Article Text
Abstract
Objective: To compare the long-term outcomes of photorefractive keratectomy (PRK) and laser in situ keratomilesis (LASIK) for myopia between −6 and −10 D.
Methods: A retrospective, control-matched study including 68 eyes, 34 which underwent PRK and 34 LASIK, with myopia between −6 and −10 D, operated using the VISX 20/20 excimer laser, was performed. Optical zones of 5.5 to 6 mm were used. All PRK-treated eyes were matched with LASIK-treated eyes of the same age, spherical equivalent within ±1.25 D, sphere within ±1.5 D and cylinder within ±2.5 D. All patients were evaluated 3 months, 1 year, 2 years, 5 years and 10 years after surgery. The main outcomes measures were refractive predictability and stability, safety, efficacy and re-treatment rate.
Results: At 10 years, 20 (71%) and 23 (88%) were within ±1.00 D after PRK and LASIK respectively. The re-treatment rate was 35% and 18% respectively. No eye lost more than two lines of BSCVA in both groups. The efficacy was 0.90 for PRK and 0.95 for LASIK.
Conclusions: Both PRK and LASIK were safe for moderate myopia. LASIK demonstrated slightly better efficacy, predictability, and less rate of re-treatment after 10 years. The technical improvements should be taken into account when comparing these results with those obtained more recently.
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Photorefractive keratectomy (PRK) to correct myopia was introduced in the late 1980s.12 Because of severe postoperative pain and relatively slow recovery after PRK, laser in situ keratomilesis (LASIK) was introduced in the early 1990s and became the most performed refractive surgery modality in the 2000s with claimed advantages over PRK such as quick visual rehabilitation, higher predictability, minimal postoperative discomfort and absence of corneal haze.345 Although, studies with short-term follow-up reported that the risks associated with LASIK were considered to be low, postoperative flap-related complications and corneal ectasia can be sight-threatening.67 Consequently, excimer laser superficial keratectomy techniques such as photorefractive keratectomy (PRK), laser subepithelial keratectomy (LASEK) and epithelial laser in situ keratomileusis (Epi-LASIK) have gained popularity in recent years to correct myopia to refrain from possible complications of LASIK such as corneal ectasia.489
Given that refractive surgery is mostly performed on young and healthy eyes of patients with high expectations, long-term safety and efficacy are the greatest concerns.410 Despite millions of procedures having been performed, there is a great lack of data about the long-term comparison of PRK and LASIK.11 Previous studies comparing PRK and LASIK outcomes, up to 1 year after surgery, found similar or slightly better safety and efficacy outcomes for LASIK.1213141516 The aim of the present study is to perform a comparative analysis of the evolution of the corneal curvature and the refractive stability 10 years after myopic PRK and LASIK for moderate myopia by means of a control-matched retrospective study.
Methods
Patient population
A total of 4800 charts of eyes that underwent PRK or LASIK between April 1992 to December 1995 at the Instituto Oftalmológico de Alicante (Spain) were reviewed. Our database was compiled including 509 eyes of 356 patients treated with myopic PRK and 294 eyes of 178 patients treated with myopic LASIK that returned for follow-up at 3 months, 1 year, 2 years, 5 years and 10 years after the initial procedure, either spontaneously or after telephone calls (particularly at 10 years). Among this group, 34 (17 right, 17 left) PRK-treated eyes of 33 patients and 34 (15 right, 19 left) LASIK-treated eyes of 32 patients who had a preoperative spherical equivalent between −6 and 10 D were matched using the following criteria: (1) same age, (2) preoperative spherical equivalent (SE) within ±1.25 D, (3) preoperative sphere (S) within ±1.50 D and (4) preoperative cylinder (C) within ±2.50 D. Patient demographics, refraction, mean optical zone and ablation depth at the time of treatment are given in table 1. The study was approved by the institutional review board (Ethical Committee of Clinical Investigation of Instituto Oftalmológico de Alicante) and followed the tenets of Helsinki Declaration.
