Treatment of post-keratoplasty astigmatism by topography supported customized laser ablation

Refractive surgery after deep anterior lamellar keratoplasty: a review of the literature

PURPOSE

The main objective of this work is to present an updated review of the different surgical procedures for the correction of residual refractive errors following deep anterior lamellar keratoplasty (DALK) surgery.

METHODS

A review of the literature was conducted using PubMed, Web of Science, and Scopus databases. The search was conducted in January 2022 and was limited to articles published in peer-reviewed journals. The information extracted from each publication included sample size, mean follow-up time, pre- and post-operative uncorrected (UDVA) and corrected distance visual acuity (CDVA), pre- and post-operative refraction and spherical equivalent (SE), safety and efficacy indexes and complications.

RESULTS

Residual ametropias, mainly high astigmatism and myopia, and the resulting anisometropia are likely to occur following DALK. They become a limiting factor and may lead to unsatisfactory visual restoration, therefore affecting patients' quality of vision and life. Alternative surgical interventions may be required to treat this residual ametropia, such as corneal refractive surgery or intraocular lens implantation. A total of 47 relevant articles were studied in detail. Different refractive surgery techniques have been shown to be effective and safe for the correction of ametropia following the DALK procedure and to improve the patient's quality of vision, although more research is needed to confirm long-term results.

CONCLUSION

The final refractive technique will depend on different factors, such as the amount of ametropia, the condition of the cornea or the patient's individual needs, economics, and occupational demands.

Topography-Guided Trans-Epithelial No-Touch Photorefractive Keratectomy for High Irregular Astigmatism After Penetrating Keratoplasty: A Prospective 12-Months Follow-Up

Purpose

To describe the efficacy and safety of topography-guided trans-epithelial no-touch photorefractive keratectomy (PRK) for the correction of highly irregular astigmatism after penetrating keratoplasty (PK).

Patients and Methods

A prospective study was conducted on 12 eyes of 12 patients affected by highly irregular astigmatism after PK for keratoconus. Each patient underwent a single-step topography-guided trans-epithelial ablation (CIPTA^®2 software, iVis Technologies). Corneal topography data as well as uncorrected (UDVA) and corrected distance visual acuity (CDVA) and spherical equivalent (SEQ) were collected preoperatively (T0) and at 1 (T1), 3 (T2) and 12 (T3) months after surgery.

Results

Mean UDVA and CDVA significantly improved, respectively, from 1.22±0.17 and 0.18±0.03LogMAR at T0 to 0.63±0.17 (CI 95%, P<0.001) and 0.04±0.03LogMAR at T3 (CI 95%, P<0.001). Mean SEQ significantly decreased from −3.75±1.32 to –1.60±1.32D (CI 95%, P<0.02). Mean subjective refractive astigmatism (SRAST) and mean keratometry astigmatism (SimK) significantly decreased, respectively, from 7.83±0.98 and 8.10±1.48D to 2.83±0.98 and 5.29±1.48D (CI 95%, P<0.001). Corneal Morphological Irregularity index (CMI) significantly decreased from 62.76±7.26 µm to 23.24±7.26 µm (CI 95%, P<0.001). Apart from a single episode of graft rejection 5 days after ablation, successfully reverted with topical steroids, no other complications were noted. A mild corneal haze was observed in two eyes (16.7%) at 3 months post-PRK, and no regression was observed at 12 months.

Conclusion

Our study demonstrates the safety and long-term effectiveness of a trans-epithelial topography-guided ablation in the treatment of post-PK highly irregular astigmatism.

Efficiency and Safety of Scleral Lenses in Rehabilitation of Refractive Errors and High Order Aberrations After Penetrating Keratoplasty

OBJECTIVES

Visual rehabilitation after penetrating keratoplasty (PK) has difficulties linked to the high spherical and/or cylindric refractive errors with high order aberrations (HOAs) based on the ocular surface irregularities. We aimed to present the scleral contact lens (SCL)-induced complications and improvements in refractive errors and HOA with SCL fitting in post-PK patients.

