Empirical Advanced Orthokeratology Through Corneal Topography: the University of Houston Clinical Study

Eye & Contact Lens 33(5): 224–235, 2007 © 2007 Contact Lens Association of Ophthalmologists, Inc. Empirical Advanced Orthokeratology Through Corneal Topography: The University of Houston Clinical Study Sami El Hage, O.D., Ph.D., D.Sc., F.A.A.O., Norman E. Leach, O.D., M.S., F.A.A.O., William Miller, O.D., Ph.D., F.A.A.O., Thomas C. Prager, Ph.D., M.P.H., Jason Marsack, M.S., Katrina Parker, O.D., F.A.A.O., Angela Minavi, O.D., and Amber Gaume, O.D., F.A.A.O. Purpose. Traditionally, orthokeratology has used diagnostic lenses unaided visual acuity. First described in 1962 by Jessen,1 the to determine the best fit. The purpose of this study was to orthokeratology procedure and lens designs have evolved greatly determine the efficacy of fitting empirically from corneal topog- during the last 40 years.2–39 Lenses were originally made of raphy, without the use of diagnostic lenses. Methods. Twenty-nine polymethylmethacrylate and usually fitted flatter than the flat subjects, 18 to 37 years old, with myopia of 1.00 to 4.00 diopters keratometric reading. This concept has limitations with conven- (D) and astigmatism of no more than 1.50 D, were entered into this 6-month study. Corneal topography, scanning slit topography and tional lens designs, especially when the amount of central base corneal thickness (Orbscan), confocal microscopy, ultrasound cor- curve flattening is significant. Typically, the spherical flat-fitting neal thickness, aberrometry, and biomicroscopy were used to lens would rock over the cornea and result in induced corneal assess corneal changes. Unaided logMAR high-contrast visual astigmatism. The solution was to change the base curve gradually acuity, subjective refraction, and questionnaires were used to with time as the cornea changed. However, this procedure required monitor vision and symptoms. Follow-up visits were scheduled several months of treatment to achieve only 1 or 2 diopters (D) of after 1 day, 1 week, 2 weeks, 1 month, 3 months, and 6 months. myopia reduction. Because of only modest and unpredictable Results. For 6-month data, unaided logMAR acuity improved improvements in uncorrected visual acuity, orthokeratology fell Ϯ Ϯ from 0.78 0.26 in the right eye and 0.75 0.22 in the left eye into disfavor among most eye care practitioners and was declared to 0.06 Ϯ 0.18 in the right eye and 0.04 Ϯ 0.16 in the left eye. not to be a viable option for the correction of myopia in the Myopia decreased from –2.55 Ϯ 0.87 D in the right eye and 40–47 Ϫ2.47 Ϯ 0.89 D in the left eye to ϩ0.45 Ϯ 0.74 D in the right eye optometric and ophthalmologic literature. and Ϫ0.17 Ϯ 0.69 D in the left eye. Shape factor, using corneal In 1989, Wlodyga and Bryla introduced accelerated orthokera- topography, increased from 0.85 Ϯ 0.13 in the right eye and 0.85 Ϯ tology, with a report on 15 patients fitted with a new series of 0.15 in the left eye to 1.28 Ϯ 0.32 in the right eye and 1.30 Ϯ 0.29 reverse-geometry lens designs manufactured by Contex, Inc. (Van in the left eye. Both eyes showed a decrease in lower-order Nuys, CA), called OK lenses.48 Keratometry was used to design aberrations (i.e., defocus) and an increase in higher-order aberra- the lens and monitor corneal changes. The difference between tions (i.e., spherical aberrations and coma). Conclusions. Myopia central and peripheral keratometric readings was used to establish reduction after 1 week was clinically insignificant from the the corneal shape factor and to predict the amount of myopia 1-month results, indicating that the full effect is achieved by 1 reduction. It may be argued that because of the large mire sepa- week. Neither total nor epithelial corneal thickness varied significantly from baseline measurements. ration, the keratometer is not the best instrument to use for Key Words: Confocal microscopy—Corneal topography—Opti- monitoring orthokeratology results and that the difference between cal aberrations—Optical pachymetry—Reverse geometry. a central and temporal keratometric reading is not truly represen- tative of the corneal shape or shape factor. Nevertheless, during the next 15 years, new technologies in the manufacturing of the lenses Orthokeratology is a procedure that uses gas-permeable contact and in monitoring their effects on the cornea have revitalized lenses to reshape the cornea to temporarily reduce or modify interest in orthokeratology worldwide.49–65 myopia and astigmatism to achieve a transient improvement in Modern contact lenses for overnight orthokeratology are made of enhanced oxygen-permeable materials and have a more com- plex posterior design. The back surface of newer lens designs may From Eye Care Associates (S.E.); Texas Eye Research and Technology be divided into four or more zones. Each zone influences the lens Center (N.E.L., W.M., J.M., K.P., A.M., A.G.), College of Optometry, positioning over the cornea and the resulting visual outcome. The University of Texas; University of Texas-Houston Medical Center, Her- four zones are the optical zone (central), the reverse curve(s) mann Eye Center (T.C.P.), Houston, Texas. Address correspondence and reprint requests to Dr. S. El Hage, 5320 (paracentral), the alignment curve (intermediate), and the periph- Richmond Ave, Houston, TX 77056; e-mail: [email protected] eral curve (edge lift). Depending on the specific manufacturer, the Accepted March 30, 2007. contact lens has different nomenclature for each zone and different DOI: 10.1097/ICL.0b013e318065b0dd ways of calculating the posterior curvatures of the lens. For 224 ORTHOKERATOLOGY THROUGH CORNEAL TOPOGRAPHY 225 instance, some manufacturers prefer a lens design in which the extended-wear conventional gas-permeable lenses and for daily- central optical zone is calculated first, followed by the alignment wear orthokeratology lens designs. In June 2004, Boston Equalens curve, and finally, the two segments are joined by a return curve of II material was approved additionally for overnight ortho- specific geometric shape. Other manufacturers may choose a keratology. different approach in which the alignment curve (peripheral seg- This clinical trial used a different approach for designing and fitting ment) is calculated first, followed by the optical zone (central orthokeratology contact lenses, which is based on corneal topography segment), and then, the two segments are joined by the reverse and tear layer thickness. The corneal topographer provided three curve patterned after a specific geometric shape. With the afore- coordinates (x, y, and z) of each point on the cornea and allowed mentioned design, the practitioner sends the keratometric readings reconstruction of the shape of the cornea and design of the lens from and refraction to the laboratory for lens design and fabrication. corneal topography information using a built-in software program. Lacking information about the asphericity of the individual cornea, Another consideration is the tear layer thickness between the contact the laboratory assumes an average eccentricity, usually 0.5, when lens and the cornea. In normal gas-permeable lens designs, the tear manufacturing the lens. In another fitting modality, the practitioner thickness is approximately 20 ␮m between the cornea and the back uses a diagnostic set of contact lenses to trial fit the patient. In most surface of the lens. This concept is even more important in fitting cases, the laboratory does not indicate the eccentricity used in their reverse-geometry contact lenses in orthokeratology. A change of a lens design. In yet a third fitting modality, the practitioner uses a few micrometers in the cornea–to–contact lens relationship, for ex- corneal topographer to obtain the apical radius of the cornea, the ample, in the central segment of the lens, may induce a diopter or sagittal height at a chord of 9.35 mm, and the eccentricity or shape more in refractive change. A change of only a few micrometers in the factor. From this information, a diagnostic lens is chosen for an return curve segment or in the alignment segment will affect the overnight trial before ordering the lenses. This last fitting modality centration of the contact lens over the cornea. For instance, to center has an advantage over the previous methods because of the added the contact lens over the patient’s cornea, increasing the return curve information about the shape of the cornea, mainly eccentricity or depth of a few micrometers may be needed to tighten the fit. Alter- shape factor. The authors prefer to use the term shape factor natively, to loosen the lens, the return curve depth may be decreased. because it differentiates between prolate and oblate elliptical The same applies to the alignment curve. By varying the slope of the surfaces. For example, if the shape factor is between 0 and 1.0, the alignment curve, it can be brought closer or farther away from the shape of the cornea lies on the steeper side of the ellipse (i.e., a cornea. Bringing the alignment curve closer to the cornea will tighten prolate shape). If the shape factor is greater than 1.0, the shape of the fit of the contact lens, and moving the alignment curve away from the cornea is on the flat side of the ellipse (i.e., an oblate shape). the cornea will loosen the fit. In this study, the CKR fitting program Because of these innovative designs, lenses may be fitted with base was integrated with the Focal Points software program (Advanced curves significantly flatter than those used in conventional orthok- Medical, SRL, Milan, Italy) to design and manufacture the lenses eratology to result in larger and more rapid changes in corneal (Fig.

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