Corneal hydration and central islands after excimer laser photorefractive keratectomy

Analysis of Impact of Humidity and Temperature on Excimer Laser Ablation of Polyethylene Terephthalate, Polymethylmethacrylate, and Porcine Corneal Tissue

BACKGROUND AND OBJECTIVES

To analyze the impact of humidity and temperature on excimer laser ablation of polyethylene terephthalate (PET), polymethylmethacrylate (PMMA) and porcine corneal tissue, and an ablation model to compensate for the temperature and humidity changes on ablation efficiency.

STUDY DESIGN/MATERIALS AND METHODS

The study was conducted using an AMARIS 1050RS (Schwind eye-tech-solutions) placed inside a climate chamber at ACTS. Ablations were performed on PET, PMMA, and porcine cornea. The impact of a wide range of temperature (~18°C to ~30°C) and relative humidity (~25% to ~80%) on laser ablation outcomes was tested using nine climate test settings. For porcine eyes, change in defocus was calculated from the difference of post-ablation to pre-ablation average keratometry readings. Laser scanning deflectometry was performed to measure refractive change achieved in PMMA. Multiple linear regression was performed using the least square method with predictive factors: temperature, relative humidity, time stamp. Influence of climate settings was modeled for pulse energy, pulse fluence, ablation efficiency on PMMA and porcine cornea tissue.

RESULTS

Temperature changes did not affect laser pulse energy, pulse fluence (PET), and ablation efficiency (on PMMA or porcine corneal tissue) significantly. Changes in relative humidity were critical and significantly affected laser pulse energy, high fluence and low fluence. The opposite trend was observed between the ablation performance on PMMA and porcine cornea.

CONCLUSIONS

The proposed well-fitting multi-linear model can be utilized for compensation of temperature and humidity changes on ablation efficiency. Based on this model, a working window for optimum operation has been found (temperature 18°C to 28°C and relative humidity 25% to 65%) for a maximum deviation of ±2.5% in ablation efficiency in PMMA and porcine corneal tissue. Lasers Surg. Med. © 2019 The Authors. Lasers in Surgery and Medicine Published by Wiley Periodicals, Inc.

Characterization of the iPSC-derived conditioned medium that promotes the growth of bovine corneal endothelial cells

Corneal endothelial cells (CECs) maintain corneal transparency and visual acuity. However, the limited proliferative capability of these cells in vitro has prompted researchers to find efficient culturing techniques for them. The aim of our study was to evaluate the use of conditioned medium (CM) obtained from induced pluripotent stem cells (iPSCs) as a source for the effective proliferation of bovine CECs (B-CECs). In our study, the proliferative ability of B-CECs was moderately enhanced when the cells were grown in 25% iPSC conditioned medium (iPSC-CM). Additionally, hexagonal cell morphology was maintained until passage 4, as opposed to the irregular and enlarged shape observed in control corneal endothelial medium (CEM). B-CECs in both the 25% iPSC-CM and CEM groups expressed and Na+-K+-ATPase. The gene expression levels of NIFK, Na+-K+-ATPase, Col4A and Col8A and the percentage of cells entering S and G2 phases were higher in the iPSC-CM group. The number of apoptotic cells also decreased in the iPSC-CM group. In comparison to the control cultures, iPSC-CM facilitated cell migration, and these cells showed better barrier functions after several passages. The mechanism of cell proliferation mediated by iPSC-CM was also investigated, and phosphorylation of Akt was observed in B-CECs after exposure to iPSC-CM and showed sustained phosphorylation induced for up to 180 min in iPSC-CM. Our findings indicate that iPSC-CM may employ PI3-kinase signaling in regulating cell cycle progression, which can lead to enhanced cellular proliferation. Effective component analysis of the CM showed that in the iPSC-CM group, the expression of activin-A was significantly increased. If activin-A is added as a supplement, it could help to maintain the morphology of the cells, similar to that of CM. Hence, we conclude that activin-A is one of the effective components of CM in promoting cell proliferation and maintaining cell morphology.

