Human excimer laser lamellar keratectomy. A clinical study

Do Topical Corticosteroids Have a Role Following Excimer Laser Photorefractive Keratectomy?

The article reviews the evidence for and against the use of topical corticosteroids following PRK. Two significant problems after PRK are the development of corneal haze and unpredictability of the refractive outcome. These arise from changes in the anterior stroma, the deposition of new subepithelial tissue, and individual variations in the wound healing response. In rabbits, corticosteroids reduced corneal haze by limiting the synthesis of subepithelial collagen. However, in humans, controlled trials showed that corticosteroids had no lasting effect on either haze or regression, and were associated with an unacceptably high incidence of unwanted effects. The difference between species probably related to the relative absence of collagen and predominance of glycosaminoglycans during corneal wound healing in humans. Some human studies have shown rapid changes in refraction occurring within days of stopping or restarting corticosteroids. This suggests that their transient hyperopic effect is probably mediated by changes in the hydration of the tissue. Therefore, the role of corticosteroids in PRK is very limited. There is no justification for their routine use after PRK for low or moderate myopia. If corticosteroids have a role in improving haze or refractive outcome in selected patients, a means for their early identification must be found.

An Empirical Model of Hyperopic Shift With Corticosteroid Modulation and Refractive Power Prediction After Photorefractive Keratectomy

Data from a selected sample of 158 patients who received excimer laser photorefractive keratectomy (PRK) in our center and obtained a desirable refractive outcome at 6-months were analyzed retrospectively to establish an empirical model of refractive power after PRK. This model, generated by a computer-assisted regression program, is presented as a multivariate non-linear quadratic equation. Since the coefficients of each variable (attempted correction and postoperative days) are statistically significant, the estimated influence of the variables on the postoperative refraction from this regression model is reliable and consistent. Therefore, this model has been used to direct the use of corticosteroids for modulating post-PRK refraction. The predictability of this model has also been corroborated by separate data from 1-year follow up of PRK recipients. The recipients whose refraction at 6 months agree with the calculation from this model have obtained an excellent outcome; while those whose refractive outcome deviates from this model have gotten undesirable results.

Excimer Laser In Situ Keratomileusis for Myopia

Excimer laser in situ keratomileusis was done in 97 eyes of 59 patients to correct moderate and high myopia. Mean follow-up was 150 days. Mean preoperative myopia was -10.75 diopters (D). Mean reduction in myopia was -11.281 D (106.98%) at 1 month and -10.37D (98.417%) at 3 months. At 1-month, 41 eyes (42.27%) had a refraction within -1.00 D and 70 teeyes (72.16%) had a refraction within -2.00 D. At 3 months, 41 eyes (46.59% of 88 eyes) had a refraction within -1.00 D and 66 eyes (75% of 88 eyes) had a refraction within -2.00 D. Mean preoperative keratometric power was 44.00 D; mean postoperative keratometric power was 36.39 D. The keratometric change in power was 7.61 D. Preoperatively, 73 eyes (75%) had a spectacle corrected visual acuity of 20/40 or better; postoperatively, 64 eyes (65.63%) at 1 month and 75 eyes (77.01%) at 3 months reached 20/40 or better. The uncorrected visual acuity was 20/40 or better in 33 eyes (33.72%) at 1 month and in 49 eyes (49.96%) at 3 months. Mean ablation zone diameter was 4.92 mm. Astigmatism was treated separately in 10 eyes. Complications included optical zone decentration (3 eyes with more than 1 mm, epithelial implantation (1 eye), incomplete resection of the disc (2 eyes), severe hypotension, and irregular astigmatism (1 eye).

Multizone Photorefractive Keratectomy for Myopia of 9 to 14 Diopters

We performed photorefractive keratectomy (PRK) using new multizone software of the Summit Omnimed excimer laser (Summit Technology, Inc., Waltham, Mass.) on thirteen highly myopic eyes. Average preoperative myopia was -10.94 diopters (D) (range -8.60 D to -13.50 D). Mean postoperative spherical equivalent refraction was -0.15 D (range +1.50 D to -4.00 D), at 3 months postoperatively. Uncorrected visual acuity of all the eyes improved markedly. There was no serious complication except one eye that lost two lines of spectacle corrected visual acuity possibly due to decentration. Though we need longer follow-up data and a larger sample size, the new multizone software seems to be effective for correcting high myopia.

