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CONTINUING EDUCATION AND TRAINING Gain 2 CET credits - enter online at www.otcet.co.uk or by post Figure 9 The same eye as seen in Figure 8 with a significant increment in the optic zone projection illustrating full corneal clearance. comparison to the corneal thickness, as shown in Figure 10. The fluorescence can be seen perfectly well with white light; cobalt blue filters reduce the brightness too much for the corneal thickness to be seen well. Apical clearance between 0.2mm and 0.3mm is a satisfactory target for a nonventilated ScCL, approximately half a normal corneal thickness, with the pre-corneal fluid reservoir extending just beyond the limbus. The exact measurement of corneal clearance is not critical, although if excessive it is more difficult to maintain an air-free pre-corneal fluid reservoir or for the patient to insert the lens without an air methods consequent to the introduction of RGP materials. There are many fitting nuances which could not be covered, and there are significant problem areas which have only briefly been touched upon. However, it is emphasised that the fitting processes have become straightforward and predictable in most cases, with a rapid arrival at the end-point mostly using nonventilated preformed RGP designs. ScCLs may be lens-of-choice when a result is needed more than at any other time. There are no conditions for which they cannot be considered, so it is clearly necessary to preserve the clinical and manufacturing skills required . Since the introduction of is defined according to two axially measured parameters: the OZP from the extrapolation of the scleral curve measured at the apex and at the limbus. These increments are just clinically significant enough to keep the number of lenses in the fitting system manageable. OZP is a function of the BSR, hence if the BSR is flattened, the OZP needs to be increased to compensate. The OZS or OZP for an initial trial lens is selected from an assessment of the overall corneal profile with the naked eye. If inaccurately estimated, there is no major loss as the first lens inspected in situ gives a clear indication for subsequent lens selection. Keratometry and corneal topography are of limited value as neither provides any information about the peripheral cornea or the projection from the sclera. The parameters are assessed simultaneously with the lens in situ, but the initial decision must be to determine the optimum scleral zone specifications as the scleral zone has an impact on the optic zone clearance. The lens is inserted filled with saline and fluorescein so that areas where the lens is clear of the surface can be easily distinguished from areas where the lens is in contact. Figures 8 and 9 demonstrate the effect of increasing the OZP to reduce corneal contact. Slit lamp biomicroscopy assessment of the optic zone If fluorescein is added to the saline prior to insertion, inspection of the optic zone using a cobalt blue filter shows any corneal contact zones. A slit lamp optical section can be used to measure the optic zone clearance. A pachometer is not necessary but the depth of the pre-corneal reservoir can be estimated with sufficient accuracy by Figure 10 Slit lamp optical cross section demonstrating the pre-corneal fluid reservoir. The lens, the reservoir and the cornea are all clearly seen in cross section. The depth of the reservoir is just less than the corneal thickness at the visual axis, therefore approximately 0.35mm to 0.4mm. By comparison, the cornea is 0.5mm to 0.6mm in thickness. The reservoir is thinner superiorly, and thicker inferiorly, indicating a degree of downward displacement of the lens. This is a normal feature with non-ventilated RGP ScCL fitting and does not usually have any detrimental effect (Courtesy of Scott Hau) bubble. If there is insufficient clearance, corneal contact zones may reduce comfort and tolerance. Increasing the OZS or OZP by 0.25mm alleviates a compressive central contact zone to give corneal clearance. Conclusion This is paper intended to provide an introduction to modern scleral contact lens practice, principally to outline changes in fitting RGP materials their application can be made at all grades of pathology where benefits can be seen. There remain some problems, and commitment to clinical practice is a prime requirement. The pathology may be active or progressive, so close liaison with ophthalmology is essential. A small number of practitioners are needed to maintain a functional service, but those who are not directly involved should also be aware of the significant developments in recent years. 31 | October 20 | 2006 | OT
