Refractive outcome and corneal topographic studies after photorefractive keratectomy with different-sized ablation zones

Unexpected ocular morphological changes after corneal refractive surgery: A review

Corneal refractive surgery (CRS) currently is widely used to correct refractive errors because of its efficacy and reliability. Several studies dealt with the corneal modification induced by this type of surgery, but it is still debated if CRS can induce unexpected changes namely anterior chamber depth (ACD) and axial length (AL). A literature review was performed, including all articles regarding CRS and eye-variations from 1999 to December 2021. Excluding articles about specific systemic conditions (e.g., pregnancy), pathological conditions, post-surgical complications or about only corneal flattening and thinning post CRS, we found nine studies that met the search criteria. We divided the found articles according to the type of surgery performed (radial keratotomy, PRK/LASEK, lasik) and analyzed the results about ACD and AL. Finally, according to the literature, we can conclude that CRS not only gives a corneal flattening, thinning and biomechanical changes, but also induces AL and ACD decrease. This makes the AL and ACD measurements obtained before CRS uselessness in case of IOL power calculation.

Evaluation of corneal structures in myopic eyes more than twenty-two years after photorefractive keratectomy

The aim of this study is to evaluate corneal epithelial thickness (CET), corneal densitometry (CD) in 84 myopic eyes (57 patients) more than 22 years after photorefractive keratectomy, using anterior segment-optical coherence tomography (AS-OCT) and Scheimpflug imaging system. The CET was significantly higher in all operated eyes than in unoperated eyes in central sector. A statistically significant increase in CD in corneal anterior layer of central sector was shown in groups of operated eyes with greater ablation depth respect to unoperated eyes. While there was no significant difference in CD between the operated eyes groups with lower ablation depth and unoperated eyes. A significant trend toward higher values in anterior CD with deeper ablations in central sector was found. These noninvasive imaging techniques allow to better understand the corneal remodeling process after photoablation and to monitor the patients over time.

Twenty-Year Follow-Up of Excimer Laser Photorefractive Keratectomy: A Retrospective Observational Study

Introduction

Photorefractive keratectomy (PRK) was introduced in the late 1980s to correct myopia. The purpose of this study was to assess its long-term efficacy and safety, analyzing patients with at least 20-year follow-up.

Methods

This retrospective observational study was carried out on 85 eyes of 54 patients (33 females) that underwent PRK between 1991 and 1998 (mean age 32.62 ± 9.74, range 18–55 years). Both preoperatively and postoperatively, patients underwent a complete ophthalmological evaluation, including uncorrected and corrected distance visual acuity, slit-lamp, intraocular pressure, dilated fundus, and corneal topographic examinations. The outcome assessment was made by comparing the preoperative refraction, as spherical equivalent, with the postoperative ones, taking into account the planned refractive correction. Safety and efficacy indices were also calculated. All the data were evaluated with a paired t test.

Results

The mean attempted correction as spherical equivalent was – 5.64 ± 3.01 D (range – 1.00 to – 15.00 D), while the mean achieved correction after 20 years was – 4.30 ± 3.13 D (range – 1.88 to – 14.25 D), with a significant statistical difference (p < 0.01).

The mean expected refractive outcome was – 0.27 ± 0.81 D (range – 4.00 to + 1.25 D). The mean difference between achieved and attempted treatment was 1.33 ± 1.92 D (range – 4.25 to + 6.25 D), with a significant difference (p < 0.01).

The safety index was 1.00 and the efficacy index was 0.63.

Conclusion

The results provided by this study highlight that the procedure could be considered safe, with no long-term sight-threatening complications such as late ectasia or haze.

Corneal Anterior Power Calculation for an IOL in Post-PRK Patients

PURPOSE

After corneal refractive surgery, there is an overestimation of the corneal power with the devices routinely used to measure it. Therefore, the objective of this study was to determine whether, in patients who underwent photorefractive keratectomy (PRK), it is possible to predict the earlier preoperative anterior corneal power from the postoperative (PO) posterior corneal power. A comparison is made using a formula published by Saiki for laser in situ keratomileusis patients and a new one calculated specifically from PRK patients.

METHODS

The Saiki formula was tested in 98 eyes of 98 patients (47 women) who underwent PRK for myopia or myopic astigmatism. Moreover, anterior and posterior mean keratometry (Km) values from a Scheimpflug camera were measured to obtain a specific regression formula.

