Individuals often have questions about their suitability for certain implants and how the various mechanisms of action might work.
The well-established physiology of small-aperture optics has been understood by astronomers and photographers for centuries. When applied to a camera, the lens aperture, or F-stop, adjusts the amount of light that reaches the film or image sensor.
As the aperture decreases, image sharpness improves over larger and larger distances. For example, when taking a photo, everything close in the field of view and everything in between will be sharpened. However, if you have a fixed focus or wider aperture, the lens is set in focus but everything in front of and behind the object is out of focus (figure 1).
By the 1940s, eye care was beginning to find use for small apertures, but only now has this powerful principle been incorporated into presbyopia-correcting intraocular lenses (IOLs). FDA approval of the IC-8 Apthera IOL (AcuFocus) represents the first small-bore IOL technology to be approved for use in the United States.
Features of Small Aperture IOLs
The Apthera lens is an aspheric monofocal IOL with an embedded small aperture called the FilterRing component (figure 2). This unique design filters out peripheral defocused and aberrated light and allows centrally focused light to reach the retina undisturbed, providing greater depth of focus.
Apthera IOLs are intended for contralateral implantation, with monofocal or monofocal toric IOLs implanted in the fellow eye. The eye receiving the Apthera lens has a target of –0.75 diopters (D), making the eye slightly nearsighted, while the other eye has an emmetropic target. This miniature single vision target further enhances the patient’s field of view beyond the baseline range provided by the small aperture.
In the U.S. clinical trial, patients treated with Apthera IOLs (n=343) achieved best-corrected continuous visual acuity of nearly 3.00 D (image 3). Furthermore, the implant demonstrated reliable vision results in eyes with corneal astigmatism up to 1.50 D without relying on toric correction.
The study also showed that patients with Apthera IOLs had lower levels of visual symptoms and comparable photopic and mesopic contrast sensitivity compared to patients treated with bilateral monofocal IOLs (Figure 4).1-6
patient selection
The new lenses are ideal for cataract patients who wish to reduce their reliance on reading glasses, those who have had success with monocular vision using contact lenses or corneal refraction, and those who hesitate to choose multifocal lenses due to concerns about loss of contrast sensitivity or reflections.
Other candidates include patients with low levels of astigmatism but who want enhanced glasses independence, and patients who may be dissatisfied with the range of vision after receiving a monofocal implant in their first eye. All of these types of cataract patients may be excluded from consideration by many of today’s other presbyopia correction techniques.
Optometrists will note that patients with smaller apertures, such as older adults with naturally narrow pupils, are less sensitive to diopter choices with less diopter variation.
The depth of focus stretches and flattens the depth-of-field curve, giving them 20/20 or 20/25 vision over multiple 0.25-D steps. People with large pupils or large apertures will immediately notice a 0.25-D difference.
Patients implanted with Apthera IOLs maintained normal levels of stereopsis better than monovision IOLs. Furthermore, monovision with monofocal implants does not provide the same range of visual field as phakic monovision because the natural lens still has some residual accommodation.
The small aperture design of the Apthera IOL overcomes this limitation, providing patients with the same sustained focus as phakic monovision. Therefore, the Apthera IOL is an ideal choice for patients who were previously satisfied with phakic monocular vision.
Considerations for complex corneas
Patients who have had previous refractive surgery or keratoconus may develop aberrations in the cornea that cause visual disturbances such as night halos and other distortions. This is an important consideration when selecting an IOL for cataract surgery.
In a distorted cornea, light rays passing through the natural pupil do not necessarily focus together on the retina. When a small aperture is added, it reduces or reduces the effects of peripheral aberrations, blocking stray light from damaging central tissue as it falls on the retina. This reduction in the effects of peripheral aberrations is a unique feature of the small aperture and could serve as a meaningful solution for patients with corneal aberrations.
other factors
For some, there may be hesitation about small aperture technology. An optometrist may think that when they look through the small pinhole in the trial lens set, things look sharp, but everything is a little dim or darker. But the small aperture in the Apthera IOL is equivalent to a 1.6 mm pupil, which is larger than your pinhole occluder.
The aperture of the IOL is also inside the eye, while the occluder is used in front of the eye. Suppose a patient has an injury that causes them to have a small pupil and a large pupil. They adapt. Perhaps at first the smaller aperture makes the vision appear dim, but over the course of a few days or weeks, adaptation occurs and this effect diminishes.
With the Apthera IOL, a monofocal is implanted in the other eye, so initially the patient may notice a difference in vision between the two eyes. However, this improves over time, and studies have shown that the functional impact is negligible and contrast performance is maintained.1,5
One might ask whether the small aperture affects field of view or peripheral vision. In US clinical trials and global studies, visual field tests showed no scotoma and only a slight attenuation of pattern standard deviation, with results comparable to monofocal controls. Due to the optical properties of the Apthera IOL and its location in the eye, no effects on peripheral vision have been reported.
As with any cataract surgery, optimizing the ocular surface is important to achieve accurate and reproducible measurements. This applies to monofocal or multifocal implants.
For the small aperture technique, we want to make sure the central cornea is normal, unscarred, and has a good tear film. Although retinal visualization and therapy using the Apthera IOL is possible and has been successfully performed with only minor technical changes, it is not suitable for patients with retinal disease.7
in conclusion
As primary care vision providers, optometrists play a critical role in identifying lens changes and treating cataract patients before and after surgery. Therefore, it is critical that we understand this newly approved IOL design, the first in the US to utilize small aperture optics as its mechanism of action.
Editor’s note: On January 18, 2023, Bausch & Lomb announced that an affiliate has acquired AcuFocus. The acquisition has not been announced at the time of writing.
refer to
-
Blecher M. Visual outcomes at one year after implantation of a small-bore IOL in one eye. Paper presented at: AAO 2022; September 30-October 3, 2022; Chicago, IL. https://eyetube.net/meeting-coverage/aao-2022/one-year-visual-outcomes-following-monocular-implantation-with-a-small-aperture-iol. Accessed on December 15, 2022
-
Grabner G, Ang RE, Vilupuru S. Small aperture IC-8 intraocular lens: a new concept for increasing depth of focus in cataract patients. American Journal of Ophthalmology. 2015;160(6):1176-1184.e1.
doi:10.1016/j.ajo.2015.08.017 -
Dick HB, Pivilla M, Vukich J, Vilupuru S, Lin L; Clinical Investigators. Prospective multicentre trial of small-bore intraocular lenses in cataract surgery. Journal of Cataract and Refractive Surgery. 2017;43(7):956-968. doi:10.1016/j.jcrs.2017.04.038
-
Ang re. Resistance of small aperture intraocular lenses to induced astigmatism. Clinical Ophthalmology. 2018;12:1659-1664. doi:10.2147/OPTH.S172557
-
Tucker J, Charmain WN. The depth of focus of the human eye on the Snellen letters. Am J Optom Physiol Opt. 1975;52(1):3-21. Ministry of the Interior: 10.1097/00006324-197501000-00002
-
Dick HB. A small aperture strategy for correcting presbyopia. Curr Opin Ophthalmol2019;30(4):236-242. doi:10.1097/ICU.0000000000000576
-
Srinivasan S, Khoo LW, Koshy Z. Visualization of the posterior segment of the eye with a small aperture intraocular lens. Journal of Refractive Surgery2019;35(8):538-542. doi: 10.3928/1081597X-20190710-01
