use of the Noryl spherical test surface instead of polymethyl methacrylate (PMMA) resulted in small differences in the accuracy performance of the placido-disk videokeratoscopes only.

Prediction of letter contrast sensitivity using videokeratographic indices

PURPOSE

To analyze the relationship between corneal topography and letter contrast sensitivity.

METHOD

Experiments were conducted on 59 eyes of 51 patients who had best spectacle-corrected visual acuity of 20/20 or better and no ocular pathology except for the corneal shape. Thirty-nine eyes had an abnormal topographic pattern resulting from keratoconus, and the other 20 eyes showed a normal topographic pattern. Videokeratography was performed with the TMS-2 videokeratoscope, and the surface regularity index, surface asymmetry index, and coefficient of variation of power were obtained for each subject. Letter contrast sensitivity was measured with the CSV-1000LV with spectacle correction. The correlation between the number of correct letters and topographic indices was calculated.

RESULTS

The abnormal topography group had a significantly greater loss of letter contrast sensitivity (median = 20 letters) than the normal control (median = 23 letters; P =.0001). There were statistically significant correlations between number of correct letters and the coefficient of variation of power (r = -.77; P =. 001), number of correct letters and surface regularity index (r = -. 76, P =.001), and the number of correct letters and surface asymmetry index (r = -.64; P =.001). The linear regression equation between number of correct letters and the coefficient of variation of power was the number of correct letters = -0.05 x the coefficient of variation of power + 23.2.

CONCLUSIONS

Our results suggest that subtle visual deteriorations, which are barely detected by contrast sensitivity testing, can be predicted objectively by the corneal topographic indices.

Selective zonal ablations with excimer laser for correction of irregular astigmatism induced by refractive surgery

OBJECTIVE

To demonstrate the safety and efficacy of selective zonal ablations with excimer laser for the correction of irregular astigmatism induced by refractive surgery.

DESIGN

Thirty-one eyes of 26 patients. Retrospective and noncomparative case series.

METHODS

The authors reviewed the medical results of the two series of consecutive cases of irregular astigmatism induced by refractive surgery. One group showed a topography map with a defined pattern: decentered ablation, decentered steep central island, central irregularity, and peripheral irregularity. The other group showed an irregular astigmatism without defined pattern. Selective zonal ablation was performed with a broad-beam excimer laser. Laser ablations were adjusted according to the Munnerlyn formula. A phototherapeutic keratectomy (PTK) mode was used in all cases without using viscous masking solution.

MAIN OUTCOME MEASURES

Uncorrected visual acuity, best spectacle-corrected visual acuity (BCVA), and Holladay diagnostic summary (Eye Sys 2.000) were considered before and after surgery to obtain the qualitative and quantitative characteristics of the visual acuity and corneal topography. The corneal uniformity index (CUI) was used to evaluate the corneal surface changes induced by the selective zonal ablations in the correction of irregular astigmatism. It proved to be a useful tool in the understanding of astigmatic changes when it is not possible to apply the vector analysis.

RESULTS

In group 1, the irregular astigmatism was significantly improved in 96.4% of cases. The CUI was improved from 58+/-0.3% to 87+/-0.8%, P < 0.005 (Student's t-test). The improvement of the BCVA was from 20/40+/-20/100 to 20/25+/-20/100, P < 0.005 (Student's t-test). The results obtained in group 2 were not clinically significant in terms of improvement of CUI and BCVA.

CONCLUSIONS

Using the corneal topographic map as a guide, selective zonal ablations with excimer laser can be used to create a more regular corneal surface, when a defined pattern of topographic irregularities can be defined.

Comparison of topographic indices that correlate with visual acuity in videokeratography

OBJECTIVE

To evaluate the relationship between the best spectacle-corrected visual acuity (BSCVA) and two quantitative indices of the anterior corneal surface obtained by videokeratography.

DESIGN

Prospective, single center, comparative, observational study.

PARTICIPANTS

Eighty-nine normal eyes and 52 eyes with keratoconus with contact lens-corrected visual acuity of 20/20 or better.

INTERVENTION

Videokeratography was performed with the TMS-2 and the CAS system 2000.

