Ординатура / Офтальмология / Английские материалы / Orthokeratology Principles and Practice_Mountford, Ruston, Dave_2004

8 ORTHOKERATOLOGY

is also used to describe the appearance of the annulus of tears at the BOZD IRC junction. The earlier Contex three-zone lenses, for example, usually showed deep and wide tear reservoirs, whilst the newer fourand five-zone lenses exhibit narrow and deep reservoirs (Fig. 1.3).

Central touch zone

Reverse geometry lenses, when fitted correctly, show an area of "central touch" over the pupil zone. The term "touch" may be a misnomer, as the tear layer present under the lens at the apex is less than the 20 urn that is usually accepted as the minimal thickness at which fluorescein becomes visible under a lens. However, the area and centration of the touch with respect to the pupil center are used as a means of determining the relative accuracy of the lens fit.

Corneal sag and elevation

The sagittal height (z) of the cornea is the vertical distance from a line joining the common point of

interest on the corneal surface (the chord) to the apex of the cornea (Fig. 1.4). This term is then linked to the sag height of the lens (see later). Topographers, however, measure elevation, which is the measurement from the corneal apex to the chord of reference. The two terms result in identical measurements, and are basically synonyms.

Lens sag

A contact lens prescription is usually written as a means of specifying the radii and diameters of the varying curves that make up the design. Lens sag is simply the sagittal height of each of the individual zones of the lens added together (Fig. 1.5). As can be seen from the diagram, the sag of the lens is equal to the sag of the BOZR I BOZD, plus the sag of the RC over its width, and the sags of the alignment curves. The sag of a lens is commonly only measured to the diameter that represents the common chord of contact between the lens and the corneal surface (see Ch.4).

Figure 1.4 The sagittal height (z] of the cornea from the chord over which it is measured.

Figure 1.5 The sag height of a reverse geometry lens is equal to the sags of the individual curves overtheir respective diameters. BOZR, back optic zone radius; BOZO, back optic zone diameter; RC, reverse curve;

AC, alignmentcurve.

Jessen factor

Jessen's original orthofocus technique consisted of making the radius of the BOZR equal to the refractive change required, using the general rule of 0.20 mm difference in radii being clinically equivalent to 1 D. Therefore, a 3.00 D refractive error would result in the lens being fitted with the BOZR 3.00 0 (0.60 mm) flatter than the flat-K reading. All current fourand five-zone lenses, with the exception of the BE, use a variation of the Jessen factor as a means of determining the refractive change required and the BOZR of the lens. The variation is an extra increase in flattening of the BOZR as a "compression factor" and ranges from zero to 1.00 D.

Clearance factor

The term was originally coined by Noack to describe the difference in tear layer thickness between the apex of the cornea and the maximum tear layer depth at the edge of the BOZD. A

lens with an apical clearance of 5 urn and a tear layer thickness of 65 urn at the BOZO would therefore have a clearance factor of 60 urn. Clearance factor is also used to describe the tear

HISTORY AND GENERAL PRINCIPLES 9

wear topography map of an ideal response to the wear of the lens (Fig. 1.6). The map shows a well-centered area of central corneal flattening, surrounded by a zone of steepening and little or no change in the peripheral corneal shape.

Cone angles

Cone <Ingles are used to describe a tangent periphery. A tangent is a flat or straight line and does not have a radius of curvature. Instead, the angle that the tangent makes with the optic axis is termed the cone angle (see Ch. 4 for the construction of tangents). Tangents are used in some orthokeratology lens designs to control lens centration.

Bull'seye

The term "hull's eye" is used to describe the fluorescein pattern of an ideally fitting reverse geometry lens. However, it is most commonly used to describe the appearance of the post-

Smiley face

A "smiley-face" topography plot results when the lens fit is effectively too flat. The lens decenters superiorly, with the resulting topography difference map showing a crescent-shaped area of steepening within the pupil zone, and the area of apical flattening decentered upwards (Fig. 1.7). This is an unacceptable outcome of lens wear, and the fit must be adjusted.

Central island

A "central island" postwear topography plot indicates that the lens is effectively too steep. The difference map shows a perfectly centered area of central corneal steepening that is approximately 2.00 mm in diameter, surrounded by a

Figure 1.6 A bull's eye topography plot. The top left is prefit and bottom left is postwear, with the subtractive difference shown on the right-hand side.

