the contact lens exam
Weighing Wavefront Options
BY CHARLOTTE E. JOSLIN, OD,FAAO & TIMOTHY T. MCMAHON, OD, FAAO
Lately, the buzz in optical correction is custom wavefront designed correction. And, while laser vision correction systems certainly have attracted the limelight, they undoubtedly aren't the only optical correction being evaluated. Initial clinical results with both custom wavefront designed contact lenses and even spectacles have been reported at recent meetings and in journals.
As exciting as the potential is to create "super vision" through surgical and even nonsurgical means, we must first address several issues. While multiple questions are shared by both methods of wavefront guided correction, specific issues relate to custom wavefront correcting contact lenses. The $64,000 question is, can the benefits of a nonsurgical custom wavefront option outweigh its potential costs, and can it overcome present obstacles?
Appreciating the Benefits
Clearly, highly aberrated eyes such as those that have corneal disease or are post-surgical, stand maximum potential benefit with aberration-correcting lenses, particularly for those in which biomechanical limitations prevent future surgical wavefront-guided consideration.
Patients who perform a majority of visual tasks under low contrast, low luminance conditions are also especially likely to benefit. Regardless, highly aberrated eyes represent a small percentage of the population, and it remains to be seen if the benefit-to-cost ratio in normal eyes is worth the new technology.
Another significant benefit for contact lenses -- which patients frequently replace -- is that ocular aberrations are transient in nature. Because of both the short- and long-term flux of ocular components (the tear film, accommodation and crystalline lens-to-cornea compensation), the frequent replacement of contact lenses allows for continuous correction of the temporal variability in ocular aberrations, creating a more perfect match.
Understanding the Obstacles
Most obstacles to a nonsurgical custom wavefront option relate to the lens fit, capital investment or corneal physiology. Can we reduce lens movement and also appropriately register the wavefront correction to the corneal position? Presumably, custom wavefront correction would individually correct all non-rotationally symmetric wavefront aberrations, through the 5th order.
With current contact lenses, x and y plane lens movement may be substantial enough to induce additional aberrations before even correcting current aberrations. It's well known that vertical decentration of spherical aberration induces vertical coma.
Moreover, what are the potential effects on aberrations from issues such as contact lens flexure, contact lens dehydration or tight fitting contact lenses? What are the potential effects that variations in the posterior tear layer cause?
What about corneal issues, such as the variation in aberrations caused by dry eyes, lens deposit-induced keratopathy or lens-induced corneal molding? What about aberrations induced by alignment or fixation errors, or even aberrations induced by the processing technology? And finally, what about the following necessary investments:
- patients would have a recurring fee for both diagnostic procedures and materials
- doctors would require capital investments as well as training
- industry would require investment in diagnostic and lathing technology as well as tech nology to transfer between equipment, all in addition to the expenditure necessary to market to and educate consumers.
Worth the Investment?
Two overall questions exist:
1) "Is the percentage of aberrated eyes within the population sufficient to warrant investment in technology?"
2) "Is the benefit-to-cost ratio in normal eyes sufficient to warrant investment?"
Dr. Joslin is an assistant professor at the University of Illinois at Chicago Department of Ophthalmology and Visual Sciences. Dr. McMahon is a professor at the same institution.