Orthokeratology

File:Image-Efecto topografico.jpg

Topographic differences in a patient treated with orthokeratology

Orthokeratology (also referred to as Ortho-K, Overnight Vision Correction and Corneal Refractive Therapy), marketed under brand names like "DreamLens", "i-GO OVC", "GOV", "Wake and See", "CRT" and "Emerald", is the use of rigid gas-permeable contact lenses, normally worn only at night, to improve vision through the reshaping of the cornea. This method can be used as an alternative to eyeglasses, refractive surgery, or for those who prefer not to wear contact lenses during the day. The latter may be due to discomfort from working in air-conditioned or dusty environments, from extended computer usage which reduces blink rates and tear film production or from displacement or loss during sports activities.

History

The idea of correcting vision by reshaping the cornea has been around for some time. Doctors discovered the reshaping phenomena of glass lenses as early as the 1940s. This history of orthokeratology includes contributions made by Jessen, Ziff, Nolan, Paige, Gates, May, Grant, Fontana, Tabb, Freeman, Shed, Kerns, and Binder to the use of contact lenses for myopia reduction.

George Jessen created what was probably the first orthokeratology design in the 1960s made from PMMA material, which he marketed as "Orthofocus". These early designs had generally unpredictable results, leading to the belief that applied orthokeratology was more art or luck than science. Many groups and individuals claim to have been the first to develop modern orthokeratology solutions. But Dr. Richard Wlodyga, in particular, is generally credited with developing the first reverse zone lens design in the 1980s.

However, it was not until computerised corneal topography became available during the 1990s that it became possible to apply the theory to create designs with repeatable results through being able to accurately map the surface curvature of the cornea using a non-invasive, painless imaging procedure. Additionally, the development of new base materials for rigid gas permeable lenses which provided much higher levels of oxygen permeability opened up the possibility of orthokeratology becoming an overnight procedure rather than being used for daytime wear alone. Finally, the introduction of computer-controlled precision lathes meant that lens designs could be manufactured to sub-micron levels of accuracy thereby offering the prospect of high volume production becoming commercially viable.

In the summer of 2000, at an educational meeting of optometrists in Toronto, the Orthokeratology Academy of America (OAA) was formed to support, promote and advance orthokeratology. By providing quality education and scientific information on the subject of ortho-k to all interested practitioners, the OAA provides an unbiased forum for the free exchange of ideas and concepts relating to all aspects of orthokeratology. Its membership has the opportunity to advance their role in the field by applying for and passing comprehensive exams in order to achieve Fellowship status in the Academy. In parallel, the British Orthokeratology Society (BOKS) was established in the UK with similar objectives to promote orthokeratology as a new procedure to correct myopia.

A significant milestone for the US market occurred in June 2002 when the FDA granted approval for overnight wear of a type of corneal reshaping called "Corneal Refractive Therapy" (CRT). This forms the basis of the "Paragon CRT" product backed by Paragon Vision Sciences Inc. Subsequently in 2004, the U.S. Food and Drug Administration (FDA) approved a design from Euclid Systems Corporation ("Emerald"). Each FDA approval has a Premarket approval (PMA) number. The PMA number of the FDA approval for Paragon CRT is P870024; the PMA number of the FDA approval for Emerald is P010062.

However, nightwear ortho-k solutions were available to consumers in many countries outside the US much earlier as a result of different regulatory controls and bodies.

In 2005, Bausch & Lomb acquired the PMA for the Emerald lens design from Euclid (which is marketed in Europe as 'i-GO OVC') and created the "Vision Shaping Treatment" (VST) program to collectively market a number of ortho-k lens designs which were manufactured using Bausch & Lomb base material. This program currently comprises five lens designs in addition to Emerald.

In 2006 and 2007 papers presented at the British Contact Lens Association and the Global Ortho Keratology Symposium indicated the possibility of orthokeratology slowing or stopping myopic progression. This was found to be effective in children in Hong Kong and is the subject of wider study to verify this data. More recently, the first year results from the SMART Study (Stabilising Myopia by Accelerated Reshaping Technique) – a five-year longitudinal study currently underway in the Greater Chicago area – show a statistically significant difference for children in the treatment group wearing orthokeratology lenses who exhibited no overall prescription change against children in the control group wearing conventional soft contact lenses for whom there was a mean increase in myopia of 0.40 dioptres.

Mechanism

It is presumed that the lenses used in orthokeratology reshape the cornea by moving the epithelial cells that cover the surface of the cornea. Some studies indicate that the epithelial cells are compressed in some areas as well as relocated.

A common misconception in design is that the lens "presses" on the cornea, thus flattening the eye. In fact, the belief is that the forces generated by the reverse curve cause the redistribution of the epithelial cells, and not pressure on the eye. Properly designed lenses do not touch the user's cornea.

