Differences in nasal and temporal responses of the cornea after photorefractive keratectomy

Post-Ortho-K Corneal Epithelium Changes in Myopic Eyes

The study is aimed at evaluating corneal epithelial thickness changes associated with overnight orthokeratology (ortho-K). In this retrospective study, epithelial thickness was measured using optical coherence tomography (OCT) before and after 1 day, 1 week, 1 month, and 3 months ortho-K nightly lens wear. Compared with pre-orthokeratology measurements, central (2 mm) corneal epithelium thickness was significantly reduced at 1 day, 1 week, 1 month, and 3 months with ortho-K (P < 0.05). Paracentral (2 mm~5 mm annular ring) epithelial thickness was also significantly reduced at superior temporal, inferior temporal, temporal, and inferior locations after ortho-K (P < 0.05), while midperipheral (5 mm~6 mm annular ring) epithelial thickness was greater post- than pre-ortho-K at superior, superior temporal, inferior temporal, inferior, and inferior nasal locations (P < 0.05). In other zones, superior, superior nasal, nasal, and inferior nasal in paracentral annular ring and temporal and superior nasal in midperipheral ring, epithelial thickness underwent no significant change. Ortho-K lens wear caused the central corneal epitheliums to thin. The temporal half zones become thinner in paracentral zones and thicker in midperipheral zones.

Inverted lens provides reverse geometry solution for post laser vision correction (LVC) corneas

Many patients require optical correction post-laser vision correction (LVC). While mildly irregular corneal topographic patterns or asymmetry can sometimes be treated with conventional soft lenses, often this proves inadequate. This article introduces a novel technique to provide visual improvement and comfort for these patients. An inverted senofilcon A (Acuvue Oasys®, Johnson & Johnson Vision Care) lens (off-label)was inserted on a patient's eyes that reported discomfort with his current soft contact lenses, which provided improved centration as was seen with a slit lamp and high molecular fluorescein through a yellow filter. The patient achieved a visual acuity of 6/6+ in each eye and reported that the vision did not fluctuate. The post-lens tear film decreased to 35micron versus 43micron in the conventional position, as shown in OCT. The patient reported that he wore the lenses 9 hours a day. His Dry Eye Questionnaire-8 (CLDEQ-8) score decreased from 22 to 15 when wearing the lenses in the inverted position. This case demonstrates that post-laser vision correction patients with minimal asymmetric topography within the treated zone requiring refractive correction may be helped using an inverted conventional soft frequent replacement lens.

Visual acuity outcomes in eyes with flat corneas after PRK

PURPOSE

To evaluate the impact of corneal curvatures less than 35 diopters (D) after photorefractive keratectomy (PRK) on visual acuity outcomes.

METHODS

Visual acuity outcomes of 5,410 eyes that underwent PRK from January 2006 to November 2010 were retrospectively analyzed for the impact of postoperative corneal curvatures on visual outcomes. All procedures were performed on a single platform (Allegretto 200Hz excimer laser; Alcon Laboratories, Inc., Irvine, CA). Main outcome measures were postoperative corrected distance visual acuity (CDVA) and loss of CDVA.

RESULTS

Corneas with a measured or a calculated postoperative flat meridian less than 35 D and those with a measured postoperative steep meridian less than 35 D had worse postoperative CDVA than corneas with meridians of either 35 D or more (P ≤ .021). However, the preoperative CDVA was worse in the flatter curvatures in all comparisons performed (P ≤ .024). Consequently, the measured or calculated meridian curvature had no effect on CDVA loss (P ≥ .074).

CONCLUSION

Postoperative corneal keratometry values (flat and steep meridians) less than 35 D did not have a predictive effect on the risk of losing visual acuity following myopic PRK performed on the Allegretto 200Hz excimer laser.

Intraocular pressure change after myopic laser in situ keratomileusis as measured on the central and peripheral cornea

BACKGROUND

Laser in situ keratomileusis (LASIK) leads to inaccurate measurement of intraocular pressure (IOP). This study aimed to determine the efficacy and reliability of IOP measurement on the peripheral cornea after LASIK.

METHODS

The IOP was measured in 52 eyes of 26 subjects before LASIK and one week and one month after LASIK using non-contact tonometry. The measurements were made on the central and peripheral cornea and the results were subjected to statistical analysis.

RESULTS

The mean measured IOP on the central cornea was decreased by 6.425 ± 2.544 mmHg at one week and 5.752 ± 3.863 mmHg at one month post-operatively. The mean IOP measured on the peripheral cornea was decreased by only 0.921 ± 2.054 mmHg at one week post-operatively. Most notably, the mean IOP measured on the peripheral cornea was increased by only 0.158 ± 2.979 mmHg at one month post-operatively with no statistical significance (p > 0.05). Furthermore, a linear regression of ablation depth versus change in IOP measured on the central cornea was demonstrated, but the peripheral IOP did not display such a statistically significant correlation with the depth of ablation at both one week and one month after operation.

