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Post photorefractive keratectomy contact lensfitting
Journal of the British ContactLens Association, Vol. 19, No. 2, pp. 55-57, 1996
© 1996 British Contact Lens Association
Printed in Great Britain
Case Report POST-PHOTOREFRACTIVE KERATECTOMY CONTACT LENS FITTING Christine L. K. Astin* (Received 26th June 1995; in revised form lst August 1995)
Abstract--A case is described of a patient fitted with contact lenses to correct myopic astigmatism following regression of refractive effect post-photorefractive keratectomy. Rigid gas-permeable lens tolerance was limited by lid sensation so the patient was refitted with daily-wear soft lenses. Successful lens wear was achieved with minimal corneal curvature change and epithelial disturbance. Corneal changes following PRK and subsequent contact lens fitting are discussed. KEYWORDS:Photorefractive keratectomy, rigid gas-permeable contact lens, daily-wear soft contact lens
p
Introduction hotorefractive keratectomy (PRK) of the cornea is performed using argon fluoride excimer lasers which emit ultraviolet wavelengths (193nm) of high proton energy capable of breaking intermolecular bonds within proteins and other large molecules without causing thermal damage to nearby tissues. 1,2 Smooth, precise re-profiling changes in the anterior corneal surface result from this tissue ablation. Computer-generated calculations control the beam according to the resultant tissue shape required. Gartry et al. ~ found that the greater the refractive change intended in PRK for myopia, the deeper the ablation a n d the less predictable the outcome. The regression effect in some patients leads to resultant ametropia, anisometropia and irregular astigmatism, which are indications for contact lens fitting (PRKCL) once the corneal contour has stabilised. Case Report Daily-wear thin soft contact lenses had been worn for several years without problems, but the 34-year-old woman patient found lens care and handling to be a nuisance so she chose to have PRK treatment. Lens specifications were not known. Pre-PRK treatment, right eye (RE) variables were: Refraction: - 5 . 2 5 / - 0 . 5 0 axis 70, visual acuity (VA) 6/5 Central corneal thickness (CCT) 0.51ram Keratometry 7.80mm at 180/7.51mm at 90 (regular mires) (Left eye refraction: -3.75DS VA 6/5). At that time, contact lens wear was suspended 10 days before PRK aiming to allow the cornea to be measured without lens influence. In recent years patients *BSc (Hons), MPhil, FBCO, DCLP, FAAO.
have been advised to suspend lens wear several weeks before PRK. In February 1991, the right eye underwent PRK to correct the myopia. PRK was performed over a central ablation zone diameter of 5.00mm using a Summit UV 200 excimer laser. She was part of a study group of 80 patients. 4 In February 1992: Refraction: - 1 . 7 5 / - 0 . 2 5 ax 180, VA 6/6-2, CCT 0.48mm Keratometry: 8.09mm at 175/7.83 at 85 (slightly irregular mires) Keratoscopy: 43.62D at 80/42.30D at 160, astig. 1.32D within 3.0mm zone. Slit-lamp microscopy revealed no significant signs except central slight superficial corneal haze in the PRK zone. In March 1992, the patient was referred for contact lens fitting to correct the resultant myopia. From previous experience when fitting irregular corneal contours the author considered aspheric back surface rigid lens designs, e.g. Bausch & Lomb Quantum, Cibavision Persecon Elliptical and Jack Allen aspheri P. On previous patients these lens types had been found to follow the peripheral corneal contours distributing the lens pressure, allow smooth lens movement and encouraging good tear flow beneath the lens. The tear lubrication was important to ease lens movement without causing epithelial erosions, also to remove debris and to supplement the corneal oxygen supply. Therefore, good wettability of the lens surface was desired. From the rigid lenses tried, the patient in this case preferred a Quadrant Nova lens. This lens had a back optic zone radius (BOZR) 7.90mm, OD 9.20mm, back vertex power (BVP) -3.00D. This gave a VA of 6/5. The lens power included an extra -1.00D to cancel out
55
CHRISTINEL.K.AKf'IN the positive power of the central tear pool. A satisfactory fit was obtained: lens riding centrally, 1-2mm movement on blinks, slight central fluorescein pooling and alignment of lens at the mid-periphery (Figure 1). Comfort and lens handling were acceptable. After 2 weeks of adaptation, lens tolerance was limited by lid awareness of the lens edge. The patient had similar problems with RGP lens tolerance over many years.
