Human excimer laser lamellar keratectomy. A clinical study

[Influence of pregnancy on refractive parameters after LASIK surgery]

OBJECTIVES

Different ocular changes and complications of refractive surgery such as photorefractive keratectomy (PRK) due to pregnancy have been described in the last few years. However there is no information about the possible problems of laser in situ keratomileusis (LASIK) in pregnant women. Our objective was to study how physiological changes secondary to pregnancy could alter the refractive situation in pregnant women who have undergone LASIK surgery. We show the results obtained due to the changes between the first two trimesters of the pregnancy.

METHODS

A prospective and observational study was conducted in which one study group, made up of 9 patients who had undergone LASIK surgery before becoming pregnant, was compared with a control group of 9 patients with non-surgically corrected refractive problems. The following measurements were made in both groups in the first and second trimesters of the pregnancy; visual acuity, the best corrected visual acuity, tonometry, ocular anatomical characteristics by biometry, and refractive and corneal study by Pentacam(®).

RESULTS

Signicant changes were observed in the cylinder and spherical equivalent between the two trimesters in both groups. Visual acuity and spherical equivalent show a strong trend towards worsening, which was more significant in the study group. The patients of this group who had a larger pre-surgical defect showed lower modifications during the six first months of pregnancy.

CONCLUSIONS

The majority of women who require laser refractive surgery are are between 20-30 years old, thus in many cases corneal surgery is followed by at least one pregnancy, with different possible ophthalmological effects. The conclusions that may be derived from this study are that the assessment of the refractive changes in surgically operated corneas may be biomechanically weakened on being subjected to physiological hormone stimulation as happens during gestation.

Relationship between preoperative aberrations and postoperative refractive error in enhancement of previous laser in situ keratomileusis with the LADARVision system

PURPOSE

To identify factors leading to inaccuracy of spherical correction in wavefront-guided enhancement treatments.

SETTING

Department of Ophthalmology, University of California, Irvine, California, USA.

METHODS

This retrospective outcomes analysis comprised 23 eyes (20 patients) having wavefront-guided flap-lift (19 eyes) or photorefractive keratectomy (4 eyes) enhancements after conventional non-wavefront-guided laser in situ keratomileusis (LASIK) for correction of residual myopia with a minimum follow-up of 1 month. Main outcomes measures were changes in lower-order aberrations and higher-order aberrations (HOAs) related to predictability of the refractive outcome.

RESULTS

The enhancement procedures reduced HOAs. Because of a hyperopic shift, however, uncorrected visual acuity (UCVA) improved 2 or more Snellen lines in 3 of 9 eyes (33.3%) treated without a surgeon offset compared with 9 of 14 (64.3%) when a surgeon offset was incorporated into the treatment plan. The quantitative amount of preoperative HOAs correlated with the amount of hyperopic shift, particularly strongly for spherical aberration (r(2) = 0.446, P = .0005). The amount of hyperopic shift was related in a linear manner to the amount of HOAs (for spherical aberrations, Y = 1.31X - 0.30, where Y is the postoperative manifest refraction spherical equivalent and X is the wavefront error in microns root mean square).

CONCLUSIONS

Wavefront-guided enhancements can reduce levels of HOAs. Although UCVA improved in many patients, some with high levels of preenhancement spherical aberration had worse UCVA. Adjusting the nomogram for the amount of preenhancement HOAs may improve the accuracy of the lower-order correction in wavefront-guided enhancements.

Surgical Correction of Hyperopia

Surgical attempts to correct hyperopia have yielded varying results over the last 130 years. These techniques include the reshaping of the cornea through incisions, burns, or lamellar cuts with removal of peripheral tissue; the addition of central inlays; laser ablations; and the replacement of the crystalline lens. By examining the success of each surgical technique, the refractive surgeon may be able to make an informed decision on its indications and limitations, based on the specific patient's characteristics. Reporting the outcomes and complications of hyperopic surgery will help refine our approach to the management of an increasingly hyperopic and presbyopic population.

Excimer Laser Phototherapeutic Keratectomy for Granular and Lattice Corneal Dystrophy: A Comparative Study

PURPOSE

To compare the visual and refractive outcome after excimer laser phototherapeutic keratectomy (PTK) for superficial comeal opacities in granular and lattice dystrophy.

METHODS

Phototherapeutic keratectomy was performed in 62 eyes of 40 patients (granular dystrophy [n = 50] and lattice dystrophy [n = 12]) after epithelial debridement and pannus removal. Data regarding pre-and postoperative best spectacle-corrected visual acuity, changes in spherical equivalent of manifest refraction, Zeiss keratometry, astigmatism, and corneal topography were analyzed and compared between granular and lattice dystrophy. Recurrence, if any, was noted during a mean follow-up of 3.0 +/- 2.7 years.

RESULTS

Best spectacle-corrected visual acuity improved in 79% and 62% of eyes with granular and lattice dystrophy, respectively. Spherical equivalent refraction increased by a mean of 1.3 +/- 1.7 diopters (D) (median: 1.0 D) for granular dystrophy and a mean of 1.0 +/- 1.8 D (median: 0.5 D) for lattice dystrophy. The keratometric central power decreased by a mean of -0.8 +/- 4.3 D (median: -1.6 D) for granular dystrophy and a mean of -0.3 +/- 1.6 D (median: -0.7 D) for lattice dystrophy. No significant changes were noted regarding keratometric astigmatism in either granular or lattice dystrophy. The proportion of "regular" and "mild irregular" keratometry mires increased (39% vs 67% for granular dystrophy and 0% vs 50% for lattice dystrophy). In granular dystrophy, the surface regularity index/surface asymmetry index (SRI/SAI) decreased significantly from 2.14/2.24 preoperatively to 1.31/0.80 postoperatively (P = .006/P = .01). In contrast, decrease of SRI/SAI from 1.97/1.65 preoperatively to 1.35/1.16 postoperatively did not reach statistical significance in lattice dystrophy. Recurrences were observed in 10 (20%) eyes with granular dystrophy and 2 (17%) eyes with lattice dystrophy.

CONCLUSIONS

Our results suggest that PTK may be tried in all patients with superficially accentuated opacities in granular and lattice dystrophy before undergoing a more invasive procedure, such as lamellar or penetrating keratoplasty.

Late-onset corneal haze and myopic regression after photorefractive keratectomy (PRK)

OBJECTIVE

To report the incidence and clinical course of a series of patients who developed both delayed-onset, clinically significant progressive haze and myopic regression after photorefractive keratectomy (PRK).

METHODS

In this retrospective case series, the charts of 542 consecutive patients who had undergone PRK with the VISX Star Excimer or Nidek EC-5000 laser between July 1996 and October 1998 and who had a minimum of 6 months of follow-up were reviewed. Ten eyes of 8 patients developed progressive haze to greater than 1+ and myopic regression equal to or more than -1 D 3 months or more after PRK. The historical and clinical features were reviewed.

RESULTS

The incidence of combined progressive haze and myopic regression was 1.8%. The average age was 40.5 years. Three of the 8 patients were female. The median spherical equivalent (SE) attempted correction was -6.69 D (range -4.00 to -12.25 D). Five patients who underwent bilateral PRK had unilateral involvement. The mean SE regression was -2.01 +/- 0.79 D (range -1.00 to -3.00 D). Regression plateaued at a mean of 9.8 months. Haze ranging up to 4+ peaked at a mean of 7.4 months. Topical steroid treatment and/or epithelial scraping was attempted in 3 eyes but was ineffective.

