The Cornea is Not a Piece of Plastic

Laser in situ keratomileusis to correct post-keratoplasty astigmatism; 1-step versus 2-step procedure

PURPOSE

To investigate the correction of post-penetrating keratoplasty (PKP) astigmatism using laser in situ keratomileusis (LASIK). Visual and refractive outcomes were evaluated after LASIK was performed in 1 step (lamellar cut and ablation in 1 procedure) or 2 steps (lamellar cut then ablation in 2 successive procedures).

SETTING

Department of Cornea and Refractive Surgery, Vissum-Instituto Oftalmológico de Alicante, University of Miguel Hernández, Alicante, Spain.

METHODS

In this prospective observational study, 22 consecutive eyes were divided into 2 groups depending on the LASIK procedure performed to correct post-PKP astigmatism. Group 1 (1-step LASIK) included 11 eyes and Group 2 (2-step LASIK), 11 eyes. The patients were followed for 6 months.

RESULTS

A statistically significant improvement was obtained in Group 2 with a mean vector analysis result of the cylinder of -4.37 diopters (D) +/- 1.79 (SD) (P = .018). In Group 1, the mean astigmatism correction was 2.38 +/- 1.71 D. The number of reoperations and residual refractive defects were significantly better in Group 2.

CONCLUSION

The 2-step technique improved the accuracy of excimer laser correction of post-PKP astigmatism.

The Theoretical vs. Measured Laser Resection for Laser in situ Keratomileusis

PURPOSE

To measure the excimer laser resection during and after laser in situ keratomileusis (LASIK) and to determine the rate of stromal ablation and factors predicting its measurement.

METHODS

A retrospective, comparative, interventional case study of 6010 eyes undergoing LASIK surgery was performed. In vivo ultrasonic pachymetry was performed as a measure of the laser resection (1'MLR). The theoretical laser resection generated by one of three separate excimer lasers (Summit Apex Plus, Alcon LadarVision, VISX) was recorded. The change in central comeal thickness measured at enhancement surgery (2'MLR) and the change in the residual stromal thickness prior to the laser ablation at the primary and enhancement procedure (3'MLR) were recorded as separate measures of the laser resection. Paired sample t test and regression analysis was performed to determine the relationships and to develop a model predictive of laser resection for each laser.

RESULTS

For all lasers, 1'MLR produced the highest estimate compared to 3'MLR. Laser-induced stromal desiccation, microkeratome effects, and change in measured flap thickness were the most likely causes for the differences. Laser resection was overestimated in the Alcon theoretical laser resection group, but underestimated in the Summit and VISX groups. The difference between 2'MLR and 3'MLR was due to epithelial hyperplasia, which measured 8.3, 17.8, and 10.8 microm in the Alcon, Summit, and VISX enhancement groups, respectively. Stromal ablation rates were 11.79, 8.26, and 12.71 microm per spherical equivalent diopter laser setting for the Alcon, Summit, and VISX lasers, respectively, when measured with 3'MLR. Multiple pre- and perioperative factors were associated with laser resection.

CONCLUSIONS

To accurately predict residual corneal thickness, the expected laser resection for an attempted refractive change must be known. As the laser resection can be laser-specific, the surgeon needs to establish the mean and range of tissue removal for a given attempted correction.

Post-LASIK myopic shift after a trek in the North Pole

Bilateral laser in situ keratomileusis was performed in a 37-year-old male ethnic Chinese adventurer. Eight weeks postoperatively, the patient made an unassisted trek to the geographic North Pole. After 2 weeks in this environment, a myopic shift of more than -1.50 diopters occurred in both eyes. The myopic shift resolved over 2 months when the patient returned to a tropical climate. Using Orbscan corneal topography, we propose that the myopic shift was attributed to biomechanical changes in the cornea induced by corneal dehydration in the Arctic environment.

Differences in nasal and temporal responses of the cornea after photorefractive keratectomy

PURPOSE

To examine the differences in the biomechanical response of the peripheral regions of the cornea after photorefractive keratectomy (PRK).

SETTING

Department of Ophthalmology, Catholic University of Rome, Rome, Italy.

METHODS

Preoperative and 1-, 3-, 6-, and 12-month postoperative corneal topographies of 70 eyes that had PRK with the Technolas 217C excimer laser (Bausch & Lomb) were obtained. The eyes were divided into 4 groups according to the preoperative spherical equivalent refraction. Preoperative and follow-up topographic data were imported into custom software that computed the average composite corneal map and difference maps in each group to scientifically evaluate the corneal response to the surgery. The software was also used to analyze regional corneal changes after the laser ablation. Corneal peripheries up to 9.0 mm were evaluated.

RESULTS

The preoperative corneas had a flatter nasal periphery than temporal periphery. The corneal surfaces in the right eyes and left eyes showed a mirror symmetry. Significant differences in the regional response of the cornea were observed (P<.05), with a greater increase in the curvature of the nasal periphery than in the temporal periphery.

CONCLUSIONS

To refine modeling of the cornea, the different regional anatomic features and biomechanical responses must be considered. Modifying existing ablation algorithms to compensate for the differences between nasal and temporal corneal flattening of the preoperative corneal surface and between the nasal and temporal responses may improve the postoperative corneal shape and quality of peripheral optics.

Wavefront-guided LASIK with the Zyoptix 3.1 system for the correction of myopia and compound myopic astigmatism with 1-year follow-up

OBJECTIVE

To assess safety, efficacy, predictability, stability, and change in aberrations after wavefront-guided LASIK for myopia and myopic astigmatism.

DESIGN

Prospective, nonrandomized, self-controlled trial.

PARTICIPANTS

Wavefront-guided LASIK was performed in 97 eyes in a 1-year trial. Treated eyes had a mean subjective manifest spherical equivalent (SE) of -5.22+/-2.07 diopters (D), with a range of -0.25 to -9.00 D of myopia and 0 to -3.25 D of astigmatism.

