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   Table of Contents      
ORIGINAL ARTICLE
Year : 1999  |  Volume : 47  |  Issue : 3  |  Page : 167-172

Excimer laser phototherapeutic keratectomy : Indications, results and its role in the Indian scenario


Medical Research Foundation, Chennai, India

Correspondence Address:
S K Rao
Medical Research Foundation, Chennai
India
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Source of Support: None, Conflict of Interest: None


PMID: 10858771

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  Abstract 

PURPOSE: To report indications, technique, and results of excimer phototherapeutic keratectomy (PTK), and describe possible reasons for the small numbers of such procedures performed in a referral institute in India. METHODS: Retrospective review of case records of 10 patients (11 eyes) who underwent excimer PTK at our institute between February 1994 and September 1997. RESULTS: Corneal scars were the most common indication for treatment. Best-corrected visual acuity (BCVA) improved in 6 eyes (mean: 2 lines of Snellen acuity). All eyes had BCVA > or = 6/12 after treatment. None of the patients experienced loss of BCVA after treatment. Unaided visual acuity improved in 3 eyes and decreased in 2 eyes. Change in spherical equivalent refraction > or = 1 diopter occurred in 77.8% of eyes after treatment. Treating central corneal scars resulted in a significant hyperopic shift in refraction. CONCLUSIONS: Excimer PTK is a safe and effective procedure for the treatment of superficial corneal opacities. Post-treatment ametropia may require further correction with optical aids. Inappropriate referrals, deep corneal scars, and cost of the procedure could have contributed to the small numbers of PTK performed at our institute. Improved understanding of procedural strengths and limitations could lead to increased use of this procedure, with satisfying results in selected patients.

Keywords: Excimer laser, phototherapeutic keratectomy, results


How to cite this article:
Rao S K, Fogla R, Seethalakshmi G, Padmanabhan P. Excimer laser phototherapeutic keratectomy : Indications, results and its role in the Indian scenario. Indian J Ophthalmol 1999;47:167-72

How to cite this URL:
Rao S K, Fogla R, Seethalakshmi G, Padmanabhan P. Excimer laser phototherapeutic keratectomy : Indications, results and its role in the Indian scenario. Indian J Ophthalmol [serial online] 1999 [cited 2019 Jun 27];47:167-72. Available from: http://www.ijo.in/text.asp?1999/47/3/167/14922

HSV IS HERPES SIMPLEX VIRUS; RCE IS RECURRENT CORNEAL EROSION; N/A IS NOT AVAILABLE; CFCF IS COUNTING FINGERS CLOSE TO FACE; UCVA IS UNCORRECTED VISUAL ACUITY; BCVA IS BEST CORRECTED VISUAL ACUITY.

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HSV IS HERPES SIMPLEX VIRUS; RCE IS RECURRENT CORNEAL EROSION; N/A IS NOT AVAILABLE; CFCF IS COUNTING FINGERS CLOSE TO FACE; UCVA IS UNCORRECTED VISUAL ACUITY; BCVA IS BEST CORRECTED VISUAL ACUITY.

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Use of the excimer laser in ophthalmology was first proposed by Trokel et al in 1983.[1] The excimer laser utilises 193 ran ultraviolet light to disrupt intermolecular bonds in the cornea by a process termed "photoablative decomposition".[2] The process is athermal and works with submicroscopic precision with minimal damage to adjacent tissues.[3] Excimer laser corneal surgery has two major indications: photorefractive keratectomy (PRK) for the correction of refractive errors and phototherapeutic keratectomy (PTK). PTK allows the removal of superficial corneal opacities and surface irregularities. With the initial fears of corneal surface instability after excision of Bowman's layer proving unfounded, the excimer laser is now used routinely for the correction of refractive errors.[4] Excimer PTK is performed less frequently, though several studies have demonstrated its safety and efficacy in a variety of corneal conditions.[5][6][7] In these situations, PTK often helps avoid more invasive procedures like lamellar and penetrating keratoplasty.[8],[9] This is particularly relevant in developing countries like India, where corneal tissue for transplantation is scarce.[10] Though the excimer laser has been used in the cornea clinic of our tertiary care ophthalmic centre in Chennai, India, since November 1993, PTK is performed very infrequently. In this study we report indications and results of excimer PTK at our institute and describe possible reasons for its infrequent use.


