In situ collagen gel mold as an aid in excimer laser superficial keratectomy

Phototherapeutic keratectomy: Indications, methods and decision making

Phototherapeutic keratectomy (PTK) involves treating anterior corneal lesions by superficial corneal ablation using an excimer laser (193 nm). Some of the commonly treated conditions include recurrent corneal erosions (RCE), corneal dystrophies, spheroidal degeneration, keratoconus, and corneal scars. We discuss various techniques of PTK including large area PTK, focal PTK, and multifocal PTK and alternatives to PTK. Masking agents like hyaluronate, methylcellulose, and dextran are recommended to help achieve a better outcome when ablating irregular corneal surfaces. Antifibrotic agents like mitomycin C reduce the chances of recurrence of the disease, apart from minimizing the postoperative scarring. Some of the complications include induced hyperopia and irregular astigmatism, haze, recurrence, and corneal thinning. However, earlier postoperative recovery, possibility of a repeat procedure, and ability to control the depth of ablation make PTK a promising, minimally invasive alternative to keratoplasty in cases with anterior corneal pathologies.

Excimer Laser Phototherapeutic Keratectomy

Anterior corneal pathology, such as corneal scars and corneal stromal dystrophies, can be visually devastating. Over the past decade, there was a shift in treatment of these conditions from corneal transplantation to phototherapeutic keratectomy (PTK) using the 193 nm excimer laser for visual restoration. We have reviewed the recent literature on techniques for performing and refining PTK and also on various pathologic conditions that can be treated with PTK. The primary indications for PTK include anterior corneal dystrophies, such as lattice, granular, and Reis-Bückler's dystrophy. PTK can produce significant visual improvement in these patients, and corneal transplantation or retransplantation can be delayed. Corneal degenerations, such as Salzmann's nodular degeneration, keratoconus nodules, and climatic droplet keratopathy, also can be successfully treated with PTK. Additionally, anterior corneal scars from such etiologies as trauma, corneal ulcers, and prior refractive surgery can have visual improvement with PTK. In summary, PTK is a powerful tool for the management of anterior corneal pathology. In a properly selected and well-counseled patient, PTK can significantly improve vision and quality of life.

The PALM Technique: histological findings of masked phototherapeutic keratectomy on rabbit corneas

BACKGROUND

To compare the corneal healing response between conventional and phototherapeutic keratectomy through a masking agent, in rabbit corneas.

METHODS

24 adult rabbits underwent phototherapeutic keratectomy. Animals were divided in two groups: 12 received photoablation through a masking agent (PALM gel) and the remaining 12 received conventional phototherapeutic keratectomy of equal depth and served as control. Light and transmission electron microscopy was performed in specimens of both groups obtained: immediately after, four hours, one week, one, three and six months after treatment.

RESULTS

Reepitheliazation was complete within five days in all eyes. Light and transmission electron microscopy did not reveal any differences of the healing process in the experimental eyes compared to the controls.

CONCLUSION

Photoablation through the PALM technique did not result any evident alterations of the reepithelisation and stromal healing process.

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.

Treatment of corneal surface irregularities using biomask as an adjunct to excimer laser phototherapeutic keratectomy

PURPOSE

To assess the clinical changes induced by the use of BioMask as an adjunct to phototherapeutic keratectomy (PTK) using the excimer laser.

METHODS

A nonrandomized, prospective clinical trial was conducted. Twenty-two consecutive patients were enrolled for BioMask treatment with reduced visual acuity caused by irregular astigmatism or corneal scarring. Patients were treated with a broad-beam excimer laser using the standard PTK mode. The BioMask collagen material was applied as a liquid at 47 degrees C and molded using custom-made, rigid, gas-permeable contact lenses. After allowing the material to cool for approximately 3 minutes, the lens was removed and the gel was ablated at a fluence of 130-140 mJ/cm2 at a frequency of 10 Hz, until the stromal bed appeared smooth. After surgery, patients were followed at day 1, week 1, and months 1, 3, 6, 9, and 12. The main outcome measures were best spectacle-corrected visual acuity (BSCVA) and uncorrected visual acuity (UVA).