"moat" of marked flattening, which is followed by the annulus of steepening (Fig. 1.8). The peripheral cornea usually shows an area of distortion from a

tight alignment zone. The best corrected visual acuity (BCVA) is adversely affected by central islands, and they must be resolved in order for a

correct outcome to occur. Clinical experience shows that there are two types of central island. In some cases, the island is flatter than the original corneal apex, but still steeper than the surrounding corneal area. These types may resolve within the first week of lens wear, and should be considered to be incomplete bull's-eye plots. In other cases, however, the island is appreciably steeper than the original corneal curve. These will not resolve with time, and require a refit of the lens.

HISTORY AND GENERAL PRINCIPLES 11

Figure 1.9 A smiley-face pattern with a fake central island.

Treatment zone diameter

The area of effective corneal flattening is called the treatment zone (TxZ). It is defined as the chord at which there is no change from the original corneal surface when the refractive power subtractive map is used. The cursor is moved from the center to the nasal side until the dioptric change is equal to zero, and the distance noted. The process is repeated for the temporal side. The addition of the two values is the TxZ.

Smiley face with fake central island

A lens that is excessively flat will decenter and cause a smiley-face pattern as well as central corneal staining. The resulting distortion of the topographer mires leads to inaccuracies in the reconstruction algorithm, resulting in the appearance of a steep island. In these cases, the plots will show both a smiley face and a central island (Fig. 1.9). It is the decentration of the area of paracentral steepening that confirms the diagnosis of a flat-fitting lens, with the central island simply indicating that the lens is excessively flat.

A full description of the topography outcomes following lens wear is given in Chapter 6.

Ring jam

Disruption of the epithelial surface (or tear film) leads to distortion and "crowding" of the topographer mires. This can occur centrally and, in the case of an extremely tight alignment curve, in the peripheral cornea. Excessive distortion of the mires, leading to either gaps in the continuity of the image, or adjacent mire reflections actually coming into contact with each other, leads to a breakdown in the ability of the reconstruction algorithm to determine the correct localized curvature accurately. The reconstructed map therefore shows areas of marked steepening or

12 ORTHOKERATOlOGY

flattening with respect to the adjacent corneal area, and is a totally inaccurate representation of the surface (Fig. 1.10). This phenomenon is commonly called "ring jam."

Night therapy

Night therapy is the use of orthokeratology lenses only at night, with no lens wear required for the normal waking hours. It is also known as "nightwear orthokeratology."

In other areas, the usual nomenclature used to describe the corneal response to lens wear and the associated pathological terminologies are still used in orthokeratology. The simple fact is that the ISO definitions are relatively clumsy terms, and the simple expediency of using terms such as RC, AC, and so on does tend to make communications between practitioners more immediate, without the necessity to "translate" what is meant. It would appear that the above terms have become entrenched in orthokeratology circles in preference to the ISO equivalents, but as long as a common language is used, confusion should be minimal.

Figure 1.10 Ring jam. Note the distortion of the central mires. The map shows a small area of marked central flattening, which is a totally inaccurate representation of the data.

AIMS AND OBJECTIVES OF THIS BOOK

The problem with writing a book on this subject is trying to get specific information about each of the designs, as most manufacturers and designers treat their lenses as intellectual property and proprietary information. To describe the lenses in the manufacturer's terms would simply lead to an "advertorial" on each, which is of little value. The approach, therefore, has been to describe the underlying mathematical principles involved in the construction and fitting of orthokeratology lenses in a generic manner, with specific mention of the designs as needed.

There are basic underlying concepts, such as sag fitting, surface area matching and lens construction, that are common to all the different lens types. The aim of this book is therefore to demonstrate these underlying principles from the initial mathematical basis through to the actual application of fitting. If the concepts and formulae in the text are applied, it is possible for practitioners not only to design their own lenses, but also to be able to resolve any fitting difficulties that occur when proprietary lens designs are used.

There is a pronounced emphasis on the role of corneal topography throughout the text. The authors are convinced, from both a scientific and clinical viewpoint, that topography is essential to the correct fitting and aftercare of the orthokeratology patient. The keratometer simply cannot supply the information required to fit the lenses accurately or monitor the changes, and should be treated as inferior technology. The authors consider that the practice of orthokeratology without topography may be unethical. Topography-based orthokeratology is the "gold standard" of practice and every patient deserves to receive this level of service.

