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PURPOSE: To determine the maximal ablation that can be safely performed with laser in situ keratomileusis (LASIK) to maintain long-term corneal integrity. SETTING: TLC The Windsor Laser Center, Windsor, Canada. METHODS: The pretreatment protocols for the VISX Star, Summit Omnimed, and Chiron Technolas 116 excimer lasers generally apply 1 to 2 microns per diopter (D) at an optical zone of 3.0 mm or less to avoid the postoperative central islands that can occur with broad-beam excimer lasers. The ablation depth per diopter for the VISX Star, Summit Omnimed, Chiron Technolas 116, and Chiron Technolas 217 excimer lasers ranges from 10 to 24 microns per diopter depending on the size and number of ablation zones and the excimer laser used. RESULTS: Previous experience with lamellar surgery suggests that at least 250 microns of central posterior stromal tissue should be preserved to maintain long-term corneal integrity and avoid postoperative corneal ectasia. If a 160 microns flap is created for LASIK, the average 550 microns cornea will have 140 microns of corneal stroma available for ablation. Depending on the excimer laser and ablation nomogram used, the maximal LASIK correction for the average cornea ranges from 9.8 to 15.0 D. CONCLUSION: The preoperative corneal thickness and the depth of the excimer laser ablation must be evaluated before LASIK to ensure that adequate posterior corneal stroma is preserved.

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OBJECTIVE: To evaluate the efficacy, predictability, stability and safety of a conservative approach to photorefractive keratectomy (PRK) (treating only 60% to 70% of the residual myopia) for residual myopia following radial keratotomy (RK). DESIGN: Case series. SETTING: Laser eye surgery centre in Windsor, Ont. PATIENTS: Thirty-three eyes of 27 patients with an average age of 40.1 years who underwent PRK between January 1993 and July 1995, 12 months or more after RK. All were followed for at least 12 months after PRK. OUTCOME MEASURES: Efficacy and safety were assessed by changes in the uncorrected and best corrected visual acuity. Predictability was determined by the proximity of the final result to emmetropia. The stability of the refractive outcome was assessed over the follow-up period. RESULTS: At 12 months 12 eyes (36%) had 20/20 or better uncorrected visual acuity and 29 eyes (88%) had 20/40 or better uncorrected acuity. Twenty-seven eyes (82%) were within 0.50 D of emmetropia, and 30 eyes (91%) were within 1.00 D of emmetropia. There was a significant change in the mean postoperative spherical equivalent between 1 and 3 months (p < 0.001); however, there was no significant change after this time. Six eyes (18%) had a loss of 2 or more lines of best corrected visual acuity due to corneal haze; however, retreatment reduced this incidence to 9% at 12 months. CONCLUSIONS: Our results show that conservative PRK for residual myopia following RK is efficacious and predictable and produces stable results. However, the risk of postoperative haze reduces the safety of this procedure.

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We present the preliminary refractive and visual results in 46 sighted eyes that had undergone excimer photorefractive keratectomy. Although our sample size was small and the follow-up relatively limited (3-6 months), a few important conclusions were drawn. The visual and refractive results compare quite favorably to the preliminary results with radial keratotomy. Thirty-four of 46 patients (74%) were within 1 D of emmetropia; and 45 (98%) were within 2 D at the end of the follow-up period. Forty-four patients (96%) achieved at least 20/40 vision, with more than half achieving 20/20 or better. We encountered no significant vision threatening complications. Twenty of 46 patients (43%) complained of night vision disturbances, but symptoms improved in all patients by 6 months. The majority of patients developed trace epithelial haze; however, corneal haze did not produce reductions in visual acuity. Though the preliminary results of our study of the safety and efficacy of excimer photorefractive keratectomy are promising, further study of long-term vision stability and corneal clarity is clearly needed.

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The use of topical corticosteroids following excimer laser photorefractive keratectomy (PRK) is widespread, but the role of corticosteroids remains poorly defined. The purpose of this study was to evaluate the use of different corticosteroid postoperative regimens following 193-nanometer excimer PRK in identical twins. Preoperatively, cycloplegic refraction of the left eye of each twin measured -3.00 D sphere. The full correction was treated utilizing the identical technique within the same 30 minute period. The study was conducted in a double-blind fashion and the refraction, corneal clarity, uncorrected and best spectacle corrected visual acuity, and keratometry were assessed over the 6-month study period. Both twins were placed on an identical regimen, tapering their drops over a 3-month period. Corneal clarity was remarkably well preserved with essentially clear corneas observed in both twins. Similarly, both twins achieved 20/15 vision with a cycloplegic refraction of +0.75 D at 6 months. The only difference in the healing pattern was the slightly more rapid visual recovery in twin A. The code was revealed upon the cessation of corticosteroid regimen. Twin A had received fluoromethalone 0.1% and Twin B received dexamethasone 0.1%. Keratometry demonstrated identical flattening. There were no corticosteroid induced intraocular pressure rise or other complications noted. Identical twins presented a unique opportunity to better understand wound healing following laser keratectomy. The results of this limited study indicated that for at least lower degrees of myopia the need for full strength topical corticosteroids remains questionable.

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Excimer photorefractive keratectomy (PRK) was performed on 100 consecutive patients (147 eyes) for the correction of myopia. The follow up period ranged from 3 to 16 months (mean, 8). Preoperative myopia ranged from -1.75 to -8.75 D (mean -3.90 D). Patients were divided into three groups for analysis based upon their degree of preoperative myopia: mild (< or = 2.90 D), moderate (-3.00 to -5.90 D), and severe (> or = -6.00 D). Uncorrected visual acuity was 20/40 or better in 88.4% (130/147) overall; 97% of mild, 91% of moderate, and 68% of severely myopic patients. Preoperative astigmatism remained the primary cause of reduced uncorrected visual acuity. Postoperatively, 68% of patients overall had a spherical equivalent refraction within +/- 1.00 D. The majority of treated corneas had trace (53%) to mild (29%) subepithelial haze. One patient with severe myopia developed significant subepithelial opacification postoperatively with partial regression and loss of one line of best spectacle corrected visual acuity.

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