INTERVENTION

After a microkeratome cut, a wavefront-based excimer ablation (Zyoptix 3.1) was performed. The full treatment to achieve emmetropia of an early nomogram provided by the system manufacturer was used in all procedures.

MAIN OUTCOME MEASURES

Safety, efficacy, predictability, and stability were evaluated at 1, 3, and 12 months postoperatively. Wavefront changes of higher order aberrations (HOAs) at 1 year were determined for pupil sizes of 3.5 and 6 mm.

RESULTS

At 1 year postoperatively, uncorrected visual acuity (VA) was 20/20 or better in 83% of the eyes, and 20/40 or better in 98%. The mean subjective manifest SE at 1 year was -0.25+/-0.43 D; it was within 0.50 D in 77% and within 1.0 D in 95%. No eye lost > or =2 lines of best spectacle-corrected VA (BSCVA) at 1 year postoperatively; 40 eyes gained 1 line of BSCVA, and 5 eyes gained 2 lines. The total HOA root mean square (RMS) increased on average by a factor of 1.23+/-0.57 with a 3.5-mm pupil; for the 6 mm pupil, the increase factor was 1.52+/-0.36. No change or reduction in the total HOA RMS was observed in 45.5% of the eyes for a 3.5-mm pupil and in 20.6% for a 6-mm pupil. There was a significant increase of primary spherical aberration (Z 4,0) by a factor of 4.11+/-10.17 for 3.5-mm pupils and 4.31+/-6.76 for 6-mm pupils.

CONCLUSIONS

Wavefront-guided LASIK using Zyoptix 3.1 is an effective and safe procedure for the treatment of myopia and myopic astigmatism. Although in close to half of the eyes HOAs could be reduced, there was still undercorrection and induction of HOAs with the algorithm employed.

Ocular biomechanics and biotransport

The eye transduces light, and we usually do not think of it as a biomechanical structure. Yet it is actually a pressurized, thick-walled shell that has an internal and external musculature, a remarkably complex internal vascular system, dedicated fluid production and drainage tissues, and a variety of specialized fluid and solute transport systems. Biomechanics is particularly involved in accommodation (focusing near and far), as well as in common disorders such as glaucoma, macular degeneration, myopia, and presbyopia. In this review, we give a (necessarily brief) overview of many of the interesting biomechanical aspects of the eye, concluding with a list of open problems.

Safety, efficacy, and stability indices of LASEK correction in moderate myopia and astigmatism

PURPOSE

To evaluate the visual outcomes and complications in low to moderate levels of myopia and astigmatism treated with laser-assisted subepithelial keratectomy (LASEK) with a focus on postoperative recovery.

SETTING

Massachusetts Eye and Ear Infirmary, Harvard Medical School, Boston, Massachusetts, USA.

METHODS

A retrospective analysis of a case series of eyes treated with LASEK from 1996 to July 2002 with a follow-up of 2 years was performed. The LASEK technique involved creating an epithelial flap with 25 to 45 seconds of exposure to 20% alcohol, ablating the corneal surface using 3 different excimer lasers and nomogram adjustment, and repositioning the flap and applying a bandage contact lens. The main outcome measures were uncorrected visual acuity (UCVA), efficacy index, manifest refraction, best spectacle-corrected visual acuity (BSCVA), safety index, retreatment rate, and complications.

RESULTS

One hundred seventy-one eyes (85 right eyes and 86 left eyes) of 105 patients were studied. Preoperatively, the mean spherical equivalent was -2.99 diopters (D) +/- 1.43 (SD) (range -0.38 to -7.75 D) and the mean cylinder, -0.78 +/- 0.73 D. The UCVA ranged from 20/800 to 20/32, and the BSCVA ranged from 20/63 to 20/16; the median was 20/20. One week postoperatively, 96% of eyes had a UCVA of 20/40 or better but definitive visual recovery took more than 4 weeks in some eyes. Approximately 95% of eyes were within +/-1.0 D of emmetropia after 4 to 52 weeks; the remaining 5% did not show major deviations. At 4 to 52 weeks, only 1 eye was overcorrected by more than 1.0 D of manifest refraction. The safety index remained close to 1.0 for the follow-up after 4 weeks. The efficacy index displayed a plateau at 0.9 from 1 month to 1 year. No serious complication (including recurrent erosion syndrome) was encountered. The mean follow-up was 31 weeks, and the retreatment rate was 2.9% up to 2 years of follow-up.

CONCLUSIONS

The long-term safety and effectiveness of LASEK for the correction of low to moderate myopia and astigmatism were demonstrated. The treatment effect stabilized after 4 weeks.

Flap lift for LASIK retreatment in eyes with myopia

PURPOSE

To analyze the results achieved with LASIK retreatment after lifting the original flap in a large series of patients.

DESIGN

Retrospective, noncomparative, interventional consecutive case series.

PARTICIPANTS

Two thousand four hundred twenty-two consecutive eyes undergoing LASIK surgery for myopia, including 334 eyes submitted to flap lift for LASIK retreatment.

MAIN OUTCOME MEASURES

Uncorrected visual acuity, best-corrected visual acuity, refractive error, and complications.

RESULTS

LASIK retreatment was performed in 334 eyes (14%), and the mean time between initial procedure and retreatment was 8.2+/-6.2 months. The mean spherical equivalent (SE) improved from -1.2+/-0.6 diopters (D) (range, -4.2 to +1.2 D) before retreatment to +0.2+/-0.4 D (range, -3.1 to +1.1 D) after the retreatment. The uncorrected visual acuity (UCVA) after retreatment was 20/20 or better in 58% and 20/40 or better in 92% of eyes. The mean SE was within +/-1.0 D in 96% of the patients and within +/-0.5 D in 80.5% after retreatment. Eighteen eyes (5%) lost 1 line of best-corrected visual acuity, and 4 eyes (1%) lost 2 lines.

CONCLUSIONS

LASIK retreatment surgery performed by relifting the flap was a useful procedure for correcting residual refractive errors after the primary LASIK procedure. It provided good uncorrected visual acuity, predictable results, good refractive stability, and few complications.