METHODS

In this prospective study, 38 eyes of 35 patients who underwent PK and using SCLs were included. Uncorrected visual acuity, best-corrected visual acuity (BCVA), spherical equivalence, manifest astigmatism (Cyl), keratometry, and wavefront analyses of HOAs with corneal topography of all patients were measured before SCL and after 8 hrs of SCL wearing-on. The endothelial cellular density (ECD) changes at 6 months were also recorded. Any complications related to SCL were noted.

RESULTS

All patients showed an improvement in visual acuity with SCL. Uncorrected visual acuity before SCL fitting was 1.15±0.26 log of minimal angle of resolution (logMAR) and BCVA was 0.84±0.24 logMAR. The contact lens-corrected visual acuity decreased to 0.13±0.09 logMAR. Spherical equivalence, Cyl, and keratometry parameters decreased significantly with SCL. The anterior corneal HOAs, coma, and astigmatism coefficients decreased significantly. Conjunctival prolapse (in one eye) and graft rejection episode (in two eyes) were observed during follow-up time (14.25±1.3 months) and they restarted to use SCLs after treatment. The ECD decrease was similar with those who not using SCLs after PK in the literature.

CONCLUSION

Our results indicate that scleral lenses may be a safe and effective option for the treatment of corneal astigmatism and HOAs associated with PK.

Use of Scleral Lenses and Miniscleral Lenses After Penetrating Keratoplasty

PURPOSE

To examine the clinical outcomes of scleral lenses for visual rehabilitation after penetrating keratoplasty (PK).

METHODS

A retrospective review was conducted for 34 patients (48 eyes) who had a history of prior PK and were fit with scleral lenses between October 2009 and December 2013 at the UC Davis Eye Center.

RESULTS

The most common initial indication for PK was keratoconus in 27 eyes (56%). Thirty-three eyes (69%) had previously been fit with other types of contact lenses, with small-diameter rigid gas-permeable lenses being the most common. The improvement in best-corrected visual acuity with a scleral lens compared with prior spectacle refraction or other contact lens was a mean of two best-corrected visual acuity lines. Forty-four eyes (91.7%) achieved functional vision with best scleral lens-corrected visual acuities of 20/40 or better. Patients who continued wearing scleral lenses were significantly more likely to report "good" subjective vision compared with patients who abandoned scleral lens wear (P=0.009), although change in objective best-corrected visual acuity did not differ significantly. There were no cases of infectious keratitis. Six eyes (12.5%) developed graft rejection; 3 were able to resume scleral lens wear. Nineteen eyes (39.5%) discontinued scleral lens wear for various reasons, the most common reason for discontinuation of lens wear was difficulty with scleral lens insertion or removal (8 eyes, 42.1%).

CONCLUSION

Scleral lenses are effective and safe in patients who have had PK. There was a mean gain in visual acuity, with the majority of patients achieving 20/40 vision or better. The patient's subjective perception of vision was a significant factor in determining whether scleral lens wear was continued or abandoned.

Clinical results of topography-based customized ablations for myopia and myopic astigmatism

PURPOSE

To evaluate clinical outcomes after primary treatments for myopia and myopic astigmatism with topography-based ablation profiles on the Allegretto Wave Eye-Q platform (Alcon Laboratories Inc).

METHODS

Records of patients who underwent topography-guided, Q-adjusted LASIK were retrieved and analyzed. Patients with mean refractive spherical equivalent up to -16.00 diopters (D) and cylindrical errors up to 6.00 D were included (2051 eyes). Refractive outcomes and visual acuities were analyzed preoperatively and 3 months postoperatively.

RESULTS

Manifest spherical refractive error decreased from -5.05 +/- 2.33 to +0.04 +/- 0.35 D. Manifest cylindrical refractive error decreased from 0.83 +/- 0.75 to 0.23 +/- 0.22 D. At 3 months postoperatively, 86.1% (1766 eyes) had a spherical equivalent refraction within +/- 0.50 D of targeted refraction. The number of eyes with uncorrected distance visual acuity of 20/16 and 20/20 was 851 (41.5%) and 1495 (72.9%), respectively. At 3 months, 0.10% (2 eyes) lost more than 2 lines of corrected distance visual acuity and a loss of 2 lines occurred in 0.24% (5 eyes). All patients who lost lines of visual acuity were found to have corneal erosions related to dryness and recovered vision on further follow-up.

