Corneal Wavefront Research Articles

Phototherapeutic keratectomy: 12 years of experience.

Phototherapeutic keratectomy (PTK) has been employed as a surgical tool to treat corneal disease for more than 10 years. The laser has made it possible to remove superficial corneal opacities and thereby restore vision. The 193 nm ultraviolet light separates molecules and splits molecules in biological tissue, thereby ablating it. About 0.25 microm of tissue is ablated by each pulse. The development of the excimer laser technique has been fast. It has principally focused on refractive surgery but has also benefited PTK. The ability to delay or postpone corneal grafting in superficial corneal dystrophies represents a very important achievement. Map-dot-fingerprint dystrophy or basal membrane dystrophy is a common indication for PTK. Other dystrophies such as Meesman's, Reis-Bückler's, Thiel-Benke's, granular, macular, lattice and Schnyder's can be treated, although with differing degrees of success and varying rates of recurrence. Subepithelial scarring in Fuchs' dystrophy has been ablated. Other trials have involved the removal of substantial parts of the stroma in order to reduce the load on the endothelium. Recurrent dystrophic changes can likewise be removed from corneal grafts and thus prevent the need for regrafting. Laser treatment has made it possible to manage wound-healing problems better after recurrent erosions. Recurrent erosions are the most common indications for PTK: several studies show good and persistent effects with this type of treatment. Persistent epithelial defects of various origins, among them corneal ulcers resulting from allergic disease, can likewise be treated. Scars after surgery such as pterygeum excision can be removed. Smooth muscle actin containing fibroblasts in old scars should be given special consideration in PTK. Excimer laser surgery can be successfully combined with conventional surgery to remove excessive scar tissue, Salzmann's nodules and very flaky and coarse band keratopathy. Irregular corneal surfaces following ulcers and injuries pose problems that have so far proved difficult to overcome. Thinning is often seen after bacterial corneal ulcers or after herpes simplex keratitis. A rough or uneven surface can be made smoother by using modulators during treatment by casting a new surface under a hard contact lens (PALM technique), a surface that is then projected into the stroma by laser ablation. Modern techniques linking the excimer laser with computerized corneal topography and wavefront analysis promise to further improve the smoothing capacities of lasers and to increase the quality of optical results. The most feared complication of PTK is the postoperative infection. These are rare. Haze is usually not prominent but scar tissue formation of a more persistent type has been noted after laser surgery in eyes with pre-existing surgical scars. Keratectasia has been described after PTK. Failure due to deep opacities or a surface that is too uneven is a more common frustration. This paper reviews advances in excimer laser treatment of corneal disease.

Corneal aberrations and reading.

To investigate the effects of eyelid pressure on corneal shape and corneal aberrations during reading. Twenty young subjects with normal ocular health were recruited for the study. The experiment was conducted early in the morning, with subjects instructed not to perform any prolonged reading before the experiment. Corneal topography of one eye was measured with a videokeratoscope before reading and then again after a 60-min reading task. The natural position of the eyelids was photographed in primary gaze and during the reading task. Twelve of the 20 corneas showed significant changes in central topography immediately after reading. The location of the changes corresponded closely to the position and angle of the subject's eyelids during reading. The change in shape was best described as a wave-like distortion that significantly altered some corneal wavefront Zernike coefficients. There was a significant correlation between the changes in primary vertical coma and trefoil (along 30 degrees). Within the central 6 mm of the cornea, there were significant changes in the root mean square error, overall refractive power, and astigmatism. The changes we observed in corneal topography appear to be directly related to the force exerted by the eyelids during reading. Because the cornea is the major light-refracting surface of the eye, the optical characteristics of some eyes can be significantly changed during reading by the force of the eyelids. These findings may have important implications for the definition of refractive status and may also aid in the understanding of the relationship between reading and the development of refractive errors.

Wavefront and Topography: Keratome-induced Corneal Changes Demonstrate That Both Are Needed for Custom Ablation

To examine the effects of laser in situ keratomileusis (LASIK) flap incision and healing on the shape of the cornea and the wavefront error of the eye. Four weeks prior to bilateral LASIK, study subjects had a flap cut using a keratome in one eye. The fellow eye remained untouched as a control. Corneal topography and wavefront errors were measured at 1 day, 1, and 4 weeks after the flap was created. After 4 weeks, the flap was lifted and LASIK was performed. The control eye also had LASIK at this time. Differences in corneal shape and wavefront error consistent with a mild hyperopic shift were seen as a result of the keratome incision. Cutting the flap in LASIK causes subtle changes to corneal shape and the optics of the eye that may affect customized treatments. Additional work is needed to quantify these changes so that their effect can be incorporated into future treatments.

