Small Aperture Technology: A Simple Solution for Complex Corneas

Patients who present with atypical corneal irregularities, diverse ocular histories, and corneal shapes are considered to have ‘complex corneas’. These irregular corneas can present challenges with regard to a variety of aspects of cataract surgery, particularly the choice of IOL and achieving good post-operative visual results.

The day after surgery, the patient was 6/9 and practically in tears

Causes of irregular corneas can be iatrogenic or naturally occurring. Examples of the former include patients with surgically-induced corneal complexity, such as post-laser insitu keratomileusis, photorefractive keratectomy, radial keratotomy, astigmatic keratotomy, cataract surgery, pterygium surgery, corneal transplants, and trabeculectomy.

In the latter group are patients with genetically-derived corneal aberrations and those with disease or trauma-induced corneal shape irregularities. Examples of diseases include keratoconus, epithelial basement membrane dystrophy, Terrien’s marginal degeneration, Salzmann nodular degeneration, and pellucid marginal degeneration. Full or partial penetrating eye injuries can cause an irregular cornea, as well as subsequent surgical treatment that may result in scarring and oedema. Additionally, corneal infections, of any kind (viral, fungal or bacterial), can lead to scarring and irregularity of the corneal surface.

HOW COMMON ARE THESE CORNEAS?

It is estimated that around 12% to 14% of patients presenting for cataract surgery present with a complex cornea.¹ A recent panel, convened to create consensus on corneal irregularity, reported that approximately 24% of the members’ preoperative cataract patients have irregular corneas.² A review of 200 eyes in 400 patients found that approximately 25% of patients scheduled for cataract surgery had abnormal corneal topography without a history of previous corneal surgery.³ Additionally, the review found that 10% had irregular astigmatism or dry eye disease; 9% had borderline pellucid marginal degeneration, forme fruste keratoconus, or superior steepening; and the remaining 6% had topographic findings consistent with forme fruste keratoconus, pellucid marginal corneal degeneration, or keratoconus.³

Based on general population studies from the early 1980s, keratoconus was estimated to occur in at least one out of every 2,000 patients, according to Keratoconus Australia. With improved screening techniques, however, it is now believed that figure may be as high as one in 750 patients worldwide.⁴ A recent Australian study among 20-year-olds showed a prevalence of one in 84.⁵ Researchers note that this higher prevalence is likely due to better diagnostic technology rather than a rising incidence of keratoconus.

IN PRACTICE: COMMON CLINICAL PRESENTATIONS

The majority of irregular corneas I see in my practice are in patients with corneal transplants due to keratoconus. Keratoconic patients have inferior coning and more irregular astigmatism in one axis. I also see post-RK patients who have very irregular corneas; in fact, some of the cuts can bulge more than others and in different axes, resulting in odd topography. Other patients have had corneal issues such as a severe ulcer or large pterygium coming close to the central cornea or pupillary edge.

Patients with such corneal irregularities cannot be managed with standard toric lenses: they have higher order – and sometimes even lower order – aberrations that compromise their vision. Most anterior segment surgeons are rightfully squeamish about using multifocal IOLs in patients with serious corneal irregularities. The only option then is a single-focus lens, yet many of these individuals are quite keen to have some glasses-free near visual acuity.

For these complex cornea patients, I favour the IC-8 IOL (AcuFocus) which provides good distance vision, as well as depth of perception and reading vision.

UNIQUE PINHOLE TECHNOLOGY

An aspheric monofocal lens with an embedded mask, the IC-8 IOL applies the small aperture principle to extend the depth of focus, providing an increased range of vision from far to near.^6,7 The wavefront-filtering design eliminates unfocused peripheral light rays so that only the central rays focus on the retina.⁸ The small aperture design provides an extended depth of focus that makes the implant more tolerant of sphero-cylindrical residual refractive errors compared with multifocal implants. The IC-8 IOL can tolerate up to 1.50D of corneal astigmatism.⁹

Studies have shown the IC-8 IOL can provide up to 3.00D of extended depth of focus and tolerate up to 1.00D deviation from the target manifest refraction spherical equivalent.10 In contrast, with multifocal technology anything over 0.75D of residual sphere will compromise visual acuity and spectacle independence, leading to unhappy patients.¹¹ The IC-8 IOL has also been found to enhance vision at all distances, with improved visual acuities and a broad range of continuous functional vision. The standard approach to implanting the IC-8 IOL is, in general, to put it in the eye with worse visual acuity i.e., higher astigmatism or more severe irregularity in the cornea.

As an investigator for many clinical trials, I like to examine post-operative contrast sensitivity when I use new IOLs. To satisfy my curiosity, I have studied contrast sensitivity across a variety of spatial frequencies in IC-8 IOL patients and found it to be on par with other multifocal IOLs. This has made me even more confident in implanting this lens for suitable patients.

