Ординатура / Офтальмология / Учебные материалы / Orthokeratology Principles and Practice 2004

There are many benefits to gas permeable rigid lenses, including quality of vision and eye health, not to mention their uses in presbyopia and keratoconus. However, one of their most exciting applications is in the rapidly growing area of orthokeratology. Although orthokeratology has been a process that has been available for over 40 years, it has only been in the last decade that the interest has greatly escalated. This has been the result of several new developments including:

•a better understanding of how orthokeratology results in myopia reduction and how to predict the specific amount of reduction

•sophisticated lens designs which can often reduce the patient's baseline myopic correction without the need for a series of lenses

•overnight therapy and retainer wear to allow the patient the convenience of not wearing lenses during their waking hours, and

•introduction of corneal topography instrumentation and orthokeratology software to assist in lens design and monitoring of corneal change.

The individual most responsible for these developments has been the 'father' of modern orthokeratology, John Mountford. Along with Helen Swarbrick, he has increased our understanding of the relationship between orthokeratology lens design and the concurrent changes in the tear layer and corneal tissue resulting in myopia reduction. Lacking from the rapid growth of ortho-

keratology, however, has been a contemporary clinical text that describes such important topics as patient selection, corneal topography, lens design and fitting, aftercare and problem solving. Orthokeratology, authored by John Mountford and noted orthokeratology specialists Trusit Dave and David Ruston, provides this information and more.

It is quite evident that the ability to improve a patient's unaided vision and, therefore, their quality of life via orthokeratology is one of the most exciting developments in the eyecare field today. It is important for every practitioner seriously to consider incorporating orthokeratology into everyday practice. Not only is this a great practice-builder, but the opportunity to improve patients' ability to function without correction - and without surgery - makes eyecare practice much more fulfilling for the orthokeratologist.

The important new text will provide any practitioner, ranging from the practicing orthokeratologist to the novice clinician, with the important information to successfully implement orthokeratology into their practice.

Edward S. Bennett 00, MSEd, 2004 Executive Director of the RGP Lens Institute; Associate Professor of Optometry and Co-Chief of the Contact Lens Clinic, University of Missouri School of Optometry, St. Louis, Missouri, USA

CHAPTER CONTENTS

The practitioners 2 The academics 4

The computer programmers 5 The material companies 5 Terminology 6

Aims and objectives of this book 12 References 13

Appendix

The first meeting of the International Orthokeratology Society. 13 October 1962 14

Orthokeratology is, all of a sudden, a "hot topic" once again. However, it has been in a constant state of evolution for 40 years. In 1962, George Jessen caused a degree of controversy at the International Society of Contact Lens Specialists conference in Chicago when he first described his "orthofocus" procedure. The report of the meeting was reprinted in full in Contacio (Nolan 1995) and makes for interesting and delightful reading. The amazing thing is to read Newton Wesley's almost prophetic comments on the research that would need to be done to place the technique on a solid scientific basis. The research that he outlined 40 years ago is just starting to be carried out now. The National Eye Research Foundation has kindly given permission for a full reprint of the paper to be included in the appendix to this chapter.

The name "orthokeratology" was coined by Wesley at the meeting, and was adopted as the recognized term for the procedure. Kerns (1976) defined orthokeratology as "the reduction, modification or elimination of a refractive error by the programmed application of contact lenses." Modern marketing has attempted to put a different spin on the process by using alternative descriptors, but there is something special about orthokeratology having such a long history that demands some respect from the profession.

Orthokeratology was discovered and developed by optometrists, and is the one area of contact lens practice to which we can claim total ownership in terms of research and development. It is ours. As a sign of respect for all those practitioners who spent

a lifetime dedicated to the development of the procedure, this textbook will only use the term "orthokeratology" to describe the design, fitting, and science behind the process of applying contact lenses with the deliberate intention of altering the refractive state of the eye.

Traditional orthokeratology did not have the advantages that are currently available, like corneal topography, computer-aided design and computer numeric-controlled (CNC) lathes: the successes were totally due to individual practitioner skill and experience. Hindsight is always 20/20, but it is amazing to realize that a great deal was accomplished in those early days, and the reader is encouraged to take the time to review the work of those early pioneers. Coon (1982) was the first to publish a thorough review of the literature on the various techniques and philosophies in his study of the Tabb method. Carney (1994) also published an excellent analysis of the traditional techniques, whilst Mountford (1997) reviewed the controlled studies of Kerns, Coon, Brand, and PoIse.

