Orthokeratology Simplified – Start Fitting Your First OrthoK Lens

In my previous article titled “Myopia Management and Orthokeratology”, I had written about what Orthokeratology is and how it works to manage Myopia. In this article I will be talking about Orthokeratology from an Optometrist’s perspective and this article will mainly help young Optometrists who wants to start Orthokeratology practice.

Orthokeratology has not been widely practiced because very few about this topic has been taught and practiced in the field of Optometry. With this article I want to simplify the process of starting Orthokeratology practice by an Optometrist.

How to get started with your Orthokeratology Practice?

If you are an Optometrist and want to start an Orthokeratology practice the first thing you can do is start fitting the OrthoK lenses and learn by yourself. You can also get help from the OrthoK practitioner community while you start to fit the OrthoK lenses. There is no better way to learn Orthokeratology than learning it by practice. The problem can arise if you do not have a trial set with yourself. Luckily you can now get an OrthoK trial set from Eagle Eye Ophthalmic Enterprises who are the official distributors of SEED Contact Lens in Nepal. They will also give you all the information you should know before starting your OrthoK practice.

Things to know before fitting an Orthokeratology Lens

Breath-O Correct OrthoK Lens by SEED, Japan

The above picture is of Breath-O Correct Orthokeratology lens from SEED, a Japanese Contact Lens Manufacturing Company which is easily available in Nepal and India along with most of the Asian Countries as these OrthoK lenses are mainly designed for the Asian Population. You can see 5 parameters mentioned in the lens which are F.T, T.P, B.C, P and S.

F.T is the Fitting curve which gives base curve of the alignment zone and is given in Diopter and mm.
T.P is the Target Power

B.C is the Base Curve of the Treatment Zone

P is the Power of the Lens

S is the size or the diameter of the Lens

Anatomy of Orthokeratology Lens

Anatomy of Orthokeratology lens
Anatomy of OrthoK Lens

The OrthoK lenses usually comes with 4 curves which are (from inner to outer):

1st Curve or BOZR (Back Optic Zone Radius):

This is the main optical area of the lens through which a patient can see. It is also called Treatment zone or the Treatment Curve. In an optimal OrthoK fit it should sit right infront of the pupillary area.

2nd Curve or the Reverse Curve:

This curve comes right after the BOZR which is responsible for creating that pulling force by which the reverse geometry design of the OrthoK lens actually work. It accumulates the tear reservoir and helps to flatten the Optic zone of the cornea.

3rd Curve or the Alignment Curve:

The alignment curve is where the OrthoK lens rests onto the cornea and is mainly responsible for centration of the OrthoK lens into the cornea

4th Curve or the Peripheral Curve:

This curve provides the edge lift and helps in the comfort and movement of the lens. It also helps in exchanging of tear and debris.

How to Fit an OrthoK Lens?

If you have fitted RGP lenses before then fitting an OrthoK will not be very challenging. Also, do not worry if you have not fitted any Rigid lenses before because with Breath-O Correct lenses it is very easy to fit an OrthoK lenses.

Let’s take the following example for an OrthoK fit

Right Eye:

  • Refractive Error -2.00Dsp

  • K1= 42.82 D @153˚ K2= 43.04 D @63 ˚

  • AVG=42.93 D

Fitting an OrthoK Lens:

Orthokeratology Lens by SEED
OrthoK Lens by SEED, Japan
  1. Choose a trail lens
    • Determine the fitting curve or the FT. The FT is the Average K value of the patient which is 42.93 D as our previous example suggests.

    • Determine the target power of the TP. TP = Patient Spherical Value + (Flat K – FT). In this example TP will be -2.00 + (42.82 – 42.93) i.e. -2.11 equivalent to -2.00

    • Hence, the parameters of first trial lens will be FT = 43.00, TP = -2.00 D. The rest of the parameters will already be mentioned on the trial lens which we do not have to calculate.

  2. Fitting Rules
    • While changing the Power, Base Curve will remain the same.
      Fitting Rule One

      While changing the Base Curve the power will also change:

      Fitting Rule Two
  3. The Final Fit:
Orthokeratology Fit
Final Fit, The Bull’s Eye


Orthokeratology is a booming topic and one of the specialty services in Optometry. With practice and time you can master this art of Orthokeratology. This is a very good time to start your Orthokeratology practice because unlike before it is not really expensive to get your hands on the Orthokeratology trial lens set. Feel free to contact the author for more information related to Orthokeratology.

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