Surgical procedure
Inclusion criteria for surgery were: no contact lens wear 4 weeks before the surgery and stable refractive error for at least 6 months before surgery, normal peripheral retina or treated with photocoagulation when necessary, no previous ocular surgery, and no corneal diseases, glaucoma or history of ocular trauma. Exclusion criteria for surgery were: evidence of keratoconus or keratoconus suspect as evidenced by corneal topography, active ocular or systemic disease likely to affect corneal wound healing, pregnancy and nursing.
The day before surgery, diclofenac sodium 0.1% drops (Voltaren, Novartis, Basel, Switzerland) and trimethoprim and polymyxin B eye-drops (Oftalmotrim, Cusi, Barcelona) were instilled. The procedure was done using topical anaesthesia of oxybuprocaine 0.4%.
All surgeries were performed by three surgeons using the same technique and same protocol.1718 A 193 nm VISX 20/20B excimer laser, software version 3.2, (VISX, Santa Clara, California) was used both for PRK and LASIK. During surgery, patients fixated on the laser’s helium–neon fixation light. Ablation was achieved using a beam with fluence of 160 mJ/cm2 at an ablation rate of 5 Hz. Optical zones of 5.5 to 6 mm were used. Astigmatism was corrected by sequential ablation with an area of 6.0×4.5 mm in both techniques.
For LASIK, a 8.5- to 9.0 mm diameter nasally hinged anterior corneal flap was created using the Automated Corneal Shaper (ACS, Chiron Vision, Irvine, California) microkeratome with either 130 or 160 µm head, in every patient. Flaps were not measured ultrasonically.
For PRK, epithelial debridement was performed up to 7–8 mm using manual debridement. Following this, the Bowman membrane was dried with a sponge, and the PRK was performed without further drying of the ablated area
Postoperatively, tobramycin (Tobrex, Alcon Laboratories, Ft Worth, Texas) and diclofenac 0.1% (Voltaren, Novartis) drops and were used. Dexamethasone 0.1% was used four times a day during the first week. Subsequently, fluorometholone 0.25% was applied four times daily for a minimum of 4 weeks based on the refraction and intraocular pressure. The steroid dose was tapered gradually (three times and twice daily for 2 weeks each).
Re-treatments
Re-treatments after PRK were performed by reablating the stromal bed after manual or laser de-epithelisation. The stromal bed was ablated using one of the excimer laser systems: VISX 20/20 excimer laser (10 re-treatments), NIDEK EC-5000 (NIDEK Co. Gamagori, Japan; one re-treatment) and Technolas 217 (Bausch & Lomb, Rochester, New York; one re-treatment).
Re-treatments after LASIK were performed by lifting the flap and reablating the stromal bed. Before the surgery, the edge of the flap was marked with gentian violet on the temporal side. A flat spatula was used to lift the corneal flap. The stromal bed was ablated using one of the excimer laser systems: VISX 20/20 excimer laser (five re-treatments), and Technolas 217 (Bausch & Lomb; one re-treatment). After ablation, the flap was replaced to its original position, and the interface was irrigated copiously.
Postoperative evaluation
All patients were evaluated 3 months, 1 year, 2 years, 5 years and 10 years after surgery, including measurement of manifest refraction, cycloplegic refraction, uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity (BSCVA), slit-lamp biomicroscopy, dilated funduscopy, applanation tonometry and corneal thickness using the DGH-500 pachymeter (DGH Technology, Exton, Pennsylvania), or Alcon Ocuscan RxP Ophthalmic Ultrasound System (Alcon Laboratories, Forth Worth, Texas). Visual acuity was measured using a standard Snellen acuity chart at 6 m. All patients had corneal topography evaluated using EyeSys topographer (EyeSys Corneal Analysis System, Houston, Texas; preop to 5 years of follow-up), Orbscan I slit-scanner (Bausch & Lomb; only at 5 or 10 years of follow-up) and CSO corneal topography system (CSO, Florence, Italy; only at 10 years follow-up).
