contact lens Q&A

A Unique Hydrogel Lens Phenomenon

BY CHRISTOPHER SNYDER, OD, MS
MARCH 1998

Patient S.R. presented in 1991 as a healthy, presbyopic 58-year-old man requesting to be fitted into contact lenses for the first time. His spectacle prescription was OD +0.50-0.50 x 093, 20/20 and OS +0.75 -0.25 x 120, 20/20, with an add of +2.25D for good near vision correction. For the left eye only, we prescribed a +2.75D low water, ultrathin, polymacon hydrogel lens to manage his presbyopic condition, which he successfully wore for an average of 10 hours each day. He used a multipurpose lens care system to clean, disinfect and store the lens.

After five years of regular contact lens wear on a traditional replacement schedule, we decided to refit S.R. with a +3.00D power, low water, hyperthin, polymacon hydrogel disposable lens. This change to a two-week replacement contact lens resulted in a thinner lens, which is usually desirable for a plus powered soft lens because it maximizes oxygen transmissibility.

Encountering the Phenomenon

Figure 1 illustrates an unusual lens performance phenomenon that was evident only in secondary gaze positions with this very thin disposable lens. It appears that the lens buckled as a result of its thinness at the lenticular junction. We first noted the buckling at the first progress evaluation, but we discovered that it was evident immediately upon insertion. We were able to repeat the phenomenon on the patient's eye using a subsequent lens having the same parameters but from a different manufacturing lot.

What was happening here?

When the thickness at the lenticular junction of a hydrogel lens is made below a critical minimum amount, the lens becomes prone to buckling at this junction. This is presumably a consequence of shear forces created when the eyelid moves over the differential between center of the lens and the lens lenticular "skirt." In addition, the thin, weak junction and the associated extra thinness of the peripheral portion of the lens can make it particularly challenging to handle. S.R.'s former lens design was marginally thicker than the disposable lens and was consequently easier for him to handle.

What was the patient's reaction?

Despite the curious phenomenon we noted, the contact lens provided good wearing comfort and good vision. However, the lens was particularly difficult for S.R. to handle.

While other disposable lens brands with different designs and materials most likely would not have shown this buckling phenomenon and may have handled more easily, S.R. chose not to pursue any other new contact lenses. Besides increasing the power to +3.00D, we made no change from his original lens type. Because we didn't prescribe the disposable lens, any ocular or visual consequences that may have occurred from the wear of this misbehaving lens are unknown.

Dr. Snyder is a professor of optometry and serves as chief of contact lens patient care at the School of Optometry at the University of Alabama at Birmingham.