Refractive Index of the Human Corneal Epithelium and Stroma

BACKGROUND

The refractive index of the cornea must be determined to optically perfect keratorefractive procedures. There are very few empirical measurements of the human corneal refractive index described in the literature. Throughout its depth, the cornea demonstrates regional variations in physiological properties such as swelling/de-swelling characteristics. These properties suggest there may be a difference in the refractive index between the anterior and posterior corneal surfaces.

METHODS

The refractive index of the human corneal epithelium of 10 eyes was measured, in vivo, using a modified hand-held refractometer. The refractive indices of the anterior and posterior surfaces of the bare stroma of fresh human corneas were measured using a bench model Abbe refractometer.

RESULTS

The mean refractive index of the epithelium, stromal anterior and posterior surfaces were 1.401 (SD +/- 0.005), 1.380 (SD +/- 0.005), and 1.373 (SD +/- 0.001) respectively.

CONCLUSIONS

The refractive index of the cornea is not uniform. The calculated dioptric power of the corneal epithelium is approximately -1.40 diopters (D). The varying refractive index does not significantly affect the total dioptric power of the cornea. The varying refractive index of the cornea has the potential to significantly contribute to the overall optical performance of the eye in relation to refractive surgery. The results should be incorporated into mathematical models, comparing and contrasting the optical performance of the eye before and after surgery.

The shape of the corneal apical zone after excimer photorefractive keratectomy

Applying an experimental photo-keratoscope, which assesses the shape of cornea within the pupillary region, to a group of subjects who have undergone excimer laser photorefractive keratectomy over a central 4 mm chord diameter of the cornea, we report the shape of the typical cornea within the ablated zone conforms to a steepening ellipse (average shape factor, 1.25). A statistically significant difference in the mean shape factor (asphericity) between the photoablated and the normal cornea (average shape factor 0.89) was not confirmed. However, there is more variability in the shape factors found in the photorefractive keratectomy group compared with normals, within the same distance from the corneal apex. Using the criterion of overlap within two standard deviations, averaging the vertical horizontal meridians, 75% of photorefractive keratectomy eyes fall within the shape factor limits of the normal eye group. In all cases the post-ablated corneal surface was found to be regular in terms of surface quality alone. The excimer photorefractive keratectomy technique is therefore a clinically acceptable method of refractive surgery.

Results one year after using the 193-nm excimer laser for photorefractive keratectomy in mild to moderate myopia

As part of a clinical trial, photorefractive keratectomy using the VISX 2015 193-nm excimer laser was performed on 91 healthy eyes of 91 patients. Preoperative refractive errors (spherical equivalent) ranged from -1.00 to -7.50 diopters (mean, -4.16 +/- 1.41 diopters). No patient had more than 1 diopter of refractive astigmatism. Six months postoperatively, the average residual refractive error was +0.09 +/- 0.63 diopters (range, -2.13 to +1.63 diopters). Correction within 1 diopter of that attempted was attained in 85 eyes (93%). Uncorrected visual acuity of 20/40 or better was attained in 86 eyes (95%) and was 20/25 or better in 67 eyes (74%). At one year, follow-up information was available on 85 eyes of 85 patients. The average residual refractive error was -0.15 +/- 0.65 diopters (range, -2.50 to +1.63 diopters). Correction within 1 diopter of that attempted was attained in 85 eyes (93%). Uncorrected visual acuity was 20/40 or better in 83 eyes (98%) and was 20/25 or better in 68 eyes (80%). One patient lost three lines of best-corrected visual acuity because of corneal haze, dropping from 20/15 to 20/30, whereas all other patients returned to best-corrected visual acuity within one line of their preoperative best-corrected visual acuity. Photorefractive keratectomy with the 193-nm excimer laser appears to be a useful treatment modality for the reduction of mild to moderate myopia.