  Materials and Methods Top


The case records of 10 patients (11 eyes), who underwent excimer PTK at our institute, between February 1994 and September 1997 were analysed. Information obtained from the records included: age and sex of patients, cause of corneal pathology, pre- and post-treatment unaided visual acuity (UCVA), manifest refraction and best-corrected visual acuity (BCVA), slitlamp biomicroscopic findings (before and after PTK), and laser parameters used during treatment. Extent of corneal pathology was determined using a slitlamp biomicroscope (Haag-Streit, Bern, Switzerland). Depth of corneal scarring was measured using optical pachometry (Haag-Streit, Bern, Switzerland). Written informed consent was obtained from all patients prior to treatment.

Indications for treatment are listed in [Table - 1]. In 6 eyes with preoperative BCVA ≤6/ 18, surgery was performed to improve visual acuity. In 4 eyes (1a, 2, 6, 7), preoperative BCVA was ≥6/9 and treatment was performed to reduce glare and ghosting induced by the corneal opacity and improve quality of vision. Case 8 underwent treatment for recurrent erosion unresponsive to medical therapy.

A Summit excimer laser (Omnimed laser till May 1997, Apex Plus laser thereafter) was used to perform PTK. The laser uses 193 nm ultraviolet light with a fluence of 180 mJ/ cm[2] and a pulse rate of 10 Hz. The ablation rate of normal corneal stromal tissue with these parameters is 0.25 microns per pulse. The laser was calibrated before each use according to manufacturer's guidelines.


  Surgical technique Top


All procedures were performed using topical anaesthesia with 4% lidocaine. The fellow eye was patched to allow fixation with the eye undergoing treatment. A sterile wire speculum was used to separate the eyelids. Patients were asked to fixate on coaxial red and green target lights within the delivery system positioned above the patient's head. In eyes with irregularly elevated scars, epithelium was debrided mechanically using a blunt scarifier. Elevated scar tissue was removed using a fine-toothed forceps to hold and strip tissue. Dissection was facilitated by the use of a dry week-cell sponge. After adequate excision of scar tissue, the irregular corneal surface was masked using 2% methylcellulose to produce a more uniform surface profile. Laser ablation was performed using an appropriately sized beam. A polishing effect was achieved by gently rotating the patient's head during ablation. Masking fluid was replaced as required during the procedure. The patient's eye was examined at an adjacent slitlamp at intervals during treatment, to ensure adequate depth of ablation and complete removal of the corneal opacity.

In eyes with a smooth epithelial surface and subepithelial opacity, the epithelium was removed by laser ablation. The laser was focused precisely on the epithelial surface using aiming Helium-Neon beams. Corneal epithelial ablation was performed using an appropriately sized excimer beam. Disappearance of blue fluorescence associated with epithelial ablation was considered the endpoint for epithelium removal. After epithelium removal, the laser beam was used to ablate the corneal opacity. Treatment continued in a staged fashion, with intermittent slitlamp biomicroscopy to monitor depth of ablation and improvement in corneal clarity. After adequate treatment, a 3 mm beam was applied to the periphery of the treatment area using the polishing technique described earlier to blend the treatment zone with the peripheral cornea.

In one patient with recurrent corneal erosion, loose epithelium in the affected area was carefully removed using a fine non-toothed forceps. Fifty pulses were delivered using a 2 mm beam to perform a "Bowmanectomy".

At the conclusion of the procedure, eyes were patched after instilling 0.3% ciprofloxacin eye ointment. Patients were examined everyday, and patching was repeated, till epithelialization was complete. All patients received 0.3% ciprofloxacin eye drops 6 times daily, 0.1% betamethasone eye drops 4 times daily, and 0.7% hydroxypropyl methylcellulose eye drops 6 times daily. Ciprofloxacin eye drops were discontinued after one week. Topical betamethasone was tapered over a period of 1-3 months, and the tear substitute was continued for 3 months. Patients were scheduled for review at 1, 3, 6, and 12 months following treatment.