Epithelial hyperplasia in ablation
EyeSys keratoscopy indicated that the central corneal zone remained relatively flat, but the peripheral corneal contour was similar to that found preoperatively, and the zone transition had a smooth contour. In June 1993, to avoid the nuisance of lens wear and care, the patient elected to undergo repeat PRK, which she found to be satisfactory. She obtained at 4 weeks later, unaided VA of 6/9 Refraction: + 0.50/- 0.50 at 120, VA 6/9 +, CCT 0.48ram Keratoscopy: 41.15 at 76/40.27 at 166, astig. 0.88D within 3.0ram zone.
~tact
Discussion
"nea
Figure 1. Rigid gas-permeable contact lens bridges over the photorefractive keratectomy zone.
She was refitted with daily-wear soft hydrophilic lens (DWSCL) Omniflex lens (Allergan), BOZR 8.40mm, OD 14.30ram, BVP -2.50D, which gave a VA of 6/6+1. The soft lens draped over the corneal contours with a smaller tear layer beneath the lens, and so a lens of weaker power than the rigid lens could be used. Comfort improved and wearing time increased to 14h a day, without problems. In June 1992, after 3h wear, VA was 6/5, the lens was of good fit and condition, with good lens tolerance of 15h a day; on slit-lamp microscopy nothing significant was noted except the central PRK slight haze area. Refraction post CL wear was -1.50DS, VA of 6/6-1; CCT 0.49mm. Keratometry: 8.05mm at 180/7.8mm at 90 (slightly blurred mires as a result of reflection from greasy tears). These measurements indicated negligible changes resulting from lens wear on the PRK flattened cornea. In March 1993, lens wear was satisfactory for 15h per day. At aftercare, after 4h of wear, the fit and condition of the lens were good, VA 6/6 part, an extra +0.50D gave VA of 6/5 part. A new lens (Omniflex, BOZR 8.40, OD 14.30, BVP -2.00D) was given. On slit-lamp microscopy there were no signs of conjunctival disturbance nor of corneal staining or reduction in transparency except for the central PRK slight haze. Unaided VA 6/12. Refraction post CL wear -1.50DS, VA 6/5, CCF 0.48mm Keratometry: 8.14mm at 180/7.83mm at 90 (slightly blurred mires) Keratoscopy: 43.88D at 76/42.34D at 166, astig. 1.54D within 3.0ram zone. 56
Following PRK for myopia, the healing response resulted in the thickening of the ablated zone due to subepithelial deposition and the growth of new layers of epithelial cells? The corneal thickness remains less than that measured pre-PRK. It is now known that if the perimeter of the PRK ablated zone has a graduated edge this discourages excess epithelial growth over the area, which would otherwise reduce the power change induced on the cornea. Ficker et al. 4 noted that PRK for myopia often resulted in a hypermetropic power shift during the first postoperative weeks then a partial regression of the refractive effect over a variable period ranging from a few weeks to several months. This regression was faster and more frequent where the attempt correction was greater than 5.0D? ,6 A number of studies have investigated the corneal response to PRK for myopiaY-9 Lakkis and Brennan 1° gave a good overview of the current literature and described advantages and disadvantages of PRK. Several grades of subepithelial haze have been noted, generally more apparent at 8 weeks postoperatively. Concern as to whether this haze would increase light scatter and decrease vision contrast has been put in perspective by Lohmann et al. 11 They found a decrease at 3 months but by 12 months PRK gave similar vision contrast results to spectacle correction, better than those shown by soft contact lens correction. Wilson et al. 6 found that two thirds of the patients obtained a stabilised corneal power by 3 to 7 months, but the remainder continued to change power by at least 0.50D up to 12 months. Sher et al. 8 monitored PRK on myopia of 8.0 to 14.0D, noting that 75% reached to within 2.0D of their intended refraction and 12% suffered a reduction in best corrected vision of at least one Snellen line. Resultant ametropia, anisometropia and irregular astigmatism were indications for contact lens correction. Although not keen on either method of correction, most PRK patients would prefer contact lenses to spectacles. Waiting 6 to 12 months for the corneal contour to stabilise before lens fitting would be advisableY Contact lenses causing orthokeratology or undue pressure on the cornea would not be recommended. Contact lens fitting after PRK for myopia was easier when the ablation zone was relatively small, e.g.