193-nm excimer photorefractive keratectomy in high myopia

PURPOSE

To evaluate the refractive results of 193-nm excimer laser photorefractive keratectomy (PRK) performed on 48 highly myopic eyes in a multicenter study.

METHODS

A Visx 2015 or 2000 argon-fluoride excimer laser and a single-zone ablation technique were used. Postoperatively, eyes were treated with topical fluoromethalone for up to 5 months. Most eyes were treated with a 6.0- to 6.2-mm beam diameter after undercorrections and increased regression were noted with a 5.5-mm beam in earlier studies. Forty-eight eyes were treated for myopia, which was between -8.0 and -15.25 diopters (D) (spherical equivalent). The mean preoperative refraction was -11.2 D. Retreatment was performed after 6 to 16 months on 11 eyes for undercorrection. All eyes not retreated were followed for at least 12 months.

RESULTS

At 6 months, follow-up was available on 47 eyes. Of these eyes, 40% and 64% achieved corrections within 1 and 2 D of attempted correction, respectively. At 1 year, 60% of eyes attained 20/40 visual acuity or better uncorrected. Eleven patients (23%) were retreated between 6 to 16 months for undercorrection and/or regression. After retreatment, 47% and 81% of eyes achieved corrections within 1 and 2 D of attempted correction, respectively. At 1 year, 15% of eyes lost two lines of best-corrected visual acuity, and no eyes lost more than two lines. There was slightly more corneal haze seen in this group compared with the haze seen in patients undergoing PRK for low and moderate myopia.

CONCLUSIONS

These data show that excimer PRK can correct high amounts of myopia with reasonable stability after 6 months. Excimer PRK is an effective surgical treatment of severe myopia, but long-term follow-up is still needed to assess the stability of its effect.

Phototherapeutic keratectomy in herpes simplex keratitis. Clinical results in 20 patients

Twenty eyes in 20 patients with herpetic keratitis were treated with excimer laser photoablation. Sixteen eyes underwent phototherapeutic keratectomy because of scars and surface irregularities, with the goal to improve vision and 1 eye for cosmetic reasons. Two eyes had slight scars but were treated because of a refractive error (PRK). One eye had a metaherpetic ulcer where the wound surface was ablated with the intention to improve healing. Twelve of the 16 eyes showed improved visual acuity of 2 lines or more on the Snellen chart. The refractive outcome was acceptable in the 2 PRK eyes. The cosmetic result was good and the metaherpetic wound epithelialized within 3 weeks. Of the 20 patients 5 had 1 or more recurrences during the 16.8 months of follow-up. In the 17 months preceding the laser surgery 9 eyes had 1 or more recurrences. Three of these eyes still showed improved vision. Phototherapeutic excimer laser surgery is advantageous in selected cases and should be considered before corneal grafting. The procedure does not seem to increase the incidence of recurrences.

Human excimer laser keratectomy. Clinical and histopathologic correlations

PURPOSE

To understand the healing capabilities of the diseased human cornea after excimer laser photoablation by morphologic analysis of laser-treated corneas.

METHODS

Twelve corneal specimens were obtained 5 to 16 months after lamellar or full-thickness keratoplasty following phototherapeutic keratectomy for undercorrected myopic epikeratoplasty (2 eyes), corneal leukomas (2 eyes), herpes zoster corneal scarring (1 eye), band keratopathy (2 eyes), adenoviral subepithelial opacity (1 eye), keratoconus (1 eye), herpes simplex corneal scarring (2 eyes), granular corneal dystrophy (1 eye), and recurrent lattice dystrophy (1 eye). The morphology of the corneas was examined by light and electron microscopy.

RESULTS

Epithelial hyperplasia, abnormal epithelial attachment, and disorganized stromal matrices were observed. Evidence of residual disease frequently observed in these specimens indicated that the pathology either was not excised at the time of laser keratectomy or was recurrent.