  Selected Case Reports

Case 4
 Top


A 17-year-old male sustained a traumatic corneal laceration in his left eye. The laceration was treated medically and healed with a dense corneal scar [Figure - 1]a. At presentation UCVA in the affected eye was 3/60. A retinoscopic reflex could not be obtained due to central corneal scarring. Slitlamp biomicroscopy revealed a vertical, triangular nebulomacular scar involving the visual axis. Corneal thickness was 515 μcentrally and the depth of the scar, measured by optical pachymetry was 200 μ. Corneal topography revealed an irregular corneal surface. Excimer PTK was performed with a 5mm beam, using 409 pulses and a polishing technique, after mechanical removal of the epithelium and the underlying scar [Figure - 1]b. One month after treatment, UCVA improved to 6/24. The best-corrected visual acuity with +4.00x105 was 6/12. The central cornea had a persistent nebular opacity [Figure - 1]c.


  Case 5 Top


A 36-year-old female developed a nebulomacular corneal opacity involving the visual axis of the right eye, after an episode of viral keratitis. The unaided visual acuity in the affected eye was 6/36 and improved to 6/ 18 with +1.50/-1.50x180. Slitlamp biomicroscopy revealed subepithelial nebulomacular opacity involving the visual axis [Figure - 2]a. Simulated keratometry from corneal topography was 45.50 x 122/44.0 x 32, with a symmetric bow-tie pattern. Corneal thickness in the affected area was 520 μ. Excimer PTK was performed with a 3 mm beam, using 400 pulses and a polishing technique, after laser epithelial removal. Six months later, UCVA in the treated eye was counting fingers but improved to 6/12 with +3.50D. The treated corneal area revealed a mild subepithelial haze [Figure - 2]b.


  Case 7 Top


A 56-year-old male presented with complaints of unsatisfactory spectacle-corrected vision in the left eye, following pterygium excision. On examination, vision in the left eye with +7.00-4.00x145 was 6/5. Slitlamp biomicroscopy revealed a subepithelial nebulomacular opacity in the temporal cornea, partly involving the visual axis. Following excimer PTK, central corneal clarity improved and UCVA 3 months later was 6/12, improving with +4.50/-3.50x140 to 6/5. The patient reported improved quality of vision after laser PTK.


  Case 8 Top


A 43-year-old male had recurrent episodes of epithelial erosion in the right eye since 1 year. Treatment with tear substitutes was ineffective. A bandage contact lens was fitted but was poorly tolerated. Visual acuity in the right eye was 6/5 with -2.00D. Slitlamp biomicroscopy revealed a 2 mm epithelial defect with unhealthy margins, in the inferior paracentral cornea. Corneal topography revealed localised irregularity in the region of the recurrent corneal erosion. Excimer PTK was performed with a 2 mm beam, using 50 pulses as described earlier. During an 8 month follow-up visit, he was asymptomatic with no recurrence of the corneal erosion. Visual acuity in the right eye remained 6/5 with no change in refraction.


  Case 9 Top


A 50-year-old male presented with gradual reduction in vision in the left eye since 8 years. He complained of poor vision in the right eye since 10 years. On examination, visual acuity in the right eye was counting fingers and in the left eye with +11.0D was 6/24. Slitlamp biomicroscopy revealed aphakic corneal decompensation in the right eye. The left cornea revealed 2 large paracentral bluish subepithelial nodules with nebulomacular superficial stromal opacity involving the central cornea. There was a regressed pannus with Herbert's pits at the superior limbus and the eye was aphakic. The upper palpebral conjunctiva was scarred but there was no tarsal distortion or entropion. On pachymetry the central corneal thickness was 480 μ. Keratometry was 41.00x90 x 45.50x180 (distorted mires). Excimer PTK was performed as described earlier, after mechanical removal of the nodules. Postoperatively good central corneal clarity was obtained with a smoother surface. Visual acuity in the left eye with +11.0D was /12, 18 months postoperatively.


  Results Top


The indications, clinical features and treatment details, and results are described in [Table - 1]. Excimer PTK was performed in 11 eyes of 10 patients, 8 of whom were males. The mean age of the study group was 42.8 years (17-68 years). Corneal scars (healed infections 3 eyes, post-trauma 2 eyes, post-pterygium surgery 2 eyes,) were the most common indications for treatment in this series, and were present in 7 of 11 eyes. Three eyes were treated for Salzmann's degeneration and 1 eye for recurrent corneal erosion (RCE). Treatment was performed to improve visual acuity in 10 eyes, and to improve ocular comfort in 1 eye (RCE). The number of pulses used to perform PTK ranged from 50 to 409 (mean 171.5). Diameter of the laser beam used varied from 2 mm to 5 mm depending on extent of corneal pathology. The average followup was 6.5 months (1-18 months).

