ORTHOKERATOLOGY (ortho-k) retainers utilize a variety of curves to achieve target correction and induce peripheral plus power onto the retina for myopia control. Each curve is unique and important in ortho-k design and in overall treatment of the cornea. In the June issue of Contact Lens Spectrum (Pillai, 2024), there was substantial discussion of the peripheral curve. In this issue, let’s dive into the alignment curve.

The alignment curve is just anterior to the peripheral curve in the ortho-k architecture, nestled between the reverse curve and the peripheral curve. The alignment curve is crucial for proper centration and, of course, alignment of the ortho-k retainer. This is the widest area of the ortho-k design and one of the most important.

The alignment curve, as mentioned, is in between the reverse curve and peripheral curve of the ortho-k retainer. Its width can vary from 0.70mm to 1.0mm (Gidosh et al, 2017). Some designs have multiple alignment curves within this area that can be modified independently. The alignment curve also will be a significantly flatter curve compared to the reverse curve, as it will land on the corneal surface.

For ortho-k success, 360º of proper alignment in this zone is essential. In cases of corneal toricity, especially limbus to limbus, toric alignment curves are required to have proper alignment in all meridians. A careful analysis of corneal topography and elevation differences can determine whether toric alignment curves are required. Typically, an elevation difference of greater than 25mm at a 7mm to 8mm chord will require toricity in the alignment curve.

Suitable alignment of the ortho-k retainer is crucial for success. The alignment curve, when properly designed, should land smoothly on the midperipheral cornea. This landing allows for good centration and midperipheral alignment, ensuring that adequate vision correction is achieved.

Improper landing of the alignment curve will cause decentration issues that can induce astigmatism and aberrations. Initial topography selection is crucial for alignment curve selection. This curve is typically around 6mm to 8mm in the midperipheral cornea, thus central K values are not relevant for alignment curve selection. Having midperipheral corneal data and eccentricity can be crucial for proper initial design.

In my experience, the alignment curve is one of the most frequently modified curves in ortho-k troubleshooting. Centration issues, undercorrection, and central islands can all be caused by alignment curve mismatches with the cornea.

Given its importance in overall ortho-k success, it makes sense that it is a common curve to be modified when things go astray. A proper seal of the alignment curve allows for the negative pull force to occur in the reverse curve to achieve target correction but also create a zone of peripheral plus. The peripheral plus is crucial for myopia control, as this is what is theorized to provide the myopic defocus signal to the retina to slow down myopic progression in ortho-k.

Ortho-k can be a huge benefit to patients for vision correction, and especially to our pediatric patients for myopia control. A proper understanding of ortho-k architecture will help clinicians not only in initial design, but also for modifications when troubleshooting is required.

References

1. Pillai A. Keep the Periphery in Mind. Contact Lens Spectrum. 2024 Jun;39:32. Available at clspectrum.com/issues/2024/june/orthokeratology-today. Accessed 2024 Aug 20.

2. Gidosh N, Morgan B, Norman C. Elevate Your Ortho-k Fitting to the Next Level. Contact Lens Spectrum. 2017 Dec;32:32-34,36,37. Available at clspectrum.com/issues/2017/december/elevate-your-ortho-k-fitting-to-the-next-level. Accessed 2024 Aug 20.