CONTINUING EDUCATION AND TRAINING Gain 2 CET credits - enter online at www.otcet.co.uk or by post References 1) Ezekiel D. Gas permeable haptic lenses. J Br Contact Lens Ass, (1983), 6(4), 158 - 161. 2) Bier N (1948). The practice of ventilated contact lenses. Optician 116, 497-501. 3) Pullum KW, Hobley AJ and Parker JH. Dallos Award Lecture part two. Hypoxic corneal changes following sealed gas permeable impression scleral lens wear. J Br Contact Lens Assoc 1990; 13(1): 83 - 87. 4) Pullum KW, Hobley AJ and Davison C. 100 + Dk: Does thickness make much difference? J Br Contact Lens Assoc 1991; 6: 158 - 161. 5) Pullum KW and Stapleton FJ (1997). Scleral lens induced corneal swelling: what is the effect of varying Dk and lens thickness? The CLAO Journal. 23(4), 259 – 263. 6) Pullum KW, Parker J H and Hobley AJ. Development of gas permeable impression scleral lenses. The Josef Dallos Award Lecture, part one. Trans BCLA Conf J. Br. Contact Lens Ass. 1989; 77-81. 7) Schein OD, Rosenthal P and Ducharme C. A gas-permeable scleral contact lens for visual rehabilitation. Am J Ophthalmol, (1990), 109, 318 - 322. 8) Kok JHC, and Visser R. Treatment of ocular surface disorders and dry eyes with high gas-permeable scleral lenses. Cornea, (1992), 11, 518 - 522. 9) Tan DTH, Pullum KW and Buckley RJ. Medical Applications of Scleral Contact Lenses: 2. Gas-Permeable Scleral Contact Lenses. Cornea, (1995), 14(2), 130 – 137. 10) Pullum KW and Buckley RJ. A study of 530 patients referred for rigid gas permeable scleral contact lens assessment. Cornea, (1997), 16(6), 612 – 622. 11) Cotter J and Rosenthal P. Scleral contact lenses. J Am Optom Ass, (1998), 69, 33 - 40. 12) Swann PG and Mountford J. Facial nerve palsy and scleral contact lenses. Br J Optom and dispensing, (1999), 7, 85-87. 13) Ramero-Rangel T, Stavrou P, Cotter JM, Rosenthal P, Baltatzis S, Foster CS. Gas-permeable scleral contact lens therapy in ocular surface disease. Am J Ophthalmology, (2000), 130, 25 - 32. 14) Rosenthal P, Cotter JM, Baum J. Treatment of persistent corneal epithelial defect with extended wear of a fluid-ventilated gas-permeable scleral contact lens. Am J Ophthalmology, (2000), 130, 33 - 41. 15) Tappin MJ, Pullum KW, and Buckley RJ. Scleral contact lenses for overnight wear in the management of ocular sur face disorders. Eye, (2001). 15, 168-172. 16) Rosenthal P, Cotter J. The Boston scleral lens in the management of severe ocular surface disease. Ophthalmol Clin North Am (2003);16:89-93 17) Fine P, Savrinski G, Millodot M. Contact lens management of a case of Stevens- Johnson syndrome: a case report. Optometry (2003) Oct; 74:659-64 18) Looi AL, Lim L, Tan DT. Visual rehabili tation with new-age rigid gas-perme able scleral contact lenses - a case series. Ann Acad Med Singapore (2002);31:234-7 19) Contact Lenses, edited by Anthony J Phillips and Lynne Speedwell, Publishers Elsevier, Chapter 15, Scleral Contact Lenses, by KW Pullum, pp 333 - 353. Acknowledgements The author is grateful for the assistance of the cornea consultants at Moorfields and Oxford Eye Hospitals, and for their support over many years in a long-term project modernising scleral lens practice and developing new clinical and manufacturing techniques. Figure 10 is courtesy of Scott Hau, senior optometrist, Moorfields Eye Hospital. For further information contact address: Ken Pullum, 73 Railway Street, Hertford, Hertfordshire, United Kingdom, SG14 1RP, (kenpullum@btinternet.com, kenpullum@tiscali.co.uk) Continuing education and training initiative For all CET Queries please contact: Jessica Buckingham CET, McMillan-Scott, 9 Savoy Street London, WC2E 7HR Telephone: 0207 878 2412 Fax: 0207 379 7118 E-mail: jessica@mcms london.co.uk 32 | October 20 | 2006 | OT
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