These three controlled studies led to the sudden and prolonged demise of published articles on orthokeratology (Bara 2000). Interviews with practitioners who were actively involved in the procedure as background information for the chapter in Phillips & Speedwell's Contact Lenses made it clear that there were strong disagreements between what they had achieved in practice and what was reported in the studies (Mountford 1997). As a result, the developments continued to be made, as they still are today, by practitioners. The advancement in orthokeratology has also been accelerated by active academic involvement, along with the support of the two major companies that produce the materials from which the lenses are made. The following brief history therefore deals mainly with the individuals involved and their contributions to this ever-evolving emerging science. The reader will find references throughout this book that cite the practitioners' published works as well as those of the academics.

Bryla. Richard Wlodyga is an experienced and well-read practitioner. He basically adapted an earlier concept that was described by Fontana, and started the reverse geometry revolution in orthokeratology.

Fontana described a "one-piece bifocal" lens, with the central 6.00 mm optic zone being cut 1.00 0 flatter than the peripheral section of the lens, which was fitted on-K. The lens had a major advantage over its contemporaries in that centration was better. The common problem with traditional orthokeratology was control of lens centration. Superior decentration was responsible for induced with the-rule astigmatism (Kerns 1976), and the addition of a relatively steeper peripheral area on the lens tended to decrease the problem.

There are some who consider the Fontana lens to be a true reverse geometry lens, but it was, in fact, a recessed optic lens. With the lathing technology available at the time, it was simply impossible to produce a true reverse curve lens. The difference between the two design concepts is shown in Figure 1.1.

Wlodyga made the logical assumption that if a lens was made with a very flat base curve or back optic zone radius (BOZR), the first back peripheral radius (BPRj ) would need to be more than 1.000 steeper than the base. His initial concept was to make a lens with a 3.00 0 steeper secondary curve in order to control centration, and he needed someone who was prepared to make it for him. He eventually found Nick Stoyan, who produced specialty rigid lenses in his Californian laboratory

THE PRACTITIONERS

The first report of a new development in orthokeratology was published in 1989 by Wlodyga &

Figure 1.1 The difference in construction between (A) "recessed optic," asdescribed by Fontana, and (B) a true reverse geometry lens.

(Contex). Stoyan had always had a reputation as an innovator and manufacturer of extremely highquality lenses. His perfectionist nature made him one of the first to use the new CNC lathes that offered much better quality in terms of finish and reproducibility. The relationship forged between Wlodyga and Stoyan is what originally set modern orthokeratology on course.

The Contex OK-3 was a 9.6 mm total diameter (TD) lens with a 6.00 mm back optic zone diameter (BOZD), 3.00 D steeper reverse curve (forming the tear reservoir or TR) and an aspheric peripheral curve that was 0.50 mm wide. If the lens diameter was altered, the optic zone (BOZD) and peripheral curve (PC) remained constant, with the TR width being varied. This was the first three-zone orthokeratology lens, and Stoyan was granted a patent for the design and use of the lens for orthokeratology. Siviglia had earlier been granted a patent for a lens with aLSO D steeper than BOZR lens for use in postrefractive surgery conditions, but no mention of orthokeratology W,lS made in the patent documents, so Stoyan can still rightfully claim precedence in the area.

Wlodyga and Stoyan were recognized for their contribution to the field by being awarded the Founders Award at the first Global Orthokeratology Symposium in 2002.

The fitting philosophy of the Contex lens was based on central and temporal keratornetry readings, with the initial lens being fitted 1.50 D (0.30 mm) flatter than the flat-K reading. The accepted methodology of a series of progressively flatter lenses was then fitted until the maximum refractive change had been achieved. A totally unforeseen result of the new design was that the refractive changes were, on average, twice that achieved with the traditional techniques, and in half the time. The epithet "accelerated orthokeratology" was used to describe the technique. The range of lenses available eventually encompassed BOZDs from 6.00 to 8.00 mm in 0.50-mm steps and reverse curves from 1.00 to 15.00 D steeper than the BOZR.

Sammi EI Hage applied topography data to the design and fitting of the lens. The BOZR was based on the Jessen factor, where the liquid lens provided the refractive correction, i.e., the higher the degree of myopia, the flatter the lens is fitted.