Central Islands and Visual Outcomes of Phototherapeutic Keratectomy Using the Photorefractive Keratectomy Mode

PURPOSE

To compare the rate of central island (CI) and visual acuity after phototherapeutic keratectomy (PTK) with and without the CI program.

METHODS

This retrospective study comprised 147 eyes of 89 consecutive patients (mean age ± SD, 71.1 ± 9.6 years) undergoing PTK with and without the anti-CI program [photorefractive keratectomy (PRK) mode and PTK mode] using the VISX Star S4 excimer laser system (Johnson & Johnson Vision, Santa Ana, US) for treatment of band keratopathy (BK) or granular corneal dystrophy (GCD). The rate of CI formation, defined as a steepening area of 3 D, 1.5 mm in diameter, on each corneal videokeratographer (ATLAS 9000; Carl Zeiss Meditec, Jena), and best spectacle-corrected visual acuity (BSCVA) were assessed 3 months after PTK.

RESULTS

We found CI formation in 17 eyes (22%) in the PRK mode group and 45 eyes (73%) in the PTK mode group at 3 months postoperatively (P < 0.001, χ test). Postoperative logarithm of the minimal angle of resolution BSCVA was 0.10 ± 0.14 in the PRK group and 0.15 ± 0.13 in the PTK group (P = 0.019, Student t test). The rate of CI was 13 eyes (33%) with BK and 6 eyes (15%) with GCD in the PRK mode group and 25 eyes (83%) with BK and 20 eyes (71%) with GCD in the PTK mode group.

CONCLUSIONS

The use of the anti-CI program for PTK significantly decreases the rate of CI formation and significantly improves BSCVA in post-PTK eyes, suggesting its viability for treatment of BK and GCD in clinical setting.

Central island formation after cataract surgery in a laser in situ keratomileusis eye: New etiology

We report a case of central corneal steepness formation after cataract surgery in a post-laser in situ keratomileusis (LASIK) eye. A 51-year-old woman with traumatic cataract and a history of myopic LASIK surgery had uneventful phacoemulsification cataract surgery. Postoperatively, the corneal topography difference map showed a central island formation of 2.1 diopters (D) compared with the preoperative level, which correlated with a clinical myopic shift of 2.00 D from the targeted emmetropia. At 4 months, most of the central island had resolved spontaneously. At 15 months, topography returned to preoperative levels. We speculate that wound hydration in the flap interface during surgery resulted in a pond-like fluid accumulation in the center of the cornea, resulting in a myopic refractive shift. We report this case because of the high probability that this phenomenon could be confounded by an erroneous intraocular lens (IOL) calculation and the potential for a serious iatrogenic complication from a premature IOL exchange or touch-up procedure.

Central islands: rate and effect on visual recovery after phototherapeutic keratectomy

PURPOSE

To retrospectively assess the proportion of patients affected by a central island (CI) and its effect on visual recovery after phototherapeutic keratectomy (PTK).

METHODS

This retrospective study evaluated 30 eyes of 21 consecutive patients (mean age ± standard deviation, 69.6 ± 6.8 years) undergoing PTK for the treatment of band keratopathy or granular corneal dystrophy. We investigated the rate of CI formation, which was defined as a steepening area of 3 D, 1.5 mm in diameter, on each corneal videokeratograph (ATLAS 9000; Carl Zeiss Meditec), and its effect on visual recovery at 3 months and at 1 year postoperatively.

RESULTS

A CI was found in 22 of 30 eyes (73%) 3 months postoperatively and in 14 of 25 eyes (56%) 1 year postoperatively. The degree of CI was significantly correlated with the change in logMAR corrected visual acuity (Spearman correlation coefficient r = 0.445, P = 0.026). The degree of CI in eyes with band keratopathy was significantly larger than that in eyes with granular dystrophy 1 year postoperatively (Mann-Whitney test, P = 0.045). The degree of CI was not significantly correlated with the ablation depth (Spearman correlation coefficient r = 0.116, P = 0.582) or the residual corneal thickness (r = -0.235, P = 0.278).