CONCLUSIONS

The results of this retrospective study demonstrate refractive predictability, efficacy, and safety comparable to other treatment modalities such as wavefront-optimized and wavefront-guided treatments. Results are also similar to those reported with other topography-guided systems.

Corneal higher-order aberrations after Descemet's membrane endothelial keratoplasty

PURPOSE

We compared corneal higher-order aberrations (HOAs) in eyes after Descemet's membrane endothelial keratoplasty (DMEK), Descemet's stripping automated endothelial keratoplasty (DSAEK), and penetrating keratoplasty (PK), and in a control group that had not undergone surgery.

DESIGN

Retrospective analysis of clinical data.

PARTICIPANTS

Thirty eyes of 30 patients who had undergone standard DMEK, 20 eyes of 20 patients after DSAEK, 20 eyes of 20 patients after PK, and 20 eyes of 20 controls were analyzed.

METHODS

In addition to standard postoperative examinations, each participant was analyzed with the Pentacam high-resolution rotating Scheimpflug imaging system (Pentacam HR, Oculus, Wetzlar, Germany). Data were compared between groups.

MAIN OUTCOME MEASURES

Visual acuity and HOAs.

RESULTS

The mean follow-up was 6.5 ± 1.2 months after DMEK, 22.6 ± 11.8 months after DSAEK, and 103.1 ± 74.2 months after PK. There were no statistically significant differences for the anterior 4.0-mm zones between the DMEK group and the controls or between the DMEK and DSAEK groups. The DMEK procedure compared with PK showed statistically significant differences in all terms for the 4.0-mm zones. All combined Zernike terms for mean posterior aberrations of the central 4.0-mm zones showed statistically significant higher aberrations for DMEK compared with controls. The DMEK procedure compared with DSAEK showed statistically significant lower mean values for all combined Zernike terms, except for coma and coma-like terms in the central 4.0-mm zones of the posterior corneal surface. Compared with PK, DMEK showed statistically significant lower mean values for all combined Zernike terms for the central 4.0-mm zones of the posterior corneal surface, except for spherical aberration (SA) and SA-like terms. Best spectacle-corrected visual acuity (BSCVA) after DMEK was statistically significantly better than after DSAEK (P=0.001) and PK (P=0.005). There was no statistically significant difference when BSCVA was compared with controls (P=0.998).

CONCLUSIONS

Both DSAEK and PK exhibit increased posterior corneal HOAs even years after surgery. Patients receiving DMEK display only slight changes in posterior corneal HOAs.

Evaluation of Deep Lamellar Endothelial Keratoplasty Surgery Using Scatterometry and Wavefront Analyses

OBJECTIVE

To determine if postsurgical corneal interface abnormalities after deep lamellar endothelial keratoplasty (DLEK) cause increased light scattering or wavefront aberrations that may help to explain decreased best-corrected visual acuity in DLEK patients compared with penetrating keratoplasty (PK) patients.

DESIGN

Prospective comparative case series.

PARTICIPANTS

Clinically clear corneas of 4 eyes that had undergone DLEK surgery and 4 eyes that had PK were studied. Normal control data for light scattering was collected from 12 right eyes and 11 left eyes with normal corneas.

METHODS

Corneal light scattering was measured with a scatterometer designed at the Johns Hopkins Applied Physics Laboratory, and wavefront analysis was performed using standard methods with a Hartmann-Shack wavefront sensor.

MAIN OUTCOME MEASURES

Corneal scattering measurements were normalized by taking the ratio of the subject's corneal light scattering to a reference material. A scattering index was calculated as the ratio of the normalized scattering for a given patient's cornea to the average scattering of normal corneas. Astigmatism and higher-order aberrations were analyzed using standard data output from wavefront analysis and Zernike polynomial decomposition.