Astigmatism and LASIK.

Although laser in situ keratomileusis (LASIK) enjoys a high success rate, postoperative residual or induced astigmatism may limit uncorrected visual acuity and cause starbursts and glare at night. Irregular astigmatism can also cause loss of best-corrected visual acuity, monocular diplopia, and ghosting of images. Astigmatism may be measured by keratometry and refraction, while corneal topographic techniques help to define irregular astigmatism, in particular. Further information may be obtained regarding induced higher-order aberrations with aberrometry. Because astigmatism has both direction and magnitude, its analysis is more complex than that of the spherical component of the treatment. There are multiple approaches to the analysis of surgically induced astigmatism, including vector analysis, conversion to a Cartesian coordinate system, matrix formalism, and linear optics. Both excimer laser and incisional techniques may be used to correct astigmatism after LASIK, but the treatment of irregular astigmatism requires selective zonal ablation techniques or customized corneal ablations, using topographic or wavefront derived data.

Treatment of post-keratoplasty astigmatism by topography supported customized laser ablation.

To evaluate the clinical and optical efficiency of topography modulated customized corneal ablations for irregular corneal astigmatism. Sixteen eyes of 16 patients with iatrogenic corneal astigmatism (post keratoplasty) were consecutively included. Based on preoperative corneal topographic measurements height deviations from a spherical corneal shape were calculated and transferred to a flying-spot excimer laser. Photo-refractive keratectomy of the topographic irregularities was then performed. Clinical and optical efficiency was evaluated by best corrected visual acuity and by computation of corneal wavefront aberrations before and up to one year after treatment. Wavefront aberrations were decomposed by Zernike polynomial analysis. Before treatment the average best-corrected visual acuity was 0.23. Three and 12 months after PRK the average best-corrected visual acuity had increased to 0.37 (p<0.05) and 0.45 (p<0.05), respectively. Corneal wavefront aberrations (root-mean-square) were 3.35 before surgery and 1.88 (p<0.05) and 1.51 (p<0.05) at three and 12 months after treatment. Zernike polynomial decomposition of the wavefront aberrations revealed that regular corneal astigmatism was the most important aberration component before and after surgery. Regular astigmatism was significantly decreased by the procedure, whereas coma, spherical aberrations, and higher-order aberrations were not reduced significantly. Topography modulated photorefractive keratectomy of highly astigmatic corneal grafts can improve best corrected visual acuity and reduce corneal wavefront aberrations. Even in apparently irregular topographic astigmatism, regular astigmatic wavefront aberration may be the most important contributor to wavefront errors.

Computerized corneal topography and its importance to wavefront technology.

To review the development and application of corneal topography in refractive surgery. Review of the literature and discussion of recent developments in corneal topography and wavefront technology. Analysis of corneal topography provides critical information for the preoperative examination of patients before refractive surgery and for the evaluation and treatment of patients with complications after surgery. Corneal topography will continue to be a critical diagnostic modality for refractive surgery. Even with the advent of wavefront analysis designed to detect refractive error and aberrations of the eye, it will be necessary to have detailed corneal topographic information to understand the contribution the cornea makes to vision so that custom alteration of that surface can be used to optimize vision. This will be true of the normal eye, but it will be of special importance in eyes with abnormalities that were induced by corneal surgery.

Quantitative Evaluation of Corneal Irregular Astigmatism and Wavefront Aberrations

Quantitative Evaluation of Corneal Irregular Astigmatism and Wavefront Aberrations

Quantitative Evaluation of Corneal Irregular Astigmatism and Wavefront Aberrations

Quantitative Evaluation of Corneal Irregular Astigmatism and Wavefront Aberrations

Corneal first surface optical aberrations and visual performance.

