Central staining with poor centration is seen when a Forge Myopia orthokeratology lens has an insufficient sagittal height or minimal to no central clearance.
The bearing of the lens on the cornea should always be in the alignment zone and never in the centre. The ideal central tear film thickness (cTFT) for correcting low myopia (< -5.00D) is 10-20 microns and for high myopia (> -5.00D) is 20-30 microns.
To control the cTFT adjust the Z-Zone value. The numeric value of the Z-Zone in microns equals the sag height of this region of the lens. Altering the Z-Zone will change the overall sag of the lens and therefore change the apical clearance. The greater the numeric value of the Z-Zone, the more apical clearance. For example, 285 microns has 10 microns less apical clearance than 295 microns.
Central staining can occur more often in high myopia cases due to the extra amount of peripheral moulding required which alters the peripheral corneal shape and causes the lens to sit lower on the cornea after the ortho-k mould completes.
In the example below a Forge Myopia lens is designed to correct for -6.00D of myopia. The z-zone is adjusted to allow for a cTFT of 13.7 microns.
However, due to the high myopia moulding process, the peripheral epithelium thins and causes the lens to sink lower on the cornea. In the EyeSpace simulation below, you can see what happens to the central clearance if the lens sinks down a further 15 microns. The central clearance is now insufficient (cTFT -1.3 microns) which will cause poor centration, central staining and under correction.
The solution is to increase the overall sag of the Forge Myopia lens. In this example, we need to increase the overall sag by at least 10 microns to achieve central clearance after the moulding process is completed. To increase the overall sag of the lens by 10 microns, we simply increase the Z-Zone value by 10 microns.
The increased sagittal height of the new Forge Myopia lens now allows for adequate central clearance and should resolve the central staining and decentration problems.
You can use the following table as a guide to decide by how much to increase the cTFT when troubleshooting central staining issues.