CONCLUSIONS

Combined delayed-onset progressive haze and myopic regression can occur after PRK. In such cases, the amount of haze appears to correlate with the magnitude of attempted initial correction (r = 0.639, P = 0.046) although not with the magnitude of subsequent regression. Patients may need at least 10 months of follow-up to achieve a stable refraction and level of haze. These observations suggest a need for improved understanding of corneal wound healing following PRK and of biologic factors that may contribute to variability in outcomes.

Interaction between injured corneal epithelial cells and stromal cells

PURPOSE

To review the effects of injured corneal epithelial cells on myofibroblastic cell formation in corneal stroma after excimer laser surgery.

METHODS

Denudation of epithelium alone, photorefractive keratectomy (PRK), laser in situ keratomileusis (LASIK), and LASIK with denudation of epithelium were performed in rabbit eyes. Postoperative anterior stromal haze was assessed using a standard scale. Immunohistochemical methods were used to detect alpha-smooth muscle actin (alpha-SMA), a marker of myofibroblastic cells, and type III collagen in subepithelial corneal tissue. Rabbit corneal fibroblasts were cultured on collagen gels with or without cocultured corneal epithelial cells, or with partially scraped epithelial cells, on a companion plate separated by a permeable membrane. Gel thickness was measured daily to evaluate fibroblast-induced gel contraction. The total number of fibroblasts per gel was determined. Myofibroblasts were counted using immunocytochemical identification with alpha-SMA. Transforming growth factor (TGF)-beta was assayed in media on days 3 and 6; these procedures were also carried out in the presence of anti-TGF-beta antibody.

RESULTS

Three weeks after surgery, the presence of alpha-SMA-positive long-extended and spindle-shaped stromal cells as well as synthesis of type III collagen were observed in the subepithelial stromal layer, corresponding to corneal haze, in eyes that underwent PRK and LASIK with denudation of epithelium, but not in those that underwent denudation of epithelium alone or LASIK. Gel contraction, number of alpha-SMA-positive cells, and total cell number were significantly greater on gels with injured epithelial cells than on those without epithelial cells or with uninjured epithelial cells, as was TGF-beta concentration in media. Anti-TGF-beta antibody eliminated these differences.

CONCLUSIONS

The intact corneal epithelium might play an important role in curbing differentiation of myofibroblasts in corneal wound healing. Injured epithelial cells stimulate fibroblast myodifferentiation through one or more soluble factors, including TGF-beta.

Persistent Haze and Disorganization of Anterior Stromal Collagen Appear Unrelated Following Phototherapeutic Keratectomy

PURPOSE

The theoretical effects on corneal transparency induced by changes in collagen fibril packing following phototherapeutic keratectomy were compared to changes in objective measurements of haze.

METHODS

Phototherapeutic keratectomy was performed on the right eyes of four young rabbits; left eyes were used as controls. Postoperative slit-lamp measurements of haze were taken at regular intervals up to 19 months. Wounded stromas were studied by synchrotron x-ray diffraction to calculate the average interfibrillar spacing of the collagen fibrils. These data were combined with transmission electron microscope measurements, and the summation of scattered fields method was used to predict the transmission of visible light.

RESULTS

Objective measurements of haze were higher than the baseline control throughout the study. Electron micrographs of anterior stroma in 8-month-old wounds displayed irregularly spaced and poorly organized fibrils and x-ray diffraction indicated larger mean interfibrillar spacing compared to the controls. However, the predicted transmission of visible light through the anterior stromal scar tissue was not significantly different than normal.

CONCLUSIONS

Following phototherapeutic keratectomy, anterior corneal collagen fibrils were more widely spaced and unevenly organized than in the normal rabbit cornea. However, this did not cause a significant loss of transparency and was therefore unlikely to contribute to haze.

Acanthamoeba keratitis after photorefractive keratectomy

A 37-year-old women developed severe suppurative keratitis immediately after having photorefractive keratectomy in her left eye. The keratitis was unresponsive to intensive topical antibiotic agents and topical and systemic steroids. Although the differential diagnosis included nonmicrobial and fungal keratitis, the clinical course and confocal microscopy suggested, and subsequent histopathologic examination confirmed, a diagnosis of Acanthamoeba keratitis. The amebic contamination probably resulted from exposure of the deepithelialized cornea to contaminated freshwater in a northern Wisconsin marsh. This case emphasizes the importance of encouraging patients with epithelial defects and bandage soft contact lenses to avoid exposure to contaminated freshwater until reepithelialization is complete.

Treatment of corneal surface irregularities using biomask as an adjunct to excimer laser phototherapeutic keratectomy

PURPOSE

To assess the clinical changes induced by the use of BioMask as an adjunct to phototherapeutic keratectomy (PTK) using the excimer laser.

METHODS

A nonrandomized, prospective clinical trial was conducted. Twenty-two consecutive patients were enrolled for BioMask treatment with reduced visual acuity caused by irregular astigmatism or corneal scarring. Patients were treated with a broad-beam excimer laser using the standard PTK mode. The BioMask collagen material was applied as a liquid at 47 degrees C and molded using custom-made, rigid, gas-permeable contact lenses. After allowing the material to cool for approximately 3 minutes, the lens was removed and the gel was ablated at a fluence of 130-140 mJ/cm2 at a frequency of 10 Hz, until the stromal bed appeared smooth. After surgery, patients were followed at day 1, week 1, and months 1, 3, 6, 9, and 12. The main outcome measures were best spectacle-corrected visual acuity (BSCVA) and uncorrected visual acuity (UVA).

RESULTS

Comparison of the preoperative and postoperative BSCVAs showed a statistically significant change in vision (p = 0.0356). Before surgery, the mean Snellen BSCVA was 20/90, whereas after surgery, BSCVA increased to 20/60. Sixty-five percent of the patients had an improved BSCVA, with an average gain of 3.7 lines per patient (range, +2 to +10). Fifteen percent of the patients lost an average of 3.0 lines (range, -2 to -6), and 20% had no change. UVA averaged 20/180 before surgery and 20/123 after surgery, but the change was not statistically significant. With respect to UVA, 63% improved by an average of 4.25 lines per patient (range, +2 to +9). Twenty-six percent (26%) lost an average of 5.6 lines (range, -2 to -8), and 10% had no change. The average follow-up time was 8 months for both BSCVA and UVA. When patients were stratified by their treatment indications and ranked in order of BSCVA outcome, patients with Salzmann's nodular degeneration did the best, followed by those with prior refractive surgery, corneal dystrophies, and corneal scars. Even though BSCVA did not improve as much in the corneal scar group, 50% of the patients reported an improvement in the symptoms they experienced before surgery based on questionnaires. Conclusion. BioMask appears to have potential as an ablatable mask material when used in conjunction with PTK for the treatment of corneal surface irregularities.

Stress wave amplitudes during laser surgery of the cornea

PURPOSE

To determine the stress wave amplitudes generated during photoablation of the cornea using an argon fluoride excimer laser.

DESIGN

Experimental study using porcine eyes.

METHODS

Profiles of the stress wave amplitudes and enucleated human eyes along the axis of symmetry of porcine eyes and enucleated human eyes were measured using a miniature piezoelectric transducer. The ablation parameters, fluence, and ablation diameters were varied within the range of clinical application.