CASEREPOt~
4-5ram in diameter, as the lens could more readily bridge over the zone. Obtaining a good fit lens was also helped when the corneal curvature at the PRK zone margin changed gradually, as described by Klyce e t al. 1~ Shovlin1'~ mentioned that the problems of excess edge clearance and instability of movement of the RGP lens, found when fitting after radial keratotomy16were reduced in the PRK patient. After PRK, the peripheral corneal region retained a normal contour. By the 6month stage the epithelial hypertrophy over the PRK zone usually became stabilised and the central corneal thickness settled at a value slightly less than that measured preoperatively. Therefore tear pooling beneath the lens centre would not be excessive. In the case described here, the contact lenses easily bridged over the central PRK zone, allowing the corneal periphery to support the lens. This avoided pressure or friction from the lens which may have disturbed the new area of epithelium. If the ablated zone had been wider, e.g. 6-7ram or very irregular, problems with lens fit and stability could have arisen. Visual blur during lens wear may result from lens surface greasing but in some cases the reduction in best corrected visual acuity is due to persistent reticular corneal haze resulting from PRK?~ GartryTM noted that the side effects of regression, corneal haze and loss of best corrected visual acuity of one or more Snellen lines are more commonly found in patients treated for higher degrees of myopia.This patient was typical of those electing for PRK who have limited tolerance of contact lenses or who object to the time and expense involved in their care. Therefore, although a contact lens was successfully fitted and worn, she chose to undergo repeat PRK to aim for improved unaided vision and to escape dependence on an optical appliance.
Acknowledgement I thank Mr A. D. McG. Steele for permission to describe this patient. Thanks are due to Bausch & Lomb International for funding assistance.
Address for Correspondence Christine L. K. Astin, Contact Lens Department, Moorfields Eye Hospital, City Road, London EC1V 2PD.
REFERENCES Trokel, S.L., Srinivasan, R. and Braren, B. Excimer laser surgery of the cornea. Am. J. OphthalmoI., 96, 710-715 (1983). Marshall, J., Trokel, S.L., Rothery, S. and Krueger, R.R. Long term healing of the central cornea after photorefl-active keratectomy using an excimer laser. Ophthalmology, 95, 1411-1421 (1988). Gartry, D.S., Kerr Muir, M.G. and Marshall, J. Excimer laser photore#active keratectomy--18 month follow-up. Ophthamology, 99, 1209-1219 (1992). 4 Ficker, L.A., Bates, A.K., Steele, A.D. McG., Lyons, C.J., Milliken, A.B., Astin, C., Slattery, K. and Kirkness, C.M. Excimer laser photorefractive keratectomy for myopia: 12 month follow-up. Eye, 7, 617-624 (1993). Tuft, S.J., Zabel, R.W. and Marshall, J. Corneal repair following keratectomy, nvest. Ophthalmol. Kis. Sci., 30, 1769-1777 (1989). Wilson, S.E., Klyce, S.D., McDonald, M.B., Hu, J.C. and Kaufman, H.E. Changes in corneal topography after excimer laser photor~ fractive keratectomy for myopia. Ophthalmology, 98, 1338-1347 (1991). 7 Stevens, J.D., Ficker, L.A., Steele, A.D.McG., Astin, C., Milliken, A. and Claoue, C.M.P. Excimer laser treatment of myopia with primary astigmatism. Invest. OphthalmoL Vis. Sci., 34(4), 799 (1993). s Sher, N.A., Barak, M., Daya, S., DeMarehi, J., Tucci, A., Hardten, D.R., Frantz, J., Eiferman, R.A., Parker, P., Telfair, W.B. and Lane, S.S. Excimer laser photorefractive keratectomy in high myopia. Arch. Ophthalmol., 110, 935-943 (1992). Tengroth, B., Epstein, D., Fagerholm, P., Hamberg-Nystrom, H. and Fitzsimmons, T.D. Excimer laser photorefractive keratectomy for myopia. Ophthalmology, 100, 739-745 (1993). Lakkis, C. and Brennan, N.A. Photorefractive keratectomy. Clin. Exp. Optom., 76(2), 