RESULTS

The mean scattering index was significantly higher after DLEK (1.78+/-0.29, mean+/-standard deviation [SD]) than after PK (1.03+/-0.27; P = 0.043). The higher-order root mean square (RMS) wavefront error was significantly higher after PK (0.71+/-0.11 microm, mean+/-SD) than after DLEK (0.44+/-0.12 microm; P = 0.029). Zernike polynomial decomposition of the wavefront aberrations revealed that regular corneal astigmatism was the most important aberration component for both PK and DLEK, but the PK patients had significantly more regular astigmatism (1.7+/-0.45 diopters [D], mean+/-SD) than did the DLEK patients (0.84+/-0.27 D; P = 0.029).

CONCLUSIONS

Our data quantitatively support subclinical corneal haze as an explanation for the limited visual acuity after DLEK as compared with PK. Intraoperative or postoperative modifications to reduce stromal haze after DLEK may result in better visual acuity outcomes.

[LASIK treatment for graft astigmatism]

OBJECTIVE

To analyze the advantages of LASIK as a treatment for postkeratoplasty residual astigmatism.

PATIENTS

and

METHOD

Retrospective, noncomparative, monocentric study on 14 patients with penetrative keratoplasty. Visual acuity, objective refraction, intraocular pressure, slit lamp biomicroscopy, videotopography, pachymetry, and patient satisfaction were studied before and after surgery.

RESULTS

Refraction was stable 6 months after treatment in 93% (13/14) of the patients. The mean uncorrected visual acuity increased from 0.16 to 0.4. Two out of four patients (14.3%) were retreated because visual acuity did not improve after the first treatment; 35% did not need any optical correction after the intervention. The measured astigmatism showed a significant decrease, from 65% greater than 5 D before treatment to 82% under 2 D after treatment. Only two out of 14 patients (14.3%) were not satisfied. No intraocular or postoperative complications were reported in this study.

CONCLUSION

LASIK seems to be an interesting option in the treatment of postkeratoplasty residual astigmatism. This study has shown its efficacy and safety in this indication. Its originality lies in the one-step procedure, with the flap and photoablation done in a single step. However, it is still dependent on the operator and most particularly the equipment (laser, microkeratome, aberrometry).

Incidence of Rigid Gas-Permeable Contact Lens Wear After Keratoplasty for Keratoconus

PURPOSE

To review retrospectively the charts of all 190 patients who underwent penetrating keratoplasty for keratoconus in one hospital during a 5-year period (1995-2000), with special attention paid to contact lens fitting.

METHODS

The frequency of postoperative contact lens use, the time to fit lenses after grafting, tolerance and visual acuity, and postoperative risks for the graft were studied.

RESULTS

The authors successfully fitted large-diameter (12 mm) tricurve rigid gas-permeable contact lenses for 90 (47%) of 190 penetrating keratoplasty patients with good tolerance. There were nine dropouts, and 91 eyes were corrected in another way. Fitting contact lenses after grafting usually started after 8.5 months (range, 1-60 months). The average tolerance was 9.2 hours a day (range, 2-12 hours), and best-corrected visual acuity was 20/25 (range, 20/16-20/200). The average follow-up was at least half a year. The average age of the patient at the first lens fitting was 36.2 years (range, 14-75 years). There was no increased risk in graft rejection.

CONCLUSIONS

Twelve-millimeter rigid gas-permeable contact lens wear was successful in 47% of patients who underwent penetrating keratoplasty for keratoconus. It does not interfere with the use of chronic postoperative topical medication, nor does it increase the risk of corneal graft rejection. It is necessary to recommend likely use of contact lenses to patients who have undergone grafting surgery.

Refractive surgery after corneal transplantation

PURPOSE OF REVIEW

Many patients who have undergone corneal transplantation are unable to achieve satisfactory visual acuity with spectacle and contact lens correction alone. For these patients, refractive surgery becomes a viable option to reduce the post-keratoplasty ametropia. With the many recent advances in refractive surgery for naturally occurring refractive error, new possibilities arise for application to this complicated set of patients. This review discusses key recent developments in refractive surgery after corneal transplantation.

RECENT FINDINGS

The biomechanical effects of incisional keratotomy on post-keratoplasty corneas continue to be studied, and these techniques remain a common and simple method of reducing astigmatism. Photorefractive keratectomy, previously problematic for regression and haze formation, is gaining new prominence as early experience with the adjunctive use of mitomycin C has demonstrated good results. Long-term studies with laser in-situ keratomileusis (LASIK) have continued to show good safety and efficacy. Modern developments in cataract surgery appear to have lower incidences of graft rejection and failure. Developments in lens implantation technology continue to offer expanding options for intraocular refractive surgery.

