Ortho-k has its origins from the early days of polymethylmethacrylate (PMMA) corneal contact lenses. Because the lenses did not allow any oxygen to pass through, clinicians often made their lenses flatter than the cornea to allow more tear exchange under the lens, increasing the available oxygen to the cornea. Clinicians observed that this caused a flattening of the corneal tissue and, usually, “spectacle blur” was the result. Patients noted that their vision was not correct when looking through their glasses for a period of time after removing their contact lenses. Some patients with low myopia reported they saw quite well without any correction at all for some time after removing their lenses.

The first literature reports of this effect were published in the 1950s.¹,²,³ In 1962, George Jessen published a paper on what he called “orthofocus.”⁴ This was the first report of a deliberate attempt to mold the cornea to reduce myopia. Jessen later called the technique “orthokeratology.” Jessen’s method was to use a standard PMMA lens that was flatter than the cornea by the amount of refractive error. If successful, this would flatten the cornea enough to eliminate the myopia. The lens itself had plano power, as the tear film provided all the visual correction. Unfortunately, lenses fitted this flat were very uncomfortable and likely would decenter, creating irregular astigmatism.

Although there were other techniques advocated in the early years of ortho-k, the technique of Grant and May was the most utilized by clinicians.⁵ Their method was to use a series of gradually flatter and flatter large-diameter lenses, starting at about 0.12 D to 0.50 D flatter than K. As the cornea flattened, slightly flatter lenses were used until the myopia was reduced to the desired level. This process could take many months.

Proponents of ortho-k published many reports of success, but controlled studies using conventional lens designs were not published until the mid-1970s and early 1980s.⁶,⁷,⁸,⁹,¹⁰,¹¹ These studies varied in their design and results, but they all reached very similar conclusions: they found that (a) in general, ortho-k was as safe as standard contact lens fits with PMMA lenses;
(b) reduction in myopia was about 1.00 D on average, although significantly higher amounts were reported; (c) flat-fitting lenses may cause induced with-the-rule and irregular astigmatism; (d) no patient characteristics were found that allowed for prediction of success; (e) improvement in unaided visual acuity did not necessarily match the amount of corneal flattening or the amount of myopia reduced; and (f) the changes were not permanent and required wear of the lenses for some portion of each day to maintain the effect. After these studies were published, orthokeratology was mostly rejected by the eye care community.¹²,¹³,¹⁴ Interest in the procedure did not evaporate completely, and a handful of proponents continued to advocate and refine the fitting techniques over the following years.

A major advance in lens design resulted in a resurgence of interest in ortho-k. In contrast to early ortho-k using standard rigid lens designs, the modern procedure uses a completely different lens design known as a reverse geometry (RG) lens. Standard rigid lenses have curves that flatten from center to periphery. When fit flat for ortho-k, there is nothing to prevent the lens from decentering and causing poor results. In contrast, RG lenses are designed such that the secondary curve is steeper than the base curve. This secondary curve is often known as the reverse curve. The peripheral area of the lens then flattens out again to align with the peripheral cornea; this area is often called the alignment curve. These design changes allow the lens to maintain centration, even when the base curve is fit very flat to correct the myopia. RG lens designs also produce much faster results and can treat greater amounts of myopia, on average.

Although the first use of RG lenses was reported in 1972,¹⁵ manufacturing technology at the time made them difficult to produce. Improvements in the RG design and the introduction of computer-driven lathes in the early 1990s led to better ortho-k outcomes.¹⁶,¹⁷ The improvement in treatment with the new designs was remarkable enough to lead to the term accelerated orthokeratology to denote the use of these lenses and to differentiate it from the original ortho-k procedures. Results were achieved within weeks rather than months, as with the old procedure. There are now numerous contact lens laboratories that manufacture RG lenses and clinicians now have far more lens options for ortho-k (see section on lens designs).

The use of corneal topography also differentiates modern practice from old. Corneal topography is much more useful than keratometry because of the much greater area of the cornea measured. This allows for better lens designs, more accurate prediction of treatment success, and better monitoring of post-treatment effects. Although some RG ortho-k lenses could be fit without the use of a topographer, it would be very difficult to monitor progress of treatment and know how to change lens parameters if treatment is not optimal.

Rigid lens materials with high oxygen permeability (Dk) are now available that allow the ortho-k lenses to be worn during sleep (often referred to as “overnight orthokeratology”). This makes the procedure much more convenient for the patient, as there is no need to wear the ortho-k lens during waking hours. The lens is applied before bedtime and removed in the morning. When optimal results are achieved, the patient has good vision during all waking hours. Overnight wear of any contact lens can increase the risk of complications, and patients must be monitored carefully for any signs of problems related to lens wear.

Recent research supports the clinical observations that RG lenses lead to faster results and higher potential corrections than standard lens designs, particularly with overnight wear.¹⁸,¹⁹,²⁰,²¹,²² In general, these studies show an average treatment of about −2.50 D within 7 to 14 days of treatment, with corrections up to about −4.00 D possible. Preliminary studies of structural changes in the cornea that result from RG lens wear showed mostly central epithelial thinning and midperipheral epithelial thickening,²³ although more recent studies suggest stromal thickening may also play a role.²⁴,²⁵ In a cat eye model, this change has been shown to be primarily compression rather than loss of cell layers.²⁶ Helen Swarbrick has conducted an extensive, excellent review of research on ortho-k effects on the cornea for the interested reader.²⁷

Modern orthokeratology practice uses RG lenses almost exclusively. The focus of this section will be a description of basic techniques applicable to most RG lens designs.

To flatten the central cornea and reduce myopia, the base curve of an RG lens is made flatter than the corneal curvature. Exactly how flat is dependent on the lens design, but most make the
base curve flatter than the cornea by the amount of treatment desired. This technique is sometimes called the Jessen method as this is how George Jessen chose the base curve for his standard lens “orthofocus” procedure.⁴ For example, if the patient’s spherical refractive error is −3.00 DS, the base curve radius is made 3.00 D flatter than the flat K reading. Most ortho-k lens design manufacturers will recommend selecting a base curve radius about 0.50 to 0.75 flatter than this value to slightly overcorrect the eye to a low amount of pseudohyperopia. This allows for a slight regression of the effect during the day so that the eye is close to plano by bedtime, allowing clear vision for all waking hours. A nice effect of this is that the tear film provides all the visual correction needed by the eye, and the lens power is close to plano. It should be noted that Mountford³¹ argues that selecting the base curve radius in this manner has no scientific basis (see below).