MAIN OUTCOME MEASURES

Stress wave amplitudes generated during photoablation.

RESULTS

The stress waves pass through a pressure focus located in the posterior lens and anterior vitreous, where amplitudes of up to 100 atm were measured with a 6-mm or larger ablation zone. Posterior to this focus, the stress wave amplitudes rapidly decrease to less than 10 atm at the retinal site. Small diameter excimer laser spots (< or =1.5 mm) produce a declining stress wave with no pressure focus at the lens and anterior vitreous.

CONCLUSIONS

Stress waves may be potentially hazardous to anterior structures of the human eye, including the corneal endothelium, lens and anterior vitreous face. They peak at the lens and vitreous with a broad beam, but not with small spot laser ablation. At posterior retinal and subretinal structures, they may be considered harmless.

Short-term impact of corticosteroids on hyaluronan and epithelial hyperplasia in the rabbit cornea after photorefractive keratectomy

PURPOSE

To investigate the impact of corticosteroids on subepithelial hyaluronan deposition and corneal epithelium thickness in the first 10 days after photorefractive keratectomy (PRK) and to analyze a possible contralateral effect of corticosteroids.

METHODS

Thirty-two New Zealand white rabbits were assigned into two groups and had a transepithelial 5.0-mm diameter, 8.00-diopter myopic PRK performed on one eye. The corticosteroid treatment group (16 animals) received 0.1 mL of methylprednisolone 4% subconjunctivally on the operation table, followed by 0.1% dexamethasone eye drops six times a day during the postoperative period. The sodium chloride (NaCl) treatment group received topical isotonic NaCl eye drops six times a day. In each treatment group, eight animals were killed after 3 and 9 days, respectively. The harvested specimens were stained for hyaluronan and the epithelial thickness was measured.

RESULTS

In contrast to the epithelial thickness, the subepithelial hyaluronan did not show a significant increase during the observation period. The corticosteroid treated group showed at both time-points significantly less subepithelial hyaluronan formation as well as a significantly thinner epithelium, when compared with the NaCl-treated group. At 9 days, the corticosteroid-treated group showed a mild epithelial hyperplasia in only one of eight eyes, whereas this was a common finding in the NaCl-treated group. We detected no hyaluronan deposits in any contralateral-untreated eye, and the epithelial thickness did not differ significantly between any of the four contralateral-untreated eye groups.

CONCLUSIONS

Corticosteroid medication during the first 10 days after operation reduces the amount of subepithelial hyaluronan production and inhibits the epithelial proliferation, and epithelial hyperplasia is prevented. Neither a contralateral hyaluronan deposition nor a contralateral corticosteroid effect could be detected.

Phototherapeutic keratectomy for decentration and central islands after photorefractive keratectomy

PURPOSE

To determine visual outcomes after treatment of decentration and central islands occurring after photorefractive keratectomy (PRK).

DESIGN

Retrospective, noncomparative case series.

PARTICIPANTS

Patients (n = 14) who exhibited decentration or central islands after PRK and photoastigmatic keratectomy (PARK).

METHODS

Fourteen eyes with post-PRK decentration (group I) or central islands (group II) were treated by transepithelial phototherapeutic keratectomy guided by epithelial fluorescence without modulating agents, and subsequently were treated with PRK or PARK. Mean follow-up time was 9 months (range, 45 days-21 months).

MAIN OUTCOME MEASURES

We analyzed pre- and postoperative keratometry, refractive errors, uncorrected visual acuity (UCVA), best-corrected visual acuity, and haze. In group I, we also measured pre- and postoperative decentration; in group II, we compared pre- and postoperative central island power.

RESULTS

Group I showed improvement in centration (P = 0.003). Group II showed decreased central island power (P = 0.18). -LogMAR UCVA improved from 0.59 (20/80(+1)) to 0.17 (20/30) (P = 0.03) and from 0.74 (20/100(-1)) to 0.21 (20/30(-1)) (P = 0.01) after retreatment of groups I and II, respectively.

CONCLUSIONS

Retreatment of patients having decentration and central islands after PRK results in improved visual outcomes.

Intact corneal epithelium is essential for the prevention of stromal haze after laser assisted in situ keratomileusis

AIMS

To determine the effect of intact corneal epithelium on stromal haze and myofibroblast cell formation after excimer laser surgery.

METHODS

Denuded epithelium alone, photorefractive keratectomy (PRK), laser in situ keratomileusis (LASIK), or LASIK with denuded epithelium was performed in rabbit eyes. Postoperative anterior stromal haze was assessed employing a standard scale. Immunohistochemical methods were used to detect alpha smooth muscle actin (alpha-SMA), a marker for myofibroblastic cells, and type III collagen in subepithelial corneal tissue.

RESULTS

Three weeks after surgery, the presence of alpha-SMA positive long extended and spindle-shaped stromal cells, and synthesis of type III collagen were observed in the subepithelial stromal layer corresponding to corneal haze in PRK and LASIK with denuded epithelium, but not in denuded epithelium alone and LASIK.

CONCLUSION

The intact corneal epithelium may play an important part curbing subepithelial haze and differentiation of myofibroblasts in corneal wound healing.

Mitomycin C reduces corneal light scattering after excimer keratectomy

PURPOSE

To evaluate the effect of intraoperative mitomycin C (MMC) on corneal light scattering after excimer laser keratectomy.

METHODS

Phototherapeutic keratectomy (PTK) was performed in 24 rabbit eyes. After 40-microm epithelial ablation, animals were divided into three groups. In group 1, filter paper discs soaked with MMC (group 1A, 0.5 mg/mL; group 1B, 0.25 mg/ml) were applied for 1 minute. In group 2, annular filter papers soaked with MMC (group 2A, 0.5 mg/mL; group 2B, 0.25 mg/mL) were applied for 1 minute. Controls received vehicle only (group 3). Six-millimeter diameter 100-microm deep PTK was performed. Corneal light scattering was measured weekly from 1 to 6 weeks, at 10 weeks, and at 8 and 13 months using a scatterometer. A corneal light scattering index (SI) ranging from 0 to 10 was calculated; SI of 1 represents normal scattering.

RESULTS

A statistically significant decrease in mean SI was noted in group 2A (annular MMC 0.5 mg/mL; p<0.05) as compared with the control group at 2 weeks. At 10 weeks, SI approached baseline levels in group 2 and the control group but showed significant increase in group 1 (MMC disc; p < 0.05). At 8 and 13 months, SI showed no statistical differences between groups.

CONCLUSIONS

Controlled application of 0.5 mg/mL MMC in the corneal midperiphery transiently reduces corneal light scattering after excimer keratectomy in this rabbit model.

A Model to Explain the Difference Between Changes in Refraction and Central Ocular Surface Power After Laser in situ Keratomileusis

PURPOSE

After refractive surgery, changes in central ocular surface power are usually less than actual changes in refraction. The aim of this paper was to examine the theoretical changes in the radius of the epithelial-stromal interface and corneal stromal refractive index which could take place to account for some of the empirical findings.

METHODS

The parameters of an aspheric human eye model featuring gradient index optics was modified using data from a clinical study evaluating effects of laser in situ keratomileusis (LASIK) for correction of moderate to high myopia. Computations were performed to determine: 1) theoretical postoperative radius of the epithelial-stromal interface and hence distribution of epithelial thickness, and 2) refractive index of the stroma when the epithelium was of fixed uniform thickness.