MAIN OUTCOME MEASURES

The relationship between the BSCVA recorded in log minimal angle of resolution (logMAR) units, the surface regularity index (SRI), and the predicted corneal acuity (PCA) were assessed by linear regression analysis.

RESULTS

The BSCVAs for all eyes ranged from 0.82 to -0.30 logMAR units. BSCVA was highly correlated with the SRI (r = 0.70, P < 0.0001) and the PCA (r = -0.61, P < 0.0001). There was no statistical difference in the regression slopes and the intercepts for the estimated BSCVA using the SRI and measured BSCVA, and the estimated BSCVA using PCA and measured BSCVA.

CONCLUSIONS

Two quantitative topographic indices, SRI and PCA, are useful for estimating the effect of irregular astigmatism on visual acuity even though both indices quantify different aspects of the anterior surface of the cornea.

Corneal topography of small-beam tracking excimer laser photorefractive keratectomy

PURPOSE

To evaluate the topographic characteristic of photorefractive keratectomy (PRK) for low myopia performed with a small-beam (0.9 mm) tracking excimer laser.

SETTING

Department of Ophthalmology, LSU Eye Center, Louisiana State University Medical Center School of Medicine in New Orleans, and the Refractive Surgery Center of the South at the Eye, Ear, Nose, & Throat Hospital, New Orleans, Louisiana, USA.

METHODS

Sixty-seven eyes of 47 patients had PRK with a small-beam tracking laser. Of these, 49 eyes had data permitting evaluation of ablation centration; usable data for topographic analysis were available for 59 eyes preoperatively, 54 eyes at 1 month, 42 eyes at 3 months, and 25 eyes at 6 months, permitting measurement of various topographic parameters, including the cylinder (CYL), average corneal power (ACP), surface regularity index (SRI), surface asymmetry index (SAI), corneal eccentricity index (CEI), and coefficient of variation of corneal power (CVP).

RESULTS

Preoperatively, all eyes were topographically normal. Postoperatively, no eye exhibited a "central island" by even the least-restrictive definition, and all eyes had best spectacle-corrected visual acuities (BSCVAs) of 20/20 or better at all follow-ups. Mean decentration of the ablations from the pupil centers was 0.42 mm +/- 0.28 (SD) (n = 49). There was no correlation between measured decentration and BSCVA (P = .46). The central cornea was flattened (decreased ACP; P < .001) and made oblate (decreased CEI; P < .001) as expected. There was no increase in SRI or SAI (irregular astigmatism) at 6 months compared with preoperative values (P = .91); however, CYL and CVP (varifocality) increased slightly (P = .04 and .02, respectively).

CONCLUSION

The absence of significant regular or irregular astigmatism 6 months after PRK with the small-beam laser is an improvement over published results achieved with wide-beam lasers and is consistent with the excellent visual acuity results in this cohort.

Natural history of corneal topography after excimer laser photorefractive keratectomy

PURPOSE

To investigate the natural history of corneal topography after excimer laser photorefractive keratectomy (PRK).

DESIGN

A prospective, single center clinical study.

PARTICIPANTS

A total of 40 eyes of 34 patients with myopia were studied.

INTERVENTION

Excimer laser photorefractive keratectomy and computer-assisted videokeratography were performed.

MAIN OUTCOME MEASURES

Preoperative, 1 week, and 1, 2, and 3 months postoperative topography patterns were compared and changes assessed by averaging defined sectors of the ablation zone in individual maps to produce composite "average" topography maps.

RESULTS

Corneal topography was generally not homogeneous at 1 week after PRK. By 3 months, there had been considerable smoothing of corneal contour. A general "central island" effect early in the postoperative period flattened over time. When right and left eyes were evaluated independently, a tendency toward a keyhole-semicircular pattern was seen on average; the maximum flattening was nasal and the least flattening was inferotemporal for both right and left eyes.

CONCLUSIONS

A tendency, on average, toward central island and keyhole-semicircular patterns is seen early in the postoperative course after PRK. Central islands tend to evolve into the keyhole-semicircular pattern, and the corneal topography in general after PRK tends to smooth considerably with time.

