Knowledge Base
2. Linking EyeSpace to your topographer

2. Bespoke Alignment Fit Advanced Parameters

8. Bespoke Alignment Fit Charl Laas

The EyeSpace Bespoke Alignment Fit lens is a daily wear rigid gas permeable lens designed and ordered purely from corneal topography data and entered spectacle prescription. To begin the design process follow the steps as listed in the Bespoke Alignment fit Quick-Start guide.

With the initial lens design complete, EyeSpace provides the more experienced contact lens fitter with options to further modify the lens design as desired.

Basic Parameters

In the basic parameters window, the drop-down menu provides the option to interchange the Bespoke Alignment Fit lens design between a Rotationally Symmetric-, Toric- or Quad design.

The Back Optic Zone Radius (BOZR), Alignment Curve (AC) and Lens Diameter values are automatically calculated by EyeSpace based on the imported topographical data, HVID and spectacle prescription.

Bespoke Rotationally Symmetric

Bespoke Toric

Bespoke Quad

Advanced Parameters

To access the advanced parameters, activate the Show Advanced Parameters checkbox.

With the checkbox activated, the follow advanced parameters become available to modify:

1.Back Optic Zone Diameter

The default size is 7.00 mm. However, the BOZD can be increased or decreased depending on the clinical requirement. For instance, keratoconic corneas, taking cognisance of pupil size and cone location, generally require lenses with a smaller BOZD to minimise the mid-peripheral tear volume around the cone and provide a more efficient tear pump .

2. Alignment Curve Eccentricity (AC e)

The AC eccentricity controls the asphericity of the alignment curve. The default value is 1. The AC eccentricity value should be the same or slightly more than the corneal eccentricity to match the rate of flattening found on the peripheral cornea.

3. Edge Width

Changing the edge width value will increase or decrease the outer edge width of the Bespoke Alignment Fit lens. Along with the Edge Radius, modifying this value will affect the edge clearance. It is usually not necessary to change the edge width value.

4. Edge Radius

Adjusting the edge radius affects the edge clearance of the lens (visualised by the edge tear profile). Flattening the edge radius will increase the edge clearance, and conversely steepening the edge radius will decrease the edge clearance. Achieving an optimal edge clearance is essential for lens comfort, optimal corneal physiology and lens movement .

For EyeSpace Bespoke Alignment Fit lenses fitted on regular corneas, aim for a horizontal edge tear profile between 50 and 75 microns and a vertical edge tear profile between 75 and 100 microns.

For irregular corneas, like keratoconus, increase the horizontal edge tear profile to between 75 and 100 microns and the vertical edge tear profile between 100 and 150 microns.

Irregular cornea

Optimizing the fit

Changing any of the Advanced Parameters potentially alter the lens-to-cornea fit relationship. Click the Optimize AC button, and EyeSpace will redesign the alignment curve radius to re-optimise the lens-to-cornea fit relationship.

Note: After optimizing the the alignment curve, It might still be necessary to change the BOZR to achieve an optimal cTFT of between 2 to 15 microns.

As always, remember to order the lens using the Review option if you doubt the accuracy of your lens design. Your troubleshooting consultant will review your case and if necessary, make the required modifications and will then notify you of these changes. The review option provides you with a safety net and at the same time allows you to gain valuable experience, enabling you to grow in confidence designing EyeSpace Bespoke Alignment Fit lenses.

References

  1. Fink BA, Hill RM, Carney LG. Influence of rigid contact lens overall and optic zone diameters on ter pump efficiency. Optom Vis Sci. 1990;67(8):641-644.
  2. 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.
  3. Gundel RE, Libassi DP, McMahon TT. Standardized rigid contact lens fitting protocol for keratoconus. Optom Vis Sci. 1996.
  4. Mannis MJ, Zadnik K. Contact lens fitting in keratoconus. CLAO J. 1989; 15: 282-289
  5. Mandell RB. Contemporary management of keratoconus. Int Contact Lens Clin. 1997; 24: 43-58
  6. Leung K.K. RGP fitting philosophies for keratoconus. Clin Exp Optom. 1999;82:230–235.
  7. Edrington TB, Gundel RE, Libassi DP, et al. Variables affecting rigid contact lens comfort in the collaborative longitudinal evaluation of keratoconus (CLEK) study. Optom Vis Sci. 2004;81(3):182-188