RESULTS

Within the central 2-mm diameter refractive zone of the cornea after LASIK for myopia, either of two factors could account for the difference between changes in central ocular surface power and the actual change in ocular refraction: 1) a steepening of the epithelial-stromal interface resulting from a 400% centrifugal increase in epithelial thickness, or 2) a reduction in stromal refractive index from an average of 1.376 to 1.364.

CONCLUSION

The difference between the observed changes in refraction and central ocular surface power could be explained by reduction in stromal refractive index and this could be secondary to a 6.5% increase in water content of the stroma during the postoperative period. The epithelial-stromal interface after LASIK is unlikely to steepen by the extent predicted by our model. This surface is not the source of the difference between the change in refraction and change in central ocular surface power.

Cooling effect on excimer laser photorefractive keratectomy

PURPOSE

To evaluate the effect of cooling on pain, corneal haze, and refractive outcome after excimer laser photorefractive keratectomy (PRK).

SETTING

Tokyo Medical and Dental University Hospital, Tokyo, Japan.

METHODS

The corneal surface was cooled before, during, and after laser ablation using a method called cooling PRK. Thirty-eighty highly myopic eyes of 38 patients whose spherical errors ranged from -8.00 to -18.75 diopters (D) were randomized into 2 groups: 16 eyes with conventional PRK and 22 eyes with cooling PRK. Postoperative pain was measured using the Visual Analogue Scale (VAS). Refraction, visual acuity, and complications were followed for up to 2 years. All data were analyzed and compared between groups to evaluate the cooling effect on PRK.

RESULTS

One day postoperatively, patients in the cooling PRK group had significantly less pain (P < .01). At 3 months, the haze score in the cooling PRK group was significantly less than in the conventional PRK group (P < .01). The residual refractive error was not significantly different between the 2 groups until 2 years, when it was greater in the conventional PRK (mean -5.09 D +/- 2.11 [SD]) than the cooling PRK group (-4.64 +/- 2.27 D). Ten eyes (62.5%) in the conventional PRK group and 15 eyes (68.2%) in the cooling PRK group were within +/- 1.00 D of the intended refraction. There were no serious complications in the cooling PRK group. Two eyes in the conventional PRK group had severe corneal haze and lost 2 Snellen lines of best corrected visual acuity.

CONCLUSION

Corneal cooling on PRK effectively reduced postoperative pain, corneal haze, and myopic regression.

Confocal microscopy reveals persisting stromal changes after myopic photorefractive keratectomy in zero haze corneas

AIMS

Micromorphological examination of the central cornea in myopic patients 8-43 months after excimer laser photorefractive keratectomy (PRK), using the slit scanning confocal microscope.

METHODS

Patients were selected from a larger cohort of individuals on the basis of full corneal clarity (haze grading 0 to +1; mean 0.3) and their willingness to participate in the study. 15 eyes of 10 patients with myopic PRK (-4 to -11 D; mean 6.7) and an uneventful postoperative interval of 8-43 months (mean 26) were examined. Contact lenses had been worn by eight of the 10 patients for 4-11 years (mean 6.7) before surgery. Controls included the five untreated fellow eyes of PRK patients, 10 healthy, age matched volunteers without a history of ocular inflammation or contact lens wear, and 20 patients who had worn rigid gas permeable (n = 10) or soft contact lenses (n = 10) for 2-11 years. Subjects were examined with a real time flying slit, scanning confocal microscope using x25 and x50 objectives.

RESULTS

In PRK treated patients and contact lens wearers, basal layer epithelial cells sporadically displayed enhanced reflectivity. The subepithelial nerve plexus was observed in all individuals, but was usually less well contrasted in the PRK group, owing to the presence of a very discrete layer of subepithelial scar tissue, which patchily enhanced background reflectivity. Within all layers of the stroma, two distinct types of abnormal reflective bodies were observed in all PRK treated eyes, but in none of the controls. One had the appearance of long (> = 50 microns), slender (2-8 microns in diameter) dimly reflective rods, which sometimes contained bright, punctate, crystal-like inclusions, arranged linearly and at irregular intervals. The other was shorter (< 25 microns), more slender in form (< 1 micron in diameter), and highly reflective; these so called needles were composed of crystal-like granules in linear array, with an individual appearance similar to the bright punctate inclusions seen in rods, but densely packed. Both of these unusual structures were confined, laterally, to the ablated area, but were otherwise distributed throughout all stromal layers, with a clear predominance in the anterior ones. These rods and needles were observed in all PRK treated corneas, irrespective of previous contact lens wear. On the basis of qualitative inspection, the incidence of rods and needles did not appear to correlate with either the volume of tissue ablated or the length of the postoperative interval. In contact lens wearing controls, highly reflective granules, reminiscent of those from which the needles were composed, were found scattered as isolated entities throughout the entire depth and lateral extent of the corneal stroma, but rods and needles were never encountered. The corneal endothelium exhibited no obvious abnormalities.

CONCLUSION

Confocal microscopy 8-43 months after PRK revealed belated changes in the corneal stroma. These were manifested as two distinct types of abnormal reflective bodies, which had persisted beyond the stage when acute wound healing would have been expected to be complete. The clinical significance of these findings in the context of contrast visual acuity and long term status of the cornea is, as yet, unknown.

Evaluation of axis alignment system for correction of myopic astigmatism with the excimer laser

PURPOSE

To evaluate the efficacy of in situ axis alignment for the treatment of myopic astigmatism with an excimer laser.

METHODS

In this prospective clinical trial, 71 eyes with stable astigmatism of -1.00 to -6.00 diopters (D) with or without stable myopia of up to 10.00 D were randomized to receive excimer treatment of their astigmatic component aligned to the axis determined by subjective refraction (control group) or to have the alignment verified and altered is necessary by in situ alignment (axis alignment group). A VISX Twenty-Twenty excimer laser was used for the treatment. Patients were followed for 6 months.

RESULTS

In 85% of eyes in the axis alignment group, the axis of cylinder was different from that calculated by routine refraction. Pretreatment mean cylinder was -1.81 D +/- 0.91 (SD) in the axis alignment group and -1.75 +/- 0.60 D in the control group. At 6 months, an undercorrection of cylinder was seen in both groups; the mean residual cylinder was -0.70 +2- 0.56 D in the axis alignment group and -0.59 +/- 0.50 D in the control group. More than 80% of all eyes were within +/- 1.00 D of plano and achieved an uncorrected visual acuity of 6/12 or better. A loss of two or more lines of best corrected visual acuity was recorded in three eyes.

CONCLUSION

The in situ alignment technique did not produce better results of photoastigmatic refractive keratectomy than the routine axis alignment technique.

Complications of photorefractive keratectomy for myopia: two year follow-up of 3000 cases

PURPOSE

To evaluate the results and complications of photorefractive keratectomy (PRK) for myopia.

SETTING

Alicante Institute of Ophthalmology, University of Alicante, Spain.

METHODS

This prospective study evaluated the results and complications of the first 3000 cases of PRK performed with a VISX Twenty-Twenty excimer laser. Myopia ranged from -1.0 to -14.0 diopters (D) and astigmatism, from -1.0 to -5.0 D. Follow-up was 2 years.