Wherever possible, the results of controlled research have been used to specify factors such as refractive change and visual acuity improvements. There are anecdotal remarks, but they are mainly concerned with the approaches to problem-solving that have been found to work in practice. Anecdotal remarks concerning fitting, aftercare, problem-solving, and other areas were only included when all authors agreed on the statement, with further corroboration sought from other experts in the field as required.

The author, like most optometry students of the 1970s, was taught that orthokeratology was a strange American pastime that "really didn't work." In 1992, Harris & Stoyan published a report on the "new" accelerated technique which

REFERENCES

caught the author's eye. A project was planned to fit 20 volunteers with the lenses, prove that it didn't work, and publish the results. Of the 20 volunteers, 11 achieved unaided acuity of 6/5 (20/15) in both eyes. The question then arose: why didn't it work on the other nine? One could simply have published a paper on the results and moved on, but there is something indescribable about the experience of having a patient remove a lens and read the 6/5 line unaided, with a look of absolute amazement, that is somehow addictive. Ten years on, and we still don't know all the answers to the myriad of questions that orthokeratology raises. Why does it work? How does it work? What changes in the cornea? What are the physiological, visual and physical effects, the limits, the dangers? How is it controlled?

What is known is that the attempt to answer these and other questions leads to areas of study concerning the mathematical basis of corneal shape, lens design, and corneal topography that have had a massive effect on all other areas of contact lens practice. Trying to understand orthokeratology and its lens designs opens new vistas to the possibilities of advanced lens designs for other applications such as postgraft and refractive surgery cases that were not available before. All this benefits the patient. There is no more satisfying specialty than orthokeratology.

Bara C (2000) Mechanism of action of the reverse geometry gas permeable contact lens in orthokeratology. M.5c. Thesis, University of Melbourne

British Standards Institute (1995) Contact lens standards. ISO 8320

Carney L G (1994) Orthokeratology. Chapter 37. In Rubin M, Guillon M (eds) Contact lens practice. Chapman and Hall Medical, London

Coon L (1982) Orthokeratology, Part 1: Historical perspectives. Journal of the American Optometric Association 53: 187-195

Harris 0, Stoyan N (1992) A new approach to orthokeratology. Contact Lens Spectrum 7(4): 37-39

Kerns R (1976) Research in orthokeratology, Part 7. Journal of the American Optometric Association 48: 1541-1553

Mountford J A (1997) Orthokeratology. In: Phillips A J, Speedwell L (eds) Contact lenses: a textbook for students and practitioner, vol. 4. London, Butterworths

Nolan J (1995) Flashback: the first ortho-K meeting. Contacto 38(4): 9-14

Soni P S, Horner D J (1993) Orthokeratology. In: Bennett E S, Weissman B A (eds) Clinical contact lens practice. J B Lippincott, Philadelphia, pp 49-1-49-7

Wlodyga R J, Bryla C (1989) Corneal molding; the easy way. Contact Lens Spectrum 4(58): 14-16

As chairman of this first meeting, Dr. George N. Jessen called the session together at 7.30 p.m. After some introductory remarks, he reported on his experience with the techniques of "orthofocus." In his opinion there were many instances in which hyperopia, myopia, and astigmatism of the cornea could be changed towards emmetropia by attempting to mold the cornea with a contact lens.

His technique was to fit steeper in the hyperope and flatter in the case of myopia. Dr. Jessen also stated that he feels in most patients with ammetropia of a noninflammatory nature the eyelids are largely responsible for the myopic pressure exerted on the eye.

The eyelid pressure can increase the intraocular pressure to 40-60 mmHg as compared with 6-7 mmHg increase due to the extrinsic ocular muscles upon strong convergence. Myopes who continue to progress after being fit with contact lenses are those who still squint, despite the lenses.

Those who do not progress have their eyes wide open. Most younger people have astigmatism with-the-rule, while most older people have against-the-rule. Why? Is this due to the change in eyelid condition due to the pressure as muscle tissue ages and tone decreases or the increased scleral rigidity? The lids do not bend the cornea as much as previously.

One millimeter of bend or change in corneal radius equals 6 O. Those who have done scleral buckling in retinal surgery have changed the eye as much as 4, 5, and 6 mm, proving that the eye is malleable.

