Corneal power, thickness, and stiffness: Results of a prospective randomized controlled trial of PRK and LASIK for myopia

Abstract

Methods One eye of each of 45 patients with myopia ranging from −6.00 to −8.00 diopters (D) (spherical equivalent spectacle refraction [SER]) was randomized to LASIK (n = 25; mean SER −7.12 D ± 0.57 [SD]) or PRK (n = 20; mean SER −6.91 ± 0.57 D). Data were collected prospectively before and 1, 3, 6, 12, and 36 months after surgery. Measurements included corneal topography (TMS-1, Tomey), corneal thickness (ultrasound pachymetry), and apparent intraocular pressure (IOP) (pneumotonometry). Retreatments were not performed during the first year, and retreated eyes were excluded from the 3-year follow-up. Changes in corneal power and aberrations, thickness, and apparent IOP were calculated in a pair-wise manner for 3 time periods: short term (preoperative to 1 month after surgery), medium term (1 to 12 months after surgery), and long term (1 to 3 years after surgery). Results In the short term, corneal power decreased equally in LASIK and PRK eyes. Spherical aberrations and coma-like aberrations increased equally, while corneal thickness decreased significantly less in LASIK eyes than in PRK eyes. The apparent IOP decreased more in LASIK eyes than in PRK eyes. In the medium term, corneal power increased significantly in both groups. Spherical aberrations decreased significantly in PRK eyes but not in LASIK eyes. From 1 to 12 months, corneal thickness increased more in PRK eyes than in LASIK eyes. During this period, the apparent IOP increased significantly in LASIK eyes. In the long term, corneal power and corneal aberrations did not change significantly in either group. Corneal thickness increased slightly but significantly in both groups. The apparent IOP increased significantly more in PRK eyes. Conclusions Differences between LASIK and PRK related to time-dependent events affecting corneal shape and structural integrity were present. Peripheral changes in flap hydration in LASIK eyes and epithelial and/or stromal thickening in PRK eyes appeared to be the most important factors in optical power changes in the first year after treatment. The changes in apparent IOP suggest that some interlamellar healing occurred during the first year after LASIK. After LASIK and PRK, corneal bending stiffness seemed permanently decreased, although some restiffening may occur in PRK eyes in the long term. Excimer laser-based photoablation of the cornea is the most widely used surgery for the correction of myopia. Photorefractive keratectomy (PRK) has been performed since the late 1980s 1 and laser in situ keratomileusis (LASIK) since the mid-1990s. 2 Factors that may influence the refractive outcome after excimer laser refractive surgery include the wound-healing response and biomechanical factors due to the altered structure of the corneal tissue. 3–5 Changes in the central optical power of the cornea and in corneal thickness can be used to characterize these processes. For many years, it has been known that some tonometers are influenced by corneal thickness. 6,7 A thin cornea bends more easily than a thick cornea. However, if the corneal thickness and the real intraocular pressure (IOP) are the same over a period of time, a change in tonometry is indicative of changes in the mechanical properties of the corneal tissue. This prospective randomized controlled trial looked at whether there are significant differences between PRK and LASIK in the induced changes in corneal shape and corneal integrity. Data from the study focused on short-, medium-, and long-term changes in corneal power including corneal spherical aberration and coma-like aberrations, corneal thickness, and pneumotonometry. Patients and methods From July to October 2000, approximately 100 patients referred to the department for refractive surgery were invited to participate in this prospective randomized study. Inclusion criteria were a monocular best corrected visual acuity of 0.8 or better in both eyes; myopia between −6.0 and −8.0 diopters (D) in subjective spherical equivalent refraction; a stable refraction (less than 0.5 D change) for the previous 2 years; and regular, subjective refractive astigmatism less than 1.5 D. Patients were excluded if they had had ocular disease or eye surgery; were susceptible to keloid formation, younger than 19 years, or pregnant; or had unrealistic expectations. Forty-six of the invited patients fulfilled the criteria and were willing to participate. The study was approved by the ethics committee of Århus County, Denmark. All patients provided informed written consent. Randomization was performed using a random number system. One eye of each patient was included in the study. Retreatments were not allowed during the first year after surgery. Twenty-five patients were randomized to LASIK and 21 to PRK. One patient randomized to PRK did not want to complete the planned follow-ups from 1 month after surgery. Data from this patient were excluded from all analyses, leaving 25 eyes in which LASIK was performed and 20 in which PRK was performed. The patient groups were comparable in age, preoperative spectacle correction, corneal thickness, IOP, and corneal refractive power ( Table 1 ). Significantly more women than men were randomized to PRK than to LASIK ( P <.05, chi-square test). Surgical technique Surgery was performed under topical anesthesia (3 drops of oxybuprocaine 0.8% at 5-minute intervals). Two drops of pilocarpine 2% applied at 10-minute intervals were used to constrict the pupil. The same surgeon (J.Ø.H) performed all procedures. In PRK procedures, the central 8.0 mm diameter epithelium was scraped after alcohol 96% was applied for a few seconds. After laser treatment, 1 drop of cyclopentolate (Cyclogyl), 1 drop of diclofenac (Voltaren), and chloramphenicol ointment were applied and the eye was patched. Chloramphenicol 0.5% antibiotic eyedrops were prescribed 3 times daily starting the day after surgery for 1 week. Eyedrops...