Article

PRESCRIBING FOR PRESBYOPIA

WHAT ARE EXTENDED DEPTH OF FOCUS (EDOF) LENSES?

Recently, contact lenses providing an extended depth of focus (EDOF) have been introduced as an option for presbyopic correction. EDOF optics aim to elongate the depth of focus so that vision from distance through intermediate and near remains above a threshold of good image quality. An EDOF is potentially more desirable because it mimics the pre-presbyopic state in which accommodation naturally extends depth of focus from distance through near.

The optics of lenses having an EDOF differ from other commonly available soft multifocal designs in several fundamental ways. A standard multi-zone multifocal lens has distinct zones of distance and near power. When such a lens is placed on a presbyopic eye, a distance and a near focus is created, but very little depth of focus is present, leading to blur at the intermediate distance.

Lens Designs

To improve intermediate vision, previous designs have incorporated aspheres or progressive power transitions or blending or stepping of the power profile to increase the focal depth. But, these designs achieve only a limited depth of field and are not able to achieve an EDOF with good image quality from distance through near (Bakaraju, Ehrmann, et al, 2018). Recently, zonal designs with very small central optic zones surrounded by large plus-power zones have created “optical pinholes” to increase focal depth, but these are also more limited in their range of clear vision (Griffen, 2007).

Sophisticated EDOF designs have recently been developed that generate true extension of the depth of field (Bakaraju, Ehrmann, et al, 2018). These designs utilize combinations of several higher-order aberrations (HOAs) on the front surface of the lens to elongate the focal point (Bakaraju, Ehrmann, et al, 2018). These lenses create unique power profiles that distinguish them from traditional annular or aspheric designs that have more limited focal depths. We see that the power profile is constantly varying (non-monotonic), rapidly increasing and decreasing across the optical zone (Figure 1). Also, power changes are very narrow and indiscrete (aperiodic) and not the wider square wave zones typically seen in annular designs. Thus, it is the non-monotonic and aperiodic power distribution from the combinations of HOAs (Zernike coefficients) that deliver a more elongated depth of focus (Bakaraju, Ehrmann et al, 2018).

Figure 1. Illustrative power profile of a non-monotonic, aperiodic EDOF lens.
Image courtesy of Mark’ennovy

The EDOF in these lens designs would be expected to mitigate the impact on vision of several factors, including individual aberrations of the eye, pupil size, and contact lens decentration (Bakaraju, Ehrmann et al, 2018).

One study of 43 presbyopic patients between the ages of 42 to 66 years reported that subjects wearing EDOF lenses have improved visual performance at intermediate and near distances without degrading distance vision (Bakaraju, Tilia, et al, 2018). Another study of 57 presbyopes reported improved clarity at intermediate and near, reduced ghosting, and improved visual stability in EDOF lenses compared to a center-near aspheric soft lens design (Sha et al, 2018).

Although early results are impressive, time will tell how EDOF lenses serve the greater population of presbyopes. Regardless, it is nice to have yet one more option to assist us in delivering the best possible contact lens care for our patients. CLS

For references, please visit www.clspectrum.com/references and click on document #294.