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IOL materials, optics and the happy patient
The optical performance of IOL materials is related to refractive index, reflectance and chromatic aberration.
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When evaluating a patient who is unhappy with his or her cataract surgery yet has good Snellen visual acuity, we should approach the patient like one who is unhappy with a new pair of glasses. Although we cannot simply “remake” the IOL, we first look for treatable causes, such as uncorrected refractive error, ocular surface disease, IOL decentration, posterior capsular opacification, macular disease and even personality issues.
If none of these explains the patient’s symptoms, consider the role of the IOL material and optics. IOLs share common optical characteristics with spectacle and camera lenses, including the fact that optical performance is dictated by lens material, with its associated index of refraction, reflectance and chromatic aberration (CA).
Index of refraction
The index of refraction is a measure of how much light slows as it travels through a material (Table). High-index materials slow light more. In accordance with Snell’s law, IOLs made of high-index materials require less convexity, which allows for thinner lenses and smaller incisions. Conversely, IOLs with lower-index materials, such as silicone, require greater lens convexity and thickness, thus necessitating larger incisions. Mid-index materials can also be implanted through smaller incisions, but they require a slight reduction in optical zone size to maintain a thin profile.
In addition to optic curvature, the refractive index determines an IOL’s surface reflectance. Defined as the percentage of incident energy that is reflected at an interface, reflectance is exponentially related to the difference in refractive index of the two materials comprising the interface. When implanted, high-index acrylic IOLs can have approximately eight times the reflectance of a low-index silicone IOL. A mid-index acrylic IOL has almost 60% less reflectance than high-index acrylics. Furthermore, because high-index IOLs require less optic convexity, the relative intensity of their internal and external reflections and glare can be hundreds or thousands of times greater than the reflection caused by the crystalline lens, resulting in bright, “cat’s eye”-type reflections (Figure 1).
Figure 1. Intraoperative image of +21 D high-index (1.55) acrylic IOL with 0.58% reflectance. Note the dimming of the periphery due to the auto light-leveling feature of the camera.
Images: Chang DH
Spectacle and camera lenses have antireflective coatings to reduce their reflectance, but IOLs do not. Therefore, when considering IOL materials, the ability of high-index polymers to create thinner IOLs must be balanced by the susceptibility of those polymers to produce bright internal and external reflections.
Chromatic aberration
The optical advantages of treating spherical aberration with aspheric lenses are well-established. However, CA has received less attention. CA is the dispersion of visible light into its component wavelengths (or colors) based on the wavelength-dependent nature of the refractive index (Figure 2a). In accordance with Snell’s law, faster-traveling longer wavelengths bend less than slower-traveling shorter wavelengths, creating multiple points of focus along the optical axis. This focus shift is called axial, or longitudinal, CA and can cause blurry or “waxy” vision.
Because index of refraction is wavelength-dependent, the value for yellow light (589 nm) is used by convention. Dispersion describes the difference between the refractive index for red (656 nm) and blue (486 nm) light. Materials with low dispersion minimize CA because all wavelengths slow (and thus bend) a similar amount (Figure 2b). A material’s dispersion relative to its refractive index is described by its Abbe number. The Abbe number is inversely related to dispersion and is generally inversely related to the index of refraction — higher index materials usually have lower Abbe numbers.
Figure 2. A ray-tracing diagram depiction of chromatic aberration (CA). A. Axial/longitudinal CA. The various wavelengths focus at different points along the optical axis. B. Reduced CA due to the use of a low dispersion/high Abbe number lens.
Opticians are familiar with the Abbe numbers of spectacle lens materials, which can vary from about 59 for crown glass and CR-39 to 30 for polycarbonate. Thus, while polycarbonate is lightweight, durable and shatter- resistant, its higher dispersion can result in poor optical quality. That is the reason why patients with higher corrections and astigmatism often cannot tolerate rimless polycarbonate glasses.
Modern IOLs have a similar range of Abbe numbers, from 58 to 37. The Abbe number of the human lens is 47. Theoretical and clinical studies suggest that minimizing CA can improve visual performance. The use of a posterior surface diffractive pattern, such as that on the Tecnis multifocal IOL (Abbott Medical Optics), can reduce CA, similar to the diffractive optics used in high-performance camera lenses.
Weighing material factors
Putting this all together, we can see that PMMA is a great mid-index material with a high Abbe number. It is easily manufactured and manipulated, and it remains clear in the eye. Unfortunately, PMMA’s rigidity precludes its use as a foldable lens. Low-index silicone IOLs are foldable with low reflectance and dispersion (high Abbe number) and thus provide good quality optics. However, low-index materials necessitate thicker IOLs, which is addressed by second-generation “high-index” silicones that reduce the necessary incision size at the expense of a lower Abbe number.
High-index acrylics were developed for thinner lens designs and a one-piece format, but their high reflectance and low Abbe number can cause problems in visual quality. AMO’s mid-index acrylic requires thicker IOLs or slightly smaller optics, but it has lower reflectance. Its low dispersion and high Abbe number provides optical quality comparable to that of PMMA.
Just like in spectacle and camera lenses, all IOL materials come with tradeoffs, some of which can be mitigated by IOL design. In evaluating IOLs, surgeons should consider the relative importance of the refractive index, reflectance and chromatic aberration. Other factors such as spherical aberration, manufacturing processes, glistenings, chromophores, edge design and haptic configuration can also help differentiate which IOL is best at maximizing our number of happy patients.