Dispense to Challenging-Fits
contact lens management
Dispense to Challenging-Fits
GP, mini-scleral contact lens may alleviate ocular issues.
STEPHEN P. BYRNES, O.D., F.A.A.O.
Optometrists are often faced with contact-lens patients who have ocular issues that make wear difficult and uncomfortable. Among them:
► Keratoconus patients. These patients struggle with fits that don't center on the eye or dislodge with eye movement.
► Irregular-cornea patients. These patients have difficulty with lens wear due to ocular trauma, penetrating keratoplasty, radial keratometry and less-than-optimal post-operative LASIK results.
► GP-intolerant patients. These patients desire the stable optics of a GP lens, but can't achieve wear due to chronic lens awareness.
While no one magic lens exists to solve all the aforementioned issues, the msd Mini-Scleral Design lens, from Blanchard Contact Lens Company, may satisfy many of these patients.
Mechanism of design
The msd lens is designed to vault the cornea and fit on the sclera, or more accurately, on the conjunctiva. Because the conjunctiva and episcleral tissues are soft and therefore compress with the presence of a lens, the lens settles into the conjunctiva. This reduces the net-vault of the contact lens over the cornea. (To accommodate for this potential decrease in vault, you must adjust the fit.) The space between the cornea and the back surface of the msd fills with tears. This creates a tear layer that minimizes the optical distortion irregularities of the cornea and allows the msd lens to float above the cornea's epithelial surface. The result: A reduction in the chance of contact-lens-induced corneal-surface irritation, and a centering lens that offers comfort and movement similar to that of a soft lens with the optic stability (needed to correct for corneal irregularities) of a hard lens.
|msd Mini-Scleral Design|
|LENS MATERIAL: Boston XO|
WEARING SCHEDULE: Daily wear
DK/T: 100 (ISO/Fatt), 141 (gas to gas)
BASE CURVE: None. You fit this lens using sagital depth.
SAGITAL DEPTH: 3.70 mm to 4.80 mm
SPHERE POWER: +/- 20.00D
COST: $195.00 per lens, with one free exchange and cancellation of $55.00. Additional exchanges are $55.00.
To ensure successful lens wear with the msd, utilize the product's fitting system, which employs the sagital depth of the contact lens as the primary fitting parameter. Sagital (sag) depth is the measurement from a flat plane at a given diameter to the apex of the inside curve of the lens. The msd lens includes a 36-lens-fitting set comprised of 12 vaults with sagital depths ranging from 3.70 mm to 4.80 mm at 0.1 mm (100 micron) increments. Each vault has three mid-peripheral/limbal zone clearance values (increased - standard - decreased). Lens diameter is fixed at 15.8 mm.
Trial fit the patient using progressively deeper vaults until you achieve corneal clearance. Now, the lens is floating on the tear layer and resting on the sclera/conjunctiva. If you fit the lens to rest on the cornea, the lens surface could damage the corneal epithelium.
To ensure lens clearance over the cornea, leave the trial lens in place for at least one hour. Then, re-evaluate lens placement for minimal corneal clearance using sodium fluorescein to make sure there's no corneal bearing. Light feather touch at the apex of a cone may be an acceptable fit after the lens has fully settled on the eye, provided on follow-up lens evaluations, no evidence of epithelial damage at the apex is present at the end of a typical full-day wearing period. Upon lens removal, you may notice a compression ring where the lens settled into the conjunctiva. This appears as a ring of fluorescein pooling in the conjunctiva during fluorescein evaluation after lens removal. It's a common and harmless finding after a full-day wearing period with the final lens, and it usually resolves in 30 minutes or less after lens removal.
Once the patient achieves apical clearance, evaluate the mid-peripheral cornea and limbal area to ensure an adequate tear layer between these areas and the lens. If mid-peripheral or limbal touch exists, increase the mid-peripheral zone of the msd lens to move the surface of the lens off the cornea. If the mid-peripheral space between the lens and the cornea is too great, an air bubble will form, which may interfere with vision or corneal health (i.e., epithelial-surface drying/desiccation). Decreasing the mid-peripheral zone alleviates or minimizes these issues. Small air bubbles in the lens periphery that move during lens wear don't interfere with vision or cause epithelial surface-drying/desiccation.
The msd lens on a post-penetrating keratoplasty eye.
Once you select the proper sag height and mid-peripheral/limbal zone, over-refract to determine lens power. The tear lens, created by the tears that collect between the corneal surface and the back surface of the msd lens, minimize any visual issue that results from corneal irregularity. Currently, the msd offers spherical lens powers, but this may change in the future.
Because the design and fitting process of the msd allows for the optimum and comfortable wear of our difficult-to-fit patients, I believe it belongs in the fitting toolbox of any practitioner who fits contact lenses. OM
DR. BYRNES MAINTAINS A PRIVATE OPTOMETRIC PRACTICE IN LONDONDERRY, N.H. HE IS AN ACADEMIC CONSULTANT TO THE SCHOOLS AND COLLEGES OF OPTOMETRY FOR BAUSCH & LOMB AND POLYMER TECHNOLOGY. HE LECTURES INTERNATIONALLY ON SPECIALTY GP CONTACT LENSES. E-MAIL HIM AT BYRNES579@AOL.COM.
Optometric Management, Issue: January 2008