CONCLUSIONS

CI formation was found in as many as 73 and 56% of patients 3 months and 1 year after PTK, respectively, using the VISX Star S4 excimer laser system, and significantly affected the improvement of visual acuity. The anti-CI program should be applied by the manufacturer, not only for corneal refractive surgery but also for PTK in a clinical setting.

Twenty-year follow-up of a randomized prospective clinical trial of excimer laser photorefractive keratectomy

PURPOSE

To investigate the 20-year efficacy and safety of photorefractive keratectomy (PRK).

DESIGN

Long-term observational case series.

METHODS

In the setting of a university hospital, a study population of 42 patients (42 eyes) who had, as part of a randomized prospective trial, undergone PRK 20 years previously were studied. All had received -3.0 or -6.0 diopter (D) corrections with either 5.0 or 6.0 mm optical zones or a multizone treatment. The mean preoperative spherical equivalent refractive error (SEQ) was -5.13 D (range, -2.75 to -8.0 D). The outcome measures included visual acuity, refractive error, corneal topography and axial length.

RESULTS

Between 1 and 20 years there was an increase in mean myopic SEQ of -0.54 D (P < 0.02). In patients younger than 40 years of age at time of correction, this increase was -0.92 D (P < 0.002) with an accompanying increase in variance (P < 0.02), whereas in those older than 40 it was -0.08D (P = 0.8). In female patients the change was -0.69D (P < 0.01), while in males it was -0.26D (P = 0.6). The efficacy index at 20 years was 0.49, and the safety index was 0.97. Corrected distance visual acuity improved between 1 and 20 years (P < 0.01); 93% of corneas were clear at 20 years; 3 eyes had trace haze. There was an improvement in haze scores between 1 and 20 years (P < 0.02). Cornea power remained unchanged between 6 months and 20 years (P = 0.4). Axial length increased by a mean of 0.84 mm (P < 0.0001). There was no ectasia.

CONCLUSIONS

There was a slight but significant increase in myopic SEQ after PRK between 1 and 20 years, particularly in those under 40 at the time of treatment and in female patients. Corneal power remained unchanged, but axial length increased. The procedure was safe, with no long-term sight-threatening complications and with improvements in corrected distance visual acuity and corneal transparency with time.

Influence of stromal refractive index and hydration on corneal laser refractive surgery

PURPOSE

To evaluate the influence of the stromal refractive index and hydration on postoperative outcomes in eyes that had corneal laser refractive surgery using the Amaris laser system.

SETTING

Augenzentrum Recklinghausen, Recklinghausen, Germany.

DESIGN

Comparative case series.

METHODS

At the 6-month follow-up, right eyes were retrospectively analyzed. The effect of the stromal refractive index and hydration on refractive outcomes was assessed using univariate linear and multilinear correlations.

RESULTS

Sixty eyes were analyzed. Univariate linear analyses showed that the stromal refractive index and hydration were correlated with the thickness of the preoperative exposed stroma and was statistically different for laser in situ keratomileusis and laser-assisted subepithelial keratectomy treatments. Univariate multilinear analyses showed that the spherical equivalent (SE) was correlated with the attempted SE and stromal refractive index (or hydration). Analyses suggest overcorrections for higher stromal refractive index values and for lower hydration values.

CONCLUSIONS

The stromal refractive index and hydration affected postoperative outcomes in a subtle, yet significant manner. An adjustment toward greater attempted correction in highly hydrated corneas and less intended correction in low hydrated corneas might help optimize refractive outcomes.

FINANCIAL DISCLOSURE

Mr. Magnago and Dr. Arba-Mosquera are employees of and Dr. Diego de Ortueta is a consultant to Schwind eye-tech-solutions GmbH & Co. KG. Mr. Rüden has no financial or proprietary interest in any material or method mentioned.