CONCLUSIONS

The response of the diseased cornea to excimer laser treatment has similar characteristics to the responses previously observed in animal studies. Incomplete ablation of diseased tissue and/or recurrence of the initial disease was the major reason for failure of the treatment. Possible causes for the inability to remove diseased tissues and superficial scars with the excimer laser include (1) insufficiently achieved ablation depth and/or diameter and (2) decreased laser ablation rates of scarred cornea.

Ultraviolet solid-state laser (213-nm) photorefractive keratectomy. In vivo study

BACKGROUND

The pulsed ultraviolet 213-nm solid-state laser has been demonstrated as an alternative to the gas argon-fluoride 193-nm excimer laser for photorefractive keratectomy (PRK). The authors studied the clinical course and histopathologic changes occurring in rabbit corneas after PRK with a 213-nm solid-state laser.

METHODS

The 213-nm output of neodymium:YAG frequency-quintupled laser was used to create 5-mm optical zone ablations in seven pigmented rabbit corneas. The radiant exposure was 250 mJ/cm2 delivered through a computer-controlled scanning delivery system with a spot size of 0.5 mm. The target ablation was 4.0 diopters with an estimated ablation depth of 40 microns. A clinical estimate of corneal epithelial healing and stromal haze was made at intervals over the 3-month study period. Animals were killed immediately after ablation, or at 10 days, 1 month, or 3 months after ablation. Corneal tissue was preserved for light microscopy and transmission electron microscopy at each study interval.

RESULTS

All corneas re-epithelialized within 10 days postoperatively. Anterior stromal haze was clinically visible at 3 days, increased until approximately 1 month, and then gradually decreased over the succeeding 2 months. Residual subepithelial haze was visible at 3 months. Results of histopathologic study documented normal epithelium healing over time; the basement membrane retained its regular thickness and hemidesmosomes were abundant at 3 months. The anterior stroma had an increased number of fibroblasts at 10 days, many of which remained until 1 month. A mild, transient, cellular reaction occurred throughout the thickness of the stroma and the endothelium.

CONCLUSION

Using the 213-nm ultraviolet solid-state laser with a scanning delivery system, PRK shows a similar clinical course and histopathologic findings to the 193-nm excimer PRK study in rabbits. It is a clinically viable procedure for refractive surgery and requires further human clinical trails to determine its efficacy.

Specular microscopic evaluation of the corneal epithelium after excimer laser photorefractive keratectomy

Nineteen eyes underwent photorefractive keratectomy to correct myopia. Using the specular microscope, we observed the central corneal epithelium of each eye preoperatively and at one, three, and six months after surgery. A normal epithelial pattern was observed in 18 eyes (94.7%) at one and three months and in 19 eyes (100%) at six months after surgery. There was no statistically significant difference in the pre- and postoperative mean cell area and corresponding coefficient of variation. These results suggest that the destruction of Bowman's layer produced by photorefractive keratectomy does not affect the morphologic characteristics of the most superficial layer of the corneal epithelium.

Use of the 193 nm excimer laser for photorefractive keratectomy in low to moderate myopia

As part of a Phase III clinical trial, photorefractive keratectomy using the VISX 2015 193 nm excimer laser was performed on 91 sighted eyes of 91 patients. Preoperative refractive errors (spherical equivalent) ranged from -1.00 diopters (D) to -7.50 D (mean -4.11 D +/- 1.43 D). At six months, average residual refractive error was 0.02 D +/- 0.64 D (range -2.21 D to +1.38 D). Ninety-three percent of eyes were within 1.00 D of attempted correction, 93% had uncorrected visual acuity of 20/40 or better, and 72% achieved uncorrected visual acuity of 20/25 or better. All patients returned to their best corrected visual acuity within one line of their preoperative best corrected visual acuity. Photorefractive keratectomy with the 193 nm excimer laser appears to be useful in reducing low to moderate myopia.

