In radial keratotomy, the most widely used technique to reduce myopia, the biomechanics of the cornea are altered by deep peripheral incisions that induce indirect flattening of the center.1 Undercorrection and overcorrection are common; 4 years after surgery, approximately 20% of the operated eyes were undercorrected by 1.00 D or more in the PERK study.2 By repeated operations, such as deepening, adding, or lengthening incisions, additional reduction of myopia may be obtained. However, the nonlinear characteristics of corneal biomechanics often limit the amount of central flattening that can be achieved.3 Therefore, a significant portion of patients (4.5% to 23%)4'5 may have to live with a residual myopic refractive error.
Photorefractive keratectomy is an alternative technique to flatten the central cornea.6"8 McDonnell et al9 presented two eyes undercorrected by radial keratotomy that were treated with photorefractive keratectomy with a follow up of 3 months. Herein, we present five eyes of four patients undercorrected by radial keratotomy that underwent excimer laser photorefractive keratectomy with follow up 6 months to 1 year.
METHODS AND PATIENTS
Photorefractive keratectomy was performed by means of commercially-available medical excimer laser (ExciMed UV200, Summit Technology, Waltham, Mass) using a standard nontouch photorefractive keratectomy technique published previously.8 The patients were asked to fixate a target mounted inside the delivery system central to the beam. During the test ablations on the intact epithelium, we noted that in all cases the clear zone of the radial keratotomy procedure was decentered relative to the excimer laser ablation zone. We attempted to remove epithelium in one sheet with a blunt spatula. Most of the incisions were not fully healed and opened slightly during removal of the epithelium. Careful irrigation and drying was necessary to ensure that all epithelium had been removed. The excimer laser ablation zone of 5 mm in diameter was positioned concentric with the pupil, regardless of the location of the incisions. The postoperative medical treatment was identical to that of other photorefractive keratectomy patients:10 gentamicin ointment until the epithelium was closed, dexamethasone drops (0.1%) five times a day for 2 months, and decreasing doses during the next months.
Refraction* (Spherical Equivalent, D) of Eyes After Radial Keratotomy and Photorefractive Keratectomy
FIGURE 1 : typical silt-lamp photography 6 months after surgery. The haze is of grade 0.5 + . Note the brownish iron deposit In the center.
The standard pre- and postoperative examination of our photorefractive keratectomy patients included manifest refraction, uncorrected and best corrected visual acuity, manual and autokeratometry, autorefraction, low-contrast and glare visual acuity, photokeratography, applanation tonometry, and slitlamp microscope inspection (anterior and posterior segments). In addition, we performed corneal topography in all cases at all follow ups. All examinations were done between 4:00 PM and 6:00 PM. In one patient, we investigated the diurnal variation of refraction from 8:00 AM to 8:00 PM preoperatively and 1 year after photorefractive keratectomy.
Included in this study were five eyes from four patients (one female and three male). The age of the patients was 29, 31, 41, and 49 years. Three eyes had eight-incision radial keratotomy for correction of simple myopia. One eye had combined eight radial and two T-incisions for correction of myopic astigmatism. One eye had asymmetric six radial incisions for correction of myopic astigmatism. Two patients were anisometropic by 2.00 D (no. 4) and 1.00 D (no. 3). The time between radial keratotomy and photorefractive keratectomy ranged from 15 months to 27 months. The pre- and postoperative refractions are listed in the Table. Glare visual acuity was maximal (0.8 = 20/25) in three eyes and was reduced in two eyes (0.5 = 20/40). All patients were contact-lens intolerant. All patients reported diurnal fluctuations in vision.
Starting from a mild preoperative average myopia of - 2.25 D (range, - 1.25 to - 2.75) all eyes were still hyperopic 1 month after surgery ( + 1.87 D; range, + 1.00 to + 3.00 D). At 3 months, the patients were still on corticosteroid drops and the hyperopia had decreased to 0.92 D (range, +0.75 to + 1.25 D). This myopic shift continued during the following 3 steroid-free months. At 6 months, the refraction was close to piano ( + 0.12 D; range, -0.15 to +0.5 D). The three eyes with a follow up of 1 year show different refraction evolutions: in one eye, the myopic shift continued, while the other two eyes held a constant refraction (!able). The preoperative refractive astigmatism ranged from 0.75 to 1.25 D and was not changed by photorefractive keratectomy. In one eye, refraction fluctuation was examined preoperatively and 1 year after photorefractive keratectomy and showed a variation in refraction of - 1.00 D from morning to evening, identical to the preoperative fluctuation.
FIGURE 2: Corneal topography of eye no. 2. (A) Preoperatively. (B) One month after surgery. (C) Three months after surgery. (D) Nine months after surgery. The preoperative astigmatism of 1.25 D was unchanged during the follow up of 9 months. Comparing the central zone pre- and 9 months postoperatively the refractive difference is about 1 .30 D.
Best-spectacle corrected visual acuity ranged from 20/25 to 20/16 preoperatively and was unchanged 1 month after photorefractive keratectomy. We considered the visual acuity to be constant at two examinations if the difference in the number of recognized letters was 2 or less.
Preoperative uncorrected visual acuity ranged from 20/400 to 20/30. After 1 month, the range was 20/40 to 20/20. Uncorrected visual acuity increased to the range of 20/30 to 20/20 at 3 months and to 20/25 to 20/16 at 6 months. At 1 year, uncorrected visual acuity dropped in eye no. 1 to 20/30 and was constant at 20/25 in eye nos. 2 and 5. Visual acuity under glare conditions increased to preoperative values after 6 months in three of the five eyes. At the 1-year follow up available in three eyes, glare visual acuity was identical to preoperative values in two eyes; in one eye, we found a loss of one line.
In four of the five eyes, the epithelial defect was healed on the 3rd day; the fifth eye required 4 days for reepithelialization. At no time after surgery did haze exceed a grade of 1.5+ (scale from 0 to 4 + ). The haze followed the typical evolution: it was maximal after 3 months and gradually decreased during the following months. At 6 months, all corneas showed a subepithelial haze of 1 + or less (Fig 1). The three eyes with a complete 1-year follow up had cornea with trace haze.
Figure 2 shows a series of topographic maps from eye no. 2. The central zone of flattening is easily recognized and is decentered inferonasally by 0.9 mm relative to the corneal vertex. During the followup period, the flattening was observed to decrease gradually.
As with other photorefractive keratectomy patients, manual and autokeratometry underestimated the refractive effect achieved and was, therefore, of only limited clinical importance.8
None of the operated eyes showed an IOP increase of greater than 3 mm Hg above preoperative values.
Radial keratotomy and photorefractive keratectomy are alternative techniques for the correction of myopia. Radial keratotomy indirectly flattens the central cornea by weakening the periphery of the cornea, while photorefractive keratectomy removes tissue directly from the central region of the cornea. Therefore, it may be possible to combine both techniques to increase the refractive effect. The purpose of the present study was to investigate if the healing response after photorefractive keratectomy in eyes that had undergone radial keratotomy before was different from eyes following photorefractive keratectomy alone. We could not detect any differences concerning subepithelial haze and glare visual acuity. All corneas showed a temporary haze gradually decreasing between 3 months to 1 year. During the first 6 months after surgery, the evolution of refraction was typical for eyes treated with photorefractive keratectomy only. However, one eye (no. 1) out of three with a 1-year follow up showed a slight continued myopic regression.
This study confirms the results of McDonnell who reported good short-term refractive results in two eyes that had photorefractive keratectomy following radial keratotomy.9 Long-term stability still remains unknown.
Photorefractive keratectomy shows very good results in corrections of myopia not greater than 6.00 to 7.00 D.10 The results of this study might indicate that combining photorefractive keratectomy and radial keratotomy may allow expansion of this range to higher amounts of myopia. However, the side effects may also be additive. Diurnal fluctuation of refraction after photorefractive keratectomy is minimal11 but quite frequent after radial keratotomy.12 These fluctuations seem to be still present after both procedures. Also, long-term drifting refraction toward hyperopia seen after radial keratotomy13 would presumably be unaltered by the addition of a photorefractive keratectomy in the same eye. An intended combination of the two techniques may, therefore, not be an optimal way to correct high myopia. However, eyes significantly undercorrected by radial keratotomy may benefit from a subsequent photorefractive keratectomy, leading to a refraction close to emmetropia.
Alternatively to the mechanical removal of the epithelium, it could be photoablated to avoid causing the incisions to open up. However, this technique would lead to an appropriate starting surface for photorefractive keratectomy only, if the epithelium would be homogeneous in thickness all over the ablated area. This might not be correct because the central flattening of the cornea results sometimes in a thickened epithelium in the center.
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Refraction* (Spherical Equivalent, D) of Eyes After Radial Keratotomy and Photorefractive Keratectomy