Influence of patient age on high myopic correction in corneal laser refractive surgery

PURPOSE

To evaluate the influence of patient age on the postoperative outcomes in eyes with myopia higher than 5.00 diopters (D) that had laser in situ keratomileusis using the Schwind Amaris laser system.

SETTING

Private practice, Utrecht, The Netherlands.

DESIGN

Case series.

METHODS

At the 1-year follow-up, right eyes with preoperative myopia higher than 5.00 D were analyzed. The effect of the patient's age on postoperative status was assessed using univariate linear and multilinear correlations.

RESULTS

The study analyzed 612 eyes. Univariate linear analyses showed that residual refraction was correlated with patient age. Univariate multilinear analyses showed that spherical equivalent (SE) was correlated with the attempted SE and patient age, whereas cylinder was correlated with attempted cylinder only. Analyses suggested overcorrections and higher residual astigmatism values for older patients.

CONCLUSIONS

Patient age affected postoperative outcomes in a subtle, yet significant manner. An age-dependent adjustment toward greater attempted correction in younger patients and less intended correction in older patients may help optimize refractive outcomes.

FINANCIAL DISCLOSURE

Drs. Ewering and Arba-Mosquera are employees of Schwind eye-tech-solutions GmbH. Dr. Luger has no financial or proprietary interest in any material or method mentioned.

Customized ablation algorithm for the treatment of steep central islands after refractive laser surgery

Steep central island (SCI) formation after photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) represents a major drawback in the visual rehabilitation of patients after refractive laser surgery. Because of the small size of SCIs, current ablation algorithms are unable to properly calculate an ablation pattern for customized retreatment. We present the use of a new ablation algorithm for the treatment of SCIs that occurred after PRK or LASIK surgery. This algorithm uses a smaller zone of approximation and takes into account the spherical shift induced by removal of the SCI. In all 3 eyes treated, best spectacle-corrected visual acuity increased to 20/16 and remained stable at the 1- and 3-month follow-up, with disappearance of the SCI in corneal topography. This new treatment algorithm may be of benefit to patients experiencing visual side effects due to SCI formation after PRK or LASIK surgery.

Influence of laser polarization on ocular refractive parameters after refractive surgery

We provide an adjustment factor for ablation algorithms used in photorefractive laser surgery that takes into account how laser polarization in reflection losses affects the cornea. We evaluate the influence of this factor on corneal radius and asphericity after surgery, showing that it is significant for visual performance (effective visual acuity is reduced) and for the correction of eye aberrations. Our data indicate that this adjustment factor should be included in the ablation algorithms (depending on the polarization state of each laser device) that are proposed for customized corneal ablation, which need great accuracy for minimization of eye aberrations.

Proper Positioning of the Plume Evacuator in the VISX Star3 Excimer Laser Minimizes Central Island Formation in Patients Undergoing Laser in situ Keratomileusis

PURPOSE

To identify risk factors in a series of patients who developed steep central islands after laser in situ keratomileusis (LASIK).

METHODS

We analyzed and compared the refractive and topographic outcome of a study group composed of 83 eyes of 44 patients who underwent LASIK using the VISX Star3 excimer laser with a refraction-matched control group of 83 eyes treated later. The vacuum aspirator of the excimer laser was abnormally positioned during the surgeries performed in the study group.

RESULTS

Mean preoperative spherical equivalent refraction in the study group was -6.75 +/- 2.50 D. Four eyes with a mean preoperative spherical equivalent refraction of -9.27 +/- 2.29 D developed steep central islands. Thirty-three (38%) of 83 eyes treated needed retreatment for residual myopia or myopic astigmatism. In the control group, mean preoperative spherical equivalent refraction was -6.76 +/- 2.50 D. Ninety-three percent of eyes were within +/- 1.00 D of target refraction. Five (6.02%) of 83 eyes required retreatment and no eyes developed central islands.

CONCLUSION

The abnormally positioned vacuum aspirator coupled with the higher preoperative refractive correction were the likely causative factors for central island formation and the increased incidence of undercorrection in these patients.

