Innovations in intraocular lenses (IOLs) and refractive cataract surgery have provided eyecare practitioners (ECPs) with a broad array of options to tailor the treatment to your patient’s individualized visual needs and goals.
Selecting an IOL that addresses patient concerns with minimal complications requires investing in pre- and post-operative testing to evaluate your patient for optical aberrations and to assess their corneal topography. In particular, spherical aberration (SA) is a notable factor in choosing the best IOL type for your patient.
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An introduction to aspheric lens technology
Dr. Raviv highlighted that over the course of his career, he has witnessed significant changes in the IOL market. Around 20 years ago, spherical IOLs were considered the norm for treating patients; however, following the introduction of aspherical IOLs 15 years ago, the treatment paradigm quickly shifted. These lenses showed promising results in improving contrast sensitivity and compensating for spherical aberration, allowing surgeons to perfect their preoperative calculations to potentially enhance postoperative visual outcomes.
Further, in 2007, Dr. Rocha led a randomized prospective study measuring the spherical aberration and depth of focus (DOF) in 60 patients (120 eyes) with bilateral cataracts who were implanted with both a spherical and aspherical IOL. She noted that the aspherical IOLs corrected for focal length and higher-order aberrations and simultaneously degraded distance-corrected near and intermediate visual acuity.
While aspherical IOLs can provide many patients with quality functional vision, they may not address each patient’s specific concerns, which is why preoperative imaging and testing, such as wavefront analysis and corneal topography, are vital to selecting the right IOL.
The link between monofocal IOLs and depth of focus
In contrast to conventional wisdom, eyes implanted with monofocal IOLs may exhibit extended-depth-of-focus (EDOF) or pseudoaccomodative characteristics. At first, it was believed that a pinhole effect caused by small pupil diameters was the root of this anomaly, but a study by Nakazawa et al. linked this phenomenon to a greater depth of field.
With time it became accepted that several ocular factors, including visual acuity, pupil size, retinal eccentricity, ocular aberrations, and age, are linked to an increase in overall DOF.
Dr. Rocha’s research on optical aberrations and monofocal IOLs
Dr. Rocha led a study in 2020 to further elucidate the relative effects of factors that may enhance the depth of focus and improve near vision in pseudophakic patients with aspheric monofocal IOLs.
The study assessed 74 patients (111 eyes) that underwent phacoemulsification with monofocal IOL implantation. Within this cohort, 91 normal eyes received aberration-free or negative-spherical aberration IOLs, and 20 post-hyperopic femto-LASIK eyes received aberration-free IOLs.
To assess the outcomes, investigators analyzed the following:
- Subjective measurements: To check for residual astigmatism or myopia, clinicians assessed distance-corrected near visual acuity (DCNVA) of J3 or better and subjective defocus curves up to ±4.0D.
- Objective measurements: Using ray-tracing aberrometry (iTrace, Tracey Technologies), clinicians measured visual Strehl ratio based on the optical transfer function (VSOTF)—a metric for contrast, the effective range of focus (EROF), sphere shift (SS), pupil size measurements at far and near, and ocular and corneal SA. Also, they evaluated corneal higher-order aberrations (such as spherical aberration, coma, trefoil, and corneal asphericity) for a 6mm pupil using Scheimpflug tomography.
Although post-refractive patients tend to have negative spherical aberration, occasionally, the ablation is decentered, causing coma-like aberrations. As a result, Dr. Rocha’s group only used aberration-free lenses for these patients due to their tolerance for decentration.
Results from the study on optical aberrations and monofocal IOLs
Using multivariate logistic regression, researchers found that the corneal profile and IOL type were key determinants of EDOF with monofocal aspheric IOLs. In addition, patients implanted with the aberration-free IOL illustrated significant improvement in DCNVA and higher total SA than the negative-SA group. In normal eyes, aberration-free IOLs were more likely to produce high positive SA and better DCNVA than a negative-SA IOL.
For post-hyperopic LASIK eyes, the aberration-free IOL provided significantly better DCNVA, higher negative spherical aberration, coma, Q value (P<0.05), and smaller pupil size (P=0.05) compared to normal eyes implanted with aberration-free lenses.
Spherical aberration and depth of focus
In a separate study led by Dr. Rocha, she used adaptive optics technology to simulate the potential effect of positive or negative SA enhancing the depth of focus, resulting in a linear shift of the center of focus by 2.6D/μm of error. However, this increase in depth of focus reached a maximum of approximately 2.0D with 0.6μm of spherical aberration and became smaller when the aberration was increased to 0.9μm.
Dr. Rocha noted that through her research, she has found that there tends to be a sweet spot somewhere between 0.3 to 0.4μm of spherical aberration to maintain distance vision.
The balancing act of IOL selection
As a relatively new entrant to the IOL market, EDOF lenses provide more range than a monofocal and slightly less range than a multifocal IOL. However, the improved range of vision can come at a cost, as there may be a reduction in modular transfer function (MTF), resulting in contrast loss.
When selecting an IOL, clinicians should keep in mind that there is a delicate balancing act of maintaining contrast/quality of vision, depth of focus, and minimizing adverse events (i.e., dysphotopsia). With current IOL technology, they can often provide positive results in two of the categories, but it can be difficult to find lenses that are successful in all three.
1. TECNIS Symfony IOL
Approved by the FDA in 2016, the TECNIS Symfony IOL features an aspheric anterior surface and posterior echelettes that create an achromatic diffractive pattern, elongating the focal point and compensating for the cornea’s inherent chromatic aberration. Dr. Rocha noted that the Symfony IOL is known for having quality contrast and high MTF, but some patients have reported dysphotopsia (i.e., halos, glare, and starbursts).
2. AcrySof IQ Vivity IOL
Approved by the FDA in March 2020, the AcrySof IQ Vivity IOL uses proprietary X-WAVE technology, a novel non-diffractive design that results in wavefront shaping. The IOL features two smooth surface transition elements that simultaneously stretch and shift light without splitting it, resulting in a smoother range of vision.
Dr. Rocha mentioned that she has noticed less noted dysphotopsia and night-related adverse events in patients implanted with the Vivity IOL. Of note, this lens provides clear vision in far and intermediate distances, but uncorrected near vision tends to be diminished.
3. TECNIS Eyhance IOL
Approved by the FDA in 2021, the TECNIS Eyhance IOL provides high image quality as well as intermediate vision. The Eyhance is an advanced monofocal with a higher-order aspheric lens, not a true EDOF IOL.
Conclusion
While there are many different IOLs currently on the market, it’s up to ECPs to accurately measure and understand their patient’s corneal topography and overall optic system to offer an IOL that will maximize their long-term quality of vision.
Using wavefront analysis and corneal topography to evaluate your patient’s cornea before and after implanting an IOL is beneficial to select the best lens based on their needs. Subsequently, it is recommended that ECPs perform due diligence by retrospectively analyzing cases to measure cataract/refractive outcomes over a set period of time to assess meaningful visual success.
