CONTACT LENS DESIGN & MATERIALS
GPs AREN’T HARD LENSES, BUT...
RONALD K. WATANABE, OD, FAAO
A 54-year-old multifocal GP lens wearer was referred for a contact lens refitting because she was experiencing blurry vision with her glasses. She was told that her astigmatism had increased and that her contact lenses needed to be changed. She loved her multifocal GP lenses because she could see both distance and near clearly. She did not seem very happy about the situation and was afraid that I would make her wear soft lenses.
My examination found visual acuities with her contact lenses of OD 20/20 and OS 20/25 at distance and OD 20/20 and OS 20/20– at near. The lenses positioned superiorly with mild apical clearance, midperipheral alignment, and moderate peripheral clearance. There was good movement and tear exchange under the lenses. Her corneas were clear without fluorescein staining.
Subjective refraction after lens removal was OD –5.25 –1.50 x 100, 20/25 and OS –4.25 –1.75 x 80, 20/25–. Her habitual spectacles were measured at OD –6.50 –0.50 x 75 and OS –6.25 –0.50 x 90. Corneal topography showed corneal distortion with superior steepening, central to inferior-central flattening, and inferior steepening (Figure 1, left).
Figure 1. Corneal molding from wearing a back-surface-aspheric multifocal lens design (left); normal astigmatic pattern after stopping lens wear (right).
A Lasting Impression
We discussed her options, and though she protested mildly, she agreed to stop wearing her contact lenses for five days. Her follow-up visit yielded a subjective refraction of OD –7.25 –0.50 x 90, 20/20 and OS –7.00 –0.50 x 120, 20/20. Corneal topography normalized to a mildly with-the-rule shape (Figure 1, right). Sim-K readings were OD 45.25/45.75 @ 95 and OS 45.50/46.50 @ 78.
Her old contact lenses had a base curve radius of 7.26mm (46.50D), 9.3mm diameter, and –8.00D power with a +2.00D add OD and OS. Though the brand/design of the lenses was unknown, based on the steep base curves, high minus powers, and apical clearance fluorescein patterns, the design was most likely a center-distance, back-surface-aspheric multifocal.
This type of multifocal design works well in providing clear distance and near vision. However, because of the high-eccentricity back surface, it has to be fitted somewhat steep to achieve good centration. It is common for the base curve to be 0.50D to 1.50D steeper than K so the lens has reasonable corneal alignment and stability of fit. However, the lens will still be steep centrally. This type of fitting relationship can cause corneal molding with an accompanying refractive change and spectacle blur. In this patient’s case, the molding seemed to decrease myopia due to the central flattening and to increase irregular astigmatism due to the overall distortion.
A Successful Refit
A few days without lens wear returned the patient to relative normalcy, and multifocal GP lenses with spherical back surfaces and aspheric front surfaces were fitted. The base curves were OD 7.45mm (45.25D) and OS 7.40mm (45.50D), which are much closer to the K-readings and are less likely to cause significant spectacle blur. The patient was able to continue wearing multifocal GP lenses and to see clearly with her new spectacles.
Though GP lenses can sometimes cause this type of mechanical change to the cornea, rethinking the lens design and selecting more appropriate parameters can keep your GP patients happy. CLS
Dr. Watanabe is an associate professor of optometry at the New England College of Optometry. He is a Diplomate in the American Academy of Optometry’s Section on Cornea and Contact Lenses and Refractive Technologies and is in private practice in Andover, Mass. You can reach him at email@example.com.