Data were obtained at the end of 10 years and evaluated retrospectively regarding the format for reporting refractive surgical data,1920 and including the safety and efficacy indexes: safety = (BCVApost/BCVApre); efficacy = (UCVApost/BCVApre), where BCVA is the best spectacle corrected visual acuity, and UCVA is the uncorrected visual acuity.
Statistical analysis
Data analysis was performed using SPSS for Windows (version 11.0, SPSS, Chicago). The Mann–Whitney U test was performed to compare PRK and LASIK groups. Differences were considered to be statistically significant when the p value was <0.05.
Results
Refractive predictability and stability
Scattergrams of attempted versus achieved correction at 3 months and 10 years postoperatively are illustrated in fig 1. Postoperatively, the mean SE decreased slightly (myopic regression) over 10 years (fig 2). The significance (p value) of difference between the means of the PRK and LASIK groups by Mann–Whitney U test was: p = 0.492, preoperatively, p = 0.001 at 3 months, p = 0.025 at 1 year, p = 0.001 at 2 years, p = 0.062 at 5 years, and p = 0.035 at 10 years.
The predictability of eyes within ±0.50 D, ±1.00 D and ±2.00 D emmetropia after 10 years is demonstrated in fig 3. The mean regression and regression per year are given in table 2.
After primary PRK, 12 (35%) of 34 eyes underwent re-treatments, whereas six (18%) of 34 eyes underwent re-treatments after LASIK. For PRK, four of 12 eyes underwent re-treatment between 3 months’ and 1 year’s follow-up, two eyes between 1 year’s and 2 years’ follow-up, and six eyes between 2 years’ and 5 years’ follow-up. In the LASIK group, four of six eyes underwent re-treatment between 3 months’ and 1 year’s follow-up, one eye between 1 year’s and 2 years’ follow-up, and one eye between 2 years’ and 5 years’ follow-up.
Vision
The safety index was 1.16 in PRK eyes and 1.11 in LASIK eyes, at 10 years. Figure 4 shows the percentage of eyes that lost or gained Snellen lines (safety). Twenty-one (62%) of 34 PRK-treated eyes and 17 (50%) of 34 LASIK-treated eyes demonstrated an increase in BSCVA after 10 years. No eye lost more than two lines of BSCVA in both (PRK and LASIK) groups.
The efficacy index was 0.90 in the PRK group and 0.95 in the LASIK group, at 10 years. Figure 5 shows the preoperative BSCVA and postoperative UCVA percentage of eyes.
Complications
LASIK
Fine wrinkles (striations) resembling fingerprint lines were observed in the flap in one (2.9%) of 34 eyes after LASIK. The wrinkles were faint, were confined to the flap, did not affect visual acuity and remained unchanged during follow-up. Corneal flap melt or necrosis of the flap edge was observed in one (2.9%) of 34 eyes postoperatively. In this eye, the flap melting developed on an epithelial ingrowth area. Both epithelial ingrowth and flap melting were located peripherally, were never wider than 2.0 mm, progressed very slowly and did not affect UCVA, BCVA or corneal astigmatism. Haze was never worse than mild throughout the follow-up. No eye developed corneal ectasia in the long term.
PRK
The mean grade of haze decreased steadily between 3 months and 10 years. Only one (2.9%) of 34 eyes had grade 0.5 haze at 10 years. Two (5.8%) of 34 eyes had a central island on the topography which was resolved spontaneously (one eye) or corrected (one eye) after re-treatment. No eye in the study group showed clinical or topographic signs of corneal ectasia.
Discussion
In the present study, a comparative analysis of the evolution of the corneal curvature and the refractive stability 10 years after myopic PRK and LASIK for moderate myopia was performed by means of a control-matched retrospective study.
Long-term refractive predictability and stability
In this study, despite higher re-treatment rates after PRK, we found better predictability rates 10 years after LASIK than that of PRK. Previous studies comparing PRK and LASIK outcomes found similar predictability rates for both groups or slightly better outcomes for LASIK.12131415 Both groups demonstrated comparable myopic regression rates and stability in refraction during 10 years of follow-up.