SUMMARY

Although visual rehabilitation after corneal transplantation remains a formidable challenge, developments in refractive surgery for naturally occurring ametropias directly translate into an improved ability to help these most challenging refractive cases. Continued research will bring about improved efficacy while maintaining a high level of safety.

Strategic Planning in Topography-guided Ablation of Irregular Astigmatism After Laser Refractive Surgery

PURPOSE

To identify an optimal customized ablation strategy in the treatment of eyes with secondary irregular astigmatism.

METHODS

Corneal anterior surface elevation maps of 50 eyes with secondary irregular astigmatism after decentered laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK) and 50 virgin eyes were used for customized ablation simulations. Two ablation simulations with targeted postoperative surfaces perpendicular to either the visual or corneal morphological axis were made for each eye. All ablations were programmed for correction of corneal irregularities, including corneal astigmatism. The manifest refractive error was not corrected. Optical diameter was 6.5 mm and total diameter was 7.5 mm. Maximum ablation depths and maximum transition zone gradients were registered and analyzed.

RESULTS

In eyes with secondary irregular astigmatism, mean maximum ablation depth was 48.21 +/- 25.96 microm and 26.31+/- 14.08 microm, whereas mean maximum transition zone gradient was 29.07 +/- 25.15 microm and 9.88 +/- 6.41 microm in ablation simulations based on the visual and corneal morphological axes, respectively. The difference between the ablation strategies was highly statistically significant for both parameters (P < .001). In virgin eyes, only a minor difference was noted between the visual and corneal morphological axis ablation simulations (P = .15 for maximum ablation depths and P=.19 for maximum transition zone gradient).

CONCLUSIONS

In secondary irregular astigmatism, ablation based on the corneal morphological axis appears to minimize corneal tissue consumption and allows a smoother transition zone.

Laser in situ Keratomileusis for High Myopia Using a Small Ablation Zone and Large Aspheric Transition Zone

PURPOSE

To evaluate the efficacy and predictability of laser in situ keratomileusis (LASIK) using the Nidek NAVEX system with a small ablation (optical, OZ) zone and a large aspheric transition zone (OATz).

METHODS

We report a prospective nonrandomized study of LASIK for high myopia using a small ablation in 23 eyes of 18 patients. The optical zone was between 3 and 3.8 mm and transition zone (TZ) was between 7 and 7.8 mm. Preoperative examination included best-spectacle corrected visual acuity (BSCVA), refraction, and corneal topography. All eyes were measured by the Nidek OPD-Scan and data were imported to FinalFit software. Aspheric profile (OATz) numbers 5 to 7 were used. BSCVA, uncorrected spectacle visual acuity (UCVA), refraction, and subjective evaluation of glare and night vision by questionnaire were recorded after surgery.

RESULTS

At baseline, mean spherical equivalent refraction was -7.03 +/- 2.39 D (range -5.50 to -15.50 D). Postoperative UCVA was better than 20/25 in 95.65% of eyes 3 months after LASIK. Postoperative refraction was within +/- 0.50 D of emmetropia in 91.3% of eyes and within +/- 1.00 D in all eyes. No eye lost lines of BSCVA; 52.17% of eyes gained 1 line and 13.04% of eyes gained 2 lines. No patient reported significant glare or night vision problems on subjective questionnaire.

CONCLUSION

LASIK with a small ablation (optical) zone and high aspheric transition zone using the Nidek NAVEX system was effective, predictable, and relatively safe for correction of high myopia.

Long Term Analysis of LASIK for the Correction of Refractive Errors After Penetrating Keratoplasty

PURPOSE

To determine the long-term safety and effectiveness of laser in-situ keratomileusis (LASIK) in the treatment of refractive errors following penetrating keratoplasty (PK).