33-49 (1993). Lohmann, C.P., Fitzke, F., O'Brant, D., Kerr Muir, M.G., Timberlake, G. and Marshall, J. Corneal light scattering and visual performance in myopic individuals with spectacles, contact lenses or excimer laser photorefractive keratectomy. Am. J. Ophthalmol., 115, 444-453 (1993). Astin, C.L.K. Considerations in contact lens fitting following photorefractive keratectomy. Practical Optometry (Canada), 6(3), 94-96 (1995). Asfin, C.L.IC Contact lens fitting after photorefractive keratectomy (PRK): a comparison of two groups. Ophthal. Phys. Opt., 15(5), 371-374 (1995). Klyce, S.D., Wilson, S.E., McDonald, M.B., Liu, J.C. and Kaufinan, H.E. Corneal topography "after excimer laser keratectomy. Invest. Ophthalmol. Vis. Sci., (suppl) 32,721-722 (1991). Shovlin, J.P. A comparison between patients wearing contact lenses following radial keratectomy and myopia photorefractive keratectomy with the excimer laser. Int. Contact Lens Clin., 19, 141-142 (1992). Astin, C.L.K. Contact lens fitting post-refractive surgery. In: Ruben, M. and Guillon, M. (Eds), Contact Lens Practice, Chapman & Hall, London, 843-854 (1994). Stevens, J.D., Steele, A.D.McG., Ritten, S., Ficker, L.A., Bates, A.tC, Kirkness, C.M., Williams, H., Claoue, C.M.P., Milliken, A.B. and Astin, C. Comparison of the Summit Excimed UV200 & VisX 20:20 Exeimer laser systems at one site. Arch. Ophthalmol., (in press). Gantry, D.S. Treating myopia with the excimer laser: the present position. BMJ, 310, 979-985 (1995).
57
© 1996 British Contact Lens Association
Printed in Great Britain
Case Report POST-PHOTOREFRACTIVE KERATECTOMY CONTACT LENS FITTING Christine L. K. Astin* (Received 26th June 1995; in revised form lst August 1995)
Abstract--A case is described of a patient fitted with contact lenses to correct myopic astigmatism following regression of refractive effect post-photorefractive keratectomy. Rigid gas-permeable lens tolerance was limited by lid sensation so the patient was refitted with daily-wear soft lenses. Successful lens wear was achieved with minimal corneal curvature change and epithelial disturbance. Corneal changes following PRK and subsequent contact lens fitting are discussed. KEYWORDS:Photorefractive keratectomy, rigid gas-permeable contact lens, daily-wear soft contact lens
p
Introduction hotorefractive keratectomy (PRK) of the cornea is performed using argon fluoride excimer lasers which emit ultraviolet wavelengths (193nm) of high proton energy capable of breaking intermolecular bonds within proteins and other large molecules without causing thermal damage to nearby tissues. 1,2 Smooth, precise re-profiling changes in the anterior corneal surface result from this tissue ablation. Computer-generated calculations control the beam according to the resultant tissue shape required. Gartry et al. ~ found that the greater the refractive change intended in PRK for myopia, the deeper the ablation a n d the less predictable the outcome. The regression effect in some patients leads to resultant ametropia, anisometropia and irregular astigmatism, which are indications for contact lens fitting (PRKCL) once the corneal contour has stabilised. Case Report Daily-wear thin soft contact lenses had been worn for several years without problems, but the 34-year-old woman patient found lens care and handling to be a nuisance so she chose to have PRK treatment. Lens specifications were not known. Pre-PRK treatment, right eye (RE) variables were: Refraction: - 5 . 2 5 / - 0 . 5 0 axis 70, visual acuity (VA) 6/5 Central corneal thickness (CCT) 0.51ram Keratometry 7.80mm at 180/7.51mm at 90 (regular mires) (Left eye refraction: -3.75DS VA 6/5). At that time, contact lens wear was suspended 10 days before PRK aiming to allow the cornea to be measured without lens influence. In recent years patients *BSc (Hons), MPhil, FBCO, DCLP, FAAO.
