Today’s contact lens practice is defined by fitting patients into different categories of lenses, whether they be soft, GP corneal, hybrid, or scleral lenses. The overall contact lens marketplace is dominated by soft contact lenses, and the majority of those lenses are molded in a mass-production fashion in an attempt to satisfy the majority of the patient population while limiting the existing parameter availability within the constraints of manufacturing capabilities and storage capacities.
Determining whether a custom soft lens design is indicated begins with an evaluation of patients’ astigmatism and identifying whether they have regular or irregular astigmatism. Patients who have regular astigmatism with a cylinder correction need beyond –2.25D at the plane of the cornea are often those who are most commonly fitted into custom soft lenses, as patients who have a cylinder error of –2.25D or below at the corneal plane have numerous molded soft lens options.
However, patients who have regular astigmatism less than –2.25D at the corneal plane may also be candidates for custom soft lenses based on the size and shape of their eye, namely, how large (visible iris diameter [VID]) or how flat or steep (central corneal curvature, keratometry value) their cornea is. VID and corneal curvature are the anatomical features that contribute to the overall sagittal height of the eye and are primarily responsible for the physical fit characteristics of a soft lens on the eye. One study illustrates this nicely by considering the anatomical features under the typical diameter of a soft contact lens and evaluating their overall contribution to the sagittal height of the eye. In this study, the anatomical feature that matters the most in terms of the overall sagittal height of the eye is the corneal diameter. Next to corneal diameter, the rate at which the cornea flattens from center to periphery (i.e., corneal eccentricity) is the anatomical feature that has the second greatest impact. Outside of these these two, surprisingly, the corneal feature that matters least is the one that we measure the most: namely, central corneal curvature.
The other category of patients for whom a custom soft lens may be indicated are those who have irregular astigmatism. The compensation or correction of irregular astigmatism is not possible in the standard mass-produced lenses because they have central thickness profiles of approximately 0.08mm for a –3.00D lens and will drape over the cornea; therefore, all corneal astigmatism will manifest through the lens. It is not until a soft lens is thickened that it begins to approach the non-flexing nature of a GP contact lens and masks some of the irregular astigmatism, converting it to more regular astigmatism that can be corrected by making the lens toric.
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