Photorefractive keratectomy to treat low, medium, and high myopia: a multicenter study

This prospective, multicenter study investigated what influence patient age, sex, and time to re-epithelialization after myopic photorefractive keratectomy (PRK) have on stability of achieved correction and intensity of corneal haze. Seven surgeons performed PRK to correct myopia on 325 eyes using the same operative procedure, type of laser, and post-PRK treatment. Follow-up was from six to 12 months. Results indicate that corneal haze intensity is influenced by the time to re-epithelialization of the treated corneal surface. Patient age and sex may also affect haze intensity, although the results from this study were not conclusive.

Results of photorefractive keratectomy on 63 myopic eyes with six months minimum follow-up

We present our experience with 63 myopic eyes that had photorefractive keratectomy with an excimer laser. Minimum follow-up was six months. Mean preoperative myopia was -3.98 diopters (D) (range: -9.00 D to 1.25 D) and mean preoperative uncorrected visual acuity, 20/220. The epithelium was removed and fixation accomplished with a suction ring. Photoablation was then performed on a 5.0 mm diameter optical zone. Mean postoperative myopia was -0.60 D (range: -6.35 D to +2.25 D) after six months. Mean uncorrected visual acuity was 20/30. All corneas had a subepithelial haze that decreased progressively. There were no major complications. Except in a few cases, predictability was satisfactory. Photorefractive keratectomy proved effective throughout the trial. We need to study more patients with a longer follow-up to determine the indications and applications of this technique.

Excimer laser photorefractive keratectomy: 12 months' follow-up

OBJECTIVE

To report the results of 12 months' follow-up of patients with myopia treated with excimer laser photorefractive keratectomy.

DESIGN

A prospective study of patients undergoing excimer laser photorefractive keratectomy for myopia was commenced in September 1991. The study included 50 eyes (38 patients) with preoperative myopia of -1.50 to -6.00 dioptres and a maximum astigmatism of 0.75 dioptres.

RESULTS

Forty-six (92%) of the 50 eyes had an uncorrected acuity of 6/6 or better at 12 months, and 49 eyes (98%) had 6/12 or better. No patients had lost their best corrected acuity at 12 months. Forty-five eyes (90%) were within 0.50 dioptres of emmetropia and 47 eyes (94%) were within 1.00 dioptre of emmetropia.

CONCLUSIONS

Excimer laser photorefractive keratectomy is a safe and predictable procedure for low to moderate myopia. It will increasingly be offered to patients as an alternative to the use of spectacles and contact lenses.

Excimer photorefractive keratectomy for myopia

PURPOSE

To study the safety, effectiveness, predictability, and stability of excimer laser photorefractive keratectomy (PRK) in 133 normally sighted eyes.

METHODS

An excimer laser was used with a fluence of 160 mJ/cm2 at a frequency of 5 Hz and an ablation zone of 5.0 mm. The effects of nitrogen purge at surgery and postoperative steroids were evaluated.

RESULTS

No significant complications occurred in any patient. After an initial over-correction, the refraction stabilized. The average results obtained at 6 months were maintained on successive examinations to 36 months. Analysis of their visual acuity in groups IIA, IIB, and III indicates that results at 6 months are predictive of final results at 1 to 3 years (P < 0.0001; r > 0.9). In myopia (1.00-6.00 diopters [D]) treated with the excimer laser, there was a trend toward improvement in results over the course of the study. In 60% in group IIA, 58% in group IIB, 71% in group III, and 88% in group III no nitrogen (no N2), +/- 1 D was obtained. For 20/40 or better uncorrected visual acuity, the outcome was 70% for group IIA, 67% for group IIB, 75% for group III, and 100% for group III no N2. Significant improvement was noted without nitrogen purge. No significant improvement was observed from steroid treatment as used.

CONCLUSION

In reducing myopia, PRK appears to be safe and effective. The results obtained are reasonably predictable and stable after 6 months. As more refinements are introduced, this procedure could become one of the most promising in refractive surgery.