have been advised to suspend lens wear several weeks before PRK. In February 1991, the right eye underwent PRK to correct the myopia. PRK was performed over a central ablation zone diameter of 5.00mm using a Summit UV 200 excimer laser. She was part of a study group of 80 patients. 4 In February 1992: Refraction: - 1 . 7 5 / - 0 . 2 5 ax 180, VA 6/6-2, CCT 0.48mm Keratometry: 8.09mm at 175/7.83 at 85 (slightly irregular mires) Keratoscopy: 43.62D at 80/42.30D at 160, astig. 1.32D within 3.0mm zone. Slit-lamp microscopy revealed no significant signs except central slight superficial corneal haze in the PRK zone. In March 1992, the patient was referred for contact lens fitting to correct the resultant myopia. From previous experience when fitting irregular corneal contours the author considered aspheric back surface rigid lens designs, e.g. Bausch & Lomb Quantum, Cibavision Persecon Elliptical and Jack Allen aspheri P. On previous patients these lens types had been found to follow the peripheral corneal contours distributing the lens pressure, allow smooth lens movement and encouraging good tear flow beneath the lens. The tear lubrication was important to ease lens movement without causing epithelial erosions, also to remove debris and to supplement the corneal oxygen supply. Therefore, good wettability of the lens surface was desired. From the rigid lenses tried, the patient in this case preferred a Quadrant Nova lens. This lens had a back optic zone radius (BOZR) 7.90mm, OD 9.20mm, back vertex power (BVP) -3.00D. This gave a VA of 6/5. The lens power included an extra -1.00D to cancel out
55
CHRISTINEL.K.AKf'IN the positive power of the central tear pool. A satisfactory fit was obtained: lens riding centrally, 1-2mm movement on blinks, slight central fluorescein pooling and alignment of lens at the mid-periphery (Figure 1). Comfort and lens handling were acceptable. After 2 weeks of adaptation, lens tolerance was limited by lid awareness of the lens edge. The patient had similar problems with RGP lens tolerance over many years.
Epithelial hyperplasia in ablation
EyeSys keratoscopy indicated that the central corneal zone remained relatively flat, but the peripheral corneal contour was similar to that found preoperatively, and the zone transition had a smooth contour. In June 1993, to avoid the nuisance of lens wear and care, the patient elected to undergo repeat PRK, which she found to be satisfactory. She obtained at 4 weeks later, unaided VA of 6/9 Refraction: + 0.50/- 0.50 at 120, VA 6/9 +, CCT 0.48ram Keratoscopy: 41.15 at 76/40.27 at 166, astig. 0.88D within 3.0ram zone.
~tact
Discussion
"nea
Figure 1. Rigid gas-permeable contact lens bridges over the photorefractive keratectomy zone.