Wavefront-optimized ablation profiles

PURPOSE

To describe a method for calculating wavefront-optimized ablation profiles to precompensate for the spherical aberration and higher-order astigmatism induced by myopic, hyperopic, and astigmatic corneal laser corrections.

SETTING

IROC-Institut für Refraktive und Ophthalmo-Chirurgie, and Institute for Biomedical Engineering, Swiss Federal Institute of Technology, Zürich, Switzerland.

METHODS

The basic ablation profile for myopic, hyperopic, and astigmatic correction is derived from the 2nd-order Zernike representation of wavefront aberrations. Including 4th-order spherical aberration and higher-order astigmatism in the theoretical calculation of the ablation profile allows precompensation for the expected amount of higher-order aberrations (HOAs). The shapes of wavefront-optimized ablation profiles are compared with the shapes of "classic" ablation profiles for myopic and astigmatic corrections.

RESULTS

The introduction of precompensating spherical aberration and higher-order astigmatism leads to a more aspheric ablation profile with a significant increase in ablation depth (up to 35%) in the midperiphery of the optical zone. The central ablation depth remains unchanged in the myopic correction but increases by 3% in cylinder correction.

CONCLUSIONS

Wavefront-optimized ablation profiles provide a simple method to precompensate for the expected 4th-order spherical aberration and higher-order astigmatism in the average eye. Further clinical studies must be performed to prove the theoretical results; demonstrate the reduction in HOAs; and predict safety, predictability, and stability of wavefront-optimized ablation profiles.

Wavefront- and topography-guided ablation in myopic eyes using Zyoptix

PURPOSE

To evaluate the results of wavefront- and topography-guided ablation in myopic eyes using Zyoptix (Bausch & Lomb).

SETTING

Eye Research Center and Dr. Agarwal's Eye Hospital, Chennai, India.

METHODS

This observational case study comprised 150 eyes with myopia and compound myopic astigmatism. Preoperatively, the patients had corneal topography with Orbscan IIz (Bausch & Lomb) and wavefront analysis with the Zywave aberrometer (Bausch & Lomb) in addition to the routine workup before laser in situ keratomileusis (LASIK). The results were assimilated using Zylink software (Bausch & Lomb), and a customized treatment plan was formulated. Laser in situ keratomileusis was performed with the Technolas 217 system (Bausch & Lomb). The patients were followed for at least 6 months.

RESULTS

The mean preoperative best corrected visual acuity (BCVA) (in decimal equivalent) was 0.83 (20/25) +/- 0.18 (SD) (range 0.33 to 1.00) and the mean postoperative (6 months) BCVA, 1.00 (20/20) +/- 0.23 (range 0.33 to 1.50). Three eyes (2%) lost 2 or more lines of best spectacle-corrected visual acuity. The safety index was 1.20. The mean preoperative uncorrected visual acuity (UCVA) was 0.06 (20/350) +/- 0.02 (range 0.01 to 0.50) and the mean postoperative UCVA, 0.88 (20/25) +/- 0.36 (range 0.08 to 1.50). The efficacy index was 14.66. The mean preoperative spherical equivalent (SE) was -5.25 +/- 1.68 diopters (D) (range -0.87 to -15.00 D) and the mean postoperative SE (6 months), -0.36 +/- 0.931 D (range -4.25 to +1.25 D). At 6 months, the UCVA was 1.00 (6/6) or better in 105 eyes (69.93%) and 0.5 (6/12) or better in 126 eyes (83.91%). The postoperative aberrations were decreased compared with the preoperative aberrations. One eye (0.66%) had a free cap during LASIK with subsequent loss of 2 lines of BCVA and induced higher-order aberrations (HOAs). Nine patients (11.2%) complained of halos at night.

CONCLUSIONS

Wavefront- and topography-guided LASIK leads to improve visual performance by decreasing HOAs. Scotopic visual complaints may be reduced with this method.

Ectasia after laser in situ keratomileusis

Eighty-five cases of post laser in situ keratomileusis ectasia were reviewed and analyzed. Cases of keratoconus or forme fruste keratoconus were eliminated; many remaining case reports lacked key information. The current literature is unable to define a specific residual corneal thickness or a range of preoperative corneal thickness that would put an eye at risk for developing ectasia. The most logical cause for eyes without preexisting pathology to develop ectasia is a postablation stromal thickness that is mechanically unstable; this "minimal" thickness is probably specific to each eye. The preoperative and postoperative corneal thickness, measured flap thickness, and microkeratome and laser parameters used in a given case are required to determine the range of residual corneal thickness that puts the eye at risk for developing ectasia. Other as yet undetermined factors may play a role in the development of this complication.

Spherical aberration after laser in situ keratomileusis and photorefractive keratectomy

PURPOSE

To assess changes in corneal asphericity after laser refractive surgery and mathematically model possible causes of the changes.

SETTING

Cornea and Laser Eye Institute, Hersh Vision Group, Teaneck, New Jersey, USA.

METHODS

The corneal topography (EyeSys 2000) of 20 eyes was measured before and after laser in situ keratomileusis, laser-assisted subepithelial keratectomy, and photorefractive keratectomy for myopia. All preoperative and postoperative maps were analyzed using the CTView 4.0, a computer software program for determining quantitative corneal spherical aberration. To define possible mechanisms of asphericity change, 2 mathematical models of corneal ablation were constructed and theoretical postoperative corneal asphericities were determined over a range of corrections from -12.0 to +6.0 diopters. Model 1 assumes homogeneous beam fluence over the ablation zone, and model 2 accounts for a theoretical ablation rate drop off peripherally as a result of the angle of incidence of the laser beam on the cornea. Postoperative clinical corneal spherical aberration was compared to the theoretically predicted asphericity values.