METHODS

A retrospective review was performed of 57 eyes of 48 patients with anisometropia or high astigmatism who were unable to wear glasses or a contact lens after PK and who underwent LASIK for visual rehabilitation. Uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity (BCVA), and corneal transplant integrity were recorded before surgery as well as up to 60 months after LASIK.

RESULTS

The mean follow-up after the LASIK was 21.4 +/- 14.2 months (range 3 to 60 months). Mean preoperative spherical equivalent (SE) was -4.19 +/- 3.38 D. The mean preoperative astigmatism was 4.67 +/- 2.18 D. Preoperative BCVA was 20/40 or better in 42 eyes (74%). At 2 years the mean SE was -0.61 +/- 1.81 D, and mean astigmatism was 1.94 +/- 1.35 D for the 28 eyes with follow-up. UCVA was 20/40 or better in 12 eyes (43%), and BCVA was 20/40 or better in 24 eyes (86%) at 2 years. A gain in BCVA of 1 line or more was seen in 8 eyes (29%). Two eyes (7%) had loss of 2 or more lines of BCVA at 2 years. There were 9 eyes (16%) that developed epithelial ingrowth. Five eyes (9%) in this series had repeat corneal transplants.

CONCLUSIONS

LASIK is effective for reducing ametropia after PK. Proper patient counseling is necessary because the results of LASIK after PK are not as good as, and complications are more frequent than in eyes with naturally occurring myopia and astigmatism. Complications are especially common in patients with mismatch of the donor and host cornea or in those with poor endothelial cell function.

Accuracy and Precision of EyeSys and Orbscan Systems on Calibrated Spherical Test Surfaces

BACKGROUND

It is important to know how well the surface topography can be measured with current devices for corneal topographic analysis. There are several applications that need an accurate and precise method to measure corneal shape and variations, such as the effect of contact lens wear and the different refractive surgery techniques.

PURPOSE

The aim of this study is to compare the accuracy and reproducibility of the measurement of the central curvature on calibrated steel balls using the EyeSys videokeratoscope and the Orbscan corneal topography system.

METHODS

The videokeratoscope (EyeSys Corneal Analysis System 2000, version 3.1) and the Orbscan corneal topography system (Orbscan II version 3.0) were used by four trained investigators to measure a series of five uniform and calibrated test surfaces with known radius of curvature ranging from 6.13 to 9.00 mm.

RESULT

No statistically significant difference was found between the videokeratoscope and Orbscan systems in relation to accuracy or precision. The 95% confidence limits showed a close agreement between both instruments. The mean bias was less than +/-0.05 mm for both devices. The precision of the instruments was found to be similar.

CONCLUSION

The EyeSys seems to measure more accurately, but the accuracy of the Orbscan was also acceptable, suggesting that both instruments are accurate and precise enough for research and clinical purposes. However, further studies of accuracy and repeatability of topographical measurements on human eyes generated by different topographers are necessary.

Comparison of Conventional Versus Wavefront-guided Laser in situ Keratomileusis in the Same Patient

PURPOSE

To verify whether there is any difference in visual outcome, including higher order aberrations, after performing laser in situ keratomileusis (LASIK) with conventional ablation and wavefront-guided ablation in myopic eyes of the same patient.

METHODS

This was a prospective randomized study of 20 myopic eyes (10 patients) who had LASIK using the Nidek NAVEX excimer laser system. Wavefront-guided customized ablation was used in the first eye of the patient (study group) and the other eye of the same patient was operated with conventional ablation (control group). Mean refractive error was similar between left and right eyes of the same patient. Preoperative examination included higher order aberration by Nidek OPD-Scan. Uncorrected and best spectacle-corrected visual acuity and higher order aberrations were recorded postoperatively.

RESULTS

Preoperative and postoperative best spectacle-corrected visual acuity was better than 20/40 (100%) after LASIK in both the conventional ablation and wavefront-guided customized ablation groups. Postoperative refraction was within +/- 0.50 D of emmetropia: 90% in the conventional group and 100% in the wavefront-guided group. No statistically significant difference in postoperative higher order aberrations was found between groups.

CONCLUSION

Postoperative visual outcome with both conventional LASIK and wavefront-guided customized ablation was not significantly different. Higher order aberrations did not significantly increase postoperatively in either group.