Assessment of transient changes in corneal hydration using confocal Raman spectroscopy

PURPOSE

The degree of corneal hydration has been linked to excimer laser corneal ablation rates. Enhanced precision with excimer laser refractive surgery may result from a better understanding of the transient changes in corneal hydration. To better understand the dynamic nature of corneal hydration, bovine corneas were investigated under different surface treatments.

METHODS

Confocal micro-Raman spectroscopy was used to quantify corneal hydration. Water and acetone solutions were used to establish a quantitative response of the relative OH/CH Raman bands, which are consistent with the water and collagen protein bands in cornea, respectively. Intact bovine corneas were manually debrided (designated MD group) or lamellar flaps were created to expose stromal tissue (designated lamellar keratectomy or LK group). Raman spectra were recorded every 30 seconds for 6 minutes while the prepared cornea surfaces were exposed to quiescent air or to a forced nitrogen gas flow across the surface.

RESULTS

The OH and CH Raman bands yielded a linear response while the percentage of acetone was varied from 0% to 100%. For the bovine cornea under forced flow drying, the OH/CH Raman band ratio was found to decrease by 41% from the initial value for both the MD and LK treatment groups. These decreases were significantly more (p = 0.0051 and 0.054, respectively) than the 26% decrease in the OH/CH band ratio measured for the control corneas. In quiescent air, the control and MD groups exhibited a 7% and 6% decrease in the OH/CH ratio, respectively, while the LK treatment group revealed a 19% decrease in the OH/CH ratio.

CONCLUSIONS

The bovine eye experiments demonstrate that significant changes in corneal hydration are realized under different drying conditions and treatment methodologies. This study elucidates the nature of transient changes in corneal hydration in a bovine model and suggests the need for further study of the role of such variations in surgical outcome for excimer laser corneal refractive procedures.

Fourier analysis of corneal astigmatic changes following photorefractive keratectomy

The purpose of this study was to evaluate the corneal irregular astigmatism following photorefractive keratectomy (PRK) for myopia. The corneal topography of 30 eyes of 26 patients was measured with the TMS-1 videokeratoscope before and 1 month after PRK. Axial dioptric data were decomposed into four components; A0 (Sphericity), C1 x 2 (Asymmetry), C2 x 2 (Regular astigmatism), and C3 (higher-order irregularity) for the central 3 and 6 mm zone by Fourier series harmonic analysis. Post-operative topographies were divided into those with an irregular and those with a homogeneous pattern, and the Fourier components were compared. In the 6 mm zone, A0 was significantly decreased (P < 0.001), and C1 x 2, C2 x 2, and C3 were significantly increased (P = 0.001, 0.005, 0.002, respectively). In the 3 mm zone, A0 decreased (P < 0.001) and C1 x 2 increased (P < 0.001) significantly. C1 x 2 was correlated with the post-operative corrected visual acuity (P < 0.001, r = 0.647). The irregular pattern group had a larger C1 x 2 component (P < 0.001). The treatment displacement was not correlated with any component. In conclusion, irregular topography due to intraoperative drift or asymmetrical wound healing may play a more important role in the post-operative corneal optical property than mild treatment displacement.

Diagnostic use of a rigid contact lens to show corneal topography abnormalities after laser refractive surgery

We describe 2 cases in which evaluation of rigid contact lens fluorescein patterns were used to delineate and characterize topography irregularities. Contact lens analysis confirmed and localized topography findings of an elevated central island in 1 patient and a semicircular pattern in the other patient. To determine a therapeutic strategy to correct topography irregularities after laser refractive surgery, it is critical to document a corneal elevation and delineate its location.

Arcuate keratotomy for asymmetrical steep islands after laser in situ keratomileusis and automated lamellar keratoplasty

PURPOSE

To determine the safety and efficacy of arcuate keratotomy (AK) to reduce or eliminate steep asymmetrical islands after laser in situ keratomileusis (LASIK) and automated lamellar keratoplasty (ALK).

