Understanding extended depth of focus IOLs
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The ideal lens for our eyes is the one that we had in our youth. The young human crystalline lens provides outstanding image quality and a large degree of accommodation, giving the widest range of vision.
For cataract surgery, our currently available IOLs do not provide a sufficient degree of accommodation, so we use IOLs that are fixed within the eye and do not change shape, position or curvature. In order to give more spectacle freedom and a wider range of vision, we can use bifocal or trifocal IOLs, which split the light into ranges. We can also implant an extended depth of focus (EDOF) IOL, which elongates the focus and provides more range than a monofocal IOL. However, these IOLs inherently have a compromise in order to deliver the wider range.
Understanding basic optics
We can explain the optics with four simple examples (Figure 1).
- An eye with a plano refraction focuses the incoming light precisely on the retina, giving great distance vision.
- For an eye with a myopic refraction, the light is focused just in front of the retina, giving a closer focal point, which would be about 67 cm in an eye with a –1.50 D refraction.
- We have all seen patients who are presbyopic but yet can function without glasses for most activities. Often these patients have mixed astigmatism such as this example in which the patient has a refraction of 0.00 –1.50 × 180. This creates two focal points, one on the retina (0.00) and one in front of the retina (–1.50) so that there is an extended depth of field. However, the untreated astigmatism causes some blur at all ranges, but the patient has grown accustomed to this over decades.
- The final example shows the ideal EDOF IOL, which provides a sharp focus over a wide range, from –1.50 D to plano, so that the patient is able to see from far away to 67 cm clearly. However, this is a theoretically ideal EDOF that does not actually exist in the real world.
Source: Uday Devgan, MD
Small aperture EDOF IOLs
Any photographer will tell you that using a small aperture will increase the depth of focus in the eye and the depth of field in front of the eye. However, this comes at the expense of letting less light into the eye, and this loss of light makes it more challenging to see in dimly lit environments (Figure 2). The pinhole effect from the small aperture IOL may also help in tolerating mild residual refractive errors or irregular corneas such as after radial keratotomy. Because of the loss of light, it is often recommended that a small aperture EDOF IOL be used in just one of the patient’s eyes, with the other eye receiving a full-sized optic, typically monofocal in design.
Diffractive EDOF IOLs
The diffractive EDOF IOL uses light-splitting rings to spread the focus along a wider range, thereby increasing the depth of focus (Figure 3). This was the first EDOF design to become widely available in the U.S., and it has come with some degree of success. The challenge is side effects from the diffractive rings, which create halos and glare, particularly at night. This diffractive EDOF IOL certainly does give a wider range of vision without glasses compared with a monofocal IOL; however, it causes side effects that are not well tolerated by all patients. It remains a good option for the right patient who has reasonable expectations.
Beam-shaping EDOF IOLs
For the past many months, we have been using some of these beam-shaping EDOF IOLs with reasonable results. These EDOF designs have a central element on the optic that elongates the depth of focus within the eye, providing better intermediate vision at about the 60-cm range (Figure 4). Because there are no diffractive rings, there is not a high degree of night glare or halos. With good lighting, the patients often have sufficient functional near vision to avoid wearing spectacles for most tasks. However, there is no way to cheat physics, and there is an optical cost in using this beam-shaping element, which comes in the form of loss of contrast, particularly at night.
This beam-shaping EDOF IOL requires a trade-off; shifting some of the light energy from far focus to the intermediate range then causes a loss of some contrast (Figure 5). The modulation transfer function (MTF) of this EDOF IOL shows the increased range of vision, which required reduction in the contrast at the distance focal point. The package insert for this IOL does warn specifically about this loss of contrast and the potential challenges with night driving. The depth of focus provided by this IOL allows most patients to be free from spectacles for computer use and cell phone viewing but may require the use of reading glasses for prolonged reading, particularly in dim light.
All of these EDOF designs have benefits and downsides, and none are as perfect as the young human lens. These EDOF designs will work best in eyes with a normal tear film, normal cornea and normal retina. These designs are not ideal in eyes with any sort of corneal or macular disease. Each design has a trade-off in order to produce the extended depth of field, and we should tailor our use of these IOLs to each specific patient depending on their needs and their ocular status. We welcome more innovative IOL designs because it is always better to have more options for our patients.
- For more information:
- Uday Devgan, MD, is in private practice at Devgan Eye Surgery, Chief of Ophthalmology at Olive View UCLA Medical Center and Clinical Professor of Ophthalmology at the Jules Stein Eye Institute, UCLA School of Medicine. He can be reached at 11600 Wilshire Blvd. #200, Los Angeles, CA 90025; email: devgan@gmail.com; website: www.CataractCoach.com.
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