CLINICAL

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Get in the Cone Zone: Part 2

Determine the best lens design for each keratoconus patient.

MELISSA BARNETT, O.D., F.A.A.O., SACRAMENTO, CALIF.

Just as one contact lens design does not work on all regular cornea patients, keratoconus patients do not achieve universal success with one lens design either.

Thankfully, several contact lens choices are now available for this patient population, increasing the likelihood of successful wear for even the most difficult of keratoconus fits.

Here, in Part 2 of this two-part series, I discuss these options, so you can determine the best lens for each patient.

Corneal GPs

These lenses provide a smooth, regular surface that masks underlying corneal irregularity, and they allow tear exchange. Corneal GPs range in size from 8.0mm to 10.0mm in diameter. In addition, they are available in a multitude of designs, including reverse geometry, and can be specially made with custom parameters. Caveats: These lenses are less stable, more likely to decenter and may be difficult for patient adaptation vs. soft, scleral or hybrid lenses. In addition, they place patients at increased risk for incident corneal scarring.

Ideal candidates: Those who have small, central or mild cones (average K readings less than 50.00D) and can adapt to GP lens wear. In advanced keratoconus cases, an ideal fit is a challenge due to the extent of corneal irregularities.

Optimum fit: The lenses should minimally vault over the corneal apex to prevent epithelial disruption. In addition, they should provide mid-peripheral bearing and moderate peripheral clearance on the cornea.

Intra-limbal lenses

Intra-limbal lenses move minimally, provide adequate tear exchange and are slightly smaller than the cornea (10.5mm to 12.0mm in diameter.) The lens size allows for increased corneal coverage, which improves lens centration. A caveat: This characteristic may make lens application and removal challenging.

Ideal candidates: Due to the diameter of these lenses, patients who have moderate keratoconus (average K readings between 50.00D to 58.00D) make ideal candidates. That said, many types of keratoconus exist with no specific topographical value for each type. So if the patient has corneal ectasia and clinical signs of keratoconus, that patient would be classified as moderate, and an intra-limbal lens could be ideal.

Optimum fit: This is central corneal vault or light-feather touch with mid-peripheral bearing and moderate peripheral clearance.

Scleral lenses

These lenses rest entirely on the sclera, regardless of diameter, providing lens centration and stable visual acuity. Multifocal, center near and center distance designs are now available. Corneal clearance is an advantage of scleral lenses. Depending on lens design, 100µm to 300µm of corneal clearance may be possible. The amount of corneal clearance varies with the condition. Examples include keratoconus with large differences in corneal sagittal height and keratoglobus. Both conditions require large sagittal height. A caveat: Scleral lenses do not provide as much tear exchange as small diameter GP contact lenses.

Ideal candidates: All patients who have keratoconus and corneal ectasia, but no significant edema from reduced endothelial cell count make ideal candidates. A total of 800 cells per mm2 is the minimal amount to fit scleral lenses with success, research shows. Both large (25.0mm) and smaller diameter (12.5mm) scleral lenses are attainable. Large and small diameter designs are equally comfortable.

Optimum fit: A fit that completely clears the cornea and rests evenly on the sclera without blanching (pressure causing whitening of the sclera) or impingement (where the lens edge pinches the conjunctiva).

Soft lenses and soft torics

Soft lenses and soft toric lenses are available in custom designs with a variety of base curves and peripheral curves. A caveat: Non-custom lenses may not provide as good vision as custom lenses.

Ideal candidates: Early keratoconus, decentered keratoconus and globus-like keratoconus patients who experienced poor comfort, reduced wearing time or contact lens intolerance with GP lenses are ideal candidates.

Optimum fit: These lenses should be well centered and stable without rotation. (Most of these lenses are toric due to the refractive error of keratoconus patients).

Piggyback lenses

These are comprised of a soft lens underneath a corneal GP lens. The latest GP and soft lens materials provide enhanced oxygen permeability, preventing cornea edema and hypoxia. Caveats: The piggyback lens system can be more difficult and inconvenient to manage because the patient could lose the GP lens or damage the soft lens, and multiple lens care systems are needed for contact lens care and hygiene.

Ideal candidates: This modality is usually indicated for patients who report poor comfort or significant epithelial disruption with GP lenses, apical epithelial nodules or accompanying epithelial basement membrane dystrophy.

Optimum fit: To achieve an excellent fit, keep in mind that the base curve of the soft lens can be modified to alter the fitting relationship of the GP lens. In addition, a plus-powered soft lens is used to flatten the GP fit, and a minus powered soft lens is used to steepen the gas permeable fit.

Hybrid contact lenses

Hybrid contact lenses have a GP center and a soft skirt. They provide vision and lens centration, even for advanced keratoconus (average K readings greater than 58.00D). A caveat: Late-term lens tightening can occur months after the initial fitting.

Ideal candidates: Those who have poor lens centration or stability, reduced wearing time or contact lens intolerance with small diameter GP lenses make ideal candidates.

Optimum fit: The lens should fit with apical clearance over the central cornea with little or no touch in the lens’ GP portion. No bubbles should be present in the lens’ central part, and light touch should be present at the lens’ rigid/soft junction. The lens should land evenly on the soft skirt without impingement or edge fluting. Both the central base curve and skirt curvature of hybrid lenses can be adjusted.

SynergEyes is currently the only hybrid lens manufacturer. Their UltraHealth lens is a patented reverse geometry vaulted design. It creates a central area of vault, which is available in several vault heights. The proprietary soft lift curve assures cornea clearance. The lacrimal lens under the GP portion corrects corneal irregularities.

Helping you choose

As Dr. Potter pointed out in “Get in the Cone Zone: Part 1,” with all the contact lens options now available for keratoconus patients, finding the “right” lens for each patient can be a daunting task. Hopefully, this article facilitates the selection process. OM

Dr. Barnett is a principal optometrist at the University of California, Davis, Department of Ophthalmology & Vision Science, where she performs primary and medical eye examinations and fits contact lenses, including specialty contact lenses, in addition to teaching optics and contact lenses to ophthalmology residents. She has worked with Acculens and Essilor. E-mail her at drbarnett@ucdavis.edu, or send comments to optometricmanagement@gmail.com.