Journal of Refractive Surgery

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Comparison of Laser and Manual Removal of Corneal Epithelium for Photorefractive Keratectomy

Howard V Gimbel, MD; Brian M DeBroff, MD; Robert A Beldavs, MD; John A van Westenbrugge, MD; Maria Ferensowicz, MA

Abstract

ABSTRACT

BACKGROUND: Photorefractive keratectomy relies on precise ablation of cornea stromal tissue to achieve a desired change in refraction. The routine technique for photorefractive keratectomy has been manual debridement of the epithelium prior to performing excimer laser ablation. We investigated whether laser ablation versus manual debridement of the corneal epithelium influences the refractive result.

METHODS: A retrospective matched controlled study analyzing the refractive outcome of 46 eyes after excimer laser photorefractive keratectomy was performed. Half of the eyes had the corneal epithelium ablated with the excimer laser, while the other half had mechanical removal. Topical postoperative corticosteroid dosing was different in the two groups. All photorefractive keratectomies were performed by the same surgeon (H.V.G.). The two groups were analyzed for statistical differences in refractive outcomes and corneal haze after 6 months.

RESULTS: The mean preoperative spherical equivalent refraction in the laser removal group was -5.11 diopters (D), and -5.09 D in the manual group. At 6 months postoperatively, the mean spherical equivalent refraction in the laser group was +0.03 D and -0.40 D for the manual group (p = .21). At no point postoperatively was there any significant difference in the mean refractive outcome or variance of the refractive results between the two groups, although there was a trend toward greater correction with laser ablation of epithelium. There was no statistical difference in the amount of stromal haze by slit-lamp microscopy in the two different debridement groups. There was no significant difference in final uncorrected visual acuity, rate of reepithelialization, or reported incidence of halos or glare between the two groups.

CONCLUSION: There was a tendency toward greater refractive correction at 6 months using the laser for corneal epithelial removal than manual debridement, although the difference was not statistically significant. The trend toward slightly higher correction emphasizes the need for care when removing epithelium with the laser to prevent concomitant stromal ablation. [J Refract Surg. 1995;11:36-41.]

Abstract

ABSTRACT

BACKGROUND: Photorefractive keratectomy relies on precise ablation of cornea stromal tissue to achieve a desired change in refraction. The routine technique for photorefractive keratectomy has been manual debridement of the epithelium prior to performing excimer laser ablation. We investigated whether laser ablation versus manual debridement of the corneal epithelium influences the refractive result.

METHODS: A retrospective matched controlled study analyzing the refractive outcome of 46 eyes after excimer laser photorefractive keratectomy was performed. Half of the eyes had the corneal epithelium ablated with the excimer laser, while the other half had mechanical removal. Topical postoperative corticosteroid dosing was different in the two groups. All photorefractive keratectomies were performed by the same surgeon (H.V.G.). The two groups were analyzed for statistical differences in refractive outcomes and corneal haze after 6 months.

RESULTS: The mean preoperative spherical equivalent refraction in the laser removal group was -5.11 diopters (D), and -5.09 D in the manual group. At 6 months postoperatively, the mean spherical equivalent refraction in the laser group was +0.03 D and -0.40 D for the manual group (p = .21). At no point postoperatively was there any significant difference in the mean refractive outcome or variance of the refractive results between the two groups, although there was a trend toward greater correction with laser ablation of epithelium. There was no statistical difference in the amount of stromal haze by slit-lamp microscopy in the two different debridement groups. There was no significant difference in final uncorrected visual acuity, rate of reepithelialization, or reported incidence of halos or glare between the two groups.

CONCLUSION: There was a tendency toward greater refractive correction at 6 months using the laser for corneal epithelial removal than manual debridement, although the difference was not statistically significant. The trend toward slightly higher correction emphasizes the need for care when removing epithelium with the laser to prevent concomitant stromal ablation. [J Refract Surg. 1995;11:36-41.]