HISTORY AND GENERAL PRINCIPLES 3

The rest of the lens construction was described in terms of a polynomial equation, even though the basic design could still be described as a threezone lens. The technique was named controlled keratoreformation (CKR).

The major problem with both the CKR and Contex three-zone lenses continued to be centration, or the lack of it. Variations to the total diameter (TD) and reverse curves were made and prism ballast applied, to little effect. Tom Reim totally redesigned reverse geometry lenses, and used a wider peripheral alignment curve to control centration. The reverse curve was narrow (0.60 mm) and steep, and the alignment curve extended to 1.00 mm in width, with a fixed relationship to the corneal surface. The lens was called the Dreimlens and the process renamed "advanced orthokeratology."

At approximately the same time, Mountford redesigned the Contex lenses and added a wide (1.10 mrn) tangent periphery in order to control centration better (see Ch. 4). Jim Day (Fargo Lens) varied the Dreimlens concept by dividing the alignment curves into two sections, eventually settling on a hyperbolic alignment zone. He also developed the fitting philosophy that matched the lens back surface area to that of the cornea, and, like Reim and El Hage, used a variation of the Jessen factor as a means of determining the BOZR and refractive change required. Further variations on the Dreimlens theme were developed by Roger Tabb (Nightmove), John Reinhart and Jim Reeves (R&R), George Glady (Emerald and Jade), Nick Stoyan (Contex E series) and Jim Edwards with the WAVE design.

Jerry Leggerton developed the Corneal Refractive Therapy (CRT) design, and changed the reverse curve construction to that of a sigmoid curve, which effectively blends the BOZR to the peripheral curve, which is a tangent. The sigmoid curve is used to alter the sagittal height of the lens, thereby exerting greater control over the fit. Tangents were also used by Mountford and Noack for the Contex T, Ideal series, and BE. The BE is the only fouror fivezone reverse geometry lens that does not base the fitting on the Jessen factor, but instead uses modeled squeeze film forces in the postlens tear layer to determine the optimal tear layer

profile for the lens, and thereby the back surface construction.

All these designs have different fitting philosophies, but the fact is that they all do the same job, and no one design is inherently superior to the other, as the underlying mechanisms that make orthokeratology "work" are similar (see Ch. 10). What is different about them is the fitting philosophy, and whether it is keratometryor topography-based.

Stuart Grant introduced the concept of overnight orthokeratology. Since the lenses required approximately 8 hi day to cause an effective change, he reasoned that the new extendedwear materials should be used and the lens slept in overnight, so that patients could be correction-free for their waking hours. This had major benefits over day-wear orthokeratology for obvious reasons, and became the common form of treatment in Australia as early as 1994. Overnight orthokeratology was a revolutionary concept.

Russell Lowe, a Melbourne-based optometrist, improved the fitting process by taking repeated topography readings on each individual patient and calculating the mean and standard deviation of error of the instrument. This lead to a marked improvement in first-fit success, and by extension, a logical method of refining the fit to correct for adverse outcomes. In the author's opinion, this application of routine experimental procedure to clinical practice was a major step forward.

Finally, no review of the practitioners involved in the development of modern orthokeratology would be complete without referring to Dr Kame.

The late Roger Kame was a universally highly respected clinician who was in constant demand for conferences and continuing education events due to his superb lecturing skills, and his ability to make complex problems easily understood. Along with Todd Winkler, he published the first textbook on accelerated orthokeratology, and coined the term "reverse geometry lenses." In an interview with the author a few years ago, he described his growing interest in the "new" orthokeratology, and his trepidation at being seen to be involved in what was then considered to be a "fringe" practice. However, as his battle with cancer later showed, his courage and determination knew no limits, and he started publicly lecturing on his

experiences with orthokeratology. By being the first mainstream researcher and clinician to do so, he not only raised the profile of orthokeratology, but also increased the growing acceptance of the procedure. The inaugural Roger Kame Memorial Award was presented to Professor Brien Holden at the 2002 Global Orthokeratology Symposium in Toronto, Canada.

THE ACADEMICS

Anecdotes are not science, and anecdotal reports are not proof. Practitioners are limited in their ability to perform research in practice due to the difficulties in ensuring control and the influence of bias. For a procedure like orthokeratology to become accepted, it must be able to prove its claims by subjecting them to independent scrutiny, and that means academic control. Fortunately, many academics have become involved in this type of research, and have added immensely to the science of the subject.

