Over the past few years, we have seen a number of new or updated monthly and two-week replacement lens options enter the market. Previously, the majority of lenses coming to market were in a daily disposable design, which is arguably the fastest growing contact lens modality. However, manufacturers are also recognizing that not all patients can be fit in the daily disposable modality, and the market has seen a number of updates to current monthly or two-week replacement lenses.

Although there have been brand new materials introduced for monthly lenses, in other cases companies are opting to simply update their current successful lens offerings. These introductions can be labeled as the exact same material, but may contain changes intended to improve patient comfort and/or vision. For example, with the increased use of electronic devices, lenses that have aspheric optics or increased wettability may be desirable. When this is the case, do these new lenses represent a totally new product for patients, or are they simply the previous lens in a shiny new package? More importantly, can providers expect them to fit in exactly the same manner as their previous iterations?

Not Necessarily

Because the lens material listed on the package is exactly the same, it is easy to assume that the updated monthly or two-week lens will fit and behave exactly the same way on a given patient. The two versions may even have the same diameter and same base curves listed. Can you simply switch the lens and send patients out the door? The answer is sometimes no, and ignoring possible fit and performance differences may not serve all patients well.

Even if a lens is labeled as having the same exact lens material as its predecessor, you should recognize that a number of changes could result in different fitting characteristics. Obviously, if either the base curve or the lens diameter has changed, the fit could be affected. Other changes may be less obvious, yet may still alter lens performance. A lens labeled with the same material, overall diameter, and base curve could have a different optic zone size, be more aspheric on the back surface, have different center or midperipheral edge thicknesses, or even a different edge design. All of these can change how the lens fits on the eye.

Furthermore, there could be changes in how the previous and the new lenses are manufactured. Differences in how the exact same chemical compound is handled, mixed, cured, etc. can result in lens performance differences. There could also be different solutions in the blister package, altering the initial wetting of the lens and possibly affecting comfort or even fit.

Same Material ≠ Same Lens

The bottom line is this—identical materials do not necessarily fit identically if other lens modifications have been made. Any number of small changes can potentially affect lens performance. Changes made to enhance the lens wearing experience may affect the initial fit or can potentially alter how the lens fits and moves later in the wearing day.

The newer iteration of what may seem like “the same lens” requires a new contact lens prescription, as it is a separate U.S. Food & Drug Administration-approved product. It also requires awareness on the part of practitioners that there may also be altered fitting characteristics, at least on some eyes. CLS