Improvement in BCVA was seen in 6 (60%) of 10 eyes treated for improvement of visual acuity. Average gain was 2 lines of Snellen acuity and ranged from 1 to 4 lines. Six eyes treated to improve visual acuity had a postoperative BCVA ≥6/12. In 4 eyes there was no change in the pre-treatment vision (pre-treatment BCVA was ≥6/9 in these eyes). All patients in this group reported improvement in quality of vision after the procedure. None of the patients in this study experienced a loss of vision. Penetrating keratoplasty was not considered necessary in any of these patients, following PTK. Unaided visual acuity (pre- and post-treatment) was available for 5 eyes [Table - 1]. Improvement was seen in 3 eyes (1.5, 2.5, and 3 lines of Snellen acuity). In 2 eyes, UCVA decreased by 1 and 2 lines of Snellen acuity (due to hyperopic refractive shift). In 2 eyes, only the post-treatment UCVA was available [1b and 7, Table:1]. However, considering the pre-treatment refraction, it is quite likely that this constitutes an improvement. The patient treated for RCE had no recurrence during an 8 month follow-up visit. Change in spherical equivalent refraction ≥ 1 diopter occurred in 7 (77.8%) of 9 eyes (preoperative refraction could not be obtained in 2 eyes with corneal scarring). Hyperopic shift in spherical equivalent was seen in 5 eyes (mean +3.25D, range-1.00 - 6.25 D) and myopic shift in 2 eyes (-2.25D, -5.00D).


  Discussion Top


Excimer phototherapeutic keratectomy has been used in clinical trials since 1990 and has been reported to be a safe and effective procedure.[7-9] This report in Indian eyes confirms the work of others in that excimer laser PTK is an effective tool for the treatment of selected superficial corneal scars that might otherwise need keratoplasty.[8],[9] All patients who underwent treatment for improvement in vision, had satisfactory results and further surgery was not required.

The efficacy of PTK is evaluated by improvement in BCVA comparable with results of lamellar and penetrating keratoplasty in a similar situation, and level of ocular comfort.[7],[11] A 2-line improvement in BCVA after PTK (provided pre-treatment visual acuity was < 6/12) has been considered significant.[7] In our study, 4 (66.7%) of 6 eyes attained this treatment success. Improvement in BCVA has been reported from 47% to 78% in three previous studies.[6],[7],[11] In a series of 26 patients with anterior corneal pathology, Hersh et al have reported improved uncorrected and best-corrected visual acuity in 20 eyes following excimer PTK. Loss of 2 lines of BCVA occurred in 2 eyes and 2 eyes required penetrating keratoplasty. The average refractive shift in this series was +1.4D.[12] The Summit PRK Study group has reported results of PTK for corneal visual loss in 232 eyes with one year follow up. In a subgroup (103 eyes) in this study, BCVA improved in 46 (45%) eyes and 9 (9%) eyes lost 2 or more lines. The average improvement in BCVA was 1.6 lines and the mean refractive shift was +0.87D.[13] The results in these larger series largely corroborate the results of our study.

Although improvement in BCVA in our small series was significant, documented increase in unaided visual acuity was seen only in 3 patients, with a possible improvement in 2 others. A similar improvement in 11 of 18 patients was reported by Campos et al.[6] Significant residual ametropia, seen in most eyes in our patients after treatment, was cause for poor unaided visual acuity. Post-treatment ametropia, anisometropia and /or irregular astigmatism can often result in poor patient satisfaction despite improvement in corneal clarity and BCVA. The need to use a contact lens after treatment may also be considered a poor treatment outcome by most patients. Significant, often unpredictable, shifts in refraction have been reported by most authors after PTK.[5][6][7],[11] In our patients change in spherical equivalent greater than 1 diopter was documented in 77.8% of the treated eyes. Worsening of astigmatism was seen in 2 patients and significant postoperative astigmatism in one patient in whom preoperative refraction was not possible.

The mechanism of such changes is related to the location and extent of tissue removal from the cornea. Similar to principles used in refractive surgery, tissue removal from the central cornea results in corneal flattening and hyperopic refractive shift. Excision of corneal tissue in the periphery has the opposite effect on refraction. In our patients, hyperopic shift was noted in eyes treated for central corneal opacity and in 2 patients treated for residual scarring after pterygium excision there was myopic shift.