Doug Horner and Sarita Soni (Indiana) were conducting research into the visual and refractive effects of radial keratotomy in the early 1990s when a practitioner told them of his experiences with accelerated orthokeratology. They included a group of orthokeratology patients in their study, and expanded the research to include studies of the short-term corneal response to the lens. They also wrote a chapter on the topic for Bennett & Weissman's Contact Lens Practice textbook. Sarita Soni is still deeply involved in orthokeratology research today.

Joshua Joe, Harue Marsden, and Tim Edrington (Southern California) fell under the influence of Roger Kame and studied the changes in visual acuity, refraction and corneal eccentricity with the Contex OK-3.

In Hong Kong, Lui and Edwards performed the first controlled study of orthokeratology with reverse geometry lenses and compared the results to those of a control group fitted with standard contour lenses. Under the leadership of Pauline Cho, the Hong Kong Poly-U continues to do excellent research into the differences in performance between lens designs, corneal topography, and its influence on the outcomes and myopia control with orthokeratology.

Helen Swarbrick, Gunther Wong, and Dan O'Leary from the University of New South Wales published the results of a breakthrough study that showed that the refractive changes were mainly due to epithelial thinning. Helen continues to be involved in orthokeratology research, with Masters student Ram Sridharan studying the changes in corneal shape, thickness, refraction, and visual acuity to short-term exposure to lens wear.

Ahmed Alharbi studied the long-term predictability and safety as wel1 as the refractive and corneal changes to overnight orthokeratology as part of his PhD thesis. In Melbourne, Christa Bara studied the effects of changes in the depth of the postlens tear layer at the BOZO on the refraction, visual acuity, and topography of a group of subjects, and in New Zealand, Helen Owens and Jennifer Craig are involved in evaluating the endothelial curvature changes and the effect, if any, of corneal bending on the outcome.

The Cornea and Contact Lens Research Unit (CCLRU) has recently become involved in orthokeratology research, with Nina Tahhan leading a group examining the differences in results between lens designs.

Joe Barr at Ohio State led the group consisting of Nicholls, Marsich, Nguyen, and Bullimore that published the first study on the efficacy of overnight orthokeratology, and is involved in the LOOK study with Marjorie Rah. John Mark Jackson, Ed Bennett, and Harue Marsden.

As Editor of Contact Lens Spectrum, Joe Barr has been at the forefront of promoting education and awareness of the rapid changes occurring in the field. Along with Ed Bennett, Pat Caroline, and Craig Norman, he organized the highly successful Global Orthokeratology Symposium in Toronto in August 2002.

In Canada, Des Form's group is studying the topographical thickness changes across the corneal surface, and in the UK, Trusit Dave is supervising research into methods of measuring the apical clearance of lenses in situ as well as the ocular aberrations induced by reverse geometry lenses. Nathan Efron's Eurolens group at Manchester has also started orthokeratology research - a most welcome development!

HISTORY AND GENERAL PRINCIPLES 5

One of the frustrations of writing this book has been knowing about the research being undertaken, and some of the results being generated, yet being unable to put anything in print due to confidentiality agreements. However, many academics freely made available the results of their research prior to publication, and the author's thanks goes to Helen Swarbrick, Gavin Boneham, and especially Pauline Cho for their generous assistance.

Orthokeratology research is also being done in China, with numerous papers being published in the Chinese Journal of Optometry and Ophthalmology. The difficulties associated with translation have prevented their wider availability, but where applicable, the results of the studies have been included in the text.

The result of all this research effort will be improved designs and, hopefully, the ability to improve the predictability of orthokeratology, as well as a much better understanding of the anatomical, physiological, and visual changes that occur. There are a lot of unanswered questions in a host of areas in orthokeratology, requiring a prolonged research effort to resolve. The areas that require future research are outlined in the final chapter.

THE COMPUTER PROGRAMMERS

The design and fitting of reverse geometry lenses require the use of advanced mathematics, which is not usually accessible to the private practitioner. Specialized computer programs are required for the calculations, and the outstanding architects of these advanced tools are undoubtedly Don Noack and Tom Geimer. Don Noack wrote the complex platform that is used to fit BE lenses, and the Free Design program, and Tom Geimer wrote Ortho-Tools. These are an excellent teaching and learning resource, and an outline of both programs is covered in Chapter 8.