Phototherapeutic keratectomy: 12 years of experience

BACKGROUND

Phototherapeutic keratectomy (PTK) has been employed as a surgical tool to treat corneal disease for more than 10 years. The laser has made it possible to remove superficial corneal opacities and thereby restore vision. The 193 nm ultraviolet light separates molecules and splits molecules in biological tissue, thereby ablating it. About 0.25 microm of tissue is ablated by each pulse. The development of the excimer laser technique has been fast. It has principally focused on refractive surgery but has also benefited PTK.

CORNEAL DYSTROPHIES

The ability to delay or postpone corneal grafting in superficial corneal dystrophies represents a very important achievement. Map-dot-fingerprint dystrophy or basal membrane dystrophy is a common indication for PTK. Other dystrophies such as Meesman's, Reis-Bückler's, Thiel-Benke's, granular, macular, lattice and Schnyder's can be treated, although with differing degrees of success and varying rates of recurrence. Subepithelial scarring in Fuchs' dystrophy has been ablated. Other trials have involved the removal of substantial parts of the stroma in order to reduce the load on the endothelium. Recurrent dystrophic changes can likewise be removed from corneal grafts and thus prevent the need for regrafting.

RECURRENT EROSIONS

Laser treatment has made it possible to manage wound-healing problems better after recurrent erosions. Recurrent erosions are the most common indications for PTK: several studies show good and persistent effects with this type of treatment. Persistent epithelial defects of various origins, among them corneal ulcers resulting from allergic disease, can likewise be treated.

SCAR TISSUE

Scars after surgery such as pterygeum excision can be removed. Smooth muscle actin containing fibroblasts in old scars should be given special consideration in PTK. Excimer laser surgery can be successfully combined with conventional surgery to remove excessive scar tissue, Salzmann's nodules and very flaky and coarse band keratopathy. Irregular corneal surfaces following ulcers and injuries pose problems that have so far proved difficult to overcome. Thinning is often seen after bacterial corneal ulcers or after herpes simplex keratitis. A rough or uneven surface can be made smoother by using modulators during treatment by casting a new surface under a hard contact lens (PALM technique), a surface that is then projected into the stroma by laser ablation. Modern techniques linking the excimer laser with computerized corneal topography and wavefront analysis promise to further improve the smoothing capacities of lasers and to increase the quality of optical results.

COMPLICATIONS

The most feared complication of PTK is the postoperative infection. These are rare. Haze is usually not prominent but scar tissue formation of a more persistent type has been noted after laser surgery in eyes with pre-existing surgical scars. Keratectasia has been described after PTK. Failure due to deep opacities or a surface that is too uneven is a more common frustration. This paper reviews advances in excimer laser treatment of corneal disease.

Optical coherence tomography for in situ monitoring of laser corneal ablation

OBJECTIVE

To improve the precision of refractive surgery, a new approach for determination of the removed corneal thickness profile in situ with laser ablation by optical coherence tomography (OCT) is developed.

STUDY DESIGN/MATERIALS AND METHODS

The traditional method for precision (less than 10 microm) measurements of intraocular distances is based on the use of the reflected component of probing radiation. This component is characterized by a small range of operating angles between a probing beam and a normal to the surface under study. To enhance this range of operating angles we suggest using a light component backscattered from a biological object. This will enable precision measurements over the entire surface of the cornea without any changes in the orientation between a probing beam and the eye, a necessary condition for in situ monitoring of laser refraction correction in the eye. We suggest a specially developed algorithm of OCT signal processing to measure the corneal thickness by the backscattered light component for a single longitudinal scan (A scan). The corneal thickness profile is obtained by a series of such A scans acquired by successively scanning a probing beam along the corneal surface. The thickness profile of removed layer is determined by changes in the corneal thickness profile in the process of ablation. When the cornea is ablated by a beam with a fixed transverse profile, we propose using integral characteristics of the ablated layer profile, for example, the maximum ablation depth, as criteria of changes in refractive power of the eye. The measurement precision by these characteristics is considerably higher than by a single A scan. Since the cornea is a poorly scattering medium, the Fourier filtering is employed to increase reliability and precision of the method. Model experiments on monitoring the ablation process in a lavsan film and ex vivo human cornea are described. Preliminary experiments on in vivo measurements of human corneal thickness are performed.