RESULTS

The mean (±SD) preoperative Km was 43.50 (±1.39) diopters (D) (range, 39.25 to 47.05 D). The mean (±SD) Km value calculated with the Saiki formula using the 6 months PO posterior Km was 42.94 (±1.19) D (range, 40.34 to 45.98 D) with a statistically significant difference (p < 0.001). Six months after PRK in our patients, the posterior Km was correlated with the anterior preoperative one by the following regression formula: y = -4.9707x + 12.457 (R² = 0.7656), where x is PO posterior Km and y is preoperative anterior Km, similar to the one calculated by Saiki.

CONCLUSIONS

Care should be taken in using the Saiki formula to calculate the preoperative Km in patients who underwent PRK.

IOL power calculation after corneal refractive surgery

PURPOSE

To describe the different formulas that try to overcome the problem of calculating the intraocular lens (IOL) power in patients that underwent corneal refractive surgery (CRS).

METHODS

A Pubmed literature search review of all published articles, on keyword associated with IOL power calculation and corneal refractive surgery, as well as the reference lists of retrieved articles, was performed.

RESULTS

A total of 33 peer reviewed articles dealing with methods that try to overcome the problem of calculating the IOL power in patients that underwent CRS were found. According to the information needed to try to overcome this problem, the methods were divided in two main categories: 18 methods were based on the knowledge of the patient clinical history and 15 methods that do not require such knowledge. The first group was further divided into five subgroups based on the parameters needed to make such calculation.

CONCLUSION

In the light of our findings, to avoid postoperative nasty surprises, we suggest using only those methods that have shown good results in a large number of patients, possibly by averaging the results obtained with these methods.

Computerized Scheimpflug densitometry as a measure of corneal optical density after excimer laser refractive surgery in myopic eyes

PURPOSE

To evaluate changes in anterior corneal optical density and the refractive index after photorefractive keratectomy (PRK) using a rotating Scheimpflug system.

SETTING

Department of Ophthalmology, University Federico II, Naples, Italy.

DESIGN

Comparative case series.

METHODS

Anterior corneal optical density was evaluated with a rotating Scheimpflug system at baseline and 3 months and 12 months after PRK in eyes with a refractive error between -6.00 diopters (D) and -12.00 D (study group). A control group of unoperated eyes with the same refraction range was used to calculate corneal optical density and the Gladstone-Dale constant in unoperated eyes using the Gladstone-Dale formula. In the study group, changes in the anterior corneal optical density were evaluated over time and variations in the anterior corneal refractive index were obtained using the Gladstone-Dale constant.

RESULTS

The study group comprised 37 eyes and the control group, 200 eyes. In the study group, the mean anterior corneal optical density and refractive index, respectively, were 27.71 ± 4.39 and 1.360 ± 0.05 at baseline, 37.812 ± 12.31 and 1.491 ± 0.16 after 3 months (P<.001 compared with baseline), and 26.29 ± 4.93 and 1.341 ± 0.06 after 12 months (P=.03 compared with baseline). The mean corneal optical density in the control group was 27.71 ± 4.31 (SD), and the resultant Gladstone-Dale constant was 0.013.

CONCLUSION

An early increase and a subsequent reduction in anterior corneal optical density and the refractive index were present in myopic eyes during 1 year after PRK.

New factor to improve reliability of the clinical history method for intraocular lens power calculation after refractive surgery

PURPOSE

To determine whether the refractive error in an eye developing cataract after refractive surgery represents actual regression or is cataract related and whether the method to gather this information would allow the use of history-related formulas in intraocular lens (IOL) power calculation after refractive surgery.

SETTING

Second University of Naples, Naples, Italy.

DESIGN

Case series.

METHODS

The refractive effects, axial length (AL), and mean keratotomy (K) values were evaluated in eyes before and 6 months after photorefractive keratectomy for myopia or for myopic or mixed astigmatism.

RESULTS

The study evaluated 257 eyes of 166 patients (93 women). Before surgery, there was a high correlation between refractive error and the product of AL and K (AL × K) (r(2) = 0.8213). In patients with refractive results close to emmetropia, the mean AL × K was 1005.91 ± 25.88 (SD), meaning that in the range of 954 and 1058, there was a 95% possibility that the patients were almost fully corrected. The following regression formula was obtained to calculate the amount of refractive error independent of cataract onset: Refractive error = -0.0157 × (AL × K) + 16.437.

CONCLUSIONS

The regression formula determined whether the refraction depended on the onset of cataract and estimated the amount of undercorrection or overcorrection that occurred after refractive surgery, leading to improved estimation of the power of the IOL to be implanted. It may allow the use of history-related formulas in IOL power calculation for eyes that have had corneal refractive surgery.