Excimer laser keratectomy after radial keratotomy

Radial keratotomy to treat myopia has been characterized by a lack of predictability. Until recently, patients with undercorrected myopia who were unsatisfied with their visual outcome could only be offered secondary augmentation procedures, which were equally unpredictable. We performed excimer laser keratectomy in six eyes of five patients whose myopia was undercorrected after radial keratotomy. The average residual spherical equivalent refractive error after radial keratotomy was -2.40 diopters, and this was reduced to -0.48 diopters after laser treatment. Final uncorrected visual acuity ranged from 20/80 to 20/20, and visual acuity was corrected to within one line of the preoperative best-corrected value in all patients. Epithelialization of all eyes occurred within four to six days. There were no incidences of corneal neovascularization, including no vascularization of the radial keratotomy incisions. Our findings suggest excimer phototherapeutic keratectomy offers a safe and more controlled method of augmenting undercorrected myopia after radial keratotomy.

Optics for photorefractive keratectomy

Matrix calculations are applied to Gaussian optics in the study of myopic correction by photorefractive keratectomy. The Colliac matrix formula determines the postoperative anterior curvature radius of the treated cornea. It also calculates the maximal depth of the removed corneal tissue and the ablation zone diameter needed to achieve emmetropia.

Excimer laser photorefractive keratectomy for myopia. Clinical results in sighted eyes

PURPOSE

To evaluate the refractive results of excimer laser photorefractive keratectomy (PRK) performed on normal, sighted myopic eyes; to assess the role of postoperative topical steroid treatment in patients with PRK; and to study the regression of effect.

METHODS

An argon fluoride 193-nm excimer laser was used. Photorefractive keratectomy was performed on 420 eyes with preoperative refraction ranging from -1.25 to -7.50 diopters (D). Minimum follow-up time was 12 months, and 194 of the eyes were followed for 15 months. Postoperative treatment generally consisted of topical dexamethasone for 3 months, but in a sub-study, some eyes were treated for only 5 weeks.

RESULTS

Mean refraction (spherical equivalent +/- standard deviation) at 12 months was -0.04 +/- 0.84 D and at 15 months -0.22 +/- 0.78 D. At 12 months postoperatively, 86% of the eyes were within 1.00 D of emmetropia, at 15 months 87%. At 12 months, 91% of the eyes had an uncorrected visual acuity of at least 20/40, at 15 months 87%. Eyes treated with dexamethasone for 3 months regressed significantly less than those treated for only 5 weeks (P < 0.01). Dexamethasone also was effective in reversing regression later in the postoperative course. Eyes with preoperative myopia up to 4.90 D had significantly better refractive results at 12 months than eyes with myopia ranging from 5.00 to 7.50 D (P < 0.01).

CONCLUSION

These data show that excimer laser PRK can correct myopia with good predictability. Results at 12 and 15 months tend to suggest stability of postoperative refraction. Regression of effect was more common in higher myopes. Topical steroids postoperatively seem to play a crucial role for the refractive result.

Corneal topography of excimer laser photorefractive keratectomy

The application of the 193 nm excimer laser for keratorefractive surgery promises to deliver a higher degree of precision and predictability than traditional procedures such as radial keratotomy. The development and evaluation of keratorefractive surgery have benefited from the parallel advances made in the field of corneal topography analysis. We used the Computed Anatomy Topography Modeling System (TMS-1) to analyze a Louisiana State University (LSU) Eye Center series of patients who had photorefractive keratectomy for the treatment of myopia with the VISX Twenty/Twenty excimer laser system. The excimer ablations were characterized by a relatively uniform distribution of surface powers within the treated zone. In the few cases that exhibited marked refractive regression, corneal topography analysis showed correlative changes. With topographical analysis, centration of the ablations relative to the center of the pupil could be evaluated. Marked improvement in centration occurred in the patients of LSU Series IIB in which the procedure to locate the point on the cornea directly over the pupil's center during surgery was refined. Corneal topographical analysis provides objective measures of keratorefractive surgical results and is able to measure the precise tissue removal effect of excimer laser ablation without the uncertainties caused by measuring visual acuity alone. Our observations forecast the need for improved aids to center the laser ablations and for the development of a course of treatment to prevent post-ablation stromal remodeling.

