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Today’s IOLs are designed to minimize visual compromise
Extended-depth-of-focus designs adjust chromatic and spherical aberrations to provide visual quality comparable to an aspheric monofocal.
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When patients are fit in multifocal contact lenses, they are typically educated that gaining near vision means giving up some of their quality of vision, as incoming light gets split between distance and near. Similarly, early multifocal IOLs also split light, which resulted in trade-offs in visual quality.
The first multifocal IOLs used zonal refractive technology (more akin to the design of multifocal contact lenses) to split the light to distance and near foci. More recently, diffractive IOL technology has improved quality of vision, but shortcomings still exist, including night vision symptoms and a dip in vision between the near and distance focal points.
The Tecnis Symfony IOL (Johnson & Johnson Vision), which has recently been introduced for correcting presbyopia, is an extended-depth-of-focus (EDOF) IOL with a different approach to providing an extended range of vision. While diffractive technology is used, unlike multifocal IOLs it does not split the light into two focal points. Instead, a proprietary echelette design produces a novel pattern of light diffraction that elongates the focus. In this way, Symfony uses the incoming light to create a continuous range of vision without multiple foci.
Conceptually, we can think of the difference between a multifocal and an EDOF IOL like the difference between lined bifocals and progressive bifocals. In the latter case, the continuous range of vision of an EDOF IOL feels more natural to patients. However, there are important differences between EDOF technology and progressive bifocals. Most notably, diffractive EDOF IOLs do not require patients to tilt their head or angle their gaze to find the “sweet spot” in the lens.
Quality of vision
Many patients have been willing to sacrifice some quality of vision to enjoy the benefits of progressive bifocals, monovision or multifocal contact lenses. However, we ideally would like to provide those benefits without a loss in visual quality.
With IOLs, the quality or sharpness of vision is largely determined by the presence (or absence) of optical aberrations, which can be introduced by the material, design or manufacturing processes. Although we often talk about monofocal IOLs as the standard for quality, there is a range of visual quality even among monofocal IOLs.
In particular, aspheric optics have been a major factor in improving visual quality in recent years. All Tecnis IOLs, including the Tecnis Symfony EDOF IOL, have aspheric optics. They provide -0.27 µm of (negative) spherical aberration (SA) to balance the positive SA of the cornea, leading to sharper vision.
Chromatic aberration
The Tecnis acrylic material also has a low level of chromatic aberration (CA). CA is the dispersion of visible light into its component wavelengths (or colors) based on the wavelength-dependent nature of the refractive index. Faster-traveling long wavelengths bend less than slower-traveling short wavelengths, creating CA and, thus, reducing the quality of vision.
The concept of CA may be more intuitive to optometrists who deal with spectacle lenses than it is to ophthalmologists who implant IOLs. High-index materials require less convexity, so they can be thinner and lighter, but they also have more dispersion and more CA, identified by a lower Abbe number.
For example, while polycarbonate spectacle lenses (Abbe number: 30) are lightweight and durable, their relatively poor optical quality can make a material such as CR-39 (Abbe number: 58) a better choice for patients with higher corrections and astigmatism. All Tecnis IOLs are made from the lowest dispersion hydrophobic acrylic material on the market with an Abbe number of 55.
The Tecnis Symfony EDOF IOL takes the concept of minimizing CA one step further. In this lens, diffractive achromatic technology similar to that used in high-end diffractive-optics camera lenses actually corrects the overall CA of the optical system. This helps enhance contrast sensitivity and improves retinal image quality — especially when combined with SA correction and a low-dispersion material.
Increased depth of focus with EDOF technology decreases visual quality slightly. However, because the baseline quality of vision after CA and SA correction is so high, the net result for the Symfony is visual quality comparable to an aspheric monofocal and significantly better than that of a non-aspheric monofocal with higher CA.
Symfony vs. monofocal
In the U.S. clinical trial that led to approval of the Tecnis Symfony IOL, Symfony subjects had binocular uncorrected visual acuity results for distance and intermediate of approximately 20/20 and near results between 20/25 and 20/32.
Monocular and binocular distance visual acuity for the Symfony subjects (n=148) was clinically comparable to that of the monofocal control lens group (n=150). There also was no clinically significant difference in contrast acuity between the two groups.
The percentage of patients reporting night vision symptoms with Symfony was low compared to that reported for monofocal IOLs. In our personal experience, night vision symptoms are typically only mildly bothersome when they do occur. Nevertheless, we still counsel patients about the potential for these symptoms.
Sometimes it’s difficult to predict who will have night vision symptom complaints. However, as a trend, patients’ satisfaction with their postoperative night vision is related to their preoperative visual experience. The more glare, halos and starbursts a patient has preoperatively, the less they are bothered by the photopsias they may see postoperatively. Therefore, patients with more advanced cataracts, particularly those of a cortical and posterior subcapsular nature, tend to be more satisfied with their postoperative night vision.
Recent published studies have confirmed the high quality of vision from the FDA trial. For example, Petrotti and colleagues found no significant difference in contrast acuity or visual quality between the Tecnis Symfony and Tecnis monofocal IOLs, while patients with the Symfony had better uncorrected distance acuity and much lower dependence on spectacles.
In summary, advances in IOL material and design have led to the development of EDOF IOLs that can correct presbyopia at the time of cataract surgery with little to no compromise on distance visual acuity or visual quality.
- Reference:
- Pedrotti E, et al. J Refract Surg. 2016;32:436-442;doi:10.3928/1081597X-20160428-06.
- For more information:
- Daniel H. Chang, MD, and Laura K. Huggins, OD, are in practice at Empire Eye & Laser in Bakersfield, Calif.
Disclosures: Chang reported he is a consultant for Johnson & Johnson Vision. Huggins reported no relevant financial disclosures.