RESULTS

Comparison of the preoperative and postoperative BSCVAs showed a statistically significant change in vision (p = 0.0356). Before surgery, the mean Snellen BSCVA was 20/90, whereas after surgery, BSCVA increased to 20/60. Sixty-five percent of the patients had an improved BSCVA, with an average gain of 3.7 lines per patient (range, +2 to +10). Fifteen percent of the patients lost an average of 3.0 lines (range, -2 to -6), and 20% had no change. UVA averaged 20/180 before surgery and 20/123 after surgery, but the change was not statistically significant. With respect to UVA, 63% improved by an average of 4.25 lines per patient (range, +2 to +9). Twenty-six percent (26%) lost an average of 5.6 lines (range, -2 to -8), and 10% had no change. The average follow-up time was 8 months for both BSCVA and UVA. When patients were stratified by their treatment indications and ranked in order of BSCVA outcome, patients with Salzmann's nodular degeneration did the best, followed by those with prior refractive surgery, corneal dystrophies, and corneal scars. Even though BSCVA did not improve as much in the corneal scar group, 50% of the patients reported an improvement in the symptoms they experienced before surgery based on questionnaires. Conclusion. BioMask appears to have potential as an ablatable mask material when used in conjunction with PTK for the treatment of corneal surface irregularities.

Preliminary in vitro Study of the Histological Effects of Low Fluence 193-nm Excimer Laser Irradiation of Corneal Tissue

PURPOSE

To determine if moderate numbers of low fluence, 193-nm excimer laser pulses modify or damage the corneal stroma.

METHODS

The corneal epithelium of fresh bovine eyes was scraped off and the exposed stroma was irradiated with 200 low fluence laser pulses from an argon fluoride excimer laser. This process was performed on five eyes each at two laser fluences, 10 mJ/cm2 and 30 mJ/cm2. The ten irradiated and three control (unirradiated) corneas were sectioned and studied by electron microscopy. The maximum and minimum thickness of the anterior layer of randomly oriented collagen fibers was measured using electron microscopy.

RESULTS

The mean maximum thickness of the anterior randomly oriented layer of collagen was 1.23 +/- 0.45 microm in the control corneas, 0.67 +/- 0.32 microm in the corneas irradiated at 10 mJ/cm2, and 0.10 +/- 0.12 microm in the corneas irradiated at 30 mJ/cm2. The mean thickness of corneal stroma removed was 0.7 microm at a fluence of 10 mJ/cm2 and 1.1 microm at a fluence of 30 mJ/cm2. A thin, electron-dense pseudomembrane was present at both fluences.

CONCLUSION

We report removal of bovine corneal stroma at 10 mJ/cm2--below the previously reported ablation threshold of 20 mJ/cm2.

Management of irregular astigmatism

Using a liberal definition of corneal irregularity, modern videokeratoscopy may define approximately 40% of normal corneas with a toric refractive error as possessing primary irregular astigmatism. The causes of secondary forms of irregular astigmatism include corneal surgery, trauma, dystrophies, and infections. Internal refractive surface and media irregularity or noncorneal astigmatism (ocular residual astigmatism) contribute to irregular astigmatism of the entire refractive path of which crystaline lenticular astigmatism is usually the principal contributing component. Treatment options have increased in recent years, particularly, though not exclusively, through the advent of tailored corneal excimer laser ablations. However, discussion continues concerning the systematic approach necessary to enable treatment to achieve an optimal optical surface for the eye. Discussion also continues as to what constitutes the optimal corneal shape. Some refractive procedures may increase higher order aberrations in the attempt to neutralize refractive astigmatism. The way to further refinement of the commonly performed refractive techniques will ultimately lie in the integrated inclusion of a trio of technologies: topographic analysis of the corneal surface, wavefront analysis of ocular refractive aberrations, and vector planning to enable the appropriate balance in emphasis between these two diagnostic modalities. For the uncommon, irregularly roughened corneas, the ablatable polymer techniques show some promise.

Excimer laser phototherapeutic keratectomy. Surgical strategies and clinical outcomes

PURPOSE

To assess the different strategies and analyze clinical outcomes of excimer laser phototherapeutic keratectomy (PTK) for a variety of superficial corneal disorders.

METHODS

Twenty-eight eyes of 26 patients with one of five general categories of corneal disorders were treated with a variety of PTK techniques. Patients were assessed for preoperative and postoperative corneal thickness, uncorrected and best spectacle-corrected visual acuity, refractive changes, corneal topography and astigmatism, and results of a subjective patient questionnaire. Follow-up ranged from 6 to 30 months.