It is postulated that myopic regression is a universal phenomenon in eyes that underwent excimer laser correction for myopia, greater for higher corrections.2122 In our study, the rates of myopic regression slowed down in eyes that did not undergo re-treatment after LASIK and PRK without significant difference in regression at any of the follow-ups. These data suggest that myopic regression stabilise between 2 and 5 years after both PRK and LASIK for moderate myopia. Many reasons may lead to myopic regression such as epithelial hyperplasia, corneal steepening because of corneal thinning, change in corneal biomechanics, increase in axial length and lenticular sclerosis.2122 Our results suggest that similar factors may affect the regression after PRK and LASIK.
Re-treatments
The re-treatment rate in the present study was higher for PRK than for LASIK. Previous studies with short-term follow-up also found higher re-treatment rates after PRK than that after LASIK.1213 In our study, the main reason for re-treatments was undercorrection in both groups, and a higher rate of initial non-attempted overcorrection may have resulted in less re-treatment in LASIK-treated eyes. As stated previously, re-treatments were safe and effective in both groups.1223
Visual outcome
In the present study, both PRK-treated eyes and LASIK-treated eyes demonstrated good safety.12131415 No eye lost more than two lines of BSCVA in any of the groups. Both groups demonstrated a high improvement rate in BSCVA during follow-up, which may be explained by an increase in the size of the image on the macula.24 Although PRK treated eyes demonstrated a decrease in BSCVA at 3 months due to haze, BSCVA recovered in 1-year and remained stable.
The efficacy index was slightly better in LASIK-treated eyes than that of PRK-treated eyes at 10 years. Although both groups had a similar rate of eyes that demonstrated 20/20 or better UCVA, the rate of achieving 20/40 or better was better in LASIK-treated eyes. Some studies found a similar efficacy after PRK or LASIK,121314 and others found slightly better efficacy results after LASIK.15 Improvements in the nomograms to prevent undercorrection and compensate myopic regression may even lead to a better efficacy after LASIK for moderate myopia.
Late complications
No significant sight-threatening complication was observed after PRK or LASIK. Although haze was a problem for PRK-treated eyes, particularly at 3 months, similar to previous reports,172526 it recovered in 1 year and remained stable thereafter. At 10 years, only one PRK-treated eye had visible haze (trace) at the slit-lamp. Central islands can be a frequent problem with broad-beam lasers. However, it has not been shown that they always affect vision or the refractive outcome.27 In our study, seven (3.6%) eyes of five patients had a central island on the topography after PRK but resolved spontaneously (four eyes) or corrected (three eyes) after re-treatment. Previous reports2829 identified high myopia, forme fruste keratoconus (FFKC), low RSB and multiple enhancements as risk factors for the development of ectasia after LASIK. In the present study, no eyes developed corneal ectasia in 10 years, but no eyes had preoperative FFKC on the topography, and thin residual stromal beds were avoided.
In conclusion, our findings showed that LASIK for moderate myopia is a more effective and predictable procedure than PRK for moderate myopia with a lower rate of re-treatment than that of PRK in the long-term. Both procedures were safe with similar regression rates. No eye lost more than two lines of BSCVA attributable to PRK or LASIK. No eye developed corneal ectasia in the long term. Recent advances in corneal profiles and technologies, like the optimisation of ablation diameters and optical zones treatment sizes,3031 which improved the predicatbility for surface ablations, should be considered when comparing these results with those of more recent procedures.
REFERENCES
Footnotes
Funding This study has been supported in part by a grant of the Spanish Ministry of Health, Instituto Carlos III, Red Temática de Investigación en Oftalmología, Subproyecto de Cirugía Refractíva y Calidad Visual (C03/13).
Competing interests None.
Provenance and Peer review Not commissioned; externally peer reviewed.
Ethics approval Ethics approval was provided by the institutional review board (Ethical Committee of Clinical Investigation of Instituto Oftalmológico de Alicante).
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