Indications

The US FDA overnight orthokeratology approval for Paragon CRT is for procedures up to −6.00 dioptres of myopia and up to −1.75 dioptres of astigmatism whilst the approvals for Euclid (and thus for the other five lens designs offered under the Bausch & Lomb 'Vision Shaping Treatment' portfolio) cover procedures up to −5.00 dioptres of myopia and up to −1.50 dioptres of astigmatism.

In the United Kingdom the procedure is offered primarily for myopic correction up to −5.00 dioptres and up to −1.50 dioptres of astigmatism. Fitting evidence for the leading lens designs indicates that procedures undertaken within these parameters have the highest probability of success. Some patients with higher degrees of myopia are successfully treated by specialist practitioners with "off-label" uses of these same lenses.

In countries like South Africa, Australia and Taiwan, practitioners using the GOV orthokeratology system have achieved successful fits as high as −10.00D of myopia and +5.00D of hyperopia. Not every patient within these parameters will be suitable for the procedure and, for example, conditions such as flat or steep corneas may result in the procedure being less successful.

Types of lenses

Orthokeratology lenses are made by a number of companies globally, primarily using one of the two FDA approved technologies. All use special gas permeable lenses to reshape the cornea. The lens material – and especially its oxygen permeability measured by its 'Dk' rating (the higher the value, the greater the degree of oxygen permeability) – is important for maintaining eye-health during the treatment.

Paragon Vision Sciences manufactures a lens trademarked "Paragon CRT" (Paragon Corneal Refractive Therapy) and marketed as "Accelerated Orthokeratology" (AOK) using base material manufactured in-house. Euclid Systems and the other five overnight orthokeratology lens designs offered under the Bausch & Lomb "Vision Shaping Treatment" (VST) portfolio are manufactured by various contact lens laboratories in the US and internationally using Bausch & Lomb base materials and in North America may only be fitted by VST certified practitioners.

The six designs in the VST portfolio comprise (alphabetically):

• "BE Retainer" backed by BE Enterprises Inc
• "CKR" backed by Eye Care Associates
• "Contex OK-E System" backed by Contex Inc.
• "DreamLens" backed by Dreimlens Inc
• "Emerald" designed and manufactured by Euclid Systems Corporation
• "NightMove" backed by Advanced Corneal Engineering Inc

No other design of ortho-k lenses has been approved by the FDA for overnight wear and currently there are no other studies underway for any new lens designs.

Non FDA approved designs are available, such as Menicon Z Night which uses fenestrations in the reverse curve to maximize fluid flow while maintaining centration with a high Dk material. Menicon Z material is FDA approved for up to 30 day wear in regular GP form.

Some practitioners have also designed their own orthokeratology lenses typically by using various software packages which combine corneal topographical data with the practitioners' own knowledge and experience to determine the appropriate lens parameters for individual patient prescriptions. Wave/Z-Wave is one such software package which allows for a CNC lathe manufacturing method of lenses customized by the practitioner with a wide variety of material choices.

Comfort

Lenses are generally considered to be not as comfortable as Soft contacts and can feel similar to an eyelash trapped in the wearer's eye, especially during initial wear periods and while the eye is open/blinking. This is due to the shape requirements of the lens and edge design which has to serve more purposes than a traditional lens. Generally the feeling is reduced as the body becomes used to the lens, though it may never become as comfortable as traditional soft or rgp contact lens designs which are designed to be used for active vision correction.

Overnight Ortho-K is generally very well accepted though since the main feeling is due the eyelids moving over the lens edges. When the eyes remain closed for sleep, there is generally no residual feeling of the lens after the adaptation period.

Cautions

During the first month of lens wear when the treatment zone on the surface of the cornea is in the process of becoming fully formed, some users may experience vision issues such as ghosting, double vision, contrast problems and/or starbursting, especially at night. These issues are generally resolved by the end of the first month of lens wear. If these issues persist beyond this initial period, the cause may be due to lack of centration of the lens on the eye and/or overly large pupil size (in light or dark). Resolution may be possible through redesign, material changes, better eye moisture retention (night eye masks, duct blocking, Restatis etc.) or other techniques. All issues should be immediately discussed with the Practitioner.

Orthokeratology has very occasionally had severe side-effects, even blinding complications although the latter is confined to a March 2004 report of a very small sample of cases in China where supply of lenses at that time was not subject to any regulatory regime covering safety and efficacy. More typically, complications can occur due to the patient's failure to follow appropriate hygiene recommendations when handling or cleaning the lenses including the use of tap water to rinse (although some systems allow for or suggest the use of "clean" tap water) or store. Complications may also be due to relative corneal hypoxia (lack of oxygen) with prolonged or overnight contact lens wear in lenses made from the wrong material. However, the use of high or hyper oxygen-permeable materials as approved by the US Food and Drug Administration and manufactured by leading contact lens companies such as Bausch and Lomb and Paragon Vision Sciences significantly reduces hypoxia, and these are the materials that are normally used in orthokeratology.

References

• Research in Orthokeratology – University of New South Wales (Sydney, Australia).
• Orthokeratology: part I historical perspective. Journal of the American Optometric Associationde:Orthokeratologie

zh:角膜塑形術