Jessen started using the "ortho-focus" technique following a request from a friend - a recruiting officer for the Merchant Marine

Academy. Some of the applicants for the academy qualified except for visual acuity of 20/40 or 20/50. What could be done to correct the acuity? Some were corrected by ortho-optics. On another occasion, a potential football player and also an applicant to the academy had a -3.00 0 correction and an acuity of 20/400. He had to have 20/25 unaided. What could be done?

Jessen did not promise but went ahead and fitted the boy flatter than K and plano correction. After 3 months of wear, he had obtained the needed 20/25 acuity. Jessen wrote the Navy telling them of what he was doing. They did not object. The boy is now attending the academy. Whenever he has to take a test he wears his "ortho-focus" for 2 or 3 days, then takes his test and has 20/20 acuity. The boy was certified before and after the original fitting by an ophthalmologist.

Jessen said he finds it easier to work with hyperopes and astigmats than with myopes. The hyperopes and astigmats are fitted with spherical bases steeper than K. He can then get a swelling of the cornea into the lens and they see very well. The myope is more difficult and should be fitted younger. Jessen cited the case of Dr. Joseph Cinefro's son. He put on "orthofocus" lenses when he manifested 0.500 of myopia. Lenses should be put on as young as possible: 7, 8, or 9 years of age.

A typical example is a child who fails an eye test at school. Parents tell him they don't want glasses on their child. Jessen is then able to offer them this alternative of "ortho-focus" contacts and hold a normal curvature like braces on the teeth, or the child can be fitted with glasses. It is the parents' choice to make.

A -2.00 0 myope can be fitted 2.00 0 flatter than K with plano correction. A plus 2.00 0 hyperope would be fitted with 2.00 0 steeper than K with plano correction. An astigmat could be fitted with a Cycon with prism, of opposite curves, or the astigmatic cornea could be fitted with a spherical lens.

Jessen said we are finally approaching the possibility of wiping out low refractive errors. He did not think it would be possible to correct high amounts of myopia or correct aphakic patients. He estimated that 90% of ammetropic patients had less than 3.00 O. He concluded his talk by saying that we now have a good chance of correcting these conditions. He then opened the meeting for discussion.

Dr. Robert Koetting asked: why don't we try to correct the high myope, or the patient with the large refractive error? Dr. Jessen answered by stating that these are pathological: those that have -12 to -150 have signs of chorioretinitis - they show large myopic crescents. He did not think that correction should be attempted on such patients. Koetting said we should investigate the factors involved in changing the cornea so that we can proceed on a sound basis.

Another fitter stated that whenever he fitted a patient and the cornea became steeper, especially in the center, that they experienced discomfort. Jessen replied that all he knew was that when his hyperopic patients, when fitted with a deCade bifocal with a steep central zone, removed their lenses, they invariably saw better without correction than they previously did. They are confident the phenomenon is due to their contact lenses.

It was suggested that we as fitters should do some basic research, rather than wait 10 years for results to explain what happened 10 years before. We could establish where the corneal swelling with clarity ends and swelling with cloudiness begins. He estimated it would take about a year to conduct this study, that such a study could be sponsored by the National Eye Research Foundation.

Physiologists and men at the universities should partake in this study and also study the chemistry of the aqueous and the cornea that might cause one cornea to be more malleable than another.

Dr. Jessen stated that Dr. Schick of Indiana University is conducting basic research on myopia and contact lenses. [Dr. Schick replied, but the first few sentences are not discernible.] Dr. Schick asked Dr. Jessen how he could dare to change a pilot's corneas when the lives of 100 passengers are entrusted to his care? Dr. Jessen said that the pilot wears his glasses on the field and in the airplane and contact lenses only prior to testing unaided visual acuity - to achieve a temporary flattening of the cornea. Following his answer, there was a heated discussion concerning the laws and regulations as opposed to what is right. A pilot with normal acuity with glasses can be every bit as good as a pilot with 20/20 without glasses.

Dr. Sharp asked Dr. Schick: "What can we do then as an alternative?" Dr. Shick said: "There must be a better way." Dr. Sharp again asked, "What way?" Dr. Middleton then interrupted: "By the way, what are we discussing here? Morality or contact lenses?"

There was much laughter at this broadside. He chided the group that Dr. Schick was entitled to be heard. Furthermore, like it or not, when we attempt to change the corneal contour we are dealing with physiology. He continued with remarks illustrating his point. He then turned the discussion over to Dr. Schick who continued with remarks illustrating his point.