SETTING

Rancho Bernardo Laser and Vision Center, San Diego, California, USA.

METHODS

This retrospective and noncomparative interventional study comprised 8 eyes of 5 patients who received arcuate incisions to reduce topographically defined asymmetrical steep islands after LASIK (n = 7) and ALK for hyperopia (n = 1). The islands occurred unexpectedly after standard excimer laser treatment (VISX Star and S2) for myopic astigmatism that induced irregular astigmatism and unwanted optical effects. Cases occurred consecutively over 16 months, an incidence of less than 1%. Arcuate incisions extended 30 or 45 degrees in LASIK eyes based on the amount of residual refractive astigmatism and were placed at 7.0 mm on the steep axis. Radial incisions were limbal sparing to a 5.0 mm optical zone following the Casebeer nomogram. Outcome measures were uniformity of postenhancement topography, uncorrected visual acuity (UCVA), postenhancement refraction, and reduction in unwanted optical effects.

RESULTS

The preenhancement mean spherical equivalent (SE) in the LASIK eyes was -0.99 diopters (D) +/- 0.48 (SD); the astigmatism ranged from 0.50 to 1.75 D. After incisional keratotomy, the mean SE was -0.21 +/- 0.12 D and the net residual astigmatism was reduced to a range of 0 to 0.75 D. All 8 eyes had AK to reduce topographically defined steep islands that were causing visual blurring and unwanted optical effects including ghosting, halos, and polyplopia. After enhancement, there was a marked reduction in the asymmetrical steep islands by topography, fewer unwanted optical effects, less refractive astigmatism, and improved UCVA. Two patients retained small inferior steep areas that may indicate forme fruste keratoconus, although this condition was not detected preoperatively by topographical indices.

CONCLUSION

Uncorrected visual acuity improved and unwanted optical effects were reduced after AK on the peripheral axis of asymmetrical steep islands that occurred after uneventful LASIK and hyperopic ALK. This relatively simple incisional approach may be used until more advanced topography-linked ablations with rapid and precise eye tracking are perfected and widely available.

The Alcon/Summit/Autonomous Perspective on Fixed vs. Variable Spot Ablation

The LADARVision system utilizes a small fixed diameter excimer laser beam coupled with a high fidelity eye tracking system to perform wavefront-guided refractive surgery. The fixed small beam provides a consistent ablation per pulse. By delivering many identical pulses in a predetermined pattern, which includes an optimized custom blend zone, the laser is able to ablate complex (higher order) corneal shapes accurately. The closed loop eye tracking system ensures that accurate ablation is delivered to the right place on the eye.

LASIK complications: etiology, management, and prevention

Laser in situ keratomileusis (LASIK) is a rapidly evolving ophthalmic surgical procedure. Several anatomic and refractive complications have been identified. Anatomic complications include corneal flap abnormalities, epithelial ingrowth, and corneal ectasia. Refractive complications include unexpected refractive outcomes, irregular astigmatism, decentration, visual aberrations, and loss of vision. Infectious keratitis, dry eyes, and diffuse lamellar keratitis may also occur following LASIK. By examining the etiology, management, and prevention of these complications, the refractive surgeon may be able to improve visual outcomes and prevent vision-threatening problems. Reporting outcomes and mishaps of LASIK surgery will help refine our approach to the management of emerging complications.

Computerized corneal topography and its importance to wavefront technology

PURPOSE

To review the development and application of corneal topography in refractive surgery.

METHODS

Review of the literature and discussion of recent developments in corneal topography and wavefront technology.

RESULTS

Analysis of corneal topography provides critical information for the preoperative examination of patients before refractive surgery and for the evaluation and treatment of patients with complications after surgery.

CONCLUSIONS

Corneal topography will continue to be a critical diagnostic modality for refractive surgery. Even with the advent of wavefront analysis designed to detect refractive error and aberrations of the eye, it will be necessary to have detailed corneal topographic information to understand the contribution the cornea makes to vision so that custom alteration of that surface can be used to optimize vision. This will be true of the normal eye, but it will be of special importance in eyes with abnormalities that were induced by corneal surgery.