RESULTS

After excimer laser procedures, all corneas had positive asphericity within the ablation zone, generally changing from a prolate to an oblate optical contour. The mean asphericity (Q) was -0.17 +/- 0.14 (SD) preoperatively and +0.92 +/- 0.70 postoperatively. The mean change in spherical aberration was +1.09 +/- 0.67 of positive asphericity; the range of asphericity change was +0.40 to +2.73 in the direction of a more oblate corneal profile. A trend toward greater change in asphericity and more oblateness was observed among eyes receiving higher correction. A mathematical model taking into account theoretical beam fluence changes across the ablation zone was highly predictive of the actual postoperative asphericity measurements.

CONCLUSIONS

The cornea within the ablation zone becomes more oblate after laser refractive surgery. A mathematical model of the change in asphericity, which accounts for the angle of incidence of the laser beam across the ablation area, predicted this change in spherical aberration. If the model is correct, possible changes in laser algorithms, delivering more ablation to the peripheral optical zone, may better retain the native corneal prolate conformation. Moreover, wavefront-guided ablations may have to consider the effects of fluence variability across the optical zone to fully correct spherical as well as other aberrations.

Laser in-situ keratomileusis after penetrating keratoplasty

Laser in situ keratomileusis (LASIK) after penetrating keratoplasty has been used more commonly for the correction of myopia or myopic astigmatism and less so for hypermetropia or hyperopic astigmatism. The primary goal after LASIK in such cases is resolution of sufficient myopia and astigmatism to allow spectacle correction of the residual refractive error and decrease anisometropia. All sutures should be removed prior to LASIK and the interval between penetrating keratoplasty and LASIK should be a minimum of 1 year. Preoperative evaluation includes refraction, slit-lamp biomicroscopy, corneal topography, and specular microscopy. The technique of LASIK surgery after penetrating keratoplasty is similar to the standard procedure. However, many variations have been described. These include maneuvers during surgery such as augmentation with arcuate cuts on the stromal bed and topographically guided LASIK. Other variations are relaxing incisions followed by LASIK surgery and sequential treatment by LASIK, that is, raising of the flap as a first stage procedure followed by ablation if required, 4 to 6 weeks later after relifting the flap in the second stage. Improvement in both uncorrected visual acuity and spectacle-corrected visual acuity, as well as a decrease in spherical equivalent, cylinder, and anisometropia, has been reported in various studies. All grafts were clear and no occurrence of wound dehiscence has been reported. Intraoperative complications include hemorrhage, microkeratome failure, flap buttonhole, dislocation, and perforation. Postoperative complications include undercorrection, decentered ablation, and regression. Re-enhancements after LASIK following keratoplasty are possible with acceptable visual outcome.

Increase in Corneal Asphericity After Standard Laser in situ Keratomileusis for Myopia is not Inherent to the Munnerlyn Algorithm

PURPOSE

Standard refractive surgery for myopia induces a shift in corneal asphericity toward positive values, resulting in an increase of spherical aberration. Analytical studies of changes in theoretical corneal shape after application of standard algorithms have yielded controversial conclusions. This study tries to resolve this controversy and discusses causes of optical degradation after refractive surgery.

METHODS

Computationally, we subtracted from real preoperative corneas the ablation depth given by the Munnerlyn equation and the parabolic approximation of the Munnerlyn equation. We compared the predicted postoperative corneal asphericity (and corneal spherical aberration) with real postoperative corneal asphericities of the same eyes, after laser in situ keratomileusis (LASIK).

RESULTS

Corneal asphericity increased after LASIK in real eyes, with an increase proportional to the amount of correction. This increase was not predicted by the computational application of the Munnerlyn algorithm, which predicted a slight decrease of corneal asphericity. The parabolic approximation of the Munnerlyn algorithm produced an increase in corneal asphericity that correlated with the amount of correction, but was less than the clinical findings.

CONCLUSION

Potential causes for increased asphericity (radial changes in laser efficiency, epithelial healing, and biomechanical response) are discussed. These conclusions are important for the design of optimized and customized ablation algorithms, since the theoretical performance of a given ablation algorithm (ie, Munnerlyn algorithm) can differ drastically from real outcomes.

Separate effects of the microkeratome incision and laser ablation on the eye's wave aberration

PURPOSE

To study the optical changes induced by the microkeratome cut, the subsequent laser ablation, and the biomechanical healing response of the cornea in normal laser in situ keratomileusis (LASIK) eyes.

DESIGN

Prospective randomized clinical trial.

METHODS

A Hansatome microkeratome was used to cut a corneal flap in one eye (study eye) of 17 normal myopic patients and a subsequent laser ablation was performed 2 months after this initial microkeratome incision. Control eyes received conventional LASIK treatments at the latter time point. The wave aberration of both the study and contralateral control eyes were measured over a 6-mm pupil with a Shack-Hartmann wavefront sensor for all preoperative, postflap cut, and postablation visits.

RESULTS

The eye's higher order aberrations had a small, but significant increase (P =.03) of approximately 30% 2 months after cutting a flap. No systematic changes were observed in nearly all Zernike coefficients from their preoperative levels at 2 months postflap cut. A significant difference between the study and control eyes was observed for one trefoil mode, Z(3)(3) (P =.04).

CONCLUSIONS

There was a wide variation in the response of individual Zernike modes across patients after cutting a flap. The majority of spherical aberration induced by the LASIK procedure seems to be due to the laser ablation and not the microkeratome cut. In addition, the total and higher order root mean square of wavefront errors were nearly identical for both the study and control eyes 3-months after the laser ablation, indicating that a procedure in which the incision and the ablation are separated in time to better control aberrations does not compromise the outcome of a conventional LASIK treatment.

Two-stage Laser in situ Keratomileusis to Correct Refractive Errors After Penetrating Keratoplasty

PURPOSE

To evaluate the effect of a two-stage laser in situ keratomileusis (LASIK) procedure on eyes with high astigmatism and/or anisometropia after penetrating keratoplasty.