Corneal morphological changes after myopic excimer laser refractive surgery

PURPOSE

To evaluate the changes in central corneal thickness (CCT) and corneal volume (CV) in eyes that have undergone myopic photorefractive keratectomy (PRK).

METHODS

CCT and CV obtained with an Oculus Pentacam before 1, 3, and 6 months after PRK were analyzed in 84 eyes with a mean preoperative refraction of -4.93 ± 2.23 diopter. The changes were compared with the amount of refractive treatment. The differences were evaluated with the Student t test and the correlations with the Pearson index.

RESULTS

One month after PRK, CCT and CV mean differences were 73.2 ± 31.5 μm (P < 0.001) and 2.2 ± 1.7 mm (P < 0.001), respectively. Three months after PRK, CCT and CV mean differences were 66.6 ± 26.7 μm (P < 0.001) and 1.4 ± 1.3 mm (P < 0.001), respectively. Six months after PRK, CCT and CV mean differences were 65.3 ± 25.7 μm (P < 0.001) and 1.4 ± 1.3 mm (P < 0.001), respectively. The effective treatment at each follow-up point was correlated with CCT changes (R = 0.62, 0.71, and 0.73, respectively), but not with CV changes (R = 0.04, 0.04, and 0.01, respectively).

CONCLUSIONS

Our findings support the hypothesis that after myopic PRK, when a series of corneal lamellae are severed centrally, the remaining peripheral segments relax. The squeezing force on the matrix is reduced, and the distance between the lamellae expands.

Anterior Chamber Depth Measurement before and after Photorefractive Keratectomy

PURPOSE

To measure the anterior chamber depth (ACD) with 2 different devices before and after photorefractive keratectomy (PRK).

DESIGN

Noncomparative case series.

PARTICIPANTS

One hundred forty-three eyes of 143 patients who had undergone PRK with refractive errors ranging from -13.13 diopters (D) to +7 D (mean, -3.67+/-3.58) were analyzed.

METHODS

The ACD values preoperatively and at 1, 3, and 6 months postoperatively were measured with the Orbscan II and IOL Master. The results were analyzed using the Pearson correlation.

MAIN OUTCOME MEASURE

Anterior chamber depth.

RESULTS

The instruments showed good agreement between the measurements before and after surgery. A significant decrease between the preoperative and 1-month postoperative measurements was found in the ACD measured from the epithelium with Orbscan II (P<0.01) and IOL Master (P<0.01). A nonsignificant decrease with both IOL Master (P>0.01) and Orbscan II (P>0.01) was found between 3 and 6 months after surgery. The ACD measured from the endothelium using the Orbscan II showed a significant difference only between the 3- and 6-month follow-up data (P<0.01).

CONCLUSIONS

The 2 devices showed good agreement, and the changes detected postoperatively seem to be related not only to corneal thinning but also to anterior segment remodeling.

Reliability of a new correcting factor in calculating intraocular lens power after refractive corneal surgery

PURPOSE

To test the reliability of a corneal radius correcting factor (R factor) in calculating intraocular lens (IOL) power in eyes that developed cataract after refractive surgery and compare it with the clinical history (CHM) and double-K (DKM) methods.

SETTING

Department of Ophthalmology, Second University of Naples, Naples, Italy.

METHODS

Nineteen eyes from the literature that underwent cataract extraction and IOL implantation after refractive surgery were used to compare actual postoperative and expected refractive errors utilizing the R factor, CHM, and DKM. Intraocular lens powers were calculated with 3 formulas: SRK/T, Hoffer Q and Holladay 1. The differences were evaluated with the Wilcoxon test and Spearman correlation.

RESULTS

With the R factor SRK/T and Holladay 1 formulas gave the best results; 16 (84.2%) and 17 (89.5%) eyes were within +/-2 diopters (D) of emmetropia. With CHM, the best results were obtained using the SRK/T and Holladay 1 formulas; with both formulas 12 (63.2%) eyes were within +/-2 D of emmetropia. With DKM, the best results were obtained using SRK/T and Holladay 1 formulas; with both formulas 10 eyes (52.63%) were in the range of +/-2 D from emmetropia.

CONCLUSIONS

The R factor can be used with the SRK/T or Holladay 1 formula because this method seems comparable or superior to DKM and CHM.