A morphological study of rabbit corneas after laser keratectomy

We have examined the morphology of the collagen and proteoglycans in rabbit corneas that have undergone excimer laser photorefractive keratectomy using a clinical, 193 nm excimer laser. The photoablation was carried out to a stromal depth of 100 microns and a diameter of 6 mm. All ablated corneas developed a haze that was most intense between week 4 and week 8 and which showed no improvement after week 16. The corneas were stained with the cationic dye cuprolinic blue to visualise proteoglycans and were then processed for transmission electron microscopy. The ultrastructural location of proteoglycans (keratan sulphate and dermatan sulphate) was observed in the corneal wounds at different time intervals. Corneas that had undergone steroid treatment post-operatively were also examined. In the healing tissue proteoglycan filaments of abnormal size were observed, which became most prominent after 2 weeks. As healing progressed these abnormal filaments decreased but after 45 weeks some were still present, indicating that the proteoglycan content had not returned to normal.

Photorefractive keratectomy for severe postkeratoplasty astigmatism

We performed cylindric corneal ablations with the excimer laser on 12 patients to correct severe, disabling astigmatism after keratoplasty. In some patients, an additional ablation was performed to correct myopia. Patients were followed up for an average of eight months (range, six to 14 months). Uncorrected visual acuity improved in nine patients, and nine of the 12 patients had a decrease in refractive cylinder at last follow-up. The mean preoperative refractive cylinder was 7.0 +/- 3.6 diopters, which decreased to a mean of 3.1 +/- 2.6 diopters at one month (P = .0003) and 4.3 +/- 2.9 diopters at last follow-up (P = .03). Keratometric astigmatism decreased from 7.5 +/- 3.9 diopters preoperatively to 5.2 +/- 3.9 diopters at the last follow-up (P = .001). Mean spherical equivalent was reduced from -7.4 +/- 4.2 diopters preoperatively to -3.3 +/- 4.4 diopters postoperatively (P = .003). Postoperative corneal haze, when present, did not reduce visual acuity. Excimer laser superficial keratectomy thus appears to be safe when used for postkeratoplasty ametropia, although substantial regression may limit its effectiveness in some patients.

Excimer laser refractive keratectomy for high myopia. 6-month follow-up of patients treated bilaterally

Both eyes of twenty patients were treated for myopia with refractive keratectomy using an excimer laser (193 nm), and followed for six months. The 40 eyes were divided into Group I (22 eyes) with an attempted refraction change of 5-8 diopters; and Group I (18 eyes) with 9-12 diopters of attempted refraction change. In Group I the corrected visual acuity improved or was unchanged in 16/22 eyes. Two eyes with high preoperative corrected visual acuity had experienced a significant loss in corrected visual acuity after 6 months. In Group II, the corrected visual acuity decreased in 10/18 eyes, in 4 eyes significantly. Among the 40 eyes, 39 had a refraction change less than intended after 6 months. Overcorrection was seen in only 1 eye. In Group I, 20/22 eyes were corrected up to 2.5 diopters less than attempted. In Group II, 9/18 eyes were more than 2.5 diopters from the attempted correction. All eyes developed subepithelial opacification ('haze'), which, in spite of steroid treatment, was still present after 6 months. The haze was more severe in eyes treated with 9-12 diopters of attempted refraction change. The achieved refraction change in the two eyes of the same patient was found to be correlated, possibly due to an individual factor in corneal wound healing.

Keratophakia and keratomileusis: safety and effectiveness. American Academy of Ophthalmology

The purpose of the Committee on Ophthalmic Procedures Assessment is to evaluate on a scientific basis new and existing ophthalmic tests, devices, and procedures for their safety, efficacy, clinical effectiveness and appropriate uses. Evaluations include examination of available literature, epidemiological analyses when appropriate, and compilation of opinions from recognized experts and other interested parties. After appropriate review by all contributors, including legal counsel, assessments are submitted to the Academy's Board of Directors for consideration as official Academy policy.

In situ collagen gel mold as an aid in excimer laser superficial keratectomy

PURPOSE

The aim of this study is to evaluate the potential use of bovine type I collagen as an adjuvant to excimer laser keratectomy.