Dr. Schick said that most of our errors are not due to corneal changes. How do we know we are working on the right thing when we work on the cornea? What is the cause of the error? By reducing hydraulic pressure in the eyes, the myopia might be reduced. By increasing this pressure, the eyeball length could increase.

Dr. Newton K. Wesley then rose to his feet and began to speak. He told of coining the title of "orthokeratology" as opposed to the term "ortho-focus." He told of always having been afraid of corneal distortion, that his own left eye had been diagnosed as keratoconic and his right eye had not. He then wore a scleral lens 3 hand in that short time developed keratoconus in his right eye. His ophthalmologist stated that it would have developed in any event.

But how could it develop in 3 h? Wesley reminded the group that fluid lenses distort the

eye tremendously. He also cited the Indian tribe that deliberately reshaped the skull, thereby influencing the shape and growth of the brain. He then discussed the particular patients fitted with Feinbloom tangent cone lenses: a --4.00 D myope who then went to -11.00 D while wearing the lenses. Dr. Wesley took away the lenses and a month elapsed before the eye returned to normal.

He also spoke of another patient, Dave Pattis, who wore scleral fluid lenses day and night for 3 years. He developed pannus, opacities, and his vision went from 20/20 to 20/200 - his Rx went from -5 D to about -13.00 D. Upon removal of the lenses, his eyes returned to normal and were free of irregularities and his Rx was again -5.00 0 1 month later.

Dr. Wesley continued by saying we have all fitted flatter in keratoconus cases. He gave an example of where he, Dr. Zekman, and Evelyn Corral molded keratoconic eyes before and after fitting with contacts. The change in curvature averaged 1.50 0 flatter than the original curves after using base curves of 4-5 D flatter. If we could do this with weak and diseased eyes, the chances are that we could do the same with normal eyes. "Is there one amongst you who has not distorted an eye in fitting?" Wesley asked. Remember that microlenses were 2.50-6.00 0 flatter than K. This is the history of contact lenses - not just a few hundred cases, but hundreds of thousands and millions of patients.

He agreed with all that was said about basic research, but he wanted to know how much we can change the corneal curve and how we change it. What about the changes in corneal structure, not just curvature? The changes in acuity - not the letters, but the flares, dispersion, and polyopia. The pressure changes - does the pressure go up, go down or does it stay the same? The amount of edema: what happens to the corneal topography; hyperopic and myopic control; amblyopia? There are many subjects we can cover.

He told of his strong interest in myopia, that it was one of the reasons he went into optometry. When you consider that myopia can be a change of the front or back surface of the crystalline lens, you are going to have to prove which one of the surfaces change if you say that this is the cause of the myopia. Or, you must say they stay the same

and that the myopia is caused by something else. You will also have to be able to measure accurately the distance from the fovea to the front of the eye, from the front of the cornea to the back of the cornea, from the front to the back of the crystalline lens. You must be able to measure these distances, because a change in one of these distances causes the myopia. We also have index myopia - we must establish indices - we do not even have instrumentation to measure myopia in any of these forms except for the keratometer.

Continuing, Wesley said that the variables of measurement have to be established - that is basic research. He agreed with all the statements made by Schick and Koetting that this basic research must be done. Individual cases do not prove anything - where were the controls? We have too many records which prove that controls were not established. He gave an example of orthodonture, that the changes are made very gradually.

Turning the discussion to creating a new name for the group, he suggested Society of Orthokeratology as opposed to Ortho-Focus. He then suggested we should set up certain research projects, especially in the problem of myopia.

A general discussion followed on the choice of title for the group as to its correctness in describing what is accomplished by fitting using this concept.

Dr. Schick was elected president. The following men were named but no particular office was given to them. Dr. Charles May, Dr. Middleton, Dr. Victor Arias, and Dr. Koetting. Dr. Jessen agreed to be secretary. Dr. Middleton suggested that Dr. Wesley be a counselor to the society, but he should not be part of a committee. The assembled members agreed to annual dues of $5.00 per year. Jessen duly recorded each name.

The meeting concluded with a statement of our aim: that we are dedicated to the prevention and correction of ammetropia and attendant visual problems.

Editor's note: This manuscript was adapted from two tape recordings, transcribed by Dr. Sharp. Some sections of the tape were not discernible, could not be transcribed, or background noise made individual dialogue impossible toascertain.