Reliability of the IOLMaster in measuring corneal power changes after photorefractive keratectomy

PURPOSE

To test the accuracy of the IOLMaster (Carl Zeiss) in detecting corneal power changes after photorefractive keratectomy (PRK).

SETTING

Department of Ophthalmology, 2nd University of Naples, Naples, Italy.

METHODS

Two hundred twenty-five consecutive eyes that had PRK (mean -5.13 diopters [D] +/- 2.98 [SD] [range +0.25 to -16.25 D]) were analyzed. The data included preoperative and postoperative (1, 3, and 6 months) subjective refraction and computerized keratometry. Statistical analysis was performed to determine the correlation between the changes in the subjective refraction at the corneal plane and the changes in keratometry.

RESULTS

The mean difference between the changes in refraction and the measured corneal changes was 0.75 +/- 1.13 D (range -3.84 to +7.68 D) at 1 month, 0.92 +/- 1.10 D (range -0.87 to +7.93 D) at 3 months, and 0.75 +/- 0.98 D (range -1.70 to +3.85 D) at 6 months. The difference was significant (P<.001).

CONCLUSION

Automated keratometry provided by the IOLMaster did not accurately reflect the effective refractive changes after PRK, particularly in eyes that had a high dioptric treatment.

New Formula to Calculate Corneal Power After Refractive Surgery

PURPOSE

To assess the validity of intraocular lens (IOL) power calculations utilizing a theoretical variable refractive index correlated to axial length after myopic photorefractive keratectomy (PRK) in a clinical simulation and in patients who underwent cataract surgery after PRK for myopia.

METHODS

Our study included 374 eyes of 300 patients who had PRK for myopia (-2.00 to -12.00 D, mean -4.83 +/- 2.57 D), divided into three groups: Group I had 44 eyes with small ablation zones of 5 to 5.5 mm; Group II had 49 eyes with large ablation zones of 6 to 7 mm; Group III was the control group of 281 eyes (201 patients; 87 males and 114 females) with small and large ablation zones. PRK was performed using the Aesculap-Meditec MEL 60/94 and MEL 70 lasers, and the corneal power was acquired by corneal topography (EyeSys 2000) and a Nidek KM-800 keratometer.

RESULTS

There was a higher correlation between corneal power and both the change in refraction and axial length when calculated using keratometric measurements. IOL power calculated using keratometric postoperative PRK power was underestimated. The difference between the mean calculated and actual IOL power for emmetropia was 4.30 +/- 2.34 D. A theoretical variable refractive index (obtained from eyes treated with large PRK ablation zones) that correlated with axial length provided the correct keratometric postoperative PRK power: difference between mean calculated and mean actual IOL power was 0.42 +/- 1.23 D.

CONCLUSIONS

We propose a theoretical variable refractive index that is correlated to axial length. Utilizing this keratometric correct power, we calculated IOL power similar to that for emmetropia.

Correlation of Changes in Refraction and Corneal Topography After Photorefractive Keratectomy

PURPOSE

To establish which corneal power evaluation measured with corneal topography correlates best with refractive changes after photorefractive keratectomy (PRK) for myopia.

METHODS

Two hundred fifty-one consecutive eyes of 171 patients who had PRK for myopia ranging from -14.80 to -0.50 D (mean -5.43 +/- 2.978 D), calculated at the corneal plane, were included in the analysis. Data included preoperative and postoperative (1, 3, and 6-mo) subjective refraction and videokeratography with a Keratron Scout (Optikon 2000). Statistical analysis was performed to determine the correlation between the change in subjective refraction at the corneal plane and changes in six corneal power measurements: best fit sphere, simulated keratometry (Sim K), corneal apex, and center of the pupil (last two evaluated for axial and meridional curvatures).

RESULTS

The closest correlation between subjective refraction change and corneal power measurement during the three follow-up evaluations was found with Sim K (R2 = 0.904; 0.889; 0.854) and best fit sphere (R2 = 0.919; 0.909; 0.872), whereas the other measurements showed poor correlation with the different curvatures.

CONCLUSIONS

The best fit sphere corneal topography parameter correlated best with the refractive changes, primarily for low treatment amounts, whereas it showed a clear-cut underestimation in eyes that had undergone high dioptric treatments.

Technical Improvements in Photorefractive Keratectomy for Correction of High Myopia

PURPOSE

To evaluate the effects of hardware and software improvements in photorefractive keratectomy (PRK) for the treatment of highly myopic eyes.

