Many practitioners are not always comfortable fitting GP lenses, which are a great option for certain patients. Meanwhile, practitioners who have embraced GP lenses enjoy a steady stream of referrals that help build their practices. Corneal irregularities, high astigmatism, and presbyopia are typical cases in which GP lenses provide sharper and more stable vision compared to soft lenses. Two other areas of growth in the GP arena are orthokeratology and scleral lenses, both of which are great solutions for the right patients.

Practitioners who wish to build their GP practices can enhance their GP design acumen by considering various aspects of lens design as they relate to patients’ eyes. Knowledge of ocular surface topography and eyelid structure will also help create more successful fits. Here are some lens design concepts to master to make GP fitting easier.

Diameter

Diameter has a big impact on lens positioning, movement, stability, and comfort and should therefore be one of the first parameters to determine when fitting GP lenses. Corneal lenses should have a diameter about 2mm smaller than the corneal diameter to provide stable positioning while allowing room for movement with blinking and eye excursions. A corneal lens that is too large will not have enough movement, which would limit tear exchange and result in a tight fit. One that is too small will not have sufficient stability and may move and decenter more easily.

Scleral lens diameter may vary depending on the reason for fitting and the degree of corneal irregularity, but the initial diameter choice is also based on corneal diameter. If a scleral lens is too small, its weight will be concentrated in too small a zone around the limbus, increasing the chances of impingement and discomfort. A scleral lens that is too large may create excess suction or negative pressure under the lens, may not align well with the peripheral sclera, and will be more difficult to apply and remove.

Back-Surface Geometry

The way in which the back surface of the GP lens aligns with the corneal surface impacts patient comfort and ocular surface health. Both spherical and aspheric curves can be adjusted to better align with the cornea. Base curve radius is usually the first parameter to adjust, but you can change peripheral curve radii by about 0.5mm or axial edge lift by about 0.03mm to effect significant changes in the peripheral fit. In addition, changing the optic zone and peripheral curve widths by 0.2mm to 0.4mm can make noticeable fit changes.

Also, knowing when to use toric or reverse geometry back surfaces will result in better fitting relationships. Viewing the fit with fluorescein can show astigmatic patterns that require a toric back surface or an oblate pattern that may require a reverse geometry design. Analyzing the corneal topography and matching it with a similarly shaped back surface will improve fitting success.

Thickness and Edge Profiles

GP lens thickness and edge profiles should usually be as thin as possible to provide the best comfort and ocular response. With certain lens designs and higher lens powers, lenticular designs may be needed to maintain a reasonable thickness profile. Minus-carrier lenticulars will result in a thinner center, while plus carriers will decrease edge thickness. Minus carriers are also used to increase lid attachment to improve lens stability. Knowing when to specify these features can make big differences in fitting success. CLS