METHODS

Eleven postoperative penetrating keratoplasty eyes were included in a prospective, non-controlled study. All patients had at least 4.00 D of astigmatism and/or at least 3.00 D of anisometropia and were spectacle and contact lens intolerant. Two-stage LASIK was performed; in the first stage a hinged corneal flap 160 microm in thickness and 9 mm in diameter was created. After stabilization of corneal shape (1 to 3 months after keratotomy), the corneal flap was lifted and laser refractive treatment (second stage) was performed.

RESULTS

After the first stage, a statistically significant reduction in refractive astigmatism (P<.01) was recorded. In all eyes but one, best spectacle-corrected visual acuity was maintained or improved after the procedure. Three months after the second stage, refractive astigmatism in 8 of 11 eyes (73%) was within +/- 1.00 D, and spherical equivalent refraction in 9 of 11 eyes (82%) was within +/- 1.00 D of intended correction. Preoperative irregular astigmatism persisted in three patients (3 eyes) who could not be corrected within +/- 1.00 D of refractive astigmatism and/or +/- 1.00 D of intended spherical equivalent refraction. In one eye, an interface infiltrate developed shortly after creation of the flap, and resulted in limited melting.

CONCLUSIONS

A two-stage LASIK procedure improved visual acuity and refraction in postoperative penetrating keratoplasty eyes with high astigmatism and/or anisometropia. Complications were uncommon but can lead to loss of vision.

Correlation of visual and refractive outcomes between eyes after same-session bilateral laser in situ keratomileusis surgery

PURPOSE

To determine whether between-eye refractive and visual outcomes after same-session laser in situ keratomileusis (LASIK) surgery are correlated, and to determine whether suboptimal visual and refractive outcomes in one eye are predictive of poor results in the fellow eye.

DESIGN

Observational case series.

METHODS

Retrospective chart review. A total of 484 eyes of 242 patients met inclusion criteria of having undergone same-day LASIK surgery and having 3-month refractive and visual acuity outcomes. Statistical comparisons of outcomes were performed, including between-eye Pearson correlation analysis and logistic regression models for predicting second-eye outcome. Main outcome measures were 1-month, 3-month, and 6-month uncorrected visual acuity and manifest refraction.

RESULTS

Refractive outcomes at 1, 3, and 6 months between first (right) and second eyes were not found to be significantly different. No difference in uncorrected or best spectacle-corrected visual acuity was observed between first and second eyes Refractive outcomes of spherical equivalent, sphere, and cylinder and visual outcomes of uncorrected and best spectacle-corrected visual acuity were highly correlated between the two eyes. Analysis of cases with suboptimal outcomes (> or =1 diopter from intended correction or uncorrected acuity equal to or worse than 20/40) suggests that a poor refractive or visual outcome in the first eye increases the risk of a poor outcome in the second eye by approximately 20-fold.

CONCLUSIONS

Retrospective analysis of refractive and visual outcomes from patients undergoing same-session bilateral LASIK demonstrates a high correlation of refractive and visual outcome between the eyes.

Early Results of Nidek Customized Aspheric Transition Zones (CATz) in Laser in situ Keratomileusis

PURPOSE

Standard ablation profiles for myopia and myopic astigmatism are spherical in shape and usually induce spherical aberration, often resulting in glare, halo, and other night vision problems. New ablation algorithms with aspheric transition zones may help reduce the amount of induced spherical aberrations in refractive surgery.

METHODS

Between September and November 2002, 52 eyes were treated with bilateral laser in situ keratomileusis (LASIK) using a new ablation profile with customized aspherical transition zone (CATz). Results were analyzed from examinations performed 1 day, 1 and 3 months (if available) after surgery, and compared to a standard LASIK patient group.

RESULTS

Refractive outcome of patients was within +/- 0.50 D of intended correction in 72% on day 1, and 82% at 1 month after surgery. At 1 month after LASIK, mean spherical equivalent refraction was changed from preoperative mean -6.73 +/- 2.04 D to postoperative mean -0.43 +/- 0.80 D (range -2.00 to +1.50 D). No eye lost more than 1 line of visual acuity; 10% gained 1 line and 80% remained unchanged 1 month after surgery. The topographically visible effective optical zone size was larger than in comparable treatments with standard ablation profiles. Spherical aberration was reduced and no patient complained about night vision problems after surgery.

CONCLUSIONS

Customized aspheric transition zones may help reduce induction of spherical aberration in refractive surgical correction of myopia and myopic astigmatism.

LASIK vs LASEK vs PRK: advantages and indications

The advent of the excimer laser as an instrument for use in reshaping the corneal stroma was a great step forward in refractive surgery. Laser energy can be delivered on the stromal surface in the photorefractive keratectomy (PRK) procedure or deeper on the corneal stroma by the means of a lamellar surgery in which a flap is created with the microkeratome in the laser in situ keratomileusis (LASIK) procedure. LASIK is currently the dominant procedure in refractive surgery. The main advantage of LASIK over PRK is related to maintaining the central corneal epithelium. This increases comfort during the early post-operative period, allows for rapid visual recovery, and reduces the wound healing response. Reduced wound healing correlates with less regression for high corrections and a lower rate of complications such as significant stromal opacity (haze). PRK, however, remains as an excellent option for mild to moderate corrections, particularly for cases associated with thin corneas, recurrent erosions, or a predisposition for trauma (Martial arts, military, etc.). Recently, a modification of PRK, laser subepithelial keratomileusis (LASEK), was introduced. In the LASEK procedure, an epithelial flap is created and replaced after the ablation. The benefits, if any, of the creation of an epithelial flap compared to traditional PRK are not fully appreciated. Advocates of LASEK suggest that there is less discomfort in the early postoperative period, faster visual recovery, and less haze compared to standard PRK for correction of similar levels of refractive error. Additional long-term clinical studies, along with laboratory research, will be crucial to validate these potential advantages of LASEK procedure.

Repeatability of ocular wavefront measurement

PURPOSE

To assess the repeatability of measurements of ocular aberrations using wavefront sensing in a small group of observers and to assess the potential effect of measurement error on custom corneal correction due to this variability.

