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6. Z-Zone - Central Tear Film Thickness

5. Forge Ortho-KCharl Laas

The Z-Zone or reverse curve controls the sagittal height and therefore the apical corneal clearance of the Forge orthokeratology lens. On the design overview page, EyeSpace displays the apical corneal clearance as the central tear film thickness or cTFT.

Adjusting the Z-Zone

Increasing the Z-Zone value will increase the sagittal height of the lens and therefore increase the apical clearance resulting in an increased cTFT value. In the example below increasing the Z-Zone to 275 microns resulted in a cTFT of 17.3 microns.

Decreasing the Z-Zone value will reduce the sagittal height of the lens and therefore decrease the apical clearance causing the cTFT value to fall. In the example below by decreasing the Z-Zone to 255 microns resulted in a cTFT of -1.7 microns.

A Forge Myopia lens should never have central lens bearing as shown in the image above, as this will cause corneal staining, and an unstable fit resulting in nasal or temporal decentration.


The cTFT should always be the final check as changing any other lens parameter will affect the overall sagittal height of the lens. The ideal central tear film thickness of a Forge Ortho-K Myopia 2 lens is:

  • 10 - 20 microns for Myopia under -5.00D
  • 20 - 30 microns for Myopia above -5.00D

The image below graphically depicts this: