Correct presbyopia with corneal inlays

Distinguishing the developmental pipeline of this emerging surgical treatment.

By George O. Waring IV, MD, FACS

Presbyopia, or the loss of accommodation characteristic of an aging eye, a ubiquitous disorder that affected nearly 140 million presbyopes in the United States in 2013, will reach an estimated 2 billion presbyopes worldwide by 2020, according to Marketscope research.

As a result, growing interest in the surgical management of presbyopia (lens and corneal-based refractive surgery) may lead to consideration of its own emerging subspecialty. The number of available presbyopia surgical options rivals other surgical subspecialties that require fellowship training, such as glaucoma, which stands to reason that a fellowship in surgical management of presbyopia could be developed.

While corneal inlays have gained traction recently, the technology is nothing new — keratophakia was described in late 1940s by Dr. José Barraquer. In the last 65 years, corneal inlays and the surgical treatment for presbyopia underwent a significant evolution, such as material, design, surgical technique and name changes. As a result, much confusion exists over nomenclature in distinguishing the different types of inlays.

Therefore, we have proposed a characterization schema based on the mechanism of action, which I describe here.

INLAYS IN BRIEF

An overview

In general, all inlays are implanted in one eye only, classically the non-dominant eye.

Inlays have the benefit of being resistant to the progressive nature of presbyopia. Traditional corneal-based procedures, such as blended vision or monovision, use a fixed focus point, and the lens naturally ages over time with loss of accommodation. Additionally, patients still have mid and distance vision, so they largely retain stereopsis, while traditional monovision reduces stereopsis.

The preoperative map for centration guidance of the AcuTarget HD.

PRIMARY MECHANISMS OF ACTION

Space-occupying lenticule

The Raindrop (ReVision Optics, Inc.) inlay, formerly known as the Vue+, is a permeable hydrogel inlay with the same index of refraction as the cornea. The lenticule is designed to be implanted at 120 microns either under a LASIK flap or into a corneal pocket.

This inlay elevates the central cornea, creating a hyperprolate, or “pro-focal” central zone thereby increasing depth of focus by induction of spherical aberration. Accommodative miosis creates a pseudo-accommodative state, utilizing the steepened, hyperprolate central cornea. Paracentral light rays are bent less, remaining focused on the retina, resulting in minimal distance acuity change.

Refractive annular add lenticule

The Flexivue Microlens (Presbia) is one of two inlays in the developmental pipeline with refractive power. This inlay has a higher index of refraction than the cornea and is designed to be implanted in a stromal pocket in the mid stroma. The Microlens has separate distance and near focal points, utilizing a central zone for distance, which is free of refractive power, and a peripheral zone with +1.25D to +3.00D add power for reading, similar to a multifocal contact lens.

The ICOLENS (Neoptics) is the most recent corneal inlay to be developed, utilizing a multifocal principle.

Small aperture inlay

The Kamra inlay (Acufocus) works on the small aperture principle, similar to the f-stop of a camera (hence the name). Non-bent light rays to pass through the central small aperture of 1.6 mm to broaden depth of focus while blocking bent light rays.

The Kamra inlay is the furthest down the regulatory pipeline and has the most data available.

Surgical techniques

Each corneal inlay requires a different strategy or technique based on the intended mechanism of action. As mentioned, the pro-focal inlay, which change the curvature of the cornea, should be implanted more superficially than a refractive or small aperture inlay, where one would want to avoid surface curvature changes.

Techniques for corneal inlay procedure have also evolved, including placement into a femtosecond enabled pocket, under a LASIK flap or a combination approach. A pocket procedure has multiple benefits, including preservation of the peripheral corneal nerve plexi and less biomechanical impact on the cornea. In addition, patients seem to have quicker visual recovery with the pocket approach, perhaps due to less dry eye. A dual interface procedure takes advantage of the thin flap LASIK to treat congenital near-sightedness or far-sightedness, then utilizes the benefit of a pocket at least 100 microns below the flap where the surgeon implants the inlay.

Corneal inlays are quite flexible as well, as they can be easily combined with other refractive procedures, such as staged or laser refractive treatments, prior laser refractive treatments or prior IOL implantation. This makes corneal inlays an attractive treatment modality for patients with prior surgery, such as LASIK for distance vision, who develop presbyopia. In this example, the surgeon would create a femtosecond laser-enabled pocket with sufficient space under the prior LASIK flap. Corneal inlays are also a great solution for patients who have undergone cataract surgery for distance vision and no longer want to rely on readers.

OCT-based image-guided surgery for corneal refractive surgery is emerging, similar to the usage of OCT-guided femtosecond lasers for lens surgery. However, femtosecond lasers behave differently in the anterior stroma compared to the posterior stroma because the anterior tissue is more tightly woven and compact relative to the posterior cornea. Therefore, creating mid-stromal lamellar plane in the mid stroma requires a tighter spot in line separation to have a more regular interface, leading to a more predictable refractive outcome.

PRACTICE ADOPTION

Determining candidates

In general, an ideal candidate for a presbyopic corneal inlay would have a healthy eye, normal topography and minimal to no dryness. Furthermore, the crystalline lens should be clear or with minimal opacity evidenced by a low ocular scatter index and a decreased amplitude of accommodation (Stage I dysfunctional lens syndrome). In addition, pseudophakes or post-LASIK patients may be great candidates as well.

Corneal inlay surgery fits naturally into any ophthalmology practice in terms of the corneal refractive surgical procedures that many general ophthalmologists already perform. The primary duty of ophthalmologists considering referrals parallels the normal decision-making process regarding which patients are candidates for corneal refractive surgery.

Currently available corneal inlays for presbyopia separated by mechanism of action.

CONCLUSION

The future of presbyopia management?

Although corneal inlays are still in the developmental pipeline, these technologies represent an important technology for the surgical management of presbyopia.

With the growing interest in surgical correction of presbyopia, inlays may become a mainstay of treatment for this ubiquitous disorder. OM

About the Author
	George O. Waring IV, MD, FACS, is the medical director at Magill Vision Center, an assistant professor of Ophthalmology and the director of Refractive Surgery at the Medical University of South Carolina (MUSC) Storm Eye Institute. His e-mail is georgewaring@me.com. Financial disclosure: Dr. Waring is a consultant for AcuFocus.