Topographic assessment of irregular astigmatism after photorefractive keratectomy

OBJECTIVE

To correlate new quantitative topographic indexes of corneal irregular astigmatism to best spectacle-corrected visual acuity (BSCVA) following excimer laser photorefractive keratectomy (PRK).

SETTING

Department of Ophthalmology, LSU Eye Center, and Refractive Surgery Center of the South, Ear, Nose & Throat Hospital, New Orleans, Louisiana; Manhattan Eye, Ear and Throat Hospital, New York, New York, USA.

METHODS

Videokeratography data (TMS-1) were obtained preoperatively and 1, 3, 6, 12, 18, and 24 months postoperatively from 100 eyes having PRK for low to mild myopia. Algorithms measured fine local irregularity with the surface regularity index (SRIp), varifocality with the coefficient of variation of corneal power (CVPp), and central islands with the elevation/depression magnitude (EDM).

RESULTS

The SRIp and CVPp increased after surgery and remained significantly higher than the preoperative levels throughout the 24 month follow-up (P < .05). The increase in EDM was significant from 1 to 6 months (P < .05) but not thereafter. Multiple regression analysis revealed that variables having a statistically significant relationship with postoperative BSCVA were CVPp and EDM at 1 month, CVPp at 3 months, and CVPp, haze, and age at 6 months. No statistically significant correlation between any measures of irregular astigmatism and BSCVA was found after 1 year of follow-up.

CONCLUSION

The quantitative measures used in this study are sensitive methods by which irregular astigmatism after keratorefractive procedures can be classified, evaluated, and compared.

Corneal topography of photorefractive keratectomy versus laser in situ keratomileusis. Summit PRK-LASIK Study Group

OBJECTIVE

This study aimed to compare qualitative patterns of corneal topography early in the postoperative course after excimer laser photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) when used for the treatment of myopia of 6.0 to 15.0 diopters.

DESIGN

The study design was a prospective, multicenter, randomized clinical trial.

PARTICIPANTS

A total of 64 eyes were treated with PRK and 54 eyes were treated with LASIK.

INTERVENTION

Using the Summit Apex excimer laser, patients received either PRK or LASIK using a single pass, multizone excimer laser ablation. Computer-assisted videokeratography was performed at designated postoperative examinations.

MAIN OUTCOME MEASURES

Videokeratography maps at 1 and 3 months after surgery were classified using a standard classification scheme. The association of topography patterns to loss of spectacle-corrected visual acuity was tested.

RESULTS

At 1 month, for the PRK (n = 60) and LASIK (n = 51) groups, respectively, 63.3% and 19.6% of eyes fell into one of the four optically irregular groups (central island, keyhole, semicircular, or irregularly irregular; P < 0.001). At 3 months, for the PRK (n = 49) and LASIK (n = 39) groups, respectively, 36.7% and 10.3% of eyes fell into one of the optically irregular groups (P = 0.004). Comparing the 1- and 3-month examination results in the PRK and LASIK groups, respectively, 19 (42%) of 45 eyes and 11 (31%) of 36 eyes had a change in topography, generally to an optically smoother pattern. The irregular groups, taken together, were associated with a greater tendency toward loss of spectacle-corrected visual acuity of two or more Snellen lines (P = 0.01). There also was greater tendency toward loss of spectacle-corrected visual acuity in the PRK group that diminished with time (P < 0.01 at 1 month, P = 0.05 at 3 months).

CONCLUSIONS

After treatment for moderate-to-high myopia, LASIK topography patterns generally are more regular than are PRK patterns. This may be a result either of masking of underlying topography perturbations by the lamellar corneal flap, thus mitigating induced topography changes, or differences in surface wound healing. This study suggests that more rapid return of spectacle-corrected visual acuity found in patients treated with LASIK may be a result of more regular topography patterns early in the postoperative course.

Comparison of objective methods for quantifying the refractive effect of photo-astigmatic refractive keratectomy using the MEL-60 excimer laser

PURPOSE

To investigate the accuracy and precision of automated keratometry, automated refractometry, and computerized corneal topography in estimating the subjective refractive outcome of photo-astigmatic refractive keratectomy six months postoperatively.

