The Alignment Curve or AC of the Forge Ortho-K lens is the area where the lens bears on the cornea and is responsible for centration of the lens on the eye.
It is essential that the maximum lens-corneal bearing of the Forge Ortho-K lens must occur under the Alignment Curve (AC) of the lens. The area of lens bearing can be best visualized as a blue zone of touch with the NaFl simulation in the EyeSpace design overview page.
For an optimized orthokeratology fit, the majority of the lens bearing must be distributed equally over the horizontal meridian of the cornea as shown in the image below. Should this not be the case, the lens can be tilted until the lens bearing is equalized over the horizontal meridian.
A variation to the example above is the 3-point bearing pattern, as shown in the example below. With this pattern, the lens bearing is not over the horizontal meridian. Instead, the lens-bearing is distributed equally in a triangular pattern over the alignment curve, still providing the necessary lens stability and resistance to lens tilt.
The Alignment Curve angle is the key concept to achieving perfect lens centration. The AC angle can be best visualized using the cross-section tear layer profile in the EyeSpace design overview page.
Eccentricity (e) describes the rate of flattening of the cornea. A higher eccentricity value means the cornea flattens at a faster rate when moving from the center to the periphery. For optimal centration, comfort, and corneal safety, the Forge Alignment Curve eccentricity (AC e) must be more than the corneal eccentricity at the area where the AC bears on the cornea.
For Forge Ortho-K lenses the default AC e value is 1.2. For most corneas, the standard AC e value will be optimal, and when visualized in the cross-section tear layer profile, the AC should have a U-shaped appearance as seen in the example below.
The diagram below illustrates how the shape of the Alignment Curve changes as the AC e value increases:
Some exceptional corneas have a very high eccentricity value requiring an increase from the default AC e value. In the example below the default AC e value of 1.2 is not sufficiently high, resulting in a flat or straight-line appearance of the AC in the cross-section tear layer profile (area highlighted by the circles).
If the AC eccentricity is in alignment with or even worse, less than the cornea, the Forge Ortho-k lens is more likely to cause a corneal indentation ring (image below), peripheral circular staining, and lens binding.
To correct the lens design, increase the AC e value until the AC has a more U-shaped appearance in the cross-section tear layer profile. In the example above, it was necessary to increase the AC e from 1.2 to 1.5 to achieve a more optimal AC profile as shown in the example below.
In some troubleshoot cases the AC e can also be increased to further assist in challenging reshaping cases.
In situations where the eyelids are very tight causing decentration, for example in Asian eyes, or when the cornea is very oval resulting in the Forge Ortho-K lens bearing on the vertical limbus the following adjustments can be made to the lens design: