CLINICAL EXPERIENCE has demonstrated that overnight orthokeratology (ortho-k) is incredibly useful in correcting ametropia and for myopia control. The patient wears a reverse geometry lens overnight, which gently reshapes the cornea from a prolate to an oblate surface. When the patient removes the lens upon awakening, the overall length of the eye is temporally shortened, thus decreasing the amount of myopia. This is repeated on a nightly basis where the lens functionally serves as a retainer to maintain these shape changes.

Patient selection, baseline and ongoing corneal topography, and follow-up schedules are all very important in this process. But, what about the design itself? Let’s discuss the architecture of an ortho-k retainer to properly achieve a successful fitting result.

While ortho-k designs can vary from manufacturer to manufacturer, most have these four to five curves: base curve (BC), reverse curve (RC), alignment curves (ACs), and peripheral curve (PC).

The BC in ortho-k is unlike the BC in other designs as it has nothing to do with the overall fit, but is purely for refractive error correction. If there is an under- or over-treatment of the refractive error, this is the curve that is manipulated. But if there is a centration or fit issue, the BC is not the curve to modify. The BC is typically selected to be flatter than the flat K of the cornea with an additional overcorrection, sometimes referred to as a Jessen Factor (Jessen, 1962).

This overcorrection amount will vary depending on the design but is typically between +0.50D to +1.50D depending on the amount of correction and the design. The optic zone diameter is typically between 5.0mm to 6.8mm depending on the refractive error and whether it is a myopia control case or purely a refractive correction case.

The RC is next, and is important in dictating the overall sagittal depth of the ortho-k retainer. This curve also generates the peripheral plus ring noted on corneal topography, which is important for myopia control.

The mechanism, we believe, that allows ortho-k to provide myopia control for patients is by creating peripheral myopic defocus image shell to the retina. This peripheral myopic defocus signal is generated by the peripheral plus created by the reverse curve (Erdinest et al, 2023). This curve is where the term reverse geometry derives from as it is stepper than the two curves adjacent to it.

Next is the AC, responsible for properly aligning the ortho-k retainer as the main curve to manipulate when there are centration issues. Proper alignment in the mid-periphery of the cornea is crucial for achieving success in ortho-k, thus proper design of this curve is very important.

Last is the PC, which is found at the outermost edge of the ortho-k retainer and serves to maintain a healthy ocular environment as well as good centration in conjunction with the AC. Proper edge lift at this curve is essential to allow for tear exchange and to prevent any discomfort.

These curves are each vital for proper ortho-k design and each is uniquely important for ortho-k success. Good luck in your ortho-k designs! CLS