METHODS

Photo-astigmatic refractive keratectomy (Aesculap-Meditec, MEL-60 Excimer Laser) was performed on 26 eyes with a preoperative myopia ranging from -4.0 to 7.6 dioptres, and a naturally occurring astigmatism from 0.75 to 5.0 dioptres. Six months postoperatively refractive outcome was evaluated by automated keratometry, automated refractometry (Nikon NRK-8000), computerized topography (TMS-1), and subjective refraction. Estimate errors were computed as the difference between the change in subjective refraction and the change in automated keratometry, automated refractometry, and surface topography, respectively. Astigmatic changes were evaluated by the second harmonic component in the Fourier series analysis.

RESULTS

Subjective spherical as well as cylindrical values were reduced significantly six months postoperatively. The estimate error (mean +/- one standard deviation) for automated keratometry was -1.26 +/- 0.72 dioptres for the spherical equivalent and -1.36 +/- 1.02 dioptres for the cylinder; for automated refractometry it was -0.78 +/- 0.91 dioptres for the spherical equivalent and -0.66 +/- 0.92 dioptres for the cylinder. The best estimates of subjective changes were obtained when the average of ring 2 and 3 of the topographic data was used: -0.15 +/- 0.82 dioptres for the spherical equivalent and -0.78 +/- 0.80 dioptres for the cylinder.

CONCLUSIONS

The computerized topographer with the Fourier analysis was superior to automated keratometry and automated refractometry in estimating the subjective spherical refractive outcome and comparable to automated refractometry in estimating the subjective cylinder refractive outcome after photo-astigmatic refractive keratectomy.

Corneal topographic evaluation of decentration in photorefractive keratectomy: treatment displacement vs intraoperative drift

PURPOSE

To evaluate treatment displacement and movement during treatment (drift) after excimer laser photorefractive keratectomy using tangential topographic maps.

METHODS

Forty-eight eyes of 48 patients showing axial decentration of 0.30 mm or more at 1 month posttreatment were reevaluated retrospectively to determine treatment displacement of the center of the photorefractive keratectomy ablation from the center of the pupil. A drift index was calculated to determine the relative degree of movement (drift) during treatment. We subdivided patients into four groups based on the degree of treatment displacement and drift and compared the mean axial decentration and the mean best-corrected logMAR visual acuity among the subgroups.

RESULTS

Mean treatment displacement +/- SD from the center of the entrance pupil was 0.34 +/- 0.21 mm. Thirty-eight eyes (79.2%) had ablations within 0.50 mm from the center of the entrance pupil. We observed downward displacement in 27 eyes (56.2%) and upward displacement in 21 eyes (43.8%). The drift index showed a positive, statistically significant correlation with best-corrected visual acuity (r = .58, P < .0001). Patients with low displacement and low drift had mean logMAR best-corrected visual acuity of 0.91, which was statistically significantly better than patients with high displacement and high drift (r = 0.64; P = .009).

CONCLUSIONS

In patients with gross decentration by axial topography after photorefractive keratectomy, tangential corneal topography is valuable in evaluating and differentiating photorefractive keratectomy treatment displacement from movement during treatment (drift). Patients with high drift index have worse visual outcomes after photorefractive keratectomy than those exhibiting high treatment displacement.

Corneal topography of excimer laser photorefractive keratectomy using a 6-mm beam diameter. Summit PRK Topography Study Group

OBJECTIVE

The purpose of the study is to define qualitative patterns of corneal topography after excimer laser photorefractive keratectomy (PRK) using a 6-mm beam diameter, investigate changes in patterns over time, and identify associations of topography patterns with clinical outcomes.

DESIGN

Multicenter, prospective cohort study.

PARTICIPANTS

Ninety-eight eyes of 90 patients with myopia who had undergone PRK using the Summit Technology, Inc., excimer laser with a 6-mm beam diameter.

INTERVENTION

Computer-assisted videokeratography data were analyzed for eyes having undergone PRK. Topography patterns at 3, 6, and 12 months after surgery were classified and associations with clinical outcomes assessed.