RESULTS

In model experiments the precision of measurement of laser ablation depth by one A scan was 5-20 microm, depending on the signal-to-noise ratio (SNR), whereas the precision of measurement of laser ablation depth as the integral characteristic of the ablated layer profile was 0.3-5 microm. The experimental results showed that at small SNR Fourier filtering might considerably increase reliability and precision of measurements. When SNR is high, the measurement precision does not change. The precision of measurements of the corneal thickness in preliminary in vivo experiments was higher than in ex vivo experiments. This factor is very promising for application of the method suggested herein in refractive surgery.

Utilization of refractive surgery technology in keratoconus and corneal transplants

Keratoconus is considered a contraindication for refractive surgery because of the potential corneal instability following surgery. On the other hand, penetrating keratoplasty is the usual surgical modality when the keratoconic patient cannot tolerate a contact lens. In some selected cases, when the central cornea is still transparent, refractive surgical procedures, particularly Intacs implantation, may be used to help the patient to improve visual function.

Comparison of Wavefront-guided Customized Ablation vs. Conventional Ablation in Laser in situ Keratomileusis

PURPOSE

To compare visual outcome after applying wavefront-guided customized ablation with conventional ablation in laser in situ keratomileusis (LASIK) patients.

METHODS

We performed a prospective, randomized controlled study that included 22 eyes of 11 patients who were treated with LASIK using the Nidek EC-5000 excimer laser system. We applied wavefront-guided customized ablation (study group) in the first eye of each patient. The conventional ablation (control group) was performed in the other eye of the same patient. Best spectacle-corrected visual acuity (BSCVA), spherical equivalent refraction, and high-order aberrations were evaluated preoperatively and at 1 month. High-order aberrations were measured using the Nidek OPD-Scan.

RESULTS

Preoperative and 1-month postoperative BSCVA were better than 20/40 (100%) after LASIK with both conventional ablation (control) and wavefront-guided customized ablation. Preoperatively, mean high-order aberrations in the control group were 0.572 and in the study group, 0.428 (Zernike coefficient unit). One-month after LASIK, mean high-order aberrations in the control group were 0.496 and in the study group, 0.476 (Zernike coefficient unit). We found no statistically significant difference between preoperative and 1-month postoperative high-order aberrations in either group.

CONCLUSION

LASIK with both conventional ablation and wavefront-guided customized ablation resulted in the same BSCVA 1 month after LASIK. Preoperative and 1-month postoperative high-order aberrations were not statistically different following LASIK between ablation types.

Prospective Randomized Study of Corneal Aberrations 1 Year After Radial Keratotomy or Photorefractive Keratectomy

PURPOSE

To evaluate the optical properties of the cornea 1 year after either radial keratotomy (RK) or photorefractive keratectomy (PRK) in a randomized group of patients with low myopia.

METHODS

Ninety-six patients with myopia between -0.75 and -5.00 D were randomized to either radial keratotomy (n = 46) or photorefractive keratectomy (n = 50). Topography maps were obtained 1 year after surgery and analyzed by computation of total corneal wavefront aberration and Zernike polynomial coefficients for pupil sizes of 2, 4, and 6 mm. The 4-mm pupil size was used for optimization of the model.

RESULTS

The total corneal wavefront aberrations after RK and PRK were similar and not statistically different. Wavefront aberrations arising from astigmatism or defocus accounted for approximately 70% of the total wavefront error at all pupil sizes in both groups. All types of aberrations, and in particular spherical aberration, increased significantly with increasing pupil size. Higher-order wavefront aberrations were almost twice as high after RK than after PRK at pupil sizes of 4 and 6 mm. Spherical aberration and coma were slightly higher after PRK than after RK.

CONCLUSIONS

Pupil size had a major effect on corneal aberrations after RK and PRK. The most important aberrations were sphero-cylindrical, in which eyes became significantly more myopic with increasing pupil size. The image forming properties of the cornea are better after PRK compared with RK due to the lesser amount of higher-order aberrations.