Orthokeratology (OK) is considered one of the primary optical interventions to control axial eye growth in children. However, do all OK treatment zones provide the same degree of myopia control? Hiraoka et al (2005) suggest that higher-order aberrations (HOAs) may enable practitioners to quantify OK efficacy in controlling axial eye growth. In a poster at the 2018 Global Specialty Lens Symposium, Kojima et al described the relationship between treatment zone size and its effect on corneal spherical aberration in OK.

Retrospective data was collected on 40 subjects who had a pre-fitting prescription of –3.00D (±0.25D) and had undergone successful OK treatment. All subjects were analyzed using the Medmont E300 corneal topographer both pre- and post-OK. The prescription change at a chord of 5.0mm was measured along opposing hemi-chords (2.5mm from the visual axis) using the axial subtractive map along the flat corneal meridian (Figure 1). Additionally, pre- and post-OK corneal spherical aberration values were recorded from the topographer.

The results suggest that as the treatment zone diameter decreases, the peripheral plus within the 5.0mm chord increases (Figure 2). As the peripheral plus increases, the difference in corneal spherical aberration increases (Figure 3).

These data show an inverse relationship between the treatment zone size and the resultant spherical aberration. If it is true that higher positive spherical aberration is advantageous in myopia control, then theoretically, smaller treatment zones may provide greater myopia controlling effects.

Looking Ahead

Currently, a number of important questions remain unanswered:

1. Is axial eye growth directly affected by changes in HOAs, or do other optical/environmental factors influence axial eye growth?
2. Which HOAs potentially influence axial eye growth?
3. If positive corneal spherical aberration affects axial eye growth, what is the minimum value required? Does that value vary among patients?
4. At what value does corneal spherical aberration begin to reduce distance foveal acuity?

These results appear to indicate that practitioners should be aware of the relationship between treatment zone size and peripheral plus/corneal spherical aberration when constructing OK lenses for myopia control. CLS

For references, please visit www.clspectrum.com/references and click on document #275.