MAIN OUTCOMES MEASURED

Topography patterns after PRK were determined at 3, 6, and 12 months after surgery. Associations with preoperative characteristics of age and attempted correction, and postoperative outcomes of uncorrected and spectacle-corrected visual acuity, predictability, astigmatism, corneal haze, glare, halo, and patient satisfaction were analyzed.

RESULTS

At 1 year, 21.4% of corneas showed a homogeneous topography, 27.6% showed a toric-with-axis configuration, 10.2% showed a toric-against-axis configuration, 7.1% showed an irregularly irregular topography, 24.5% showed a keyhole/semicircular pattern, and 9.2% showed focal topographic variants. From 3 to 6 months, 40.1% of maps changed; from 6 to 12 months, 53.1% of maps changed, generally to optically smoother, regular patterns. Older age and higher attempted correction were associated with the development of more irregular patterns. The irregular groups showed worse predictability than did the regular groups and a tendency for slight overcorrection. The average reported glare/halo of 1.33 (scale = 0 to 5) in this study was less than in a previous study of the 4.5- to 5-mm treatment zone. However, of six patients expressing dissatisfaction with the results of surgery, three ranked their glare or halo at the maximum level.

CONCLUSIONS

Topography patterns using a 6-mm beam diameter are identifiable, improve with time, and may affect clinical outcomes after photorefractive keratectomy (PRK). The keyhole/semicircular pattern is more prevalent with a 6 mm treatment zone than with smaller treatment zones. Although optical side effects of glare and halo appear to be reduced with the 6-mm treatment, a small number of patients still report substantial glare or halo after the procedure.

Corneal Ablation Profilometry and Steep Central Islands

BACKGROUND

Photorefractive keratectomy with large diameter ablations using a uniform laser beam has produced central undercorrections, or "steep central islands" in patients, as seen with videokeratography.

METHODS

Using a custom optical profilometer to measure corneal ablation profiles and a VISX excimer laser system, we measured the effect of ablation algorithms, diameter, depth, and dioptric correction on enucleated porcine eyes and living rabbit eyes. Our profilometer was verified using a 43.00 diopter (D) spherical surface and a 35.00 and 43.00 D bicurve test surface as a model for the ablated cornea.

RESULTS

The profilometer measured the test surfaces to within 3 microns of predicted values. Photorefractive keratectomies showed over-ablation peripherally and under-ablation centrally which increased with ablation diameter and dioptric correction. Fixed diameter ablations 2 to 6 mm in diameter and 10 to 80 microns deep showed stromal ablation rates vary spatially but not with ablation depth. These spatially variant ablation profiles were used to re-engineer the ablation algorithm and to produce photorefractive keratectomies with improved sphericity.

CONCLUSIONS

Steep central islands are caused by the spatial variance of tissue ablated with a uniform laser beam irradiance. This aberration can be corrected by modifying the laser ablation algorithm to correct for the spatial variance of stromal ablation.

Topographical analysis of ablation centration after excimer laser photorefractive keratectomy and laser in situ keratomileusis for high myopia

PURPOSE

To evaluate the ablation centration after photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) for high myopia and to assess the association between decentration and best corrected visual acuity (BCVA), glare, monocular diplopia, and halo phenomenon.

SETTING

Mater Private Hospital, Dublin, Ireland.

METHODS

Corneal topography was used to analyze centration in two groups of patients with myopia of more than 6.0 diopters: 18 had PRK and 18, LASIK. A standardized questionnaire assessed the preoperative and postoperative prevalence of glare, monocular diplopia, and halo phenomenon.

RESULTS

"Significant" ablation decentration (0.5 mm) in the LASIK group (1.33 mm) was almost twice that in the PRK group (0.75 mm). Glare increased from 27% preoperatively to 42% in the PRK group; monocular diplopia increased in the LASIK group. Halo phenomenon decreased after both procedures.

CONCLUSION

Laser in situ keratomileusis represents a step forward in the surgical correction of high myopia, but the accuracy of the corneal ablation location must be improved. Suction ring fixation of the globe or real time tracking systems may help improve centration.

Natural history of central topographic islands following excimer laser photorefractive keratectomy

PURPOSE

To assess the incidence and natural history of central corneal topographic islands following excimer laser photorefractive keratectomy (PRK) and photoastigmatic refractive keratectomy (PARK).