Corneal hydration affects ablation during laser in situ keratomileusis surgery

BACKGROUND

Studies in animal eyes indicate that the level of corneal hydration affects the ablation rate of laser surgery; the greater the hydration is, the less the ablation for a given laser pulse. Our study is an assessment in human eyes comparing the effects on ablation by blotting the corneal stromal surface under a corneal flap created for laser in situ keratomileusis (LASIK) procedures between sets of excimer laser pulses, with ablation depth in eyes not blotted between sets of laser pulses.

METHODS

We modified the surgical technique for LASIK procedures to assess the effects of the level of hydration on excimer laser ablation depth per pulse. In group 1, 40 eyes underwent LASIK surgery without any modification. Group 2 was composed of 36 eyes having LASIK procedures, but the corneal surfaces were kept relatively dry by blotting of the stromal surface between sets of laser pulses.

RESULTS

Six months after surgery, the mean spherical equivalent refractive change was from -8.38 diopters (D) to -1.44 D in group 1 and from -7.93 D to -0.09 D in group 2. For predictability, the deviation from the target refraction after surgery was assessed. Thirty-three percent (13 of 40) in group 1 and 25% (9 of 36) in group 2 were within +/-0.5 D. Forty-eight percent (19 of 40) in group 1 and 50% (18 of 36) in group 2 were within +/-1 D. Six months after surgery, 80% or more in both groups were within +/-2 D. There was myopic regression in all patients. Three months after surgery, regression averaged -0.71 D in group 1 and -1.15 D in group 2.

CONCLUSIONS

Corneal hydration levels affect the efficiency of laser ablation in LASIK procedures. With less hydrated corneas, ablation effects were greater than for corneas not blotted during the procedure, but these patients appear to undergo greater myopic regression.

Developments in corneal topographic analysis following contact lens wear and refractive surgery

There is now a wide range of devices available for corneal topographic analysis. Although most devices use the Placido disk approach, fluorescein profilometry, laser holography and scanning slit technology have also been employed. The colour-coded topographical maps have been designed for ease of clinical interpretation. The application of this technology to further our understanding of the effects of contact lens wear and various forms of refractive surgery is demonstrated. Current developments include the merging of corneal topographic analysis and ocular wavefront sensing technology to create the capability of etching sophisticated corneal shapes in the course of refractive surgery so as to provide optimal aberration control.

Natural history of central islands after laser in situ keratomileusis

PURPOSE

To assess the incidence and natural history of central islands following laser in situ keratomileusis (LASIK) and evaluate the association of central island characteristics with visual acuity.

SETTING

Department of Ophthalmology, China Medical College Hospital, Taichung, Taiwan.

METHODS

A consecutive series of 406 eyes of 212 patients who had LASIK was retrospectively evaluated. Uncorrected visual acuity (UCVA) was measured and corneal topography performed preoperatively and 1 week and 1, 3, 6, and 9 months postoperatively. Best spectacle-corrected visual acuity (BSCVA) was evaluated preoperatively and 1, 3, and 6 months postoperatively.

RESULTS

The topographic images obtained at 1 week demonstrated central islands in 23 eyes of 20 patients (5.7%). No new cases of central island formation were identified after 1 week. Of the 23 eyes with central islands, the 6 month post-LASIK maps were available in 20 eyes of 18 patients. There was a significant difference in the size and power of the central islands between 1 week and 6 months. However, the power and size decreased slowly. Within 6 months, only 5 of 20 central islands (25.0%) had resolved. Eight eyes were undercorrected, and 1 eye lost 2 lines of BSCVA. Central islands larger than 1.8 mm or 3.0 diopters (D) were significantly correlated with lower UCVA.

CONCLUSION

Most central islands that occur with LASIK persist more than 6 months. Large central islands (>/=1.8 mm or >/=3.0 D) are risk factors for lower UCVA. Preventive measures are necessary.