She was refitted with daily-wear soft hydrophilic lens (DWSCL) Omniflex lens (Allergan), BOZR 8.40mm, OD 14.30ram, BVP -2.50D, which gave a VA of 6/6+1. The soft lens draped over the corneal contours with a smaller tear layer beneath the lens, and so a lens of weaker power than the rigid lens could be used. Comfort improved and wearing time increased to 14h a day, without problems. In June 1992, after 3h wear, VA was 6/5, the lens was of good fit and condition, with good lens tolerance of 15h a day; on slit-lamp microscopy nothing significant was noted except the central PRK slight haze area. Refraction post CL wear was -1.50DS, VA of 6/6-1; CCT 0.49mm. Keratometry: 8.05mm at 180/7.8mm at 90 (slightly blurred mires as a result of reflection from greasy tears). These measurements indicated negligible changes resulting from lens wear on the PRK flattened cornea. In March 1993, lens wear was satisfactory for 15h per day. At aftercare, after 4h of wear, the fit and condition of the lens were good, VA 6/6 part, an extra +0.50D gave VA of 6/5 part. A new lens (Omniflex, BOZR 8.40, OD 14.30, BVP -2.00D) was given. On slit-lamp microscopy there were no signs of conjunctival disturbance nor of corneal staining or reduction in transparency except for the central PRK slight haze. Unaided VA 6/12. Refraction post CL wear -1.50DS, VA 6/5, CCF 0.48mm Keratometry: 8.14mm at 180/7.83mm at 90 (slightly blurred mires) Keratoscopy: 43.88D at 76/42.34D at 166, astig. 1.54D within 3.0ram zone. 56
Following PRK for myopia, the healing response resulted in the thickening of the ablated zone due to subepithelial deposition and the growth of new layers of epithelial cells? The corneal thickness remains less than that measured pre-PRK. It is now known that if the perimeter of the PRK ablated zone has a graduated edge this discourages excess epithelial growth over the area, which would otherwise reduce the power change induced on the cornea. Ficker et al. 4 noted that PRK for myopia often resulted in a hypermetropic power shift during the first postoperative weeks then a partial regression of the refractive effect over a variable period ranging from a few weeks to several months. This regression was faster and more frequent where the attempt correction was greater than 5.0D? ,6 A number of studies have investigated the corneal response to PRK for myopiaY-9 Lakkis and Brennan 1° gave a good overview of the current literature and described advantages and disadvantages of PRK. Several grades of subepithelial haze have been noted, generally more apparent at 8 weeks postoperatively. Concern as to whether this haze would increase light scatter and decrease vision contrast has been put in perspective by Lohmann et al. 11 They found a decrease at 3 months but by 12 months PRK gave similar vision contrast results to spectacle correction, better than those shown by soft contact lens correction. Wilson et al. 6 found that two thirds of the patients obtained a stabilised corneal power by 3 to 7 months, but the remainder continued to change power by at least 0.50D up to 12 months. Sher et al. 8 monitored PRK on myopia of 8.0 to 14.0D, noting that 75% reached to within 2.0D of their intended refraction and 12% suffered a reduction in best corrected vision of at least one Snellen line. Resultant ametropia, anisometropia and irregular astigmatism were indications for contact lens correction. Although not keen on either method of correction, most PRK patients would prefer contact lenses to spectacles. Waiting 6 to 12 months for the corneal contour to stabilise before lens fitting would be advisableY Contact lenses causing orthokeratology or undue pressure on the cornea would not be recommended. Contact lens fitting after PRK for myopia was easier when the ablation zone was relatively small, e.g.
CASEREPOt~
4-5ram in diameter, as the lens could more readily bridge over the zone. Obtaining a good fit lens was also helped when the corneal curvature at the PRK zone margin changed gradually, as described by Klyce e t al. 1~ Shovlin1'~ mentioned that the problems of excess edge clearance and instability of movement of the RGP lens, found when fitting after radial keratotomy16were reduced in the PRK patient. After PRK, the peripheral corneal region retained a normal contour. By the 6month stage the epithelial hypertrophy over the PRK zone usually became stabilised and the central corneal thickness settled at a value slightly less than that measured preoperatively. Therefore tear pooling beneath the lens centre would not be excessive. In the case described here, the contact lenses easily bridged over the central PRK zone, allowing the corneal periphery to support the lens. This avoided pressure or friction from the lens which may have disturbed the new area of epithelium. If the ablated zone had been wider, e.g. 6-7ram or very irregular, problems with lens fit and stability could have arisen. Visual blur during lens wear may result from lens surface greasing but in some cases the reduction in best corrected visual acuity is due to persistent reticular corneal haze resulting from PRK?~ GartryTM noted that the side effects of regression, corneal haze and loss of best corrected visual acuity of one or more Snellen lines are more commonly found in patients treated for higher degrees of myopia.This patient was typical of those electing for PRK who have limited tolerance of contact lenses or who object to the time and expense involved in their care. Therefore, although a contact lens was successfully fitted and worn, she chose to undergo repeat PRK to aim for improved unaided vision and to escape dependence on an optical appliance.
Acknowledgement I thank Mr A. D. McG. Steele for permission to describe this patient. Thanks are due to Bausch & Lomb International for funding assistance.
Address for Correspondence Christine L. K. Astin, Contact Lens Department, Moorfields Eye Hospital, City Road, London EC1V 2PD.
