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Bigger Diameters Can Be Better
BY THOMAS G. QUINN, OD, MS, FAAO
Gas permeable contact lenses fabricated in diameters of 9.5mm or greater are often more comfortable to wear. This benefit is likely a result of diminished interaction between the upper eyelid margin and the lens edge.
Lid attachment fits offer additional benefits. The eyelid holds the lens along the line of sight, stabilizing vision. Furthermore, any movement of the eyelid, even with a partial blink, provides some movement of the lens across the ocular surface. This helps promote tear exchange behind the lens and prevents corneal desiccation adjacent to the lens edge, reducing the likelihood of 3 o'clock to 9 o'clock staining.
In general, fit lenses in this diameter range 0.50D to 1.00D flatter than you'd fit smaller lenses to avoid excessive vaulting over the corneal surface.
Intralimbal GP Designs
Some lens designs employ diameters that extend nearly to the limbus when placed on the eye. Two examples include the Dyna Z Intra-Limbal lens (Lens Dynamics) and the Rose K2 IC lens (Blanchard, Figure 1), both with standard diameters of 11.2mm but offered in sizes up to 12.0mm. These lenses often provide enhanced lens centration on irregular corneas resulting from conditions such as pellucid marginal degeneration (PMD), keratoconus and penetrating keratoplasty. Reverse geometry secondary curves help prevent excessive lens edge lift.
Both of these lenses are best fit using fitting sets. Fit the central lens curves first. For example, when fitting a patient who has PMD, strive for light feather touch at the steepest part of the cornea.
Once you've achieved the desired central fluorescein pattern, evaluate the peripheral curve system, which can be modified in a step-wise fashion. Both the Dyna Z Intra-Limbal lens and Rose K2 IC offer the option of altering the peripheral curve by quadrant. When fitting irregular corneas, excessive edge stand-off at 6 o'clock is common. In such cases, you can resolve the excess lift by ordering the lens with a quadrant steeper than what is specified for the remaining portion of the lens.
Figure 1. An 11.2mm diameter Rose K2 IC in place on an eye.
Semi-Scleral and Scleral Lenses
Sometimes it's necessary to bridge over the cornea to achieve proper lens performance. Two such designs include the Jupiter Mini-Scleral lens (Med-Lens Innovations, Inc.) and the So2 Clear Corneal-Scleral Lens (Dakota Sciences). Both are offered in diameters as large as 18.5mm and are designed to vault over the cornea to rest on the sclera. Although more challenging to fit, these lenses often provide patients with exceptional comfort and extended wearing time.
The SynergEyes family of hybrid lenses (SynergEyes, Inc.), featuring a GP center and soft skirt, offer similar benefits to these semi-scleral GP designs.
Soft Lens Designs
Size contributes greatly to the comfort provided by soft contact lenses. But comfort isn't the only reason to expand soft lens diameter. Toric soft lenses often employ larger sizes to reduce possible destabilizing influences from the eyelids during the blink cycle.
Some patients are ideally suited for small diameter lenses, such as those who have central nipple cones, tiny apertures or upper eyelids that rest above the superior limbus, necessitating an intrapalpebral fitting approach.
But when faced with indecision, a larger diameter will often promote better comfort and on-eye lens performance. CLS
Dr. Quinn is in group practice in Athens, Ohio. He is a diplomate of the Cornea and Contact Lens Section of the American Academy of Optometry and an advisor to the GP Lens Institute.
Contact Lens Spectrum, Issue: March 2008