SETTING

A dedicated corneal diseases and refractive surgery unit within a National Health Service Trust ophthalmology unit in the United Kingdom.

METHODS

Corneal topographic analysis was performed in a prospective study of 100 eyes of 75 consecutive patients who had PRK. All PRK/PARK excimer laser photorefractive procedures were performed by two surgeons observing a standardized protocol using a VISX 20/20 excimer laser. Mean preoperative myopic error was 5.54 diopters (D) +/- 3.44 (SD). Corneal topographic analysis was performed on all eyes preoperatively, 1 week postoperatively, and monthly thereafter for a minimum of 6 months or until central islands, if present, resolved. All patients had a minimum 12 months follow-up.

RESULTS

Postoperatively, 29 eyes (29%) demonstrated central corneal topographic islands of greater than 3.00 D topographic power by computerized videokeratography (CVK). All central islands were identified in the first 4 weeks postoperatively. In all cases the differential dioptric power, created by the central islands within the ablation zone, decreased rapidly; within 6 months, 26 (90%) central islands had fully resolved without further treatment, and the remaining 3 (10%) resolved within 1 year of photorefractive surgery. The occurrence of central islands was related to higher preoperative myopic spherical equivalent (P = .01), greater attempted laser correction (P = .01), and greater projected depth of ablation (P = .01) (Student's two-tailed t-test).

CONCLUSIONS

Central corneal topographic islands occurred in a significantly higher proportion of eyes having excimer laser photorefractive surgery than previously believed. The islands were associated with decreased unaided vision, reduced best spectacle-corrected acuity, and other troublesome visual symptoms; however, the central islands, along with their associated visual effects, usually resolved without surgical intervention within 6 months postoperatively.

Refractive outcome and corneal topographic studies after photorefractive keratectomy with different-sized ablation zones

BACKGROUND

Discrepancies may still occur between planned and actual refractive correction in eyes undergoing photorefractive keratectomy (PRK). The authors have evaluated the use of an enlarged ablation zone.

METHOD

A computerized corneal analysis system has been used to compare the changes of the anterior surface of the cornea and the refractive changes before and 1, 6, and 12 months after PRK in 113 patients (119 eyes) treated with an excimer laser. The patients were divided into two groups: those treated with a mask with a 5-mm window (59 eyes), and those with a new mask with different window openings according to the degree of refraction at the corneal apex, starting from 5 mm in diameter for treatments less than 6.5 diopters (D) and from 7 mm in diameter for higher treatments (60 eyes). In the first group, treatment ranged from -2.5 to -16 D (mean +/- standard deviation. -8.5 +/- 3.24 D); in the second group, it ranged from -1 to -14 D (-7.8 +/- 3.06 D). Treatments were evaluated with a chi-square test.

RESULTS

In the first group of eyes, 46% were within +/-1 D at 1 month, 37% at 6 months, and 39% at 12 months. In the second group of eyes, 73% were within +/-1 D at 1 month, 60% at 6 months, and 58% at 12 months. The comparison between these data and corneal topographic changes shows that both are more stable and predictable with the new mask compared with the 5-mm mask (P = 0.002, 0.02, 0.04, at 1, 6, and 12 months, respectively).

CONCLUSIONS

The use of larger ablation zones improves the predictability and stability of refractive changes.

Corneal topographic changes after noncontact holmium:YAG laser thermal keratoplasty to correct hyperopia

PURPOSE

To analyze the changes in corneal curvature induced by laser thermal keratoplasty (LTK) to correct hyperopia.

SETTING

Cullen Eye Institute, Baylor College of Medicine, Houston, Texas.

METHODS

We performed LTK on nine hyperopic eyes using a noncontact holmium: YAG (Ho:YAG) laser. Five eyes received a single ring of eight spots at the 6 mm zone (Group A); four received a second ring of eight at the 7mm zone (Group B). Computerized videokeratography (CVK) was obtained preoperatively and 1, 30, 90, 180, and 360 days postoperatively. We calculated the net dioptric changes in the following CVK values: corneal curvature at the 1, 3, 5, and 7 mm zones; effective corneal refractive power (Eff RP); and spherical equivalent of subjective manifest refraction (SE SMR). We classified difference maps according to the pattern of induced change.