METHOD

A Shack-Hartmann wavefront sensor was used to measure the ocular wavefront in nine eyes. Head position was stabilized using a dental bite bar, and the pupil was centred using a cathode ray tube monitor and circular grating. Twenty Shack-Hartmann images were collected for each measurement. Each observer had three sets of measurements taken; the first and the second after careful alignment and the final after regrasping the bite bar in the same position as for the second measurement, but without pupil realignment. The modulation transfer functions for each set were calculated, and the effect of best-aligned custom treatments on the modulation transfer function was estimated.

RESULTS

There were highly statistically significant differences in a large number of Zernike modes between the three sets of measurements. The modulation transfer functions calculated for the residual wavefronts after aligned custom treatment were below the diffraction limit. The root mean square wavefront errors were consistently better for the residual wavefronts obtained using the realigned data than using data taken without pupil realignment.

CONCLUSIONS

Sequential measurement of ocular aberrations shows statistically significant differences in a large number of Zernike modes. If aberrations determined by a single measurement are to be used in a custom correction, the resulting modulation transfer function is likely to remain below the diffraction limit. Pupil realignment is critical in reduction of the residual root mean square wavefront values to a minimum.

Conductive keratoplasty to correct residual hyperopia after corneal surgery

Conductive keratoplasty (CK) is an electrical-current-based technique for steepening the central cornea to reduce low to moderate hyperopia. We report 4 patients who had CK to correct hyperopia after laser in situ keratomileusis (LASIK) and were followed for at least 6 months. An overcorrection was noted after the CK procedure in all patients, but no sight-threatening complications arose. Conductive keratoplasty appears to be safe and well tolerated after LASIK. However, the algorithms should be modified to increase the predictability of the CK procedure in previously treated eyes.

Progressive post-LASIK keratectasia: biomechanical instability or chronic disease process?

Progressive post-LASIK keratectasia (PPLK) is a progressive deformation of corneal anatomy that occurs rarely but may have severe consequences. Using the scientific literature and new hypotheses, we attempted to determine whether PPLK is a biomechanical result of laser in situ keratomileusis (LASIK), a chronic disease process affecting individuals predisposed to the condition, or a combination of processes. We look at whether the combination of fatigue, specifically a form of dynamic fatigue, and proteolysis provides an environment conducive to the occurrence and progression of PPLK. This review may raise more questions than it answers and in so doing may move us toward a better understanding of this occasionally serious consequence of LASIK.

Vitality of epithelial cells after alcohol exposure during laser-assisted subepithelial keratectomy flap preparation

PURPOSE

To evaluate the vitality of epithelial cells after various exposure times to 20% ethanol and epithelial flap preparation in laser-assisted subepithelial keratectomy (LASEK) using the trypan blue dye test.

SETTING

University Eye Clinic Regensburg, Regensburg, Germany, and the Rayne Institute, Department of Ophthalmology, St. Thomas' Hospital, London, United Kingdom.

METHODS

Five human cadaver eyes were exposed to 20% ethanol for 15, 30, 45, 60, and 120 seconds, respectively. After an epithelial flap (as in LASEK) was prepared, the flap was deliberately cut off. The flaps were soaked in a trypan blue 0.1% solution at 37 degrees C. After 3 washes with phosphate-buffered saline (PBS), the specimens were reincubated for 30 minutes in culture medium containing 10% fetal calf serum at 37 degrees C. After an additional wash with PBS, the cells were observed with a standard inverted light microscope.

RESULTS

After 15- and 30-second exposure to 20% ethanol, most epithelial cells were vital. This changed substantially after 45 seconds, when vital and dead cells were approximately equal. Longer exposure times (60 seconds and 120 seconds) showed predominantly dead epithelial cells.

CONCLUSIONS

Exposure to 20% ethanol should be 20 to 30 seconds as the number of vital epithelial cells rapidly decreased after that. Vitality of the epithelial flap is probably a crucial factor in the dampened wound response in LASEK compared to that in photorefractive keratectomy.

Ultraviolet Corneal Photoablation

PURPOSE

The purpose of this article is to discuss missing information on the basic physical and biological processes of ultraviolet corneal photoablation and to evaluate its potential clinical implications.

METHODS

A physical description of ultraviolet laser corneal ablation that includes photothermal, photochemical, and radiative processes is proposed.

RESULTS

Unresolved issues include the nature of the primary ablation process, the tissue and biological effects of the photothermal and photochemical components of the interaction, and the static and dynamic absorption process.

CONCLUSIONS

A better understanding of the basic physics and biology of ultraviolet corneal photoablation may help us better understand, predict, and perhaps minimize the effect of tissue hydration, plume formation, and other factors that affect the predictability of ablation and the induced tissue damage.

Wavefront and Topography: Keratome-induced Corneal Changes Demonstrate That Both Are Needed for Custom Ablation

PURPOSE

To examine the effects of laser in situ keratomileusis (LASIK) flap incision and healing on the shape of the cornea and the wavefront error of the eye.

METHODS

Four weeks prior to bilateral LASIK, study subjects had a flap cut using a keratome in one eye. The fellow eye remained untouched as a control. Corneal topography and wavefront errors were measured at 1 day, 1, and 4 weeks after the flap was created. After 4 weeks, the flap was lifted and LASIK was performed. The control eye also had LASIK at this time.

RESULTS

Differences in corneal shape and wavefront error consistent with a mild hyperopic shift were seen as a result of the keratome incision.

CONCLUSION

Cutting the flap in LASIK causes subtle changes to corneal shape and the optics of the eye that may affect customized treatments. Additional work is needed to quantify these changes so that their effect can be incorporated into future treatments.

Intraoperative Corneal Topography for Image Registration

PURPOSE

To present a method to measure the three-dimensional shape of the cornea and to use the data for registration purposes in order to optimize ablation pattern alignment during corneal laser surgery.

METHODS

The three dimensional shape of the cornea can be measured with a modified fringe projection technique using UV laser pulses. A method to register these shape images is presented. The registration is done via established algorithms that use peripheral elevation data, which is not affected during the laser treatment. The method also provides a means to control the absolute amount of tissue removal. The three-dimensional registration method is compared with conventional two-dimensional eye tracking.

