When fitting gas-permeable (GP) lenses, we worry that patients will not find the lenses comfortable and so we shy away from prescribing them. Researchers have focused on improving GP lens comfort⎯from fitting larger lenses and mini-sclerals to improving wettability. But why not consider lens thickness?
Broadening the Parameters
Lens thickness enters into the comfort equation two ways. A thinner lens is physically less noticeable to the patient, and a less visible lens means better comfort. Also, a thinner lens is less affected by gravity in the hyperope and lid forces in the myope, allowing for a better, more centrally positioned lens.
A common misconception is that labs already make the thinnest lenses possible. The truth is that lens thickness is often dependent on additional criteria, including material, lens power and lens design. Plus, if you specifically request a custom material, you may limit how thin the lens can be made.
Factors Influencing Lens Thickness
Mike Johnson, F.C.L.S.A., director of consultation services at Art Optical in Grand Rapids, Mich., explained several factors that can influence lens thickness, starting with the flexibility of the material. For example, a lens manufacturer of Boston ES—a stiffer material typically cut at 0.12mm thick—would make a 0.15mm thick cut for Boston XO2, a lens that is more flexible and prone to flexure with an astigmatic cornea. However, if flexure is not a concern and the cornea has little toricity, the same ES lens could be produced as thin as 0.08mm. Therefore, if you were fitting a patient with little or no corneal astigmatism and asked for a Boston X02 cut lens—and gave the lab no other information—you may get a lens that is nearly twice as thick as an ES lens ordered with a lab note explaining the patient has minimal cylinder and needs the thinnest possible lens.
Lens thickness can also be driven by the lens power and the specific design of the lens. Multifocals, for example, require a special power cut into the lens. Ultrathin designs have the advantage of being cut with a decreased center thickness and having optimized junction thicknesses to allow for further reduction of lens mass. These advantages are evident in the lower minus and plus powers. For these lenses, the lenticular junction thickness and the lenticular bowl will determine how the design can be made. By choosing a stiffer lens material, the lenticular junction thickness in an ultrathin design can be reduced by 0.03mm, bringing center thickness down by 0.03mm as well. In cases where corneal toricity exceeds 1.5D, this isn’t the best option as the lens will likely exhibit flexure and reduced acuity. But, in cases of mild astigmatism, ultrathin lenses are a very underutilized option.
Lastly, Mr. Johnson names the regularity of corneal astigmatism as a deciding factor in contact lens thickness. A certain amount of corneal toricity, if regular, can cause lens flexure. If the corneal astigmatism is irregular, it may still be possible to use a thinner lens—certain types of irregularities, if mild enough, may not lead to flexure. If there is no specific meridian for which the lens to flex upon, you may end up with a better visual outcome than you might otherwise expect. In certain instances of mild to moderate corneal irregularity—such as focal corneal scarring—an ultrathin may also work better than you might expect, since you may achieve better comfort while obtaining a similar visual outcome.
Cutting the Thinner Lens
Why do practitioners want lenses cut thinner, and are thin lens designs underutilized? Mr. Johnson said that thinner GP lenses are easier for patients to initially adapt to, provide increased Dk through the reduced lens mass and tend to center and remain centered better. He believes they are underutilized. Why? Typically it is the looming threat of lens flexure that causes many practitioners not to use them as the first lens of choice. In these cases, fitting consultants for the manufacturing labs can help. They will know how thin a lens can be made for any individual patient.
We all strive to find better fitting and more comfortable lenses for our patients. Lens thickness is an important issue to address. Never hesitate to provide your GP lab with all the patient information and ask for their assistance in creating the thinnest possible lenses. Your patients will thank you for it.