No toric? No problem: Small-aperture optics can mitigate astigmatism
A one-piece, hydrophobic, monofocal IOL uses the small-aperture concept to provide a good alternative to the toric IOLs currently available.
Cataract patients have a clear refractive outcome in mind, with an expectation of how their vision should function. Without question, correcting astigmatism is a part of that equation and nailing the refraction is also imperative for a premium outcome. Surgeons seek to offer the best technology available that will consistently deliver what patients demand, which means a solution that minimises the likelihood of missing the refractive target.
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More than half of cataract patients present with greater than 0.75 D of astigmatism,1 yet globally, toric IOL adoption remains below 5%.2 The reasons for this are found in the myriad of challenges with toric IOL implantation, including the limited power options with the toric IOLs currently available. Another barrier is that surgical planning for astigmatic management is complex and tools that facilitate accurate calculations may be cost-prohibitive for some surgeons.
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Surgical misalignment and post-implantation rotation make toric lenses less effective – each degree of misalignment reduces the effectiveness of astigmatic correction by 3.3%.3 One study observed that, at the 1-month postoperative mark, 19% of monofocal toric IOLs and 18% of multifocal toric IOLs had rotated by more than 10 degrees.4
Additionally, surgically induced astigmatism can reduce the effectiveness of the correction, with axis shifts inducing misalignment. In spite of surgeons’ best efforts, there remains some uncertainty in the attempted versus the achieved correction with toric IOLs.
An alternative to toric implants
Small-aperture or pinhole optics offer specific advantages in an IOL. The IC-8 IOL (AcuFocus) is a one-piece, hydrophobic, acrylic, posterior chamber monofocal IOL that applies the small-aperture principle to extend the depth of focus, which in turn provides an increased range of vision from far to near.5,6
This is achieved by the ability to eliminate unfocused peripheral light rays, allowing only the central rays to focus on the retina.7 At the same time, the small-aperture design provides extended depth of focus that makes the implant more tolerant of sphero-cylindrical residual refractive errors compared with multifocal implants.
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The IC-8 has been shown in multiple studies to tolerate up to 1.50 D of corneal astigmatism,8 whereas with multifocal lenses more than 0.75 D of residual sphere compromises visual acuity, spectacle independence and, ultimately, patient satisfaction.9 The small-aperture design is symmetrical, thereby eliminating the need for axis alignment.
This lens provides a simple solution for mitigating low to moderate astigmatism that is unaffected by surgically induced astigmatism. With a larger so-called landing zone, the IOL by nature is more forgiving than other technologies. Its design enables it to provide 3.00 D of extended depth of focus and tolerate up to 1.00 D deviation from the target manifest refraction spherical equivalent.10
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With declining reimbursement, surgeons are incentivised to offer premium options. This lens is straightforward to implant and therefore can be used with confidence by virtually any cataract surgeon, providing an entry point for those who may have less experience with premium technology.
The IOL has also been found to enhance vision at all distances, with improved visual acuities and a broad range of continuous functional vision. Implanted cataract patients enjoy high-quality, sharp vision and less concern with photic phenomena compared with multifocal and accommodating technologies.10 Importantly, patients have shown superior intermediate vision and binocular mesopic contrast sensitivity comparable to the other lenses.10
Challenges with current presbyopia-correcting implants
Bifocal, trifocal and extended depth of focus technologies seek to leverage biomechanical aspects of the ciliary body to change the shape of the lens. This approach to correcting presbyopia can come with an associated risk of glare and halo, and some patients find dysphotopsias distracting, which is why the dissatisfaction rate with these IOLs hovers around 5%.11
By virtue of its design, the IC-8 lens is not associated with significant photic phenomenon. This eliminates the need for surgeons to educate patients on the potential for postoperative visual aberrations, aiding confidence in the lens.
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Being on target with astigmatic management is critical to outcomes with multifocal IOLs. The difference between plano or no cylinder versus 0.75 D of cylinder is the difference between a patient who is reasonably happy and one who is reasonably unhappy. That is, they are unhappy and there is a reason for it!