RESULTS

Due to the parallax of the cornea with respect to the pupil center, considerable decentration of laser ablation patterns can occur when tracking just the pupil center. Registration using three-dimensional shape measurements provides a more accurate means to control ablation pattern application.

CONCLUSIONS

A new method to register corneal shapes is discussed. It should allow monitoring the real ablation rate online during the treatment and might eventually serve as an online feedback system to control the laser ablation-induced corneal shape changes.

New techniques in lamellar keratoplasty

In the past years, several lamellar keratoplasty surgical techniques have been developed, modified or improved in the past years, including microkeratome assisted anterior and posterior lamellar keratoplasty, anterior lamellar keratoplasty using air-dissection or visco-dissection, sutureless posterior lamellar keratoplasty, LASIK for postkeratoplasty astigmatism, and excimer laser assisted keratophakia for keratoconus or to manage complications after LASIK. These procedures may continue to gain interest as alternative procedures for a penetrating keratoplasty in the treatment of various corneal disorders.

Clinical Experience With the Customized Low Spherical Aberration Ablation Profile for Myopia

PURPOSE

To evaluate postoperative spherical aberration reduction and increase of visual acuity with new Flex Scan and custom ablation software.

METHODS

We created a new customized low spherical aberration ablation profile for myopic patients (true spherical ablation). This aspheric algorithm increases ablation in the midperiphery and periphery to reduce surgery-induced spherical aberration. LASIK was performed in 143 eyes, 127 with the true spherical ablation profile, and 16 with the custom true spherical ablation profile. We used the Nidek OPD-Scan and the Nidek EC-5000 excimer laser. Ablation zones of 6.0 to 6.5 mm and transition zones of 8.5 to 9.0 mm were used. Preoperative myopia was as much as -6.50 D, and astigmatism, -2.50 D.

RESULTS

In the true spherical ablation profile group, 37.8% of eyes achieved UCVA better than 1.0 (1.2 to 2.0). In custom true spherical ablation profile eyes, 62.5% achieved UCVA better than 1.0 (1.2 to 1.5). One patient had on the day after surgery (true spherical ablation profile) an UCVA of 3.0 (preoperative UCVA was 0.1 and BSCVA was 1.0 in each eye). Preoperative and postoperative OPD-Scan analysis showed reduction of spherical-like aberrations for dilated pupil after surgery or insignificant changes for most patients. No patient had any complaints concerning quality of vision.

CONCLUSION

LASIK based on our true spherical ablation profile produced good visual acuity results for myopic patients.

The effect of water content on the 193 nm excimer laser ablation

BACKGROUND

Water content of the corneal stroma may influence excimer laser ablation and may therefore affect residual refractive error following laser in situ keratomileusis. This study reports associations between water content of hydrogel materials and laser ablation depth.

METHODS

Hydrated (n = 4) and dehydrated (n = 4) hydrogel buttons of 38%, 45%, 55% and 69% water content were ablated with the Nidek EC-5000 ArF 193 nm excimer laser, set to deliver a -6.00 DS curvature. Central curvature, optical quality and water content were measured before and after ablation. Hydrated buttons were rehydrated postablation and prior to measurement, to eliminate the effect of water removal during the procedure. The ablation depth per pulse was calculated.

RESULTS

The average ablation rate for fully hydrated buttons was 0.51 +/- 0.17 microm. The ablation rate for hydrated materials (dry component ablation) reduced with increasing water content (P < 0.001). Dry hydrogel materials (0% water content) had an average ablation rate of 0.23 +/- 0.06 microm per pulse.

CONCLUSIONS

For a constant laser energy output, lower water content materials ablated to a greater extent than higher water content materials. This model provides a simple way to assess the effect of water content and dehydration on myopic laser in situ keratomileusis.

Iatrogenic keratectasia after a deep primary keratotomy during laser in situ keratomileusis

PURPOSE

To describe a case of keratectasia after a deep primary keratotomy during an aborted laser in situ keratomileusis procedure.

METHODS

Retrospective, observational case report.

RESULTS

In a 47-year-old woman, progressive keratectasia developed after a 90% depth keratotomy during an aborted laser in situ keratomileusis procedure. This case was managed with a rigid gas permeable contact lens and consideration for penetrating keratoplasty.

CONCLUSION

Keratectasia is a reported microkeratome-related complication after laser in situ keratomileusis. Appropriate microkeratome assembly and surgeon awareness are necessary to avoid this complication.

Effect of Acute Biomechanical Changes on Corneal Curvature After Photokeratectomy

PURPOSE

Unintended hyperopic shift is a common yet poorly understood complication of phototherapeutic keratectomy (PTK) that raises fundamental questions about the etiology of corneal curvature change in PRK and LASIK. We investigated the relative contributions of ablation profile and peripheral stromal thickening to intraoperative PTK-induced central flattening, and propose a biomechanical model of the acute corneal response to central ablation.

METHODS

Fourteen de-epithelialized eye bank globes from seven donors underwent either broadbeam ablation (approximately 100-microm depth, no programmed dioptric change) or sham photoablation in paired-control fashion. Peripheral stromal thickness changes and the pattern of thickness loss across each ablation zone were evaluated by optical section image analysis as predictors of acute corneal flattening.

RESULTS

Relative to sham ablation, keratectomy caused significant anterior corneal flattening (-6.3+/-3.2 D, P = .002). Concomitant peripheral stromal thickening (+57+/-43 microm, P = .01) was a significant predictor of acute hyperopic shift (r = 0.68, P = .047). Ablation pattern bias did not consistently favor hyperopia and was a poor lone predictor of hyperopic shift.

CONCLUSIONS

Unintended keratectomy-induced hyperopic shift is replicable in a human donor model and is associated with significant thickening of the unablated peripheral stroma. This biomechanical response may have a considerable impact on early refractive outcomes in PTK, PRK, and LASIK.