RESULTS

Ten eyes (0.7%) lost two or more lines of best corrected visual acuity 1 year after surgery. Retreatment for undercorrection or regression was done in 7.41% in the low myopia group and 38.69% in the high myopia group. Central islands, which could be detected only on videokeratoscopy, occurred frequently but influenced the refractive and visual outcomes in few cases. Severe haze was present in 17 eyes only after 1 year follow-up. There were no cases of progressive hyperopia. Other complications such as eccentric ablation producing astigmatism (n = 15), delayed re-epithelialization, or recurrent corneal erosion were rare.

CONCLUSION

There were few complications after PRK in 3000 eyes. With proper patient selection, PRK can be considered relatively safe compared with other refractive procedures.

Photorefractive keratectomy for residual myopia after radial keratotomy. PRK After RK Study Group

PURPOSE

To evaluate the visual outcomes in patients having photorefractive keratectomy (PRK) to correct residual myopia after radial keratotomy (RK).

SETTING

Nine refractive surgery centers in the United States and one in South Korea.

METHODS

This retrospective analysis comprised 38 eyes of 32 patients treated with PRK after RK and followed for 12 months. Analysis was based on pre-RK and pre-PRK refraction as well as response to RK (pre-RK minus pre-PRK refractions).

RESULTS

Mean pre-RK and pre-PRK refractions were -8.11 diopters (D) +/- 2.92 (SD) and -4.28 +/- 2.08 D, respectively. One month after PRK, mean refraction was +0.42 +/- 1.56 D and regressed to -0.95 +/- 1.24 D at 12 months. At 12 months, 65% of eyes had an uncorrected visual acuity of 20/40 or worse, and 11.1% lost 2 or more lines of best corrected acuity. Of eyes with an original erro of -6.00 or less, 81.8% were within +/- 1.00 D of intended correction at 12 months and of those with an original error of -9.12 to -20.00 D 50.0% (P = .004). All eyes with residual (pre-PRK) errors of -3.00 D or less and 42.9% with a residual error of -6.12 to -9.00 D were within +/- 1.00 D of intended correction (P = .07). There were no statistically significant differences in the response to PRK between eyes that had an RK response of 0 to 3.00, 3.12 to 6.00, or 6.12 to 12.00 D.

CONCLUSION

Patients with lower original and residual myopia achieved better visual outcomes after PRK than those with higher myopia. The amount of myopic correction achieved using RK was not predictive of the amount of myopic correction using PRK.

Excimer Laser Photorefractive Keratectomy for Hyperopia

OBJECTIVE

Excimer laser photorefractive keratectomy (PRK) has been shown to be an effective method in the treatment of refractive errors, especially myopia. We evaluated prospectively the efficacy, predictability, stability, and safety of excimer laser PRK in the treatment of hyperopia.

METHODS

Thirty-four hyperopic eyes were treated with an Aesculap-Meditec (MEL 60) excimer laser. The patients were divided into two groups. In the low-moderate hyperopia group, baseline spherical equivalent refraction was between +1.50 and +6.00 diopters (D) (mean, +4.20 +/- 1.30 D) and in the high hyperopia group between +6.25 and +9.75 D (mean, +7.70 +/- 1.30 D). Follow-up visits occurred 1, 3, 6, and 12 months after surgery.

RESULTS

One-year results were available for a total 27 eyes (79%): 15 eyes with low to moderate hyperopia and 12 eyes with high hyperopia. One year after PRK in the low-moderate group, six eyes (40%) had a refractive error within +/- 1.00 D of emmetropia, but in the high hyperopia group only two eyes (17%) were within +/- 1.00 D of emmetropia; three eyes (20%) and one eye (8%) were within +/- 0.50 D, respectively. The stability of the refractive change was better in the low to moderate hyperopia group; in the high hyperopia group there was still some regression after 6 months. At 12 months, 10 eyes (67%) in the low-moderate and one eye (8%) in the high hyperopia group had postoperative uncorrected visual acuity of 20/40 or better. One eye in the low-moderate hyperopia group saw 20/20 without correction. Only one eye lost two lines of spectacle-corrected visual acuity. Haze was more intense in the high hyperopia group, but it did not reduce visual acuity. No vision-threatening complications were observed.

CONCLUSIONS

When low to moderate hyperopia up to +6.00 D is treated, excimer laser PRK with the Aesculap Meditec MEL60 laser is safe and moderately effective, and refraction stabilizes after 3 months in most eyes. However, PRK is not sufficient to treat high hyperopia in an effective and predictable way.

Factors affecting epithelial hyperplasia after photorefractive keratectomy

PURPOSE

To determine the effect of patient age, postoperative time, ablation zone diameter and depth, attempted correction, and corneal topography on postoperative corneal epithelial thickness after photorefractive keratectomy (PRK).

SETTING

Private clinic and university hospital, Stockholm, Sweden.

METHODS

This retrospective, unmasked study comprised 136 myopic patients treated unilaterally with PRK. Seventy eyes had been treated with the Summit excimer laser 27 months +/- 7 (SD) earlier using ablation zone diameters of 4.1 to 5.0 mm. Sixty-six eyes had been treated with the VISX excimer laser 6 +/- 3 months earlier using a 6.0 mm zone diameter. The untreated fellow eyes served as controls. Epithelial thickness was measured at a standardized central corneal area with a modified optical pachymeter, and corneal topography was determined using computerized videokeratoscopy.

RESULTS

In the Summit group, the epithelial layer in the PRK eyes was 12.0 microns (21%) thicker than in the control eyes (P < .001; 95% confidence interval [CI] 9.35 to 14.3 microns). This thickness differential correlated significantly with increased ablation depth and attempted correction. In the VISX group, the epithelium in the treated eyes was 7.0 microns (7%) thinner (P = .0009; 95% CI -1.9 to -6.7 microns) and thickness did not correlate with ablation depth or attempted correction. There was no correlation between epithelial hyperplasia and patient age or postoperative follow-up. With the laser groups combined, epithelial hyperplasia was greater with smaller zone sizes and a greater rate of change in power at the edge of the ablation zone.

CONCLUSION

The factors associated with an increase in epithelial thickness were small ablation zones, greater attempted corrections, and deeper ablations. Larger, smoother ablation profiles may result in less epithelial hyperplasia.

Comparison of Corneal Epithelial Wound Healing after Photorefractive Keratectomy in the Rabbit with Two Types of Excimer Lasers

PURPOSE

To evaluate the differences in epithelial wound healing following photorefractive keratectomy when performed with the Summit UV 200 LA and the VISX 20/20 excimer lasers.

METHODS

Sixteen New Zealand rabbits were divided into two groups. One group was treated with the Summit laser and the other group treated with the VISX 20/20 laser. The treatment consisted of a -6.00 diopter photorefractive keratectomy with a 5-mm diameter treatment zone. Epithelial wound healing was followed by photography at 4 hour intervals for 64 hours. The length of the wound edge and the size, shape, and closure time of the wound were measured.

RESULTS

The median wound edge length at 4 hours was 18.3 mm for the Summit laser and 16.7 mm for the VISX laser. The median wound size at 4 hours was 22.0 mm2 for the Summit and 21.2 mm2 for the VISX. The median wound closure time was 53.4 hours for the Summit laser and 54.0 hours for the VISX laser.

CONCLUSION

There was no statistically significant difference in the epithelial healing of rabbit corneal wounds created by photorefractive keratectomy when performed with two current ophthalmic excimer lasers, the Summit UV 200 LA and the VISX 20/20.

