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Extended Depth of Focus Intraocular Lenses

Robert M. Kershner, MD, MS, FACS

Sir Harold Ridley, acting on the suggestion of one of his registrars (residents), Steve Parry, invented and implanted the first intraocular lens (IOL) in England on November 29, 1949. Since that time, cataract surgery has experienced an exponential worldwide growth spawning a plethora of new surgical technologies and devices. The majority of IOLs implanted worldwide have been monofocal. Monofocal IOLs have a single point of focus and are usually selected to provide the best distance vision (Figure 11-1). Multifocal IOLs utilize diffractive optics that splits light from an image into 3 separate focal points at distance, intermediate, and near. Accommodating IOLs work by responding to the movement in the ciliary body as a response to near accommodation.¹ These IOLs are dependent upon lens capsular fluidity, a state that can be difficult to achieve and maintain following cataract extraction with resultant capsular fibrosis.² Indeed, studies have shown that the actual amount of accommodative add provided is less than 1.25 diopters (D).³ The ideal accommodating IOL therefore, would be one that, mimicking the youthful crystalline lens, would alter its focal distance in response to near visual needs.⁴

One of the most significant advances in cataract surgical technology has been the modification of Ridley’s original spherical biconvex IOL into a lens that can provide not only clear uncorrected distance vision but added vision at near as well. One of the newer innovations in the surgical correction of presbyopia is the concept of extended depth of focus (EDOF), or extended range of vision, IOLs (see Figure 11-1). The strategy behind this approach is quite simple. The EDOF IOL uses elongated optics to increase the depth of field. There are several different approaches to this emerging technology. Manufacturers who are bringing these EDOF IOLs to market aim to reduce or eliminate the known optical side effects of monofocal and accommodating IOLs while creating the long-sought-after clear, uncorrected near vision that patients demand.

According to the American Academy of Ophthalmology Task Force Consensus Statement,⁵ the minimum performance criteria to categorize any device as an EDOF IOL is as follows:

1. The EDOF IOL group should demonstrate comparable monocular mean corrected distance visual acuity when compared to monofocal controls.
   
2. The monocular depth of focus curves for the EDOF IOL should be at least 0.5 D greater than the depth of focus for the monofocal IOL controls at 0.2 logMAR (20/32 Snellen).
   
3. The mean (logMAR) monocular distance-corrected intermediate visual acuity (DCIVA) needs to be tested under photopic conditions at 66 cm at 6 months and should demonstrate statistical superiority over the control (one-sided test using significance of .025).
   
4. At least 50% of eyes should achieve monocular DCIVA of 0.2 logMAR (20/32 Snellen) or better at 66 cm.
   
5. American National Standards Institute/International Organization for Standardization–compliant visual acuity charts should have a recommended nominal luminance of 85 cd/m² (80 to 100 cd/m²).
   
6. A monocular defocus curve should be obtained by using the corrected distance refraction and measuring the visual acuity between +1.5 D and -2.5 D in 0.5-D defocus steps, except in the region from +0.5 D to -0.5 D, in which case it should be in 0.25-D steps.
   
7. The defocus curve data should be stratified according to the patients’ pupil size and axial length.
   
8. The mesopic contrast sensitivity function should be performed at 1.5, 3.0, 6.0, and 12.0 spatial frequencies (cycles/degree).

The average human cornea has a net positive spherical aberration. In youthful eyes, the crystalline lens delivers an overall negative spherical aberration to the optical system of the eye, effectively neutralizing it (Figure 11-2A).⁶ As we age, the lens becomes thicker, resulting in an increased positive spherical aberration, which leads to blurred vision and reduced contrast sensitivity (Figure 11-2B). Most spherical IOLs have a net positive aberration, which makes this worse. In early 2000, Pharmacia (now Johnson & Johnson Vision) introduced the first Food and Drug Administration (FDA)–approved biconvex, wavefront-designed anterior aspheric surface IOL, the TECNIS, to correct this optical aberration and reduce the image degradation seen with spherical IOLs (Figure 11-2C).^7–13 Studies have shown that the IOL effectively decreased spherical aberration and glare and improved functional vision, contrast sensitivity, and night driving (Figure 11-3).^14–19

The refractive index of the eye varies depending upon the wavelength of light that is being focused. The visible wavelengths of light between violet (400 nm) and red (700 nm) are each bent differently by the human cornea and lens with the longer wavelengths of light being bent more than the shorter wavelengths. As a result, the various colors are focused at different focal points along the optical axis, known as longitudinal chromatic aberration (Figure 11-4A). Chromatic aberration degrades image quality, causing blurring and color fringing at edges of an image (Figures 11-4B and 11-4C). Colors that are out of focus can cause blur and reduction in contrast sensitivity.^20–25 The average human eye has approximately 2.0 D of chromatic aberration. Pseudophakia adds to this chromatic aberration.

In an effort to correct 2 of the inherent problems of presbyopic correcting IOLs, namely, chromatic and spherical aberration, the technology of the TECNIS aspheric anterior surface IOL was combined with a posterior optic achromatic diffractive surface with an echelette design (Figure 11-5). The TECNIS Symfony IOL (Johnson & Johnson Vision), the first in its class, received the CE mark, and was granted FDA approval for use in the United States on July 15, 2016. The IOL contains an ultraviolet-absorbing chromophore with an overall diameter of 13.0 mm, an optic of 6.0 mm, and powers from +5.0 D to +34.0 D and includes a toric version, indicated for adult cataract patients with less than 1.0 D of pre-existing corneal astigmatism. These IOLs provide 14% to 35% improved image contrast as a result of chromatic aberration reduction and full correction of spherical aberration.

How does a single optic IOL accomplish this degree of optical correction? While the anterior optical surface is neutralizing spherical aberration, the posterior achromatic diffractive surface takes a single focal point and elongates it. This proprietary diffractive echelette design, a novel diffraction grating designed to reflect light, creates a step structure that elongates the focus of the eye, increasing the depth of field and extending its range of vision (Figure 11-6). The extended range of vision afforded by this technology precludes the image blur associated with multifocal optics.^26,27

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