For a busy cataract surgeon, it does not take very many unhappy patients to dampen his or her enthusiasm for recommending premium implants. It is crucial that surgeons have a high level of confidence in the options they present. When patients are paying a premium price for a premium outcome, not meeting expectations—in even just a handful of patients—is detrimental to the practice’s reputation.
Challenging corneas, more eligible patients
The one-piece lens is an effective one for patients with difficult topography, such as those with corneal scars, injury or previous radial keratotomy.12,13 The small aperture mitigates both higher and lower order aberrations in irregular corneas, making the lens a viable option for patients who previously had few presbyopia-correcting choices and filling a void in the current market.
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A broad range of patients who have less than 2.00 D of astigmatism and wish to be spectacle free can benefit. One of the beauties of this lens is that there are few disqualifiers in terms of patient selection. Surgeons can also be assured that, under dilation, the posterior segment can easily be visualised.14
Conclusion
Small-aperture optics offer a simple, elegant solution to the problem of presbyopia and astigmatism up to 1.75 D. Implantation of the IC-8 IOL can be unilateral, with or without a monofocal IOL, or bilateral.
The lens extends the depth of focus and reduces naturally occurring or induced corneal higher-order aberrations while at the same time correcting refractive error and providing tolerance to residual error. It can also provide extended depth of focus and therapeutic treatment of eyes with irregular astigmatism or other corneal abnormalities such as keratoconus or scarring.
References
1. Hill W. Keratometry Database. www.doctor-hill.com/physicians/docs/Astigmatism.pdf.
2. Market Scope 2016 IOL report. Available at: www.market-scope.com/pages/reports/iol.
3. Novis C. Astigmatism and toric intraocular lenses. Curr Opin Ophthalmol. 2000;11:47-50.
4. Garzon N, Poyales F, de Zárate BO, et al. Evaluation of rotation and visual outcomes after implantation of monofocal and multifocal toric intraocular lenses. J Refract Surg. 2015;31:90-97.
5. Grabner G, Ang RE, Vilupuru S. The small-aperture IC-8 intraocular lens: a new concept for added depth of focus in cataract patients. Am J Ophthalmol. 2015;160:1176-1184.
6. Dick HB, Piovella M, Vukich J, et al. Prospective multicenter trial of a small-aperture intraocular lens in cataract surgery. J Cataract Refract Surg. 2017;43:956-968.
7. Tucker J, Charman WN. The depth-of-focus of the human eye for Snellen letters. Am J Optom Physiol Opt. 1975;52:3-21.
8. RE Ang. Small-aperture intraocular lens tolerance to induced astigmatism. Clin Ophthalmol.2018;12:1659-1664.
9. Braga-Mele R, Chang D, Dewey S, et al. Multifocal intraocular lenses: relative indications and contraindications for implantation. J Cataract Refract Surg. 2014;40:313-322.
10. Ang RE. Visual performance of a small-aperture intraocular lens: first comparison of results after contralateral and bilateral implantation. J Refract Surg. 2020;36:12-19.
11. Hovanesian JA, Lane SS, Allen QB, Jones M. Patient-reported outcomes/satisfaction and spectacle independence with blended or bilateral multifocal intraocular lenses in cataract surgery. Clin Ophthalmol. 2019;13:2591-2598.
12. Shajari M, Mackert MJ, Langer J, et al. Visual performance of a small-aperture intraocular lens: first comparison of results after contralateral and bilateral implantation. J Cataract Refract Surg. 2020;46:188-192.
13. Agarwal S, Thornell EM. Cataract surgery with a small-aperture intraocular lens after previous corneal refractive surgery; visual outcomes and spectacle independence. J Cataract Refract Surg. 2018;44:1150-1154.
14. Srinivasan S, Khoo LW, Koshy Z, et al. Posterior segment visualisation in eyes with small aperture IOL. J Refract Surg. 2019;35:538-542.
John A. Vukich, MD
E: javukich@facstaff.wisc.edu
Dr John Vukich is surgical director at the Davis Duehr Dean Center for Refractive Surgery, Madison, Wisconsin, United States. He is consultant to AcuFocus and Johnson & Johnson Vision.
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