When managing presbyopia with GP contact lenses, the most common way to incorporate multifocal optics is through the utilization of asphericity. The majority of aspheric corneal GP lenses have center-distance designs. The rate at which the add power is generated from center to periphery is dictated by the rate or amount of asphericity that is incorporated into the lens; as asphericity increases, so does the relative amount of add power (Figure 1).

Across the optic zone, the asphericity or add power can be placed on the front and/or back surface of the lens. There are optical and important fitting considerations when deciding between these options.

Back-Aspheric Designs

When the asphericity is placed on the back (posterior) surface, the lens gradually flattens from center to periphery. To align with the peripheral cornea to achieve an optimum lens-to-cornea relationship, the base curve needs to be steepened beyond the patients’ central corneal curvature.

As the asphericity or add power required for a patient increases, so does the amount of artificial steepening necessary for an optimum lens fit. This could cause molding of the patient’s cornea to the lens’ back surface curvature, thereby steepening the patient’s cornea (Figure 2). This will result in blur with the patient’s traditional spectacles after lens removal. This thankfully is a temporary effect, but it may significantly impact a patient’s lifestyle.

Front-Aspheric Designs

Another option is to place the asphericity or add power on the front (anterior) of the lens. This generates the needed add power, but leaves the back surface of the lens spherical to better align with the cornea and not alter its shape (Figure 3). To achieve optimum visual success and to better center these anterior aspheric multifocal optics over the pupil and visual axis, use slightly larger overall lens diameters (10.0mm to 11.0mm), depending on corneal diameter and lower lid margin position.

A Little of Both

As greater amounts of add powers become necessary for patients to achieve optimum visual success at near, it may be necessary to place the asphericity on both the front and back of the lens. Utilizing anterior asphericity helps minimize the amount of asphericity on the back of the lens. Ultimately, each unique patient scenario will help determine whether to incorporate the add on the front and/or the back of the lens. CLS