We attempted to reduce the extent of refractive change after PTK by removing the minimum amount of tissue necessary to achieve the desired treatment end-point. This was achieved by purposely attempting less than the full ablation initially. The patients were repeatedly examined at an adjacent slitlamp at intervals during treatment, and further ablation was performed only if needed. There was no correlation between the number of pulses used during treatment and the degree of refractive shift reported by other authors.[6],[7] The postulated reasons include: absorption of laser energy by the masking fluid, variable dehydration of corneal stroma, differential ablation rates of corneal stromal scars, and variability in post-treatment inflammation and healing response. Since these factors are not quantifiable, it is difficult in most instances to predict postoperative refractive outcome in these eyes.

Irregular astigmatism did not result in a loss of BCVA, in any of the treated eyes, in this study. In 5 patients, residual astigmatism could have contributed to non-improvement of spectacle-corrected BCVA beyond 6/12. Contact lens trial would have helped determine the extent of astigmatism related visual loss, but was not done in this retrospective study. Operative techniques used to minimize induced astigmatism included: mechanical removal of elevated nodules and scar tissue, use of masking fluid to achieve a smooth corneal surface prior to ablation, and a rotatory motion of the patients head during ablation. Mechanical removal of scar tissue reduces stromal inhomogenities in the treatment area and allows a more uniform ablation. Masking fluid fills the valleys in the irregular corneal surface and exposes the peaks to laser ablation. The masking fluid ablates at the same rate as the corneal stroma and thus produces a smoother corneal surface after treatment. Surface irregularity in a clear cornea can result in significant degradation of retinal image quality. Laser ablation in the absence of masking fluid transfers the irregular corneal surface to a deeper plane, resulting in a clearer corneal surface with persisting irregularity. No intraoperative complications occurred in this study.

There are isolated reports of recurrent herpetic keratitis following excimer laser surgery.[6] We did not use antivirals postoperatively in our patients treated for post-herpetic scarring. Neither patient experienced recurrence of keratitis in the post-operative period. However, the laser procedure is unlikely to affect the long-term course of the disease. Future recurrences of viral infection could result in further corneal scarring, and such patients therefore need close follow up after excimer PTK.[14] Recurrence of corneal dystrophies and band-shaped keratopathy after PTK is also reported.[6] A recurrence of Salzmann's degeneration in the left eye of patient 1 was noted 12 months after PTK. The opacity recurred in the periphery of the treated area and did not reduce visual acuity. Subepithelial haze similar to post-treatment haze was seen in patient 5. The haze developed a month after surgery and persisted at 6 months [Figure - 2]b. In these situations, laser treatment functions like other conventional treatment modalities such as superficial keratectomy, 1% ethylene diamne tetraacetic acid chelation, etc. However because of its submicroscopic precision, laser PTK achieves a smoother corneal surface. Anterior stromal puncture is used in the treatment of recurrent corneal erosion, but produces corneal scarring unlike the excimer laser which can therefore be considered the treatment of choice in this condition.[15]

In the three-and-half year period of this study, the cornea service at our institution has performed in excess of 1750 Excimer PRK procedures for the treatment of myopic refractive errors. The possible reasons of disparity between PRK and PTK procedures could be lack of awareness among referring physicians. A review of our common indications for penetrating keratoplasty (PK) revealed data similar to a recently published report.[12] Superficial corneal scars and anterior basement membrane dystrophies did not figure prominently among these indications. It is therefore unlikely that we were performing PK in patients who could have been treated by PTK. We may also speculate that postinfectious corneal scars in India are more extensive, and hence the procedure is less suitable. Dense corneal scars possibly due to fungal corneal infection, increased cost of PTK treatment, reduced accessability probably all contribute to less frequent use of PTK.

In summary, our small series of patients demonstrate the safety and efficacy of excimer PTK in the treatment of superficial corneal opacities. Improvement in unaided visual acuity was unimpressive and most patients had significant residual ametropia. Considering that these patients would have undergone superficial keratectomy or lamellar keratoplasty, and since the postoperative course and complications of PTK are less demanding, excimer PTK offers a viable option to keratoplasty in selected patients, particularly those with minimal reduction in visual acuity and superficial opacities. In those with severe visual impairment and more extensive corneal pathology, this procedure may allow sufficient restoration of vision and avoid or delay keratoplasty with its attendant risks and complications. Conservative guidelines for the selection of patients for this procedure include: depth of corneal opacity less than 100 m and retention of at least 300 m of corneal stroma after the procedure. Superficial, homogenous, diffuse opacities are easier to treat than focal, deep scars. Treatment with the laser, however, does not alter disease pathophysiology. A conservative approach during surgery after detailed and informed discussions with the patient regarding the goals and limitations of the procedure obviously improves the results and patient satisfaction[16].