RESULTS

At 1 year, steepening at the 1, 3, 5, and 7 mm CVK zones was 0.5 diopter (D), 0.6 D, 0.1 D, and -0.42 D, respectively, in Group A and 1.5 D, 1.5 D, 1.1 D, and 0.54 D, respectively, in Group B. Effective corneal refractive power increased 0.6 D in Group A and 1.5 D in Group B. Mean change in SE SMR was -0.6 D in Group A and -1.4 D in Group B. Most regression occurred during the first 90 days. Difference maps showed five bow-tie, two irregularly irregular, one semicircular, and one homogeneous patterns.

CONCLUSIONS

Noncontact Ho:YAG LTK produced peripheral corneal flattening and central corneal steepening. A greater change in curvature was produced using a two-ring treatment.

Assessment of Ablation Zone Centration After Photorefractive Keratectomy Using a Vector Center of Mass Formula

BACKGROUND

Adequate centration of keratorefractive surgical procedures is essential for a successful outcome. An accurate technique to mathematically describe the centration and topography of the ablation zone after photorefractive keratectomy (PRK) would be valuable in assessing the effects of these variables on subsequent visual results.

METHODS

A vector center of mass formula and computerized videokeratography were used to study the postoperative treatment zone centration and topography of 17 consecutive highly myopic patients (-6.00 to 12.00 diopters [D]). Each had undergone PRK using either a single 6.0 mm (n = 11) or three-stepped ablation zone (n = 6), with good visual results.

RESULTS

Calculations disclosed mean ablation zone decentration relative to the pupil center for all patients to be 0.20 +/- 0.16 mm using the vector center of mass formula. Areas of uniform central corneal dioptric power (mean diameter 3.4 +/- 0.8 mm) and surrounding transition zones of declining dioptric power (mean slope 1.61 +/- 0.44 D/mm) were also determined.

CONCLUSION

A new vector center of dioptric power distribution that analyzes centration and transition zone topography offers a rigorous but straightforward means to assess the effects of refractive corneal surgery procedures on central corneal topography.

Decentration of photorefractive keratectomy ablation zones after excimer laser surgery for myopia

PURPOSE

To devise a method for measuring excimer laser photorefractive keratectomy (PRK) ablation zone decentration.

SETTING

Excimer Laser Clinic, Sunderland Eye Infirmary, Sunderland Tyne and Wear, England.

METHODS

Fifty-three eyes of 53 consecutive patients having PRK for a spherical myopic refractive error were studied. Preoperative and postoperative corneal topographic maps and differential subtraction maps were produced for each cornea. Using these maps, the distance between ablation zone centers and pupil centers was measured. All measurements were repeated by a second observer.

RESULTS

Mean ablation zone decentration was 0.46 mm from the pupil center; 69% of cases decentered 0.50 mm or less. The difference between the two independent measurements was 0.05 +/- 0.06 mm (mean +/- SD). A trend toward superonasal displacement was noted.

CONCLUSION

This simple method of measuring zone decentration can be used to compare the refractive results of PRK with those of other procedures.

Corneal topography of phase III excimer laser photorefractive keratectomy. Characterization and clinical effects. Summit Photorefractive Keratectomy Topography Study Group

PURPOSE

To define qualitative patterns of corneal topography after excimer laser photorefractive keratectomy (PRK), assess changes in patterns, associations with clinical outcomes, and the accuracy of videokeratography in predicting results, and define quantitatively the optical zone contour.

METHODS

Computer-assisted videokeratography data obtained from 181 patients after PRK was analyzed. Topography patterns at two time points were characterized, and associations with clinical outcomes were tested. Power change predicted by topography was compared with refractive change, and cross-sectional power contours were analyzed.

