LASIK Refractive Surgery Research Articles

Comparison of real and computer-simulated outcomes of LASIK refractive surgery

Computer simulations of alternative LASIK ablation patterns were performed for corneal elevation maps of 13 real myopic corneas (range of myopia, -2.0 to -11.5 D). The computationally simulated ablation patterns were designed with biconic surfaces (standard Munnerlyn pattern, parabolic pattern, and biconic pattern) or with aberrometry measurements (customized pattern). Simulated results were compared with real postoperative outcomes. Standard LASIK refractive surgery for myopia increased corneal asphericity and spherical aberration. Computations with the theoretical Munnerlyn ablation pattern did not increase the corneal asphericity and spherical aberration. The theoretical parabolic pattern induced a slight increase of asphericity and spherical aberration, explaining only 40% of the clinically found increase. The theoretical biconic pattern controlled corneal spherical aberration. Computations showed that the theoretical customized pattern can correct high-order asymmetric aberrations. Simulations of changes in efficiency due to reflection and nonnormal incidence of the laser light showed a further increase in corneal asphericity. Consideration of these effects with a parabolic pattern accounts for 70% of the clinical increase in asphericity.

Cornea and external disease

New developments in cornea and external disease therapy are frequent and influence many aspects of the management of eye disease, including infectious disease, ocular surface disease, corneal surgery, and correction of refractive errors. Although media attention often highlights the novelty or potential impact of any advance, putting the new developments into proper perspective requires a more judicious and critical appraisal. The contributors to this issue of the Ophthalmology Clinics of North America provide that appraisal with respect to anti-infectious therapy, management of ocular surface disease, contact lens wear, novel surgical management of endothelial disease, and options for correction of refractive errors. The Charles T. Campbell Ophthalmic Microbiology Laboratory of the Eye and Ear Institute of the University of Pittsburgh provides a review of present-day ophthalmic microbiology. Regis Kowalski, MS, is the Technical Director of the Campbell Laboratory and reviews the experience of the laboratory with respect to antimicrobial sensitivity and emerging resistance; Francis Mah, MD, is the Associate Medical Director and reviews the scope of antibiotics and outlines new developments in antimicrobial options. A review of the present understanding of ocular surface disease summarizes dry eye therapy and management of meibomian gland disease. A summary of the new surgical approaches to treating ocular surface disease includes an overview of amniotic membrane transplantation by Thomas John, MD, and the options for limbal stem cell resurfacing by Drs. Kim and Holland. Perry Rosenthal, MD, describes the results of silicone scleral contact lens therapy for ocular surface disorders. New developments in contact lens therapy include the new generation of extended and continuous wear high-Dk materials, which are described by Peter Donshik, MD. An overview of the options for the use of therapeutic contact lenses completes an update of the evolving contact lens armamentarium. Finally, there is a presentation of novel innovations in surgical treatment of the cornea, which includes discussion and comparison of the flap (anterior) and limbal pocket (posterior) approaches to endothelial replacement. The issue concludes with a review of developments in LASIK and LASEK refractive surgery. To the authors who contributed to this issue, I extend my thanks and appreciation. To the reader, I express my hope that this review of the new developments in corneal and external disease treatment proves useful and provides perspective for optimal patient care.

Errors leading to unexpected pseudophakic ametropia.

Determination of the reasons for clinically significant unplanned ametropia following cataract surgery and the results of management of the ametropia. Retrospective review of 11 consecutive cases of tertiary referral for management of pseudophakic ametropia to the authors. Corrective surgery involved either lens implant exchange or LASIK refractive surgery. Final outcome was assessed by uncorrected and best spectacle corrected visual acuity and manifest refractive outcome. Five cases (45%) were due to significant error in axial length determination at pre-surgery biometry. Six cases (55%) were due to surgeon or surgical team error, where the surgeon implanted a lens of power at variance with that specified pre-operatively. Nine patients elected to undergo refractive surgery to correct the ametropia and 2 elected to wear a spectacle lens. Seven underwent lens implant exchange and 2 patients underwent LASIK keratorefractive surgery. Eight of nine patients were within 1 dioptre of intended spherical equivalent after refractive surgery and 1 patients was -1.5 dioptre myopic. Most cases of serious unintended ametropia after cataract surgery are avoidable. Care should be taken with the biometry and procedural checks to minimise error. When lens implant exchange or LASIK was performed the final refractive results were satisfactory.

Some variables to consider to avoid the need for LASIK surgical enhancements

Background: The predictability of LASIK refractive surgery is important both to the patient and the surgeon in obtaining an optimal result and in reducing the need for enhancement surgery. Some instruments, large ablation zones and possibly other variables may increase hyperopic overcorrection. Methods: A retrospective study was undertaken of 345 myopic LASIK eyes (175 patients) treated with a Nidek EC‐5000. The need for additional surgery in the form of an enhancement was determined after the patient had a stable refraction. The variables measured in the study were the patient's refractive correction, corneal curvature using an Alcon EH‐290 topographer, the patient's age and Nidek excimer laser ablation optic and transition zone size. The same nomogram was used for all eyes and where possible bilateral surgery was conducted on all patients. The effect of ablation sizes, refractive errors, patient age and corneal curvature on the enhancement surgeries was evaluated using SPSS 6.0. Results: The most significant variable that precipitated a LASIK enhancement was an optic zone of 6.5 mm with a transition zone of 7.5 mm (paired t‐test, p < 0.0025). Multivariate analysis indicates that the older the patient and the larger the refractive error, the greater the risk of not achieving a residual refractive error of ± 0.50 D at three months. The steeper pre‐operative corneas have a greater chance of enhancement (mean of sample 44.48 ± 1.47 D and mean of enhancements 45.30 ± 1.65 D, p = 0.01, independent sample test). The smaller optic zone was associated with a smaller refractive over‐refraction after LASIK surgery (mean for 5.5 mm optic zone, +0.71 ± 0.29 D; mean, for 6.5 mm optic +1.27 ± 0.50 D, paired t‐test p < 0.0001). Conclusions: In this study, the 5.5 mm optic zone appears to dampen or reduce the undesirable refractive results. The optic zone size of choice with the Nidek EC‐5000 laser should be 5.5 mm if the patient has small pupils and steep corneas. In this sample, the targeted emmetropic refractive state (range piano to +0.50 D sphere and piano to ‐0.50 D cylinder) was achieved in 93.3 per cent of cases after three months.