Trokel et al first suggested the use of 193-nanometer UV radiation to remove a precise amount of tissue from the anterior cornea.1·2 A number of studies have demonstrated the capability of the excimer laser to ablate the human cornea and achieve refractive changes.1"11 Studies have shown that the excimer laser ablates tissue with precision and accuracy to the micrometer level, achieving smooth edges without thermal damage of the adjacent tissue.12-14 The excimer is frequently used only for corneal stromal ablation after the epithelium has been removed by a manual technique. Manual removal of corneal epithelium using a blade is currently the preferred technique for photorefractive keratectomy by many ophthalmologists. Studies investigating epithelial removal techniques and their effects on refractive and visual outcomes in photorefractive keratectomy are limited.15-16 To determine if removing corneal epithelium manually with a blade versus the excimer affects the achieved refractive results after photorefractive keratectomy, we retrospectively analyzed the refractive outcome in 46 matched controlled eyes.

MATERIALS AND METHODS

In a retrospective, matched-controlled study, 46 eyes that received photorefractive keratectomy were reviewed to compare refractive results with epithelial removal technique. In half of the eyes (23 eyes), the epithelium was removed manually; in the other half (23 eyes), the epithelium was ablated with the excimer laser. All photorefractive keratectomy procedures were performed using the Summit UV 200 (Summit, Waltham, Mass) excimer laser by one surgeon (H.V.G.) from June 1991 to November 1991.

Patient Selection

The two groups were matched for four variables: (1) preoperative spherical equivalent refraction, (2) gender of patient, (3) age of the patient at the time of surgery, and (4) size of the ablation zone. There was no statistical difference in the mean preoperative spherical equivalent refraction of the laser removal group (-5.11 D ± 1.73 D) as compared with the manual removal group ( - 5.09 D ± 1.73 D) (p = .96). Of the 23 eyes that had undergone manual removal, the ablation zone was 5 mm in 22 and 4.5 in 1. For the laser removal group, all 23 eyes had 5-millimeter ablation zones. The mean age of patients in each group was matched (34.0 years in the laser group and 34.3 years in the manual group). There was no statistically significant mean difference in attempted correction for the laser or manual group (3.95 D ± 1.41 D and 3.90 D ± 1.52 D, respectively, p = .82).

Surgical Procedure

The pupil was constricted preoperatively, and a topical anesthetic was instilled. The patient's eye was aligned under the laser at the point where the two helium neon centering beams coincided and patient fixation on the hght-emitting diode (LED) fixation light of the laser was rehearsed. The patient was moved to a Zeiss microscope with a Keratolux fixation light; the light reflex on the cornea was marked with a 30-gauge cannula and its position relative to the entrance pupil was noted.

The eye to be treated was centered under the Excimed UV 200 LA laser to confirm the mark and align it with the helium neon beams noting their relative positions on the iris. The cornea was then dried and the epithelium removed either manually using a #57 Beaver blade or using phototherapeutic keratectomy ablation. When using the excimer to remove epithelium, once 35 to 40 µm of phototherapeutic keratectomy ablation was achieved, the room lights were turned off completely to enhance visualization of the fluorescence associated with epithelial ablation. Studies have demonstrated that the ñuorescence spectrum associated with epithelial tissue is différent from the emission associated with stromal tissue.17 Epithelium removal was stopped when 90% of the blue fluorescence was gone and the ablation area was black. Depending on the laser being used and the eye's epithelium distribution pattern, the laser ablated through the epithelium in different patterns. Most typically, a dark ring appeared in the peripheral part of the ablation zone as a crescent, and then as a ring pattern, due to the presence of thicker epithelium centrally. Often, the ring enlarged quickly and progressed centrally within a few pulses. Rarely did dark zones of Bowman's ablation presented in a geographic pattern, indicating a fairly uniform thickness of epithelium over the entire surface of the ablation area. Ablation of the epithelium was stopped when the blue fluorescence had markedly diminished in intensity, thus preventing premature laser ablation of stroma. Once the epithelium had been removed, the patient was asked to fixate the eye being treated on an LED fixation light. The excimer laser pulses were delivered at an energy level of 180 mJ/cm2 and at a repetition rate of 10 Hz. Due to our initial experience of consistent overcorrection with our excimer lasers, we set the laser to correct approximately 75% of the targeted preoperative myopia.