 
  References Top

1.
Trokel SL, Srinivasan R, Braren B. Excimer laser surgery of the cornea. Am J Ophthalmol 1983;96:710-15.  Back to cited text no. 1
[PUBMED]    
2.
Wu WCS, Stark WJ, Green WR. Corneal wound healing after 193-nm Excimer laser keratectomy. Arch Ophthalmol 1991;109:1426-1432.  Back to cited text no. 2
    
3.
Marshall J, Trokel SJ, Rothery S, Krueger RR. Long term healing of the central cornea after photorefractive keratectomy using an excimer laser. Ophthalmology 1988;95:1411-21.  Back to cited text no. 3
    
4.
Taylor HR, Guest CS, Kelly P, Alpins N. Comparison of excimer laser treatment of astigmatism and myopia. Arch Ophthalmol 1993;111:1621-6.  Back to cited text no. 4
    
5.
Sher NA, Bowers RA, Zabel RW, Frantz JM, Eiferman RA, Brown DC, et al. Clinical use of the 193 nm excimer laser in the treatment of corneal scars. Arch Ophthalmol 1991;109:491-98.  Back to cited text no. 5
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6.
Campos M, Nielsen S, Szerenyi K, Garbus JJ, Mc Donnell PJ. Clinical follow-up of phototherapeutic keratectomy for treatment of corneal opacities. Am J Ophthalmol 1993;115:433-40.  Back to cited text no. 6
    
7.
Starr M, Donnenfeld E, Newton M, Tostanoski J, Muller J, Odrich M. Excimer laser phototherapeutic keratectomy. Cornea 1996;15:557-65.  Back to cited text no. 7
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8.
Gaster RN, Binder PS, Coalwell K, Berns M, McCord RL, Burstein NL. Corneal surface ablation by 193 nm excimer laser and wound healing in rabbits. Invest Ophthalmol Vis Sci 1989;30:90.  Back to cited text no. 8
    
9.
Steinert RF, Puliafito CA. Excimer laser phototherapeutic keratectomy for a corneal nodule. Refract Corneal Surg 1990;6:352.  Back to cited text no. 9
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10.
Saini JS, Reddy MK, Jain AK, Ravindra MS, Jhaveria S, Raghuram L. Perspectives in eye banking. Ind J Ophthalmol 1996;44:47-55.  Back to cited text no. 10
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11.
Stark WJ, Charmon W, Kamp MT, Enger CL, Rencs EV, Gottsch JD. Clinical follow-up of 193 nm ArF excimer laser phototherapeutic keratectomy. Ophthalmology 1992:99:805-12.  Back to cited text no. 11
    
12.
Hersh PS, Burnstein Y, Carr J, Etwane G, Mayers M. Excimer laser phototherapeutic keratectomy. Surgical strategies and clinical outcomes. Ophthalmology 1996;103:1210-22.  Back to cited text no. 12
    
13.
Maloney RK, Thompson V, Ghiselli G, Durrie D, Waring GO III, O'Connell M, and the Summit Phototherapeutic Keratectomy Study Group. A prospective multicenter trial of excimer laser phototherapeutic keratectomy for corneal visual loss. Am J Ophthalmol 1996;122:149-60.  Back to cited text no. 13
    
14.
Vrabec MP, Anderson JA, Rock ME, Binder PS, Steinert RF, Durrie DS, et al. Electron microscopic findings in a cornea with recurrence of Herpes Simplex Virus keratitis after excimer laser. CLAO-J 1994;20:41-44.  Back to cited text no. 14
[PUBMED]    
15.
Lohmann CP, Sach H, Marshall J, Gabel V-P. Excimer laser phototherapeutic keratectomy for recurrent erosions: A clinical study. Ophthalmic Surg Lasers 1996;27:768-72.  Back to cited text no. 15
    
16.
Dandona L, Ragu K, Janarthanan M, Naduvilath JJ, Shenoy R, Rao GN. Indications for penetrating keratoplasty in India. Ind J Ophthalmol 1997;45:163-168.  Back to cited text no. 16
    


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