Consultant's Corner
Eccentricity Explained
by Kelly Indovina, OD
Director of Professional Relations
Recently, I was asked why gas permeable multifocals provide better distance and near vision than other modalities. My response? Translation is the key.
There is little to no translation with a soft multifocal lens, therefore both distance and near optics need to be provided in front of the pupil. GP multifocals are designed to provide full distance optics in front of the pupil and aspherically progress to a near periphery. As the patient looks down, the lenses should translate up, allowing the patient access to the near add.
There are many different aspheric GP multifocals with varying amounts of either front or back surface eccentricity. Some designs are a combination of both.
Eccentricity is described as the rate of flattening, the higher the eccentricity (e-value) the more quickly the lens flattens in the periphery. A spherical lens has a zero e-value and a hyperbole has an e-value of one. A higher e-value also relates to the amount of plus in the periphery. In simple terms, the higher the e-value, the higher the add power.
Many designs have the eccentricity (the add) produced on the back surface. If the eccentricity is on the back surface, the fit will need to be adjusted to compensate for the flattening due to the asphericity. If two lenses are ordered with the same base curve and one is a spherical base curve and one is an aspheric base curve, the aspheric base curve will fit flatter due to the eccentricity. Therefore, the higher the e-value, the steeper the base curve needs to be for optimal fit. Highly aspheric back surface multifocals can change or mold the cornea.
If the eccentricity is on the front surface of the multifocal, the lenses can be fit using the same technique as a standard spherical lens. When fitting this type of lens, there is usually no change to the cornea. If the patient is wearing a spherical distance design, the parameters would not change when ordering the front surface designs.
The aspheric multifocals we are most successful with are Renovation and Magniclearplus. Renovation is a front surface aspheric design and Magniclearplus is a front surface aspheric with a slightly eccentric base curve.
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