CONCLUSION

The IOP measured on the peripheral cornea is closer to the actual IOP. Therefore, it is more accurate and reliable to measure IOP on the peripheral cornea than on the centre after LASIK.

Long-term corneal wavefront aberration variations after photorefractive keratectomy for myopia and myopic astigmatism

PURPOSE

To analyze the higher-order corneal wavefront aberration during an 8-year follow-up after photorefractive keratectomy (PRK).

SETTING

IRCCS Fondazione G.B. Bietti, Rome, Italy.

DESIGN

Case series.

METHODS

Patients having PRK using the Technolas 217C excimer laser platform were divided into 3 groups according to the preoperative refraction as follows: low myopia, high myopia, and astigmatism. The preoperative and 1-, 4-, 6-, and 8-year postoperative root mean square (RMS) values of coma, spherical aberration, and total higher-order aberrations (HOAs) were calculated with 3.5 mm and 6.0 mm simulated pupils. The mean preoperative and postoperative higher-order corneal wavefront aberration maps, point-spread functions, and radial modulation transfer functions (MTFs) were represented to describe the impact of PRK on the optical quality of the anterior cornea.

RESULTS

The study enrolled 33 patients (66 eyes). Corneal spherical aberration was statistically significantly higher after PRK for simple myopia with 3.5 mm and 6.0 mm pupils (P<.05). The postoperative increase in coma was statistically significant in the high-myopia group with both pupil sizes (P<.05). Total RMS HOAs increased postoperatively with a 6.0 mm pupil in all groups (P<.05). The mean radial MTF was almost stable in all groups between preoperatively and postoperatively.

CONCLUSIONS

Higher-order corneal wavefront aberrations stabilized 1 year after PRK to treat myopia or myopic astigmatism. The effect of induced corneal HOAs tended to increase after correction of high myopia with large pupils, although without degrading the image optical quality of the cornea over the long term.

Investigation of corneal topography after deep lamellar endothelial keratoplasty

PURPOSE

To analyze the 6-month anterior and posterior topographic changes of the cornea after deep lamellar endothelial keratoplasty.

METHODS

Orbscan topographies of 22 eyes from 21 patients with corneal decompensation were retrospectively analyzed: 11 eyes received a 9-mm scleral access incision (large-incision group) and 11 eyes received a 5-mm scleral access incision (small-incision group). All the preoperative and postoperative corneal raw data were imported into custom software which computed the average composite corneal maps and difference maps for both study groups in order to evaluate the corneal response to the surgery. The software delineated 2 concentric zones of the cornea to characterize the regional response following the surgery: the central and peripheral regions.

RESULTS

There were no significant differences (analysis of variance, <0.35 D, p>0.05) between 6-month postoperative and preoperative average anterior central corneal topographies in either group. At the end of follow-up, the average posterior curvature tangential map did not significantly differ from before surgery in either group (<0.35 D, p>0.05). However, a significant increase (>0.60 D, p<0.01) in the average posterior central astigmatic power of the cornea was found in both groups.

CONCLUSIONS

Deep lamellar endothelial keratoplasty effectively preserves the preoperative keratometric corneal topography, minimizing changes in curvature and astigmatism of the cornea. The great predictability of corneal topography following deep lamellar endothelial keratoplasty is likely to be attributed to the minimal changes that occur in the anterior stroma, the portion of the cornea that appears to be mainly responsible for maintenance of corneal shape.

Upper disparity limit after LASIK

We evaluate the effect of the emmetropization technique LASIK (laser-assisted in situ keratomileusis) on stereoscopic vision. For this, we used a mirror stereoscope to measure the upper disparity limit D(max) before (with best correction) and after LASIK for 30 patients. The results show that the upper disparity limit declines from 41.1 min of arc on average to 31.3 min of arc after LASIK, being significant in 83% of the patients. This deterioration is significantly correlated with an increase in the postsurgical interocular differences in higher-order aberrations, corneal asphericity, and presurgical anisometropia. New ablation algorithms should minimize interocular differences in order to improve binocular visual performance.

Long-term optical quality of the photoablated cornea

High-order coreal wavefront analysis was performed in a population of 60 myopic eyes that underwent photorefractive keratectomy. Corneal aberration data over 3, 5, and 7 mm pupils were collected for up to three years after surgery. The optical performance of the anterior cornea was characterized by estimation of the modulation transfer function (MTF) and the point-spread function. The high-order corneal wavefront aberrations were shown to stabilize one year after surgery. Over photopic pupils, after an early slight increase, corneal RMS-high-order aberrations (HOA) tended to decrease toward preoperative values. On the other hand, over mid- and large-pupil sizes, corneal HOA significantly increased compared with the preoperative state, while the optical performance of the cornea was diminished. The MTF ratio showed a distinct decline in the optical quality of postoperative corneas at low and middle spatial frequencies over larger pupils in the range between 6 and 19 c/deg, especially for deeper ablations.

