Advanced Ortho-k Using Custom Lens Designs
BY JIM H. DAY JR., O.D., THOMAS REIM, O.D., ROBERT D.
BARD, O.D., PAT McGONAGILL, O.D., & MICHEL J. GAMBINO, O.D.
A new computer program promises more predictable orthokeratology results through individual computer-aided lens designs.
The new technologies that have energized the contact lens industry in general have also sparked advances in orthokeratology. Highly oxygen permeable RGP materials for extended wear allow corneal reshaping while a patient sleeps. Computerized corneal topographers enable us to design specific lenses for individual corneas and then monitor the reshaping progress. Computerized manufacturing lathes can produce precise new designs in reverse geometry.
In addition, new theories, such as those proposed by Charles H. May, O.D., about the contact surface area of Bowman's layer, provide the basis for a mathematical model that predicts the potential shape and volume change of the cornea before the orthokeratological procedure begins.
NEW THEORIES LEAD TO ADVANCED ORTHO-K
According to Dr. May, Bowman's layer collagen fibers are free to bend, but they do not stretch or shrink, thus the surface area of Bowman's layer stays constant. Using this constant surface theory, the surface area of a normal cornea is equal to the surface area of a spherical lens that is fit one diopter flatter than K. A two-diopter flat spherical lens does not match the cornea in surface area and should not be expected to cause a controlled change. On the other hand, a plateau or reverse geometry lens can be fit two diopters flatter than K, and because the intermediate zone of the lens is steeper than the pretreatment cornea, the surface areas can match. Control is maintained if the back surface area of the lens equals the front surface area of the cornea.
The shape of the cornea can change in an elastic sense, but tends to return to equilibrium. In clinical terms, large shape changes shorten periods of good unaided vision. We find that a well-centered, reverse geometry lens that produces the least shape change for good vision is the desired clinical result.
Only about one diopter change is created by sphericalization of the normal aspheric cornea. Oblate corneas create greater myopic corneal power changes.
BRINGING THEORY AND
PRACTICAL APPLICATION INTO FOCUS
Utilizing the new contact lens technologies and putting these new theories into practice, co-authors Drs. Day and Reim have developed a computer program that uses initial K readings and shape factors to compute the potential tissue response to lens wear. In this advanced version of orthokeratology, the practitioner provides the lab with initial patient information, and the software creates a template for the correct lens design. A computer model of the projected tissue change is then pre-fit with a corresponding lens and analyzed.
Prior methods used fixed designs so that the cornea had to change to fit the lens. The desired result would often demand unlikely changes in collagen fiber length or shrinkage. This created a moving target, difficult to understand and requiring considerable clinical skills and a large set of trial lenses.
Advanced orthokeratology utilizes reverse geometry lenses that are custom designed for a specific eye. This enables rapid, controlled changes that are stable after the first few nights of wear. Because the changes are controlled, the endpoint is predictable and the holding time is greatly enhanced.
A typical myope with -4.00D or less should be able to maintain clear, uncorrected acuity during all waking hours after only three to seven days of night-only wear. This amount of change is not dependent on the corneal eccentricity value. Advanced orthokeratology is designed not to sphericalize the cornea, but to form an oblate shape. Previous methods were designed to stop when the cornea was spherical.
ADVANCED ORTHO-K CASE REPORT
J.D., a 16-year-old student, had started orthokeratology with another practitioner who had been using reverse geometry aspheric lenses. J.D.'s original prescription was: OD �4.00 �0.25 x 160, 20/20; 43.50 @ 177/44.00 @ 087; OS �4.25 �0.50 x 014, 20/20; 43.25 @ 002/44.25 @ 092
|His first lenses were the RA 4 design: OD 8.18 base curve (BC), �1.50 power (PWR), 10.00 diameter (DIA), 1.1 reverse geometry width (RGW), 6.0 optic zone diameter (OZD), 0.24 center thickness (CT); OS 8.20 BC, �1.75 PWR, 10.0 DIA, 1.1 RGW, 6.0 OZD, 0.24 CT.|
|At his two-week follow-up visit, J.D. was wearing the lenses 16 hours a day. His unaided visual acuity went from 20/400 at 18 feet (in each eye) to 20/70 OD, 20/80 OS, 20/60 OU. Autorefraction: OD �2.50 -0.50 x 104; OS �2.00 �0.50 x 153. Keratometry: OD 42.25 @ 153/42.50 @ 63; OS 42.12 @ 165/42.75 @ 75.|
|J.D.'s next lenses were RA 5: OD 8.33 BC, �0.75 PWR, 10.0 DIA, 1.1 RGW, 6.0 OZD; OS 8.35 BC, �1.00 PWR, 10.0 DIA, 1.1 RGW, 6.0 OZD.|
|At his two month follow-up visit, J.D. was wearing his lenses 16 hours a day and his prescription was: OD �1.50 �0.25 x 80, 41.62 @ 170/41.87 @ 080, unaided VA 20/60; OS �1.50 �0.50 x 115, 41.50 @ 180/41.50 @ 090, unaided VA 20/60. Because we observed no further changes, we decided to prescribe the Reim lens design: OD 8.71 BC, +0.75 PWR, 10.0 DIA, 6.0 OZD; OS 8.77 BC, +0.50 PWR, 10.0 DIA, 6.0 OZD.|
|Three days after we dispensed the lenses, we obtained the following: OD �0.50 -0.25 x 130, 20/20; 40.62 @ 176/40.87 @ 086; OS �0.25 �0.25 x 005, 20/20; 40.37 @ 021/40.62 @ 111.|
|At J.D.'s most recent visit, findings were: OD pl �0.25 x 125, 20/20; 40.00 @ 004/40.37 @ 094; OS +0.75 �0.75 x 010, 20/20; 40.00 @ 020/40.25 @ 110. Unaided acuities OU were 20/15-. His visual acuity remains all day without correction.|
|-- Robert D. Bard, O.D., F.A.A.O.|
ADVANCED ORTHO-K EDGES OUT RK AND PRK
We believe the results of advanced orthokeratology are comparable to refractive surgery without invasion of the corneal structure or interference with the normal corneal physiology. The specificity of the new lens designs merely reshapes the anterior corneal surface through gentle pressure on the surface. We find that this gentle pressure is no greater than that applied by any properly fit rigid gas permeable lens design. When the lens is removed, the vision is much improved and the tissue memorizes the shape. Eventually, the only purpose of the lens is to help the cornea to remember and retain the new shape.
This new custom lens method may be considered the technological upgrade of classic orthokeratology. We feel it works faster with greater predictability and control. And the two-diopter limit of sphericalization does not apply. CLS
References are available upon request to the editors at Contact Lens Spectrum. To receive references via fax, call (800) 239-4684 and request Document #25. Be sure to have a fax number ready.
The computer software discussed in this article was developed by Drs. Day and Reim and is being licensed to RGP laboratories. It is not available to practitioners.
Dr. Day is an independent lens designer and a consultant for orthokeratology in private practice in Hoover, Ala. Dr. Reim is an independent orthokeratology lens designer in private practice in Indian Harbor Beach, Fla. Dr. Bard is in practice in Dallas, Texas, where he specializes in orthokeratology and dry eyes. Dr. McGonagill is an orthokeratologist in practice in Tyler, Texas. Dr. Gambino is an orthokeratologist in practice in Mesquite, Texas.