This article was originally published in a sponsored newsletter.

An eyecare practitioner (ECP) might consider the phoropter and the slit lamp as necessary and required instrumentation for the profession. However, for the orthokeratologist, the corneal topographer is considered a critical and required tool for treatment. Unfortunately, this instrument is often unfamiliar to practitioners and staff when a new orthokeratology (ortho-k) practice is established.

Let’s explore the principal considerations that contribute to optimal orthokeratology outcomes and happy patients.

A successful ortho-k fit starts with accurate topography capture (Figure 1).¹ Here are the steps:

1. To build a lens with micron accuracy, ensure the patients are comfortably seated and stable in front of the instrument. Perform the image acquisition process with slow, steady movements, maintaining critical focus to capture the required high-resolution photo.
2. Instruct patients to open their lids as wide as possible. This will maximize coverage and the collection of map data to build the large-diameter ortho-k lens.
3. Additionally, for reflective-based topography, the tear film should be smooth and even (without tear film break-up). Encourage the patient to blink regularly during capture to maintain a quality reflection. 

Now, let's discuss the importance of topography in the fitting process.

When analyzing the ocular shape, a spherical cornea is generally an ideal candidate. While astigmatic eyes are not contraindicated, treating certain types of astigmatism with ortho-k can be more straightforward, such as apical compared to limbus to limbus (Figures 2 and 3).

Using the axial interpretation in your topographer, assess whether you have apical or limbus-to-limbus astigmatism. Ortho-k can effectively reduce or eliminate corneal astigmatism when it is apical or more centrally distributed. However, limbus-to-limbus astigmatism tends to be more challenging to manipulate and, therefore, can leave the patient with residual astigmatism that may hinder success.

Additionally, consider that steeper corneas that have higher eccentricity are better candidates for ortho-k than flatter corneas with low eccentricity eyes, particularly in cases of higher myopia.²

Arguably, the most important function of a corneal topographer in ortho-k treatment is the post-wear analysis of the effect.¹ This is where ECPs learn what has changed following overnight wear.

The “subtractive,” “difference” or “comparison” map is the analysis function ECPs want to employ in their topographer’s software (Figures 4 and 5). Select the baseline, pre- ortho-k map, and the post-wear topography, then have the topography software perform a subtraction or display of what has altered.

The axial subtractive map (Figure 4) will provide analysis related to the prescription change, corneal astigmatic effects, and the position and size of the treatment zone. The tangential subtractive map (Figure 5) determines the lens position, providing the exact position of treatment while the patient was sleeping. Learning to use and understand the axial and tangential subtractive map should be a goal of every orthokeratologist.

This one function provides the practitioner with a very comprehensive understanding of ortho-k success and guides decision-making moving forward throughout treatment.

ECPs will quickly become experts at topography analysis as there’s much to learn from each case both pre- and post-ortho-k wear. Online video tutorials on topographical analysis in ortho-k are helpful, alongside training tools offered by instrument suppliers and assistance from lens consultants throughout the process.

However, the most valuable step you can take is to ensure the capture of quality topographies. This step maximizes first fit success and improves your understanding of the post-wear response.

REFERENCES

1. Mountford J, Ruston D, Dave T, Orthokeratology: Principles and Practice, Butterworth-Heinemann, 2004.

2. Mountford J, Noack D, A Mathematical Model of Corneal Shape Changes Associated with Ortho-k. Contact Lens Spectrum. 1998 Jun;13:34-40.