Postoperative Treatment

Cyclopentolate 1.0% drops and tobramycin 0.3% ointment were instilled immediately after the laser treatment. The eye was pressure patched. Corticosteroid drops were used for 3 months postoperatively in 12 eyes that underwent manual debridement, and in six eyes that had laser debridement of epithelium. This was the one variable that was not matched between the two groups (p < .05). The eyes that received corticosteroid drops were administered dexamethasone 0.1% qid for 1 month then fluorometholone tid on a tapering dose over the next 2 to 3 months. By the 3-month postoperative examination, however, corticosteroids were discontinued in all eyes. Corneal haze was determined by a physician using slit-lamp microscopy at the 6-month postoperative visit based on comparison with Summit's standardized photographs of trace, mild, moderate, and marked corneal haze. "Clear" is defined as totally clear, while "trace* corneal haze is defined as faint corneal haze seen only by indirect broad tangential illumination. "Mild" corneal haze is considered discrete haze visible with difficulty by direct focal slit illumination. "Moderate" corneal haze is a moderately dense opacity that partially obscures iris detail. "Marked" corneal haze is a severely dense opacity that completely obscures details of intraocular structures.18 The presence of glare or halos were determined by patient history. All refractions performed preoperatively and postoperatively were manifest refractions and were performed by technicians unaware of the method of corneal epithelial removal.

Table

Table 1Attempted and Achieved Change in Refraction (D)* After Photorefractive Keratectomy in the Laser and Manual Epithelial Removal GroupsTable 2Spherical Equivalent Refraction (D) After Photorefractive Keratectomy in the Laser and Manual Epithelial Removal Groups

Table 1

Attempted and Achieved Change in Refraction (D)* After Photorefractive Keratectomy in the Laser and Manual Epithelial Removal Groups

Table 2

Spherical Equivalent Refraction (D) After Photorefractive Keratectomy in the Laser and Manual Epithelial Removal Groups

Table

Table 3Distribution of Spherical Equivalent Refraction (D) at 6 Months After Photorefractive Keratectomy in the Laser and Manual Epithelial Removal Groups

Table 3

Distribution of Spherical Equivalent Refraction (D) at 6 Months After Photorefractive Keratectomy in the Laser and Manual Epithelial Removal Groups

Statistics

The paired t-test for matched samples was used to determine significance of mean differences and the F-test to determine significance of variance differences between the two groups.

RESULTS

Table 1 summarizes the mean achieved refractive correction over time for both the laser and manual removal groups. At 1 month postoperatively, the laser group achieved 6.51 D of correction as compared with 6.14 D for the manual group. By 6 months postoperatively, the laser group achieved an overall correction of 5.14 D as compared to 4.69 D for the manual group. At all three postoperative times (1, 3, and 6 months) the laser group consistently achieved a numerically greater (though not statistically significant) diopter correction than the manual debridement group. The variance in data was also consistently greater in the laser group than in the manual group (Table 1). At no time, however, were the differences in mean or variance of refractive results statistically significant.

Table 2 summarizes the mean spherical equivalent refraction over time for the laser and manual removal groups. At 6 months postoperatively, the laser group achieved a spherical equivalent refraction of + 0.03 as compared to - 0.40 D in the manual group; the 0.43-diopter difference was not statistically significant (p = .21).

Table 3 displays the distribution of refraction at 6 months postoperatively in spherical equivalents for both the laser and manual epithelial removal groups.

Figures 1 and 2 show the distribution of median spherical equivalent refractions of the treated eyes over time for both the manual and laser groups. By 6 months postoperatively, the median spherical refraction was +0.12 D in the laser group compared to -0.25 D in the manual group. The laser group achieved consistently more hyperopic correction over time as compared to the manual group. In addition, the variance in data was greater at all postoperative periods in the laser group as compared to the manual group. The differences in mean spherical equivalent refraction or variance between the groups, however, was at no time statistically significant (Table 2), although there was a tendency to greater correction with the laser. This may have been because of difficulty in determining the endpoint of epithelial fluorescence with a partial stromal keratectomy having been performed. The more frequent use of corticosteroids in the manual group may partly account for a lack of statistical significance.

Figure 1 : Distribution of median spherical equivalent refractions of the manual epithelial removal group over time. Months indicate time after surgery.Figure 2: Distribution of median spherical equivalent refractions of the laser epithelial removal group over time. Months indicate time after surgery.