Effect of Refractive Surgery Experience on Outcomes after Photorefractive Keratectomy

BACKGROUND

Excimer laser photorefractive keratectomy is effective in the treatment of myopia. It is unclear whether the refractive and ocular surgical experience of the surgeon influences outcome.

METHODS

Photorefractive keratectomy was performed on 131 consecutive myopic eyes by one experienced refractive surgeon and on 428 consecutive myopic eyes by six other less experienced surgeons. Outcome data were compared between groups.

RESULTS

The mean preoperative spherical equivalent was -5.76 +/- 2.43 diopters (D) in the single surgeon group and -5.36 +/- 2.34 D in the multiple surgeon group. Follow-up of 6 months was available on 109 eyes (83%) in the single surgeon group and 289 eyes (68%) in the multiple surgeon group. At 6 months, the average residual refractive error was +/- 0.15 +/- 1.13 D in the single-surgeon group and +0.21 +/- 1.15 D in the multiple surgeon group. This difference was not statistically significant (p = 0.68). Correction within 1 D of attempted correction was obtained in 76 eyes (70%) in the single surgeon group and in 223 eyes (77%) in the multiple surgeon group. Uncorrected visual acuity of 20/40 or better was achieved in 89 eyes (82%) in the single surgeon group and 253 eyes (88%) in the multiple surgeon group.

CONCLUSION

Refractive and visual acuity outcomes after photorefractive keratectomy are influenced minimally by the refractive surgical experience of the surgeon.

Clinical Effect of Blowing Nitrogen Gas across the Cornea during Photorefractive Keratectomy

OBJECTIVE

We evaluated the clinical effects of blowing nitrogen gas over the cornea during photorefractive keratectomy.

METHODS

We retrospectively compared clinical outcomes of 32 patients (37 eyes), divided into two groups who had photorefractive keratectomy with nitrogen gas blowing across the cornea (n = 19) and without nitrogen gas (n = 18). All photorefractive keratectomies were performed between April 1991 and August 1992. Ablation zone diameters were 5.0 or 5.5 mm. The average attempted correction was -3.47 +/- 1.76 diopters (D) in the nitrogen gas group, and -3.48 +/- 1.25 D in the non-nitrogen gas group.

RESULTS

The nitrogen gas group achieved more accurate corrections than the non-nitrogen gas group, but had a significantly greater amount of corneal haze at all postoperative examinations. Corneal haze was most evident at 1 month and gradually diminished therafter in both groups. At 6 months, videokeratography revealed a central island in 2 eyes (11.1%) of the non-nitrogen gas group. Because of undercorrection (2 eyes) or central island (1 eye), 3 out of 18 eyes in the non-nitrogen gas group were reablated 6 months later.

CONCLUSIONS

The non-nitrogen gas group had more undercorrections and a higher frequency rate of central steep islands, but nitrogen gas blowing caused a greater amount of corneal haze.

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.

Phototherapeutic Keratectomy for Granular and Lattice Corneal Dystrophies at 1.5 to 4 Years

BACKGROUND

The capability of the 193-nm excimer laser to ablate the cornea and to remove opacities and various other corneal diseases in a procedure called phototherapeutic keratectomy (PTK) has been demonstrated. In this study we evaluated the long-term results of PTK for treatment of granular and lattice corneal dystrophies.

METHODS

Four eyes with granular or lattice corneal dystrophy were treated with a mean follow-up of 47.8 months (range, 36 to 58 months). Focal ablations of the central cornea with an ablation zone of 5.5 to 6.0 mm were performed. Ablation depth was 110 microns in three eyes and 140 microns in one eye.

RESULTS

Removal of corneal opacities allowed for improvement in corrected visual acuity in all patients. Mean corneal thickness in the area of pathology decreased from 0.583 mm before surgery to 0.449 mm after surgery. Spherical equivalent of the manifest refraction measurements increased by a mean of +5.09 D. There were no major complications, but all patients developed slight haze. There was a hyperopic shift in three eyes.

CONCLUSION

Our long-term results suggest that PTK is a safe and effective alternative to penetrating and lamellar keratoplasty in patients with granular or lattice corneal dystrophies.

Effect of myopic excimer laser photorefractive keratectomy on the electrophysiologic function of the retina and optic nerve

PURPOSE

To assess by electrophysiologic testing the effect of photorefractive keratectomy (PRK) on the retina and optic nerve.

SETTING

Eye Clinic, S. Salvatore Hospital, L'Aquila University, Italy.

METHODS

Standard pattern electroretinograms (P-ERGs) and standard pattern visual evoked potentials (P-VEPs) were done in 25 eyes of 25 patients who had myopic PRK for an attempted correction between 5.00 and 15.00 diopters (D) (mean 8.00 D). Testing was done preoperatively and 3, 6, 12, and 18 months postoperatively. The contralateral eyes served as controls. During the follow-up, 3 patients (12%) developed steroid-induced elevated intraocular pressure (IOP) that resolved after corticosteroid therapy was discontinued.

RESULTS

No statistically significant differences were seen between treated and control eyes nor between treated eyes preoperatively and postoperatively.

CONCLUSION

Myopic excimer laser PRK did not seem to affect the posterior segment. The transient steroid-induced IOP rise did not seem to cause functional impairment.

Topical interferon alpha 2b for corneal haze after excimer laser photorefractive keratectomy. The Melbourne Excimer Laser Group

PURPOSE

To determine whether topical interferon alpha 2b (IFN-alpha) prevents corneal haze after excimer laser photorefractive keratectomy (PRK).

SETTING

Tertiary referral ophthalmic hospital.

METHOD

A prospective, double-blind, placebo-controlled, randomized study of 31 patients was undertaken. After surgery in a single institution, patients received a drop of either a placebo or IFN-alpha (5 x 10(6) IU/ml) four times daily for 4 weeks. The main outcome measures were corneal haze, refraction, and visual acuity.

RESULTS

The major side effect of interferon alpha treatment was a significant delay in epithelial healing by a mean of 2 days. The means of the average post-treatment clinical scores for haze in all patients up to 12 months after surgery were 0.46 +/- 0.25 for the IFN-alpha group and 0.64 +/- 0.43 for the placebo group (P = .20). Of patients with a correction of greater than 5.00 diopters (D), the IFN-alpha group had significantly less haze over the course of the study (0.39 +/- 0.23 versus 0.98 +/- 0.50; P = .03). After 12 months, the mean absolute spherical equivalent in the two groups was not significantly different (1.02 +/- 1.13 D versus 1.44 +/- 2.64 D). There was a tendency toward better uncorrected visual acuity in the INF-alpha group (P < .10, Kolmogorov-Smirnov).

CONCLUSION

Topical IFN-alpha may merit further investigation as a treatment to reduce corneal haze after excimer laser PRK for corrections greater than 5.00 D.

An in vivo investigation of the structures responsible for corneal haze after photorefractive keratectomy and their effect on visual function

PURPOSE

To make serial measurements of corneal haze and microscopic anatomy after photorefractive keratectomy (PRK) and compare the results with visual function measured at the same time points in the same single group of human subjects.

