LOW ASTIGMATISMLens Design
Jessica Conroy, OD, FAAO
Spherical GP (1)
A spherical GP lens has a single base curve radius (BCR) with one overall spherical power.
General Rules:
- A lid attached fit promotes attachment of the GP to the upper eyelid.
- The upper eyelid must overlap the superior limbus
- Begin fitting “on-K to 0.50D flatter than on-K” with a larger diameter lens
- An interpalpebral fit does not promote attachment of the GP to the upper eyelid.
- The upper eyelid will sit above the superior limbus
- Begin fitting “0.50D steeper than on-K” with a smaller diameter lens
- Overall Diameter = HVID – 2mm
- Optical Zone Diameter = pupil size in dim + 2mm
Front Toric GP (1)
Single BCR with overall spherical power
- Verify with one BCR
- Verify with two powers
- Document powers in terms of “Sphere -Cylinder x Axis”
Spherical power is determined after BCR selection (SAM-FAP).
Power of front-toricity is the amount of residual astigmatism remaining once spherical power has been applied to lens.
Prism Ballast is prism incorporated into inferior lens design.
- Prism Ballasted lenses tend to rotate 10-15 degrees nasal.
Prism Ballast and Truncation is often the most desirable lens design for front toric GPs due to good rotational stability
- Decreases edge thickness
References
- Bennett ES, Sorbara L, Kojima R. Gas Permeable Lens Design, Fitting, and Evaluation. In Bennett ES, Henry VA. Clinical Manual of Contact Lenses, 5th ed. Wolters Kluwer, Philadelphia, PA, 2020:134–185.