Effect of hinged lamellar keratotomy on postkeratoplasty eyes

PURPOSE

To evaluate the effect of a hinged lamellar keratotomy on refraction, vision, and corneal topography of postkeratoplasty eyes with high-degree astigmatism.

DESIGN

Noncomparative, interventional case series.

PARTICIPANTS

A hinged lamellar keratotomy was performed on nine eyes of nine patients at least 9 months after penetrating keratoplasty and with high-degree astigmatism. All patients were spectacle and contact lens intolerant.

INTERVENTION

A superiorly hinged lamellar keratotomy (corneal flap), 160 microm in thickness and 9 mm in diameter, was created on all eyes included in this study. Each patient was examined 1 day, 1 month, and 3 months after surgery.

MAIN OUTCOME MEASURES

Uncorrected visual acuity, best spectacle-corrected visual acuity, refraction, computerized analysis of corneal topography.

RESULTS

At each postoperative examination time, there was a significant reduction in both average spherical equivalent (P < 0.05) and average absolute value of astigmatism (P < 0.01) over mean preoperative values. The major changes were seen as early as 1 day after surgery, but both progression and regression of the effect were documented at later postoperative examinations. In all patients best spectacle-corrected acuity was maintained or improved after the procedure. Postoperatively, four patients could be successfully corrected either with spectacles (n = 2) or with gas-permeable contact lenses (n = 2). There were no surgical flap or corneal graft complications.

CONCLUSIONS

Hinged lamellar keratotomy improves vision and refraction of postkeratoplasty eyes with high-degree astigmatism. In some cases it may be so effective as to make planned excimer laser treatment unnecessary.

Broad Beam vs. Flying Spot Excimer Laser: Refractive and Videokeratographic Outcomes of Two Different Ablation Profiles After Photorefractive Keratectomy

PURPOSE

To assess the refractive outcomes and videokeratographic patterns in photorefractive keratectomy (PRK) of two last-generation excimer lasers: broad beam and flying spot lasers.

METHODS

Forty eyes were treated for a mean myopic correction of -5.59+/-2.17 D with a Ladarvision excimer laser (Autonomous Technology Corp). We compared the refractive outcomes and the videokeratographic patterns with those of 40 eyes treated with the Summit Apex Plus laser (Summit Technology Inc) for a mean myopic correction of -5.60+/-2.24 D.

RESULTS

During the 6-month follow-up, we found no significant difference in uncorrected and spectacle-corrected visual acuity, haze grade, regularity index, or decentration. The eyes treated by Ladarvision showed a lower refractive error at 1 month (P = .04), a lower incidence of central islands at 1 (P = .003) and 3 months (P = .04), a wider mean effective ablation area at 1 (P = .0004), 3 (P = .0000009), and 6 months (P = .0000000004), a less steep ablation edge at 1 (P = .0026), 3 (P = .015), and 6 months (P = .011).

CONCLUSIONS

The small beam excimer laser provides better videokeratographic outcomes, which lead to quicker refractive stability and better visual performance.

Changes in measured intraocular pressure after hyperopic photorefractive keratectomy

PURPOSE

To investigate the effect of hyperopic photorefractive keratectomy (PRK) on intraocular pressure (IOP) measurements.

SETTING

University of Ottawa Eye Institute, Ottawa Hospital, Ottawa, Canada.

METHODS

In this retrospective cohort study, IOP and central corneal thickness (CCT) were measured preoperatively and at 1, 2, 3, 6, 12, 18, and 24 months in 191 eyes that had hyperopic PRK with the VISX Star excimer laser. All corrections applied were between +1.00 and +6.50 diopters (D) of sphere and less than 3.75 D of cylinder.

RESULTS

At all postoperative examinations, the mean IOP in the hyperopic PRK group was 1.0 to 1.8 mm Hg lower than the preoperative IOP (P <.001). A large range of IOP changes was found across the population; eg, at 6 months, 49% of the eyes had a change in IOP from baseline of at least +/-3 mm Hg. A mean reduction of 19 microm of CCT was found with pachymetry after surgery (P < .001). The change in IOP readings postoperatively was not correlated with age, sex, keratometric readings, or applied correction. Changes in IOP were strongly correlated with preoperative IOP at all time points and with preoperative CCT at 18 and 24 months (P < .001). After hyperopic PRK, the measured IOP was more likely to increase in patients with preoperative IOPs less than 14.5 mm Hg and more likely to decrease in patients with preoperative IOPs above 14.5 mm Hg.

CONCLUSION

Changes in IOP after hyperopic PRK were similar to changes after myopic PRK, despite only minimal changes in the CCT. This suggests that hyperopic PRK results in biomechanical effects that modify the elastic properties of the cornea beyond the changes in rigidity expected from central corneal thinning. There was a strong negative correlation between the measured preoperative IOP and the change in IOP postoperatively that was likely the result of regression of the mean effect.

Visual Rehabilitation With Contact Lenses After Laser in situ Keratomileusis

PURPOSE

Laser in situ keratomileusis (LASIK) has become the surgical treatment of choice for moderate myopia and is in widespread use globally. Visual potential is sometimes limited due to irregular corneal topography following surgery.

METHODS

A retrospective chart review of 35 eyes of 22 patients requiring visual rehabilitation following LASIK was performed. Four contact lens designs were used and evaluated for appropriate cornea-contact lens fitting relationship.

RESULTS

Mean best contact lens-corrected visual acuity of 20/25 was significantly better than best spectacle-corrected visual acuity of 20/40. The average time from surgery to contact lens fitting was 8 months, with almost half (10/22) being fitted at 4 months. An aspheric design with 0.17 mm of axial edge lift was used most commonly. Lens diameters ranged from 9.2 to 10.9 mm, with a mean diameter of 10.2 mm. The contact lens base curve to cornea relationship would suggest an initial base curve selection to be approximately 2.1 D steeper than the mean postoperative keratometric power.

CONCLUSIONS

Rigid gas permeable contact lenses can improve visual function in patients with irregular corneal topography after LASIK.