METHODS

Ten patients underwent -6.00-diopter, 6-mm PRK. The patients were reviewed frequently for 12 months. Corneal haze was measured objectively in two ways: (1) an opacification index was determined from the variance in digitized retroillumination images; and (2) light reflected and scattered back from the cornea was assessed by gray-scale analysis of video slit images. In vivo confocal microscopy recorded the anatomic changes occurring in the cornea, and computer analysis of the images quantified the keratocytes and subepithelial deposit. Visual performance was assessed by Snellen visual acuity, contrast sensitivity, and glare-induced visual dysfunction.

RESULTS

In the first week, epithelial irregularity resulted in a transient reduction in all aspects of visual function. In the first month, keratocyte disturbances reduced contrast sensitivity at high frequencies and produced glare. Over the next couple of months, the subepithelial deposit resulted in a more prolonged loss of contrast sensitivity at low frequencies and glare-induced visual dysfunction due to the scattering of light. In several patients, these visual defects persisted after 1 year.

CONCLUSIONS

Epithelial and keratocyte disturbances only transiently affect visual function. The subepithelial deposit is more persistent and can have a lasting effect on visual performance. Therefore, attempts to improve the visual outcome of PRK must be aimed at controlling the synthesis of subepithelial material.

Astigmatic change 1 year after excimer laser treatment of myopia and myopic astigmatism. Melbourne Excimer Laser Group

PURPOSE

To evaluate the surgically induced astigmatism (SIA) 1 year after excimer laser photorefractive astigmatic keratectomy (PARK) and photorefractive keratectomy (PRK).

SETTING

Royal Victorian Ear and Eye Hospital, Melbourne, Australia.

METHODS

This study comprised 333 PARK patients and 155 PRK patients treated with a VISX 20/20 excimer laser and followed prospectively for 12 months. Vector analysis of the change in astigmatism was used to calculate the SIA in the PRK group and the percentage of astigmatism corrected in the PARK group.

RESULTS

Among patients with low cylinders astigmatic correction varied greatly, particularly in those treated for large amounts of myopia. The spherical PRK treatments yielded a mean induced postoperative astigmatism of 0.47 diopter. There was a linear relationship between this inadvertent SIA and increasing myopia.

CONCLUSION

Excimer laser surgery for myopia creates a low degree of random, unpredictable SIA that may be the result of irregular epithelial thickening during postoperative healing. This creates a background noise of astigmatic change upon which the targeted astigmatic correction is superimposed.

Corneal epithelial recovery following photorefractive keratectomy

AIMS

To further understand the morphological and functional recovery of corneal epithelium following excimer laser photorefractive keratectomy (PRK).

METHODS

The right eyes (group 1) of 15 male, New Zealand white rabbits weighing 2-3 kg underwent PRK. The left eye of each rabbit (group 2) underwent simple mechanical de-epithelialisation and were examined as treated controls. Both eyes of another eight rabbits (group 3) served as untreated controls. All eyes underwent a corneal epithelial permeability study by fluorophotometry at 2, 4, and 8 weeks after surgery. Five animals in groups 1 and 2 were sacrificed at 9, 10, and 12 weeks after surgery. The animals in group 3 were sacrificed at the end of the 12 week experimental period. Both eyes of each sacrificed animal were enucleated immediately and processed for both haematoxylin and eosin stain and electron microscopic study. The electron micrograph was magnified to 14,000x and the extent of hemidesmosome formation was quantified and analysed.

RESULTS

The corneal epithelial barrier to sodium fluorescein was subnormal and returned to a normal barrier state 4 weeks after PRK in group 1 whereas it was normal in group 2 throughout the examination period. The extent of hemidesmosome formation was abundant yet subnormal in both groups 1 and 2 up to 12 weeks, when compared with that in group 3.

CONCLUSION

The corneal epithelium regained its functional barrier 4 weeks after PRK in rabbits while the extent of hemidesmosome formation was still subnormal 12 weeks after mechanical de-epithelialisation, with or without PRK.

Role of epithelial hyperplasia in regression following photorefractive keratectomy

AIM

To determine the relation between epithelial hyperplasia and regression of effect after photorefractive keratectomy (PRK).

METHODS

Seventy unilaterally treated patients with PRK were examined. All eyes had been treated with the Summit excimer laser 27 (SD 7) months previously with zone diameters of 4.1 to 5.0 mm. The untreated fellow eyes served as controls. Epithelial thickness was measured centrally with a thin slit optical pachometer and manifest subjective refraction was performed.

RESULTS

The epithelium was 21% thicker in the treated eye (p < 0.0001). The relation between refractive regression and epithelial hyperplasia was significant (r = 0.41; p < 0.001).

CONCLUSIONS

Epithelial hyperplasia after PRK correlated with the myopic shift (including hyperopia reduction) after treatment with the Summit laser. A model is proposed suggesting that both subepithelial and epithelial layers contribute to regression in the Summit treated eyes with 18 microns of epithelial hyperplasia contributing each dioptre of regression.

Corneal Asphericity and Its Implications for Photorefractive Keratectomy: a Mathematical Model

BACKGROUND

Several clinical trials investigating myopic excimer laser photorefractive keratectomy (PRK) report an initial change in refraction from myopia to hyperopia, followed by a gradual regression toward emmetropia and occasionally to recurrent myopia. We examined the effect of corneal shape on refraction following PRK for myopia using a mathematical model.

METHODS

We calculated the volume of corneal tissue removed by PRK for -3.00-diopter (D) and -6.00-D corrections with ablation diameters of 5 mm and 6 mm. For all the operating algorithms, the central region of the cornea was considered spherical. Mathematical models were developed based on calculations of the apical radius of the ablated cornea and the final refraction for a range of corneal asphericities. Baker's equation was used to model corneal asphericity.

RESULTS

The smaller both the ablation size and desired correction, the smaller the effect of corneal asphericity on the refractive outcome. While corneal asphericity can influence the immediate refraction after PRK, the maximum effect is unlikely to be greater than +0.75 D.

CONCLUSIONS

Corneal asphericity marginally affects the initial outcome of PRK. The effect will probably be offset by the healing response of the cornea.

Effect of ablation profile on wound healing and visual performance 1 year after excimer laser photorefractive keratectomy

BACKGROUND

Early photorefractive keratectomy ablations were of limited diameter and depth to maintain the integrity of the globe and to minimise postoperative haze. This study evaluated the effects of deeper, larger diameter wounds on refractive stability and corneal haze, and investigated the effects of ablation profile on wound healing and visual performance.

METHODS

One hundred patients undergoing -3.00D and -6.00D corrections were randomised to receive 5 mm, 6 mm, or multizone treatments. The multizone treatment was 6 mm in diameter, but only the depth of the 5 mm treatment. Outcome was measured by Snellen visual acuity, residual refractive error, objective techniques for haze and halos, pupil diameter, subjective night vision, and requirement for retreatment.

RESULTS

Overall, the results of 6 mm treatments were superior to those of 5 mm and multizone treatments: they had a smaller hyperopic shift (p < 0.01), a more predictable (p < 0.001) and stable refractive outcome, less haze (p < 0.05), smaller halos (p < 0.05), fewer subjective night vision problems, and fewer patients required retreatment.

CONCLUSIONS

Analysis of these data and a literature review of corneal wound healing demonstrated that the improved outcome associated with the 6 mm beam did not relate to the depth of ablation. The factor with greatest apparent influence on the development of haze and regression was the slope of the wound surface over the entire area of the ablation. Tapering the wound edge provided no additional benefit, and contributed to night vision problems. It is, therefore, recommended that small diameter or multizone treatments should not be used in low and moderate myopia.