Figure 1 : Distribution of median spherical equivalent refractions of the manual epithelial removal group over time. Months indicate time after surgery.

Figure 2: Distribution of median spherical equivalent refractions of the laser epithelial removal group over time. Months indicate time after surgery.

The mean reepithelialization time in the laser debridement group was approximately 2.5 days, which was not statistically different from the 2.75 days mean for the manual group. In the laser group, 20 of 23 (87%) corneas were either clear or demonstrated only trace haze (7 of 23 were clear, 3 of 23 were trace) at 6 months postoperatively, as compared with 19 of 23 (82.6%) corneas (7 of 23 were clear, 12 of 23 were trace) in the manual debridement group. The incidence of mild corneal haze at 6 months was 4 of 23 (17.4%) eyes for the manual group as compared to 3 of 23 (13.0%) eyes for the laser group (p = .50). At 6 months follow up, no cornea in the study had moderate haze or greater. There was no statistical difference in the reported incidence of halos or glare at 6 months for the manual and laser groups, 6 (26.0%) and 4 (17.4%), of 23 eyes, respectively (p = .36).

The laser group had significantly fewer eyes receiving topical corticosteroids for the first 3 months postoperatively than did the manual group (6 eyes versus 12 eyes for the manual group, p = .05). There was, however, no statistical difference in refractive results between the two methods of epithelial debridement when comparing only eyes that had received corticosteroid drops (p = .86). Also, all eyes in the study had corticosteroids discontinued by the 3-month postoperative exam.

At 6 months postoperatively, all 46 eyes achieved an uncorrected visual acuity of 20/40 or better. Of the 23 eyes in the laser group, 16 (69.5%) achieved an uncorrected visual acuity of 20/20 or better as compared to 14 (60.87%) in the manual group. There was no statistically significant difference in uncorrected visual acuity between the groups. Also, there were no vision-threatening complications in either group and no case resulted in loss of two or more lines of spectacle-corrected visual acuity. Three eyes had loss of one line of spectacle-corrected visual acuity in the manual group as compared to two eyes in the laser group (p = .65).

DISCUSSION

This retrospective study did not reveal any statistically significant differences in mean refractive outcome or the variance of refractive outcomes between the laser and manual methods of corneal epithelial removal during photorefractive keratectomy at 6 months follow up. There was no statistical difference in final uncorrected visual acuity, rate of reepithelialization, haze, or reported incidence of halos or glare. No eyes demonstrated a loss of more than one line of spectacle-corrected visual acuity.

Although not statistically significant, the laser group achieved a slightly greater correction as compared with the manual group at all postoperative measurements. The variance of data was also consistently greater in the laser group as compared with the manual group. Although neither was statistically significant, the increased amount of correction and the increased variance may be attributed to the technique of laser debridement. In photoablation of the epithelium, the end point of epithelial ablation is assessed subjectively, gauged by the disappearance of the fluorescence of epithelium. Surgeon judgment is required to determine when to stop epithelial ablation. Because epithelial thickness is variable,17 some areas of epithelium are completely ablated before other areas. This results in a geographic pattern of pseudofluorescence rather than a flat, uniform pattern when stromal ablation begins. This pattern of diniinishing fluorescence will often vary from cornea to cornea due to anatomical difference in the epithelium, patient microsaccades, a varying beam energy profile, and the effect of ablation fragments. This potentially results in exposure of stroma to excimer ablation earlier in one area than another. Thus, one disadvantage of laser removal of epithelium is that the depth of the final stromal ablation may be less predictable.19"20 If during laser ablation, the surgeon notes the ring of expanding hypofluorescence to expand quite slowly, this may indicate the presence of a considerable difference in epithelial thickness from center to periphery. In such a case, it may be advisable to discontinue the laser removal and convert to a manual removal of the remaining epithelium centrally. This would prevent uneven stromal ablation which would occur if one is ablating stroma and epithelium simultaneously, as a different rate of ablation exists for epithelium versus stroma. The poorly defined endpoint of epithelial ablation may account for greater variance in that group. For consistency and comparison of results, we now recommend that laser removal of epithelium be achieved by standardizing the ablation depth to 50 µm.