RESULTS

The average number of laser pulses was 418, with a mean decrease in corneal thickness of 31 microns. Uncorrected visual acuity improved in 20 eyes and decreased in 5. Best spectacle-corrected visual acuity improved in 20 eyes and decreased in 5; two patients had two Snellen lines of best-corrected vision loss. Nineteen patients (21 eyes) noted symptomatic improvement. Corneal topography improved in 17 eyes and worsened in 1. The average refractive shift was +1.4 diopters; patients undergoing general PTK, PTK combined with mechanical superficial keratectomy, and superficial scar removal had greater degrees of hyperopic shift. Complications were rare. Two patients who did not appreciate adequate improvement in vision after PTK underwent subsequent penetrating keratoplasty.

CONCLUSIONS

A number of PTK techniques are available to treat particular corneal disorders. Planning of surgical strategy is guided by careful patient selection which will minimize optical side effects and optimize visual outcome and subjective symptomatology after the PTK procedure.

Rapidly Polymerized Collagen Gel as a Smoothing Agent in Excimer Laser Photoablation

BACKGROUND

Excimer laser photoablation effectively and precisely removes corneal tissue but may not smooth irregularities on the anterior corneal surface. An even surface might be obtained by applying a smoothing substance that fills in irregularities and ablates at the same rate as corneal tissue. Evaluation of collagen gel as a smoothing agent is reported.

METHODS

Pure, type I collagen solutions were prepared to remain soluble at physiological pH and to spontaneously gel when exposed to cationic buffers. Collagen gels were formed on the surface of enucleated porcine corneas and on human donor corneas and exposed to varying pulses of 193-nanometer excimer laser energy. Effects of collagen gel on ablation depth, corneal surface smoothness, and smoothing of roughened cornea were evaluated by examining scanning electron micrographs of control and treated specimens.

RESULTS

Collagen gels formed from 5 mg/ml collagen solutions dramatically altered the depth of photoablation. Photoablation of roughened cornea with collagen gels produced smooth corneal surfaces, whereas control eyes remained roughened and irregular. The smoothness of photoablated cornea surfaces improved when collagen gels were applied prior to photoablation.

CONCLUSIONS

The rapidly gelling collagen solution appears to exhibit the functional properties required for an effective smoothing agent for excimer laser photoablation. The substance has low viscosity when applied to the corneal surface and forms a rigid gel when exposed to cationic buffer solution. In addition, the concentration of the collagen gel can be adjusted to provide different ablation rates.

Human excimer laser keratectomy. Clinical and histopathologic correlations

PURPOSE

To understand the healing capabilities of the diseased human cornea after excimer laser photoablation by morphologic analysis of laser-treated corneas.

METHODS

Twelve corneal specimens were obtained 5 to 16 months after lamellar or full-thickness keratoplasty following phototherapeutic keratectomy for undercorrected myopic epikeratoplasty (2 eyes), corneal leukomas (2 eyes), herpes zoster corneal scarring (1 eye), band keratopathy (2 eyes), adenoviral subepithelial opacity (1 eye), keratoconus (1 eye), herpes simplex corneal scarring (2 eyes), granular corneal dystrophy (1 eye), and recurrent lattice dystrophy (1 eye). The morphology of the corneas was examined by light and electron microscopy.

RESULTS

Epithelial hyperplasia, abnormal epithelial attachment, and disorganized stromal matrices were observed. Evidence of residual disease frequently observed in these specimens indicated that the pathology either was not excised at the time of laser keratectomy or was recurrent.

CONCLUSIONS

The response of the diseased cornea to excimer laser treatment has similar characteristics to the responses previously observed in animal studies. Incomplete ablation of diseased tissue and/or recurrence of the initial disease was the major reason for failure of the treatment. Possible causes for the inability to remove diseased tissues and superficial scars with the excimer laser include (1) insufficiently achieved ablation depth and/or diameter and (2) decreased laser ablation rates of scarred cornea.

Direct chemical analysis of uv laser ablation products of organic polymers by using selective ion monitoring mode in gas chromatography/mass spectrometry

Trace quantities of laser ablated organic polymers were analyzed by using commercial capillary column gas chromatography/mass spectrometry; the instrument was modified so that the iaser ablation products could be introduced into the capillary column directly and the constituents of each peak in the chromatogram were identified by using a mass spectrometer. The present study takes advantage of the selective ion monitoring mode for significantly improving the sensitivity of the mass spectrometer as a detector, which is critical in anatyzing the trace quantities and confirming the presence or absence of the species of interest in laser ablated polymers. The initial composition of the laser ablated polymers was obtained by using an electron impact reflectron time-of-flight mass spectrometer and the possible structure of the fragments observed in the spectra was proposed based on the structure of the polymers.