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Small aperture IOLs will be helpful in patients with significant corneal higher-order aberrations
These devices will improve overall visual quality with the added benefit of improving range of vision.
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The pinhole is well-known to the practitioner to determine the potential vision of a patient without having to perform a refraction. A 1.2-mm pinhole is typical because you can achieve 20/20 vision with ametropias up to 5 D, but above this value a smaller pinhole is needed. We see this in the experiment by Miller and Johnson in 1977 on the “quantification of the pinhole effect,” seen in Figure 1, in which they concluded that a “thin, dyed contact lens may have clinical value for large refractive errors such as aphakia.” Eleven years later, Zacharia and Miller made holes of varying sizes in clear lenses and found significant visual acuity improvement with defocus.
Further developments led to the Kamra corneal inlay (AcuFocus), which was FDA approved on April 17, 2015. The pinhole aperture in the Kamra inlay was chosen to be 1.6 mm so that the limit of vision would be about 20/12.5 (Figure 2). We can also see the optical aberrations become the limiting factor with larger pupils and that between a 2-mm and 3-mm pupil size is where most people who are emmetropic achieve the best vision — the record being 20/8.9, a little over twice what we consider normal good vision (20/20).
Source: Jack T. Holladay, MD, MSEE, FACS
In Figure 3, we seen that when the aperture is one-half the size of the full aperture, the spot size is the same for twice the defocus. This is known as depth of focus. In the human eye for the average equivalent focal length (EFL) of 22.8 mm for pupil sizes of 6 mm, 4 mm and 1.6 mm, the f-stop would be 3.8, 5.7 and 14.3 (EFL/pupil size). In Figure 4, we see the depth of focus and the number of targets that can be seen clearly with each respective pupil size. The average size of the pupil at age 62 years is 4.8 mm in mesopic conditions, so with a 1.6-mm pinhole we would expect to see a significant improvement in near vision with presbyopia, and the actual improvement is from 20/40 to 20/25 (three lines better) with no sacrifice at distance vision or contrast sensitivity and maintenance of stereoacuity.
The concept of small aperture is now moving from the cornea to the IOL plane with an additional benefit besides improving range of vision. A small aperture in an IOL has the ability to reduce higher-order aberrations dramatically, much more than the pinhole effect on sphere and cylinder. For example in Figure 5, spherical aberration [Z(4,0)] and coma [Z(3,±1)], the lowest of the higher-order aberrations, are reduced by the square of the ratio of the reduction in pupil size. Reducing the pupil (aperture) from 4.8 mm to 1.6 mm (three times) would reduce the effect of these aberrations by a factor of 9 (32). For higher aberrations such as trefoil and quatrefoil, the effect is exponentially greater.
A study by McCormick and colleagues in Ophthalmology in 2005 showed that the average higher-order root mean square (RMS) corneal wavefront error over a 6-mm zone is 0.38 ± 0.14 µm in the normal population with virgin corneas. They also found following refractive corneal surgery (LASIK and PRK) that symptomatic patients had 1.31 ± 0.58 µm mean RMS and asymptomatic had 0.58 ± 0.21 µm mean RMS. Patients above 0.75 µm of higher- order RMS corneal wavefront error over a 6-mm zone would be at greater risk for dissatisfaction with multifocal IOLs because they would have an additional reduction in the optical quality of their retinal image. As a result, the small aperture IOL may be their best solution.
The AcuFocus IC-8 IOL is currently in FDA trials and is available in Europe with a CE mark. The AcuFocus IC-8 small aperture IOL incorporates a non-diffractive 3.23-mm outer diameter opaque annular mask with a 1.36-mm central aperture embedded within a 6-mm one-piece hydrophobic acrylic lens (Figure 6).
A second pinhole IOL recently reported in an article by Trindade and colleagues in the Journal of Cataract and Refractive Surgery showed the results of 21 patients who received a 1.3-mm aperture secondary piggyback IOL for irregular astigmatism from keratoconus, post-RK, post-PK and traumatic corneal laceration (Figure 7). The median corrected distance visual acuity improved from 20/200 (range 20/800 to 20/60) preoperatively to 20/50 (range 20/200 to 20/20) in the first month postoperatively and remained stable over the following months (a six-line improvement).
The use of small aperture IOLs in patients with significant corneal higher-order aberrations will not only improve their overall visual quality but also have the added benefit of improving their range of vision. These IOLs should be available in the U.S. in the near future.
- References:
- McCormick GJ, et al. Ophthalmology. 2005;doi:10.1016/j.ophtha.2005.04.022.
- Miller D, et al. Surv Ophthalmol. 1977;doi:10.1016/0039-6257(77)90114-X.
- Trindade CC, et al. J Cataract Refract Surg. 2017;doi:10.1016/j.jcrs.2017.09.014.
- Zacharia P, et al. Arch Ophthalmol. 1988;doi:10.1001/archopht.1988.01060130557035.
- For more information:
- Jack T. Holladay, MD, MSEE, FACS, is a clinical professor of ophthalmology at Baylor College of Medicine and the OSN Optics Section Editor. If you have any clinical optics questions, please send them to holladay@docholladay.com. Previous articles in the Clinical Optics 101 series may be found at http://www.healio.com/ClinicalOptics101.
Disclosure: Holladay reports he is a consultant to Abbott Medical Optics, AcuFocus, Alcon Laboratories, ArcScan, Calhoun Vision, Carl Zeiss, Elenza, M&S Technologies, Oculus and Visiometrics.