Another variable is microsaccades of the eye during laser ablation of the epithelium. This makes it difficult to achieve a straight-edge incision through the epithelium. The peripheral margin of ablated epithelium becomes sloped, creating a shielding effect during ablation which may result in a slight reduction of the size of the ablation diameter achieved and a loss of the transition zone. This variable could be eliminated if a larger zone could be used for epithelial ablation rather than for the stromal ablation. These factors may have caused the greater variability of results that were observed in the laser group.

The manual removal technique allows for uniformity of removal of epithelium; it is easy to ascertain that epithelium has been completely removed. This technique, however, is not free from variability and potential pitfalls that may influence refractive outcome. Variability in time of completion of manual removal of the epithelium can affect the amount of stromal dehydration and thus, the rate of stromal ablation. Also, manual debridement of very adherent epithelium can result in ridges or irregularities in Bowman's layer from the blade, which will be transmitted to the stroma during ablation and may increase subepithelial haze. We found, however, no statistical difference in amount of haze present between the two groups.

There are other disadvantages of the manual removal technique. The resulting epithelial defect may be substantially larger than the ablation zone of laser treatment because it is difficult to obtain clean borders with a blade without damaging the surrounding epithelium. In addition, manual removal of epithelium requires dexterity, surgical skill, learning, experience, and uniformity of technique. Also, some patients tend to be more apprehensive before and during photorefractive keratectomy knowing that a blade is used to scrape the surface of their eye, rather than the laser, yet others may become alarmed by the loud snapping noise associated with laser ablation of epithelium.

Laser ablation of epithelium has several advantages: it requires a shorter surgical time, it is technically less demanding, and it causes less damage to surrounding tissue. Most importantly, there is a standardization of debridement time and the dehydration effect to the cornea during surgery.

One confounding effect in this study may have been the variable use of postoperative corticosteroid drops. Other weaknesses of this study are its sample size, retrospective nature, and lack of randomization. A lack of statistical difference between eyes may be due to the small sample size. Despite its limitations, this study suggests that there is a trend toward overcorrection with laser ablation of epithelium, rather than manual removal. Prospective, randomized trials with larger sample sizes, and more clearly matched variables, are needed to confirm the finding that final refractive outcome may be dependent upon the epithelial removal technique.

REFERENCES

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3 . McDonald MB, Liu JC, Byrd TJ, et al. Central photorefractive keratectomy for myopia: partially sighted and normally sighted eyes. Ophthalmology. 1991;98:1327-1337.

4. Seiler T, Wollensak J. Myopic photorefractive keratectomy with the excimer laser. One year follow-up. Ophthalmology. 1991;98:1156-1163.

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10. Piebenga LW, Matta CS, Deitz MR, et al. Excimer photorefractive keratectomy for myopia. Ophthalmology. 1993;100:1335-1345.

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13. Marshall J, Trokel S, Rothery S, Krueger RR. Photoablative reprofiling of the cornea using an excimer laser photorefractive keratectomy. Lasers and Light in Ophthalmology. 1986;1:21-48.

14. Puliafito CA, Wong K, Steinert RF. Quantitative and ultrastructural studies of excimer ablation of the cornea at 193 and 248 nanometers. Lasers Surg Med. 1987;7:155-159.

15. Ishikawa T, del Cerroo M, Liang P, Kim J1 Aquavella J. Hypersensitivity following excimer laser ablation through the corneal epithelium. J Refract Corneal Surg. 1992;8: 466-473.

16. Tanelian D, Beuerman R. Responses of rabbit corneal nociceptors to mechanical and thermal stimulation. Exp Neurol. 1984;84:165-178.

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19. Casey TA, Mayer DJ. Corneal Grafting Principles and Practice, 1st ed. Philadelphia, Pa: WB Saunders Company; 1984:17.

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Table 1

Attempted and Achieved Change in Refraction (D)* After Photorefractive Keratectomy in the Laser and Manual Epithelial Removal Groups

Table 2

Spherical Equivalent Refraction (D) After Photorefractive Keratectomy in the Laser and Manual Epithelial Removal Groups

Table 3

Distribution of Spherical Equivalent Refraction (D) at 6 Months After Photorefractive Keratectomy in the Laser and Manual Epithelial Removal Groups

10.3928/1081-597X-19950101-10

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