THE MATERIAL COMPANIES

As stated before, for a new procedure to become really accepted, it must first prove itself to be effective at the hands of objective researchers,

6 ORTHOKERATOLOGY

and secondly, gain Food and Drug Administration (FDA) approval so that it is available in the US market. Also, the FDA stamp of approval has a high international standing, and can affect the acceptance of the process in other markets. The funding for this research and the costs involved in gaining FDA approval are usually borne by the companies that manufacture the materials from which the lenses will be made. Both Polymer Technology Corporation and Paragon Vision Sciences have been at the forefront of these efforts, both by developing the high-Dk materials required (Boston XO and Paragon HDS 100), and also by sponsoring much of the research currently underway. They have also been heavily involved in the FDA approval process. They are deeply committed to marketing orthokeratology worldwide, and were major sponsors of the Global Orthokeratology Symposium.

Back optic zone diameter (BOZO)

This is defined as the diameter over which the BOZR acts. Other terminologies used are the back optic zone (BOZ) or simply OZ. The BOZOs of fourand five-zone lenses range from a minimum of 5.50 mm to 6.50 mm, with 6.00 mm the most common.

First back peripheral radius (BPR,)

The curve immediately adjacent to the BOZR is BPRj • In orthokeratology terms, it is commonly known as either the "tear reservoir (TR) curve" or "reverse curve (RC)." It is usually measured by its degree of steepening compared to the BOZR, and expressed in diopters, i.e., 3.00 0 steeper than BOZR. Other lens-specific terms are return curve (CKR) and sigmoid curve (CRT). The majority of manufacturers do not disclose the value of the reverse curve.

TERMINOLOGY

The International Standards Organization (ISO) has compiled a complete list of terms that describe in full every aspect of contact lens design. Unfortunately, these terms have been largely ignored by orthokeratology lens designers and manufacturers, and substituted by other names for the various curves in order to differentiate their particular design from all the others. This leads to unfortunate and unnecessary confusion. The following section is a glossary of common terms used in orthokeratology, with reference to the ISO 8320 (British Standards Institute 1995) edition.

Back optic zone radius (BOZR)

The radius of curvature of the back optic diameter of a lens with a single refracting surface is called the BOZR. It is given the common misnomer of "base curve" or back central optic radius, which actually denotes the back central optic radius of a multifocal lens. ISO measures BOZR in millimeters, but American labs and practitioners usually denote the radius in its equivalent dioptric power. This is done by dividing 337.5 by the BOZR in millimeters.

First back peripheral diameter (BPD,)

This denotes the width of the BPRj • Equivalent terms are TR width and RC width. In the majority of fourand five-zone lenses, this value is fixed and is commonly between 0.50 and 1.00 mm wide, depending on the design.

Second and third back peripheral radii (BPR2 and BPR3)

In standard lens designs, these curves represent that part of the lens commonly referred to as the "edge lift." However, in orthokeratology designs, these curves represent curves that are designed to come into near-alignment with the corneal surface and effect control over lens centration. They are usually referred to as the "alignment curves (AC)" or zones. A four-zone lens will have a single AC that is usually 1.00 mm wide, and spherical, aspheric, or hyperbolic in construction. Tangents (flats) can also be used as alignment zones. Synonyms are "anchor zone" (CKR) and "landing zone" (CRT). Alternatively, a five-zone lens has two ACs, with the first being slightly steeper than the second. The total width is still approximately 1.00 mm, with each curve being half of the total.

Fourth back peripheral radius (BPR4)

This is the final curve on the back surface of a reverse geometry lens and is commonly referred to as the "edge lift" or peripheral curve (PC). It is commonly 0.30-0.50 mm wide on the majority of lenses.

In general, the nomenclature of a four-zone lens is the BOZR, RC, AC, and PC, whilst a fivezone lens consists of BOZR, RC, AC 1, AC 2, and

HISTORY AND GENERAL PRINCIPLES 7

the Pc. Alternatively, lenses such as the CRT have BOZR, sigmoid curve, tangent, and PC, whilst the BE has BOZR, RC I, RC2, tangent, and Pc. The construction of a five-zone lens is shown in profile in Figure 1.2.

Tear reservoir

As stated above, the TR is used to describe the steepening of the RC with respect to the BOZR. It