METHOD

A suspension of collagen with the capability to polymerize into a gel was applied to the anterior corneal surface of freshly enucleated porcine eyes, using 35.0 diopter (D), 45.5 D, or 52.0 D contact lenses as molds. Keratometry, photokeratoscopy, slit-lamp photography, scanning electron microscopy, and light microscopy were performed on the new surfaces. Furthermore, an irregular corneal surface was created and a suspension of collagen was applied to mask protruding irregularities, therefore creating a smooth surface that was subjected to excimer laser keratectomy. Ablation rates for both collagen and cornea were measured.

RESULTS

Collagen suspension placed on a cornea and molded with contact lenses created a smooth-surfaced gel that conformed to the shape of the contact lens and adhered to the anterior cornea; it was optically smooth and regular as shown by photokeratoscopy, keratometry, and scanning electron microscopy. The corneal curvature was altered in accordance with the base curvature of the contact lens used. Results of keratometry showed resolution of pre-existing astigmatism without induction of new astigmatism. The ablation rate of the gel was not measurably different than that of cornea; hence, when applied to an irregular corneal surface, a smooth surface was created after excimer laser ablation.

CONCLUSION

This study supports the potential value of collagen gel as an adjuvant to excimer laser keratectomy for removal of corneal irregularities as well as for correction of myopia or hyperopia with or without astigmatism.

Lamellar excimer laser keratoplasty: reproducible photoablation of corneal tissue. A laboratory study

We examined the depth of ablation of the recipient bed with different counts of oscillations of excimer laser beam, to determine the correlation between planned and real depth. The ablation rate per oscillation was tested preoperatively by blackened photographic paper of defined thickness and thus was calculated to be 5 microns. Forty pig eyes were used for the first study. Each eight eyes were ablated in the planned depth 100 microns, 200 microns, 300 microns, 400 microns and 500 microns. The corneal thickness was measured with an ultrasonic pachymeter before and after the procedure. The depth measured after the photoablation was 99.4 +/- 36.4 microns for 100 microns planned depth, 186.7 +/- 55.3 microns for 200 microns, 298.4 +/- 68.5 microns for 300 microns, 373.9 +/- 65.7 microns for 400 microns and 480.1 +/- 59.3 microns for 500 microns. Comparing the depth measured after the photoablation to planned depth, there was a significant correlation (correlation coefficient: R = 0.93; p < 0.0001). Five other corneas trephinated from pig cadaver eyes were ablated from the endothelial side to the desired thickness (100 to 500 microns) of lamellar graft. In a second step a donor mask was placed onto the cornea and a laser light spot was led until perforating on all sides. The lamellar keratoplasty was completed by suturing the corneal graft into the bed. Macroscopic and microscopic examination of sutured eyes after fixation showed a good fit of wound margins and stromal interface. These results indicate that excimer laser is useful for reproducible corneal photoablation in lamellar keratoplasty.

Changes in corneal topography after excimer laser photorefractive keratectomy for myopia

Computer-assisted analysis of corneal topography was performed in 17 normally sighted human eyes during the first year after excimer laser photorefractive keratectomy (PRK) for myopia. Laser ablation of the central cornea produced an optical zone with a smooth power transition to the peripheral cornea. Decentration of the ablation was noted in some eyes (less than 0.5 mm in 3 eyes, 0.5 to 1.0 mm in 10 eyes, 1 to 1.5 mm in 3 eyes, and 2.1 mm in 1 eye), suggesting that careful alignment of the laser beam is critical. Improved methods to align the ablation within the center of the entrance pupil are needed. In 12 of 17 eyes, the topographic pattern appeared to stabilize between 3 and 7 months after PRK. In the remaining five eyes, central ablation power changed by more than 0.5 diopters (D) between the 6- and 12-month examinations. Regression was more common and more pronounced in eyes with intended corrections more than 5 D, whereas the majority of eyes with intended corrections of 5 D or less showed good correspondence between the final change in central ablation power and the attempted correction. Two eyes had a loss of at least two lines of best spectacle-corrected visual acuity that was attributable to irregular astigmatism, decentration of the ablation, and/or corneal opacification.