Wavefront analysis has demonstrated that refractive surgery-induced corneal first surface aberrations are large, are dominated by symmetric aberrations (spherical-like aberrations), and are correlated to measures of visual performance. It is not clear whether the correlation between corneal first surface aberrations and visual performance can be generalized to other corneal conditions where large asymmetric aberrations (coma-like aberrations) may dominate the aberration structure. The purpose of the research reported here was to determine the general utility of corneal first surface wavefront analysis in predicting visual performance. Patients were 13 normals and 78 patients with a variety of corneal conditions including surgically removed pterygia, penetrating keratoplasty, keratoconus, radial keratotomy, laser in situ keratomileusis, and others. Videokeratographs were taken for all patients and used to calculate corneal first surface wavefront variance for 3 and 7 mm pupils. Similarly, visual performance was quantified by measurements of contrast sensitivity and high and low contrast acuities through both 3 and 7 mm pupils. Statistically significant correlations existed between all three measures of visual performance and the corneal wavefront variance. All relationships were stronger for the 7 mm diameter-pupil condition than the 3 mm pupil. Regardless of the cause, corneas with increased wavefront variance showed a quantifiable decrease in visual performance that was pupil size dependent.

Comparison of corneal wavefront aberrations after photorefractive keratectomy and laser in situ keratomileusis

PURPOSE: To compare changes in the corneal wavefront aberrations after photorefractive keratectomy and laser in situ keratomileusis. METHODS: In a prospective randomized study, 22 patients with bilateral myopia received photorefractive keratectomy on one eye and laser in situ keratomileusis on the other eye. The procedure assigned to each eye and the sequence of surgery for each patient were randomized. Corneal topography measurements were performed preoperatively, 2 and 6 weeks, 3, 6, and 12 months after surgery. The data were used to calculate the wavefront aberrations of the cornea for both small (3-mm) and large (7-mm) pupils. RESULTS: Both photorefractive keratectomy and laser in situ keratomileusis significantly increased the total wavefront aberrations for 3- and 7-mm pupils, and values did not return to the preoperative level throughout the 12-month follow-up period. For a 3-mm pupil, there was no statistically significant difference between photorefractive keratectomy and laser in situ keratomileusis at any postoperative point. For a 7-mm pupil, the post–laser in situ keratomileusis eyes exhibited significantly larger total aberrations than the post–photorefractive keratectomy eyes, where a significant intergroup difference was observed for spherical-like aberration, but not for coma-like aberration. This discrepancy seemed to be attributable to the smaller transition zone of the laser ablation in the laser in situ keratomileusis procedure. Before surgery, simulated pupillary dilation from 3 to 7 mm caused a five- to six-fold increase in the total aberrations. After surgery, the same dilation resulted in a 25- to 32-fold increase in the photorefractive keratectomy group and a 28- to 46-fold increase in the laser in situ keratomileusis group. For a 3-mm pupil, the proportion of coma-like aberration increased after both photorefractive keratectomy and laser in situ keratomileusis. For a 7-mm pupil, coma-like aberration was dominant before surgery, but spherical-like aberration became dominant postoperatively. CONCLUSIONS: Both photorefractive keratectomy and laser in situ keratomileusis increase the wavefront aberrations of the cornea and change the relative contribution of coma- and spherical-like aberrations. For a large pupil, laser in situ keratomileusis induces more spherical aberrations than photorefractive keratectomy. This finding could be attributable to the smaller transition zone of the laser ablation in the laser in situ keratomileusis procedure.

Corneal aberrations and visual performance after radial keratotomy.

Refractive surgery and videokeratography have allowed us to study the effects on visual performance of relatively large changes in corneal aberration structure induced by surgical changes in corneal shape. We quantified in one eye of nine normal and 23 radial keratotomy patients, the area under the log contrast sensitivity function (AULCSF) and corneal first surface wavefront variance for two artificial pupil sizes (3 and 7 mm). Contrast sensitivity was measured with sine-wave gratings at six spacial frequencies. Wavefront variance was derived from videokeratographs using Zernike polynomials. For normals eyes there were no significant changes over time. For eyes that had radial keratotomy, there were significant pupil size-dependent changes. For the 3 mm pupil, there were significant surgery-induced changes in the corneal wavefront variance which became large (approximately 30 times preoperative values) at 7 mm. Significant correlated changes in AULCSF for the 7 mm pupil but not for the 3 mm pupil occurred immediately following surgery and remained. Radial keratotomy, like photorefractive keratectomy, shifts the distribution of aberrations from third order dominance (coma-like aberrations) to fourth order dominance (spherical-like aberrations). Radial keratotomy-induced aberrations and loss in contrast sensitivity are reduced with increasing clear zone diameter. Radial keratotomy induces an increase in the optical aberrations of the eye and the increase for large pupils (7 mm) but not small (3 mm) is correlated to a decrease in contrast sensitivity.