It is, however, very important to counsel patients with regards to keeping expectations moderate, and because their corneas are irregular, it’s best not to overpromise and deliver more. They accept that whatever the visual result, it will be an improvement. Ultimately when they do achieve superior postoperative vision, they are absolutely thrilled.

THE CASE OF A CORNEAL TRANSPLANT WITH INFECTION

Tom,* is a man in his late 40s with poor vision associated with a previous corneal transplant due to advanced progressive keratoconus. On presentation, he gave a history of infected corneal graft sutures that had recently been managed (three months ago) at another facility. Thereafter he had noted his vision had deteriorated significantly. On examination, there was a significant amount of distortion due to scarring around the area of infection. He had subtle cortical cataract and approximately 6.00D of astigmatism, but this astigmatism was irregular; the infected area was flattish with faceting, whereas the opposite end was significantly protruded. He was offered a hard contact lens trial.

With a contact lens (RGP) trial, Tom’s vision was better but still, we could not achieve visual acuity any better than 6/24, although he was showing better vision with PAM (Potential acuity meter). I explained to him about IC-8 IOL and told him to “expect the minimum, and hope for the best”. He underwent cataract surgery with IC-8 IOL implantation. The day after surgery, Tom’s visual acuity was 6/9. He was practically in tears as not only his distance vision was so much better, but he could also read a book held at a normal distance! Tom had given up on being able to see any better with that eye, let alone being able to read a book.

I can never forget Tom’s happiness with his vision improvement and the quality of life he regained due to the IC-8 lens. This was an incredible experience for me.

I have subsequently implanted more IC-8 IOLs for many other patients. Overall, I have had terrific results with the IC-8 IOL. I intend to implant many more and, with proper patient selection, I am confident I can achieve superior outcomes.

*Patient name changed for anonymity. 

Feature image: Figures 1 and 2. IC-8 implanted in patient with previous corneal transplant due to advanced keratoconus.

Dr Uday Bhatt, MBBS, DTMH, DO, MSc(EBP), FRCSEd, FRCOphth, FRANZCO, specialises in cataract surgery and is a corneal specialist at Vision Eye Institute, Melbourne, Australia. His areas of special interest include collagen cross-linking for keratoconus and the latest techniques in corneal transplantation and grafting. Dr Bhatt may be reached at [email protected]. 

References 

1. Data on file Acufocus.
2. 2020 Global Consensus on Corneal Irregularity: ExpertPanel Offers Recommendations for Defining, Diagnosing,and Treating Irregular Corneas. Supplement to Cataract & Refractive Surgery Today / Europe. November/ December 2020. crstoday.com/wp-content/uploads/ sites/4/2021/01/0121CRST-CRSTES_Evolve-2032-Corneal- Irregularity-Consensus-Paper.pdf. Accessed July 29, 2021. 
3. Frank B, Trattler W, Mccabe S, et al. The incidence oftopographic abnormalities in patients scheduled forcataract surgery (abstract). Invest Ophthalmol Vis Sci. 2014; 55:2477. 
4. Hashemi H, Heydarian S, Hooshmand E, et al. Theprevalence and risk factors for keratoconus: a systematicreview and meta-analysis. Cornea. 2020;39(2):263-270. doi: 10.1097/ICO.0000000000002150. 
5. Chan E, Chong EW, Lingham G, et al. Prevalence ofkeratoconus based on Scheimpflug imaging: The RaineStudy. Ophthalmology. 2021;128(4):515-521. doi: 10.1016/j.ophtha.2020.08.020. 
6. Grabner G, Ang RE, Vilupuru S. The small-aperture IC-8intraocular lens: a new concept for added depth of focusin cataract patients. Am J Ophthalmol. 2015;160(6):1176- 1184. doi:10.1016/j. ajo.2015.08.017. 
7. Dick HB, Piovella M, Vukich J, et al. Prospectivemulticenter trial of a small-aperture intraocular lens incataract surgery. J Cataract Refract Surg. 2017;43(7):956- 968. doi:10.1016/j.jcrs.2017.04.038. 
8. Tucker J, Charman WN. The depth-of-focus of thehuman eye for Snellen letters. Am J Optom Physiol Opt.1975;52(1):3-21. doi:10.1097/00006324-197501000- 00002. 
9. RE Ang. Small-aperture intraocular lens tolerance toinduced astigmatism. Clin Ophthalmol 2018;12:1659-1664.
10. Ang RE. Visual performance of a small-apertureintraocular lens: first comparison of results aftercontralateral and bilateral implantation. J Refract Surg. 2020;36(1):12-19. doi: 10.3928/1081597X-20191114-01. 
11. Braga-Mele R, Chang D, Dewey S, et al.Multifocal intraocular lenses: relative indications andcontraindications for implantation. J Cataract Refract Surg. 2014;40(2):313–322