Contact Lens Design & Materials
Comfort Part 1: GP Lenses
By Ronald K. Watanabe, OD, FAAO
Many practitioners shy away from GP contact lenses primarily due to “discomfort.” Many even refer to GP lenses as “hard” lenses, presuming that the reason these lenses are uncomfortable is that they are made of rigid plastics that have a modulus many times higher than that of soft lenses. The rigid nature of GP lenses does play a role in comfort, but it is not the only critical factor. In fact, anyone who has tried wearing a well-fit scleral GP contact lens knows that it is comfortable from the beginning, so design features obviously play a major role in GP comfort.
Living on the Edge
The edge of a GP lens is the primary obstacle to initial comfort. The upper lid moves over the superior edge, and the inferior edge comes in contact with the lower lid and limbus with every blink. Over time the lids can adapt to this contact, but initially, many patients find GP lenses quite noticeable.
Also, if the edge does not have an optimal shape, which is tapered and slightly rounded at the tip, it will not be comfortable. Thin, sharp “knife” edges can dig into the lid and conjunctiva. Blunt or thick edges are bigger obstacles for the upper lid to overcome. And even an optimally shaped edge can be uncomfortable if the edge lift is not optimal. Too low an edge lift will hit and dig into the conjunctiva with every blink, while too high an edge lift will create a higher obstacle for the upper lid. By designing the ideal edge, a large portion of comfort issues can be eliminated.
The Back Surface
Another aspect of GP design that impacts comfort is the back surface of the lens. The lens-to-cornea relationship (fluorescein pattern) determines how well the back surface of the lens fits to the corneal surface. An alignment fitting relationship allows the most even distribution of the lens surface across the cornea, and it often provides the best stability while maintaining an adequate tear pump. Too steep a fitting relationship can create areas of harsh bearing on the cornea, while too flat a fit can cause excessive movement and greater interaction with the eyelids. In addition, sharp junctions between curves can rub on the corneal surface, so curve blends or aspheric surfaces are sometimes advised.
The Front Surface
Smoothness and wettability of the front surface can impact comfort greatly. The surface should be well polished and free of hydrophobic or scratchy deposits. Recommend good lens care products, a diligent cleaning regimen, and rewetting lubricants so that patients can maintain the cleanest, wettest lens surfaces possible. In addition, plasma treatment can superclean the lens surface to minimize these problems.
Finally, the diameter and thickness of the lens can impact comfort. Diameter changes can sometimes be tricky and may not provide the desired result. For example, increasing diameter by 0.4mm should theoretically improve lid attachment and decrease movement, but it also adds mass and bulk, which may cause the lens to drop more rapidly. Decreasing diameter allows for lighter, thinner designs, but this may result in an interpalpebral fit, which typically increases lid interaction. A good evaluation of the existing lens fit as well as corneal and lid dimensions will help determine the optimal lens diameter.
The Keys to Success With GPs
GP lenses can be of tremendous benefit to many patients, including astigmats, presbyopes, and those who have irregular corneas. Careful thought and selection of GP lens parameters, in addition to proper patient education and discussion of expectations, will go a long way in creating successful GP contact lens wearers. 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 firstname.lastname@example.org.