Relative Contribution of Central and Peripheral Aberrations to Overall High Order Corneal Wavefront Aberration

PURPOSE

To analyze the influence of specific combinations of corneal high order aberrations on the optical image quality of the cornea before and after photorefractive keratectomy (PRK) for low to high myopia and myopic astigmatism.

METHODS

Corneal topography was obtained for 80 eyes that underwent PRK using a scanning-spot excimer laser. The eyes were subdivided into three groups according to the preoperative refraction. The topographical data were imported into a custom software program that combined the Zernike high order terms having the same sign and angular frequency up to seventh order for 4-mm and 6-mm pupils, ie, coma and spherical aberrations, and midperipheral and peripheral high order aberrations.

RESULTS

Photorefractive keratectomy induced a significant amount of the root-mean-square (RMS) values of the combinations of midperipheral and peripheral high order aberrations over the smaller pupil size for deeper myopic ablations (P<.05). Over the larger pupil, spherical myopic ablations showed a significant increase (P<.05) of the RMS values of coma and spherical aberrations. In the astigmatism group, the combination of terms having higher angular frequency increased significantly (P<.05) after surgery both over 4-mm and 6-mm pupils.

CONCLUSIONS

After surface ablation, ablation depth and profile significantly influence the distribution and contribution of determined combinations of high order aberrations to the overall high order corneal wavefront aberration. Terms having high angular frequency were increased following large myopic correction and wide treatment zone. Quality of the whole corneal optics will be enhanced by designing future ablation profiles to compensate for peripheral high order optical aberrations.

Interocular high-order corneal wavefront aberration symmetry

The interocular symmetry of the high-order corneal wavefront aberration (WA) in a population of myopic eyes was analyzed before and after photorefractive keratectomy (PRK). The preoperative and one-year postoperative corneal aberration data (from third to seventh Zernike orders) for 4- and 7-mm pupils from right and left eyes were averaged after correcting for the effects of enantiomorphism to test for mirror symmetry. Also, the mean corneal point-spread function (PSF) for right and left eyes was calculated. Preoperatively, a moderate and high degree of correlation in the high-order corneal WA between eyes was found for 4- and 7-mm pupils, respectively. Myopic PRK did not significantly change the interocular symmetry of corneal high-order aberrations. No discernible differences in the orientation PSF between eyes were observed one year after surgery in comparison with the preoperative state over the two analyzed pupils.

Response of the Cornea for up to Four Years After Photorefractive Keratectomy for Myopia

PURPOSE

To analyze the long-term corneal topographic changes 4 years after myopic photorefractive keratectomy (PRK).

METHODS

This study comprised 15 patients (30 eyes) who had PRK surgery with a scanning-spot excimer laser (Chiron Technolas 217C; Bausch & Lomb, Dornach, Germany) and were followed up to 4 years after surgery. The eyes were subdivided into three groups according to the preoperative spherical equivalent refraction. Corneal topographic maps were obtained for all eyes with a Placido disc topographer. Preoperative and follow-up topographical data were imported into a custom software program, which computed the average composite corneal maps and difference maps for each study group to quantify the anterior corneal changes following laser ablation. The software delineated three concentric zones of the corneal surface to characterize the regional corneal remodeling following the surgery.

RESULTS

A significant central corneal steepening (approximately 0.25 D, P < .001) was calculated between the 1- and 4-year postoperative maps in all study groups. A significant steepening (P < .001) of the corneal periphery was also noted for the lower myopic ablations whereas a peripheral flattening (P < .001) was observed for the deeper ablations between 1 and 4 years after surgery.

CONCLUSIONS

The anterior corneal surface was observed to remodel for up to 4 years after surface ablation, steepening a mean of approximately 0.25 D.

Atomic force microscopy analysis of normal and photoablated porcine corneas

We showed the capabilities and accuracy of atomic force microscopy (AFM) techniques for imaging and analyzing the corneal epithelium and the photoablated corneal stroma. Eight normal porcine corneas, half of which were ablated using a scanning-spot excimer laser, were examined. All the corneas were imaged in balanced salt solution after fixation in glutaraldehyde. In the normal untreated corneas we observed the epithelial surface showing the typical polygonal cells and presenting numerous microprojections. The superficial epithelial cells were classified in three types as a result of the anterior-surface roughness measurements. AFM images of the photoablated corneal specimens showed undulations and granule-like features on the ablated stromal surface, specific to 193-nm ArF laser irradiation. Nevertheless, the quantitative analysis confirmed the precision of excimer laser surgery in removing sub-micrometric amounts of tissue. AFM showed to be a high-resolved imaging tool for the scanning of both native as well as photoablated corneal specimens. Also, this technique permits precise topographic analysis of the corneal plane, in the nanometric scale, of which smoothness is an important physical characteristic and necessary to achieve an optimal optical quality of the eye.