METHODS

A retrospective study was carried out in 554 patients (582 eyes) with myopia between -7.00 and -17.00 D (mean -11.20 +/- 3.60 D) who had undergone PRK using the Aesculap Meditec laser. Group 1 with a 5-mm-diameter single ablation zone; Group 2 with a 5-mm-diameter single zone surrounded by a 2-mm tapered transitional zone; Group 3 and Group 4, same as Group 2 but with the laser upgraded with a smoke aspiration control system (Group 3), and with a computer-controlled fluence (Group 4).

RESULTS

In Group 1 at 2 years after PRK (50 eyes), 10 eyes (20%) were within +/-1.00 D of attempted correction and 13 eyes (26%) were within +/-2.00 D. In Group 2 at 2 years (118 eyes), 42 eyes (36%) were within +/-1.00 D of attempted correction and 65 eyes (55%) were within +/-2.00 D. In Group 3 at 2 years (43 eyes), 18 eyes (42%) were within +/-1.00 D of attempted correction and 28 eyes (65%) were within +/-2.00 D. In Group 4 at 2 years (47 eyes), 25 eyes (53%) were within +/-1.00 D of attempted correction and 29 eyes (62%) were within +/-2.00 D.

CONCLUSIONS

Software and hardware improvements facilitated PRK correction of high myopia with reasonable predictability, especially if a tapered transition zone was used. At 12 and 24-month follow-up, only the use of a tapered transition zone was associated with a statistically significant improvement in predictability.

A New Method of Calculating Intraocular Lens Power After Photorefractive Keratectomy

PURPOSE

To find a method of calculating intraocular lens (IOL) power that may be independent of preoperative data, in eyes that have developed a cataract after refractive surgery.

METHODS

Prior to and 1 month after PRK, the SRK/T formula was used to calculate IOL power in 88 eyes of 65 patients with a preoperative spherical equivalent refraction between -16.25 to +0.25 D (mean -5.39 +/- 3.19 D). IOL power was calculated by utilizing the spherical equivalent refraction as target both before and after PRK. Utilizing the postoperative corneal radius measurement (R2), an underestimation of the IOL power was found. For this reason, the mean postoperative corneal radius (R3) that gave the same IOL power found before surgery was calculated for each patient. The R3/R2 ratios were plotted against the axial eye length and a linear regression formula was used to calculate R2 correcting factors that gave the new corneal radius (R4). Patients were divided into classes according to axial eye length, and the mean R3/R2 ratios for each class were calculated and used to recalculate the new mean radius (R5). IOL power for emmetropia was calculated in all patients by utilization of R3, R4, R5, the historical method, and the "true corneal power" method.

RESULTS

Within +/-0.50 D from the IOL power calculated with R3, R4 gave 35 (39.3%) IOLs, while R5 gave 40 (45.5%) IOLs; the clinical history method gave 24 (27.3%) IOLs and "true corneal power" gave 23 (26.1%) IOLs, with a statistically significant difference P<.001).

CONCLUSIONS

Our theoretical method, based on correlation between axial eye length and corneal radius correcting factors, may represent an effective method of calculating IOL power after PRK, especially if the history of the patient is unknown.

Anterior Corneal Surface After Nidek EC-5000 Multipass and Multizone Photorefractive Keratectomy for Myopia

PURPOSE

To evaluate the correlation between the presence of irregularities in corneal ablation and the number of ablation zones with multipass and multizone photorefractive keratectomy (PRK).

METHODS

The differential maps obtained from corneal topography performed before and 1 month after PRK in 62 eyes that had undergone PRK with the Nidek EC-5000 excimer laser were assessed for irregularities. PRK treatment ranged from -1.00 to -16.00 D (mean -5.25 +/- 2.72 D), and the number of zones ranged from one to five.

RESULTS

Of 62 differential maps, 27 had an irregular pattern according to the Hersh classification, with a non-significant correlation with the number of treatment zones (chi2 = 5.09, P >.1).

CONCLUSION

Our results suggest that corneal topography irregularities arising from multizone PRK were not related to the amount of treatment or to the number of ablation zones.

Correlation Between Refractive and Corneal Topographic Changes After Photorefractive Keratectomy for Myopia

PURPOSE

To compare videokeratographic and refractive data obtained before and after photorefractive keratectomy (PRK) for myopia.

METHODS

Seventy-four eyes underwent PRK for myopia ranging from -2.50 to -17.00 D (mean, -7.76 +/- 3.17 D). All patients had videokeratography with the EyeSys instrument before, and 1 and 6 months after PRK, and the changes in three corneal power measurements (center of the ablation, apex, and effective refractive power) were compared with refractive changes.