REFERENCES Trokel, S.L., Srinivasan, R. and Braren, B. Excimer laser surgery of the cornea. Am. J. OphthalmoI., 96, 710-715 (1983). Marshall, J., Trokel, S.L., Rothery, S. and Krueger, R.R. Long term healing of the central cornea after photorefl-active keratectomy using an excimer laser. Ophthalmology, 95, 1411-1421 (1988). Gartry, D.S., Kerr Muir, M.G. and Marshall, J. Excimer laser photore#active keratectomy--18 month follow-up. Ophthamology, 99, 1209-1219 (1992). 4 Ficker, L.A., Bates, A.K., Steele, A.D. McG., Lyons, C.J., Milliken, A.B., Astin, C., Slattery, K. and Kirkness, C.M. Excimer laser photorefractive keratectomy for myopia: 12 month follow-up. Eye, 7, 617-624 (1993). Tuft, S.J., Zabel, R.W. and Marshall, J. Corneal repair following keratectomy, nvest. Ophthalmol. Kis. Sci., 30, 1769-1777 (1989). Wilson, S.E., Klyce, S.D., McDonald, M.B., Hu, J.C. and Kaufman, H.E. Changes in corneal topography after excimer laser photor~ fractive keratectomy for myopia. Ophthalmology, 98, 1338-1347 (1991). 7 Stevens, J.D., Ficker, L.A., Steele, A.D.McG., Astin, C., Milliken, A. and Claoue, C.M.P. Excimer laser treatment of myopia with primary astigmatism. Invest. OphthalmoL Vis. Sci., 34(4), 799 (1993). s Sher, N.A., Barak, M., Daya, S., DeMarehi, J., Tucci, A., Hardten, D.R., Frantz, J., Eiferman, R.A., Parker, P., Telfair, W.B. and Lane, S.S. Excimer laser photorefractive keratectomy in high myopia. Arch. Ophthalmol., 110, 935-943 (1992). Tengroth, B., Epstein, D., Fagerholm, P., Hamberg-Nystrom, H. and Fitzsimmons, T.D. Excimer laser photorefractive keratectomy for myopia. Ophthalmology, 100, 739-745 (1993). Lakkis, C. and Brennan, N.A. Photorefractive keratectomy. Clin. Exp. Optom., 76(2), 33-49 (1993). Lohmann, C.P., Fitzke, F., O'Brant, D., Kerr Muir, M.G., Timberlake, G. and Marshall, J. Corneal light scattering and visual performance in myopic individuals with spectacles, contact lenses or excimer laser photorefractive keratectomy. Am. J. Ophthalmol., 115, 444-453 (1993). Astin, C.L.K. Considerations in contact lens fitting following photorefractive keratectomy. Practical Optometry (Canada), 6(3), 94-96 (1995). Asfin, C.L.IC Contact lens fitting after photorefractive keratectomy (PRK): a comparison of two groups. Ophthal. Phys. Opt., 15(5), 371-374 (1995). Klyce, S.D., Wilson, S.E., McDonald, M.B., Liu, J.C. and Kaufinan, H.E. Corneal topography "after excimer laser keratectomy. Invest. Ophthalmol. Vis. Sci., (suppl) 32,721-722 (1991). Shovlin, J.P. A comparison between patients wearing contact lenses following radial keratectomy and myopia photorefractive keratectomy with the excimer laser. Int. Contact Lens Clin., 19, 141-142 (1992). Astin, C.L.K. Contact lens fitting post-refractive surgery. In: Ruben, M. and Guillon, M. (Eds), Contact Lens Practice, Chapman & Hall, London, 843-854 (1994). Stevens, J.D., Steele, A.D.McG., Ritten, S., Ficker, L.A., Bates, A.tC, Kirkness, C.M., Williams, H., Claoue, C.M.P., Milliken, A.B. and Astin, C. Comparison of the Summit Excimed UV200 & VisX 20:20 Exeimer laser systems at one site. Arch. Ophthalmol., (in press). Gantry, D.S. Treating myopia with the excimer laser: the present position. BMJ, 310, 979-985 (1995).
57