Knowledge Base
2. Linking EyeSpace to your topographer

3. Forge Ortho-K Advanced Parameters

5. Forge Ortho-KCharl Laas

Based on the captured corneal data, measured HVID, and entered spectacle prescription, EyeSpace will either design a Forge Myopia rotationally symmetric (spherical) or Forge Toric orthokeratology lens and automatically calculate the optimal treatment zone, sagittal height, alignment curve and diameter of the Forge Ortho-K lens.

Advanced Parameters

For the experienced Orthokeratology fitter, EyeSpace offers override options to customise the Forge Myopia Ortho-K lens further. To access the advanced parameters, click the 'Lens Parameters' button on the lens design overview page.

The 'Lens Editor' page will open. To access the advanced parameters activate the 'Show Advanced Parameters' checkbox.

The following advanced parameter options will become available to modify:

1. Back Optic Zone Diameter (BOZD)

The BOZD can be increased or decreased depending on the clinical requirement. Decreasing the BOZD introduces more plus into the pupil zone periphery which can be useful for Myopia Control or Presbyopic Orthokeratology moulding. Please note by decreasing the BOZD smaller than the pupil diameter will also introduce more glare and flare into the optical system. Always review the pupil size and warn the patient of the possible visual side effects by reducing the BOZD.

2. Back Optic Zone eccentricity (BOZ e)

The BOZ eccentricity value can be modified between the values of 0 and 1.5, with the default value set at 0. The higher the e-value, the more aspheric the BOZR becomes. The higher BOZ asphericity could possibly be more beneficial for Myopia Control moulding

3. Edge width

Changing the edge width value will increase or decrease the outer edge width of the Forge Ortho-K lens, affecting the edge clearance. It usually is not necessary to change this value.

4. Edge radius

Adjusting the edge radius will affect the edge clearance of the lens (visualised by the edge tear profile) and is helpful to regulate the tear flow under the lens and to control lid attachment . Flattening the edge radius value will increase the edge clearance, and conversely steepening the edge radius will decrease the edge clearance.

For Forge Orthokeratology lenses, aim to have the horizontal edge tear profile between 50 - 75 microns and the vertical edge tear profile between 75 - 100 microns.

5. Alignment Curve eccentricity (AC e)

The AC eccentricity controls the asphericity of the alignment curve. The default value is 1.1. The high e-value allows the lens to centre better on corneas with uneven elevation contours.

Optimizing the fit

If any of the Advanced Parameters are adjusted, the lens-to-cornea fit relationship may become incorrect. Click the 'Optimize AC and Z-Zone' button, and EyeSpace will optimise the Forge Ortho-K lens parameters to restore the ideal lens-to-cornea fit relationship.

If you are unsure remember to order the lens with the ‘Review' option and your troubleshooting consultant will be happy to provide you with design feedback.

References

  1. Michaud L, Simard P. Myopia Control with Ortho-K. Eight elements to take into account when considering orthokeratology for myopia control. Contact Lens Spectrum, Volume: 32, Issue: September 2017, page(s): 20-26.
  2. Chow E. Poster presented at the Annual Meeting of the Contact Lens Association of Ophthalmologists (CLAO) 2014: The Role of Orthokeratology Lens Design in Myopia Control.
  3. Fink BA, Hill RM, Carney LG. A comparison of the effects of rigid contact lens overall diameter and axial edge lift changes on tear pump efficiency. Int Contact Lens Clin. 1991;18(7):148-153.
  4. Fink BA, Hill RM, Carney LG. Effects of rigid contact lens edge lift changes on tear pump efficiency. Optom Vis Sci. 1991;68(6):409-413.