RESULTS

Changes obtained in the three corneal power measurements at 1 and 6 months were well correlated with manifest refraction (Pearson's coefficient ranged from 0.71 to 0.84).

CONCLUSION

Power measurements obtained with corneal topography, as described above, are a reliable and objective method for the evaluation and follow-up of PRK, provided addition of an approximate 25% correcting factor.

Long-term results of multizone photorefractive keratectomy for myopia of -6.0 to -10.0 diopters

PURPOSE

To evaluate the 4 year refractive outcome of multizone photorefractive keratectomy (PRK) in eyes with high myopia.

SETTING

++SSK Okmeydani Education Hospital, Eye Clinic, Türkiye Hospital, Okmeydani, Istanbul.

METHODS

Three ablation zones were used in 92 eyes of 48 patients whose refractive errors were between -6.0 and -10.0 diopters (D) (mean spherical equivalent -7.42 D +/- 1.25 [SD]). The zones were between 4.5 and 6.0 mm based on the thickness of the cornea and the refractive correction. After the epithelium healed, dexamethasone was applied 4 times a day during the first postoperative week and then fluorometholone was applied 4 times a day for a minimum of 4 weeks. If hyperopia was found post-PRK, the steroid dose was gradually tapered. The patients were examined 1 and 3 days postoperatively, 1, 2, and 4 weeks, every 3 months for the first year, and then every 6 months.

RESULTS

All patients were overcorrected in the first postoperative week. At 2 and 3 weeks, the mean manifest refraction was closer to emmetropia. At 6 months, the refraction was stable. The mean spherical equivalent was -0.10 D at the end of the first year, and stabilization continued for 4 years. After the third month, the haze regressed gradually without requiring treatment. In 1 patient, herpes simplex keratitis developed and healed in a short time with topical antiviral therapy. Nineteen eyes regressed more than -1.0 D, 4 eyes were overcorrected, 4 eyes had central islands (at 6 months), and 2 eyes were undercorrected. Two eyes were retreated for regression; 1 eye was retreated for undercorrection and 1 eye, for central island. An uncorrected visual acuity of 20/40 or better was achieved in 79.2% of eyes, and 73.9% were within +/-1. 0 D of the intended correction.

CONCLUSION

++Photorefractive keratectomy was effective in treating high myopia between -6.0 and -10.0 D. The induced refractive changes stabilized between 6 and 9 months. In most patients, no significant regression was found after this period.

Surgical results of photorefractive keratectomy with different operative modes

PURPOSE

To compare the predictability, efficacy, and safety of photorefractive keratectomy (PRK) using different operative modes.

SETTING

National Taiwan University Hospital, Taipei, Taiwan.

METHODS

One hundred fifty-three eyes of 80 patients who had PRK for myopia with a follow-up of at least 6 months were studied. All patients were sequentially assigned to 1 of the following surgical modes: mode 1: PRK with the Summit OmniMed excimer laser; mode 2: PRK with the Summit Apex Plus laser; mode 3: PRK with the Summit Apex Plus laser with anti-central-island pretreatment.

RESULTS

Six months after treatment, a homogeneous topographic pattern was seen in 76% of mode 1 eyes, 70% of mode 2 eyes, and 88% of mode 3 eyes. In the low myopia group (< or =-6.0 diopters [D]), the mean residual refractive error was -0.79 D +/- 0.59 (SD) in mode 1, -0.94 +/- 1.02 D in mode 2, and -0.31 +/- 0.42 D in mode 3. In the high myopia group (>-6.0 D), it was -1.93 +/- 1.51 D, -1.54 +/- 0.88 D, and -0.70 +/- 0.81 D, respectively. Uncorrected visual acuity of 20/25 or better was achieved in 81% of mode 1 eyes, 56% of mode 2 eyes, and 89% of mode 3 eyes in the low myopia group, and in 48%, 28%, and 72%, respectively, in the high myopia group.

CONCLUSIONS

Photorefractive keratectomy appears to be a predictable and effective procedure. The best results were achieved with the Summit Apex Plus laser with anti-central-island pretreatment, followed by the Summit OmniMed laser. The Summit Apex Plus laser without anti-central-island pretreatment produced less satisfactory results.

Corneal topography of small-beam tracking excimer laser photorefractive keratectomy

PURPOSE

To evaluate the topographic characteristic of photorefractive keratectomy (PRK) for low myopia performed with a small-beam (0.9 mm) tracking excimer laser.

SETTING

Department of Ophthalmology, LSU Eye Center, Louisiana State University Medical Center School of Medicine in New Orleans, and the Refractive Surgery Center of the South at the Eye, Ear, Nose, & Throat Hospital, New Orleans, Louisiana, USA.

METHODS

Sixty-seven eyes of 47 patients had PRK with a small-beam tracking laser. Of these, 49 eyes had data permitting evaluation of ablation centration; usable data for topographic analysis were available for 59 eyes preoperatively, 54 eyes at 1 month, 42 eyes at 3 months, and 25 eyes at 6 months, permitting measurement of various topographic parameters, including the cylinder (CYL), average corneal power (ACP), surface regularity index (SRI), surface asymmetry index (SAI), corneal eccentricity index (CEI), and coefficient of variation of corneal power (CVP).

RESULTS

Preoperatively, all eyes were topographically normal. Postoperatively, no eye exhibited a "central island" by even the least-restrictive definition, and all eyes had best spectacle-corrected visual acuities (BSCVAs) of 20/20 or better at all follow-ups. Mean decentration of the ablations from the pupil centers was 0.42 mm +/- 0.28 (SD) (n = 49). There was no correlation between measured decentration and BSCVA (P = .46). The central cornea was flattened (decreased ACP; P < .001) and made oblate (decreased CEI; P < .001) as expected. There was no increase in SRI or SAI (irregular astigmatism) at 6 months compared with preoperative values (P = .91); however, CYL and CVP (varifocality) increased slightly (P = .04 and .02, respectively).

CONCLUSION

The absence of significant regular or irregular astigmatism 6 months after PRK with the small-beam laser is an improvement over published results achieved with wide-beam lasers and is consistent with the excellent visual acuity results in this cohort.

Photorefractive Keratectomy for -1.25 to -25.00 Diopters of Myopia

BACKGROUND

We evaluated prospectively the efficacy, predictability, stability, and safety of photorefractive keratectomy (PRK) for myopia.

METHODS

Three hundred sixty-nine eyes of 257 patients were treated with an Aesculap-Meditec MEL 60 excimer laser. Treated eyes were divided into 3 groups: low myopes (-1.25 to -6.00 D), 226 eyes; medium myopes (-6.10 to -10.00 D), 104 eyes; high myopes (-10.10 to -25.00 D), 39 eyes. Follow-up at 12 months was available for 348 eyes (94%).

RESULTS

One year after surgery the number of eyes within +/- 1.00 D of emmetropia was 182 (86.7%) for low myopes, 43 (40.5%) for medium myopes, and 12 (30.8%) for high myopes. Values for +/- 0.50 D were low: 142 (67.6%), medium: 29 (29.3%), and high: 9 (23.1%). Three eyes with low myopia (1.4%) and 5 eyes with medium myopia (5.1%) lost 2 or more lines of spectacle-corrected visual acuity. None of the high myopes lost 2 or more lines. Uncorrected visual acuity of 20/20 or better was achieved in 82 eyes (39%) with low myopia; 20/40 or better was achieved in 183 eyes (87.1%). Five eyes (5.1%) of medium myopes achieved 20/20 or better; 52 eyes (52.5%) with medium myopia achieved 20/40 or better. Zero eyes with high myopia achieved 20/20 or better; 11 eyes (28.8%) achieved 20/40 or better.

CONCLUSION

Photorefractive keratectomy proved to be an effective method to correct myopia up to -6.00 D. For myopia greater than -6.00 D, good results were achieved in most eyes when myopia was less than -10.00 D, but efficacy and predictability decrease. To avoid systematic undercorrection, slight overcorrection must be attempted with the Aesculap-Meditec MEL 60 excimer laser for the treatment of myopia.

Regression and its mechanisms after laser in situ keratomileusis in moderate and high myopia

OBJECTIVE

The purpose of the study was to evaluate the degree and mechanism of regression after laser in situ keratomileusis (LASIK) on moderate to highly myopic eyes during the first postoperative year.

DESIGN

A prospective, single-center, clinical trial.

PARTICIPANTS

A total of 52 eyes of 38 patients were entered in the study; 47 eyes had complete data available at each postoperative visit.

INTERVENTION

The intervention was LASIK using the microkeratome to create an 8.5- to 9.0-mm diameter, 130- to 160-micron-thick flap. A spherical midstromal multizone ablation (inner zone, 4.5 mm; outer zone, 5.5-6.0) was then performed using the Summit OmniMed excimer laser (Summit Technology, Inc, Waltham, MA). The mean preoperative refraction was -14.02 diopters (D). Retreatment for undercorrection and regression was performed between postoperative months 3 and 6 on 13 eyes.

MAIN OUTCOME MEASURES

Manifest spherical equivalent, mean central corneal power, and central corneal thickness were the parameters measured.

RESULTS

At 3 months, follow-up data were available on 47 eyes. The mean refractive regression was -1.07 D (7.6%) from the first week to the third month. During the first postoperative year, the mean regression of manifest spherical equivalent (MSE), increase in corneal power, and increase in corneal thickness were symmetric in magnitude and time course for the 34 eyes that did not require retreatment (-0.96 D, +1.03 D, and 15 microns, respectively).

CONCLUSION

Early regression of refractive effect after LASIK appears to be a consequence of an increase in corneal thickness associated with central corneal steepening. No evidence of progressive corneal ectasia was observed during the first year of follow-up. Longer follow-up is required to confirm these trends.

Keratometric index, videokeratography, and refractive surgery

PURPOSE

To clarify the confusion resulting from the use of slightly different refractive indices in calculations related to optical modeling of the cornea for refractive surgery, corneal diagnostics, and cataract surgery.

SETTING

Scientific Institute H.S. Raffaele, Milan, Italy.

METHODS

The cornea is represented as a centered optical system composed by 1, 2, or 3 spherical interfaces, in progression of modeling accuracy. Optical analysis is performed with the usual formulas of paraxial geometrical optics as well as with ray tracing. Simple models are also provided for corneas having both incisional and photoablative refractive surgery. Values of geometrical parameters are taken from the Gullstrand eye model.

RESULTS

Using the keratometric index of refraction of 1.3375 is validated for estimating optical power differences on untreated corneas or after incisional keratotomy. It is not as accurate in assigning absolute values of dioptric power, where the value 1.3315 is more appropriate. For photorefractive keratectomy (PRK), however, the proper stromal index of refraction, 1.376, must be used for ablation calculations and dioptric change estimates.

CONCLUSION

Videokeratographic instruments should include three distinct values of refraction index (1.3375, 1.376, and 1.3315) for an accurate and complete characterization of dioptric power maps. In cataract surgery, corrections must be introduced in the calculation of intraocular lens power for patients who have previously had PRK.

Photorefractive keratectomy. A 6-year follow-up study

OBJECTIVE

This study aimed to assess the long-term stability and efficacy of excimer laser photorefractive keratectomy.

DESIGN

Patients who participated in the first United Kingdom photorefractive keratectomy clinical trial were asked to attend a 6-year follow-up assessment.

PARTICIPANTS

Eighty-three patients (68%) of the original cohort of 120 participants were observed for 6 years. A Summit Technology UV200 excimer laser with a 4-mm ablation zone had been used with patients allocated to one of six groups according to their preoperative refraction. Each group received one of the following spherical corrections: -2, -3, -4, -5, -6, or -7 diopters (D). Within each group, all patients received an identical treatment, and thus emmetropia was not the goal in all patients.

INTERVENTION

The induced refractive change, objective corneal haze, glare, and halo measurements, together with possible late-phase complications, were analyzed.

MAIN OUTCOME MEASURES

All groups achieved a refractive undercorrection, and the magnitude of the undercorrection was related to the size of the attempted correction. The induced refraction stabilized by 6 to 12 months and has been maintained up to the 6-year follow-up stage.

RESULTS

Ninety-one percent of patients who underwent a -2.00-D correction and 76% of patients who received a -3.00-D correction were within +/- 1 D of the intended refraction at 6 years. Fifty-seven percent of the -4.00-D group and 50% of those in the -5.00-D group were within +/- 1 D, and this was reduced further to 43% in the -6.00-D group and 19% in the -7.00-D group. Six patients (7%) had evidence of residual corneal haze, which was visually significant in two patients (3%). Ten patients (12%) had significant night halos due to the small 4-mm ablation zone that was used in this early treatment trial.

CONCLUSIONS

There was no further regression of the refraction after 1 year, and, more important, there was no sign of hyperopic shift or diurnal fluctuation in the patients' refraction. In addition, corneal haze appeared to reduce further with time, with no intraocular or retinal side effects being noted. Night halos remain a significant reported problem in a small number of patients who were treated with the 4-mm ablation zone.