Scheimpflug anterior segment photography assessment of wound healing after myopic excimer laser photorefractive keratectomy

PURPOSE

To describe a method for analyzing Scheimpflug anterior segment images for a new measure of the cornea's response to excimer laser photoablation.

SETTING

Mericos Eye Institute, Scripps Memorial Hospital, La Jolla, California.

METHODS

Digitized Scheimpflug anterior segment photographs of operated and unoperated eyes were obtained in 17 patients 1 to 15 months after photorefractive keratectomy (PRK). The images were analyzed to determine their dimensions. Each imaged opacity was compared with corneal haze observed by slitlamp biomicroscopy, intended ablation depth, postoperative corneal thickness, and refractive error change.

RESULTS

All postoperative corneas displayed a nonhomogeneous, meniscus-shaped pattern in the ablated area that ranged from 17 to 40% of corneal thickness. This pattern correlated poorly with intended laser ablation depth.

CONCLUSION

This technique provides a new assessment of corneal response to PRK. Improvements in software analysis may facilitate quantitative assessment.

Double Zone PRK for High Myopia

INTRODUCTION

Although excimer laser PRK is our operation of choice for treating myopia, the correction of myopia exceeding -8.00 dpt is still a challenge as this procedure requires a deeper keratectomy. Therefore a double zone treatment may be beneficial to reduce ablation depth.

METHODS

Twenty-one eyes with myopia between -8.00 dpt and -14.00 dpt were included in this study. Twelve eyes were treated 60% of myopia by 5 mm zone and the remaining 40% in a 4 mm zone in the same session (Group 1).

RESULTS

Nine eyes achieved 60% of the desired correction treated with a 4 mm zone whereas 40% correction was achieved in a 5 mm zone (Group 2). No significant difference in postoperative healing, pain, hyperopic shift, myopic regression and haze was observed at the end of the mean follow-up time (10.4 months) but photophobic complaints were less in Group 2. Best corrected visual acuity did not decrease significantly.

CONCLUSIONS

Double zone treatment seems to be an interesting method for treating moderate to high myopia.

Corneal epithelial permeability after excimer laser photorefractive keratectomy

PURPOSE

To evaluate the effect of excimer laser photorefractive keratectomoy (PRK) on the corneal epithelial barrier function.

METHOD

Corneal uptake of 5,6 carboxyfluorescein (CF) was measured by Bernal and Ubels' method after excimer laser PRK in New Zealand white rabbits (N = 40). One cornea in each rabbit was treated, and the fellow cornea was used as a control. In both eyes, the central 7.0 mm of the corneal epithelium was removed. Myopic PRK treatments were performed at 37.75 microns (3.3 diopters [D]) and at 52.50 microns (5.0 D). The animals were euthanized and the eyes placed in CF for 5 minutes at 3 days, and at 1, 2, and 4 weeks following PRK. The corneas were then excised and dialyzed in balanced salt solution. The CF concentration in the dialysate was measured by fluorometry. Four corneas were also prepared for transmission electron microscopy using fixative containing ruthenium red.

RESULTS

Three days after PRK, CF uptake increased in all study eyes compared with normal eyes (n = 5). One week after PRK, the control corneas showed a decreased CF uptake while the study corneas still had an increased CF uptake (P < .05). Two weeks after PRK, CF uptake in corneas with a 5.0 D ablation remained increased but decreased in corneas with a 3.3 D ablation (P < .05). Four weeks after PRK, CF uptake returned to normal in all corneas. The ruthenium red penetrated into the deeper layers of the corneal epithelium 1 week after PRK; only the superficial cell layer was stained 4 weeks after PRK.

CONCLUSIONS

Excimer laser PRK affected the corneal epithelial barrier function at 1 and 2 weeks postoperatively even though the corneal epithelium covered the ablated area. Deeper laser ablations showed higher corneal CF uptake for longer periods than shallower ablations.

Clinical results of excimer laser photorefractive keratectomy: a multicenter study of 265 eyes

Efficacy, predictability, and safety of excimer laser photorefractive keratectomy were evaluated at centers in Paris and Brest, France. Photoablation was performed with the VISX laser on 265 eyes (151 at the Paris center and 114 at the Brest center). The eyes were clinically and statistically evaluated over a six month follow-up. Initial myopia ranged from -0.7 to -19.4 diopters (D) (mean spherical equivalent [SE] -5.9 D) in the Paris center and from -0.9 to -14.5 D (SE -4.5 D) in the Brest center. At both centers, the mean uncorrected visual acuity was worse than 20/200; over 90% of cases in each center had a best uncorrected visual acuity of 20/100 or worse. Results are reported globally and for subgroups of myopia: Group A, SE better than or equal to -3.0 D; Group B, SE worse than -3.0 D and better than or equal to -7.0 D; Group C, SE worse than -7.0 D. Uncorrected visual acuity was significantly improved in the patients followed for six months; 64% of Paris cases and 62% of Brest cases obtained an uncorrected visual acuity of 20/40 or better. Predictability of the treatment was good; 67% of Paris eyes and 74% of Brest eyes were less than 1.0 D from the intended correction after six months. The data suggest that the initial myopia affected the efficacy and predictability of the treatment; results in the mild to moderate myopia eyes were significantly better than results in the severe myopia eyes. One case of visual acuity regression (less than one line) was observed in the two groups. This was associated with corneal haze of moderate intensity.

Effect of Corticosteroids on Rabbit Corneal Keratocytes After Photorefractive Keratectomy

BACKGROUND

To determine the corticosteroid effect on the activity and repopulation of keratocytes after photorefractive keratectomy (PRK).

METHODS

A 193-nm excimer laser (VISX Twenty/Twenty) created a central ablation depth of 22 microns (diameter:5 nm) on 22 corneas of 16 albino rabbits. Two ablated eyes were examined 6 hours following PRK. Twelve eyes received no postoperative corticosteroids and eight were treated with topical fluoromethalone for 3 months. Corneas were examined 1, 3, 6, and 12 months after PRK by immunofluorescence and transmission electron microscopy.

RESULTS

Corticosteroids reduced haze (p=0.02), but all corneas (treated or untreated) cleared 6 months after PRK. Keratocytes were absent from the anterior 100 microns of the stroma 6 hours after PRK. However, the number and activity of keratocytes were significantly greater in this area in untreated corneas at 1 month and then gradually decreased. By 6 and 12 months, the number of keratocytes approached controls. Treated corneas had fewer keratocytes than either controls or untreated eyes (p<0.01) and by 3 months, a subepithelial acellular zone of 30 to 50 microns thickness appeared and persisted until at last 12 months after PRK.

CONCLUSIONS

Corticosteroids have a transient effect in reducing haze and seem to inhibit keratocyte movement, leading to an acellular subepithelial region beneath the ablated area.

Excimer laser photorefractive keratectomy

Excimer laser photorefractive keratectomy has been used for the correction of myopia, hyperopia and astigmatism. This laser removes tissue through a process termed photoablative decomposition, in which incident photon energy is sufficient to break molecular bonds. Selective removal of tissue across the anterior corneal surface results in a change in anterior corneal curvature. The surgical outcome may be influenced also by interindividual variability in wound healing and pharmacologic interventions. The nature of the excimer laser-tissue interaction, and clinical outcomes of predictability, stability and complications of surgery for myopia are discussed in detail.