RESULTS

Seven topography patterns were defined. At 1 year, 58.6% of corneas showed a homogeneous topography, 17.7% showed a toric-with-axis configuration, 2.8% showed a toric-against-axis configuration, 13.8% showed an irregularly irregular topography, 2.8% showed a keyhole/semicircular pattern, and 4.4% showed focal topographic variants. No central island patterns were found. Of the maps, 41% changed over time. Uncorrected vision, predictability, and patient satisfaction were best in the homogeneous group. Astigmatism increased in the irregular and toric-against-axis groups and decreased in the toric-with-axis group. There was no relation of topography pattern to best-corrected vision or subjective glare/halo. Cross-sectional power profiles showed a homogeneous power change for the central 3 mm with a diminution in correction toward the periphery. The topography unit tended to overestimate refractive change for corrections of 5 diopters or less and underestimate the change for corrections greater than 5 diopters.

CONCLUSIONS

Topography patterns after PRK are identifiable, time dependent, and may affect clinical outcomes. Understanding the actual corneal optical contour resulting from PRK may aid in improving both laser techniques and optical results in the future.

The effect of globe fixation on ablation zone centration in photorefractive keratectomy

PURPOSE

For optimal vision, the ablation zone in photorefractive keratectomy should be centered over the entrance pupil. During ablation, the globe can be immobilized by the surgeon, with a suction ring around the corneoscleral limbus. Alternatively, the globe can be immobilized by patient fixation on a target, unassisted by the surgeon. We investigated which method results in better centration of the ablation zone over the entrance pupil, by using an objective, mathematical method to determine the ablation zone center.

METHODS

Forty-eight eyes from 48 patients who underwent photorefractive keratectomy by the two techniques were studied retrospectively. The centers of the ablation zones were objectively determined by a weighted center of mass algorithm applied to the preoperative minus postoperative difference maps. The validity of the objective method was confirmed by comparison to subjective estimates of ablation zone centers made by independent human observers.

RESULTS

The 19 eyes treated by surgeon fixation had an average decentration of the ablation zone of 0.63 +/- 0.31 mm (range, 0.01 to 1.00 mm), and the 29 eyes treated by patient fixation had an average decentration of 0.41 +/- 0.23 mm (range, 0.11 to 1.18 mm) (P = .027).

CONCLUSIONS

The center of the ablation zone can be determined mathematically from the topographic map, to avoid observer bias. In this study, unassisted patient fixation during photorefractive keratectomy produced more accurate centration of the ablation zone than did surgeon fixation and has the potential for maximizing the quality of vision postoperatively.

Corneal topographic analysis after excimer photorefractive keratectomy

BACKGROUND

A 2-year experience with corneal topography after photorefractive keratectomy (PRK) is reported, specifically reviewing the correlation of surface regularity index versus best spectacle-corrected visual acuity as a function of various ablation patterns. Centration and stability issues also are discussed.

METHODS

Excimer laser PRK for myopia was performed on 502 consecutive eyes. Corneal topographic analysis was performed at the 1-, 3-, 6-, and 12-month postoperative follow-up visits. In all patients, PRK was performed with an argon fluoride laser, and a topographic analysis was done.

RESULTS

Topographic analysis at the first postoperative month was useful for determining decentration of the optical zone. The mean decentration from the pupillary center for all eyes was 0.34 mm. Four main patterns of ablation were noted by subtraction analysis. At the 1-month postoperative examination, a "uniform" ablation was present in 44% of eyes, "keyhole" ablations were present in 12% of eyes, "semicircular" ablations were present in 18% of eyes, and an unusual "central island" was present in the remaining 26% of eyes. However, on subsequent follow-up the central islands tended to resolve with time. At 3 months postoperatively, 18% of eyes in that group showed a central island, 8% showed a central island at 6 months, and only 2% of eyes showed a central island at the 12-month postoperative visit. Correlation of central island topography with visual acuity and progressive hyperopia are discussed.

CONCLUSIONS

Corneal topography is essential for evaluating surface changes after excimer PRK. The surface regularity index is a good indicator of best spectacle-corrected visual acuity and is used to evaluate irregular astigmatism after PRK. Central island topographies are correlated with poor initial visual rehabilitation. Long-term stability issues are answered with continued topographic follow-up. Understanding corneal hydration changes between the central and peripheral cornea may help us understand the etiology of central islands.