Corneal inlays have emerged as a promising surgical option for the treatment of presbyopia that could significantly improve quality of life for many people. A safe and effective solution for correcting presbyopia, a major advantage of corneal inlays is that they are relatively easy to implant, can be combined with other refractive procedures to treat ametropia and presbyopia, and are removable.
Although work on corneal inlays started more than 50 years ago, the impermeable nature of early designs resulted in complications.1,2 As a result, high water content hydrogel implants were developed due to their permeable and biocompatible characteristics.3 With better understanding of corneal physiology and optics, multiple corneal inlays with different mechanisms of action have been developed and are now in various phases of the regulatory process.
Three types of implants
There are currently three basic types of corneal inlay implants, each with a unique design and mechanism of action. None of them are FDA-approved for sale in the United States.
Small aperture inlay
The Kamra inlay (AcuFocus, Inc., Irvine, Calif.) received its name because it functions like the f-stop in a camera. The inlay utilizes a small aperture effect to increase depth of field by selecting for central light rays and minimizing refraction. It is a 5-micron thick, 3.8-mm wide opaque inlay made of polyvinylidene difluoride (PVDF). It has multiple laser-etched holes to allow passage of nutrients, fluids and oxygen. (See Figures 1 and 2.)
Figure 1: The size of the Kamra inlay compared to that of a contact lens.
Figure 2: A Kamra inlay implanted in a cornea.
Corneal reshaping inlay
Corneal reshaping inlays improve near vision by increasing the curvature of the central part of the anterior surface of the cornea. The Raindrop Near Vision Inlay, formerly known as the PresbyLens or Vue+ lens (ReVision Optics, Lake Forest, Calif.), is a transparent hydrogel implant. This inlay is 32 microns thick in the center and 10 microns thick in the periphery. (See Figure 3.) Near focus occurs when light rays pass through the central curved portion of cornea, while distance vision is preserved as paracentral light rays pass through the peripheral, flatter part.
Figure 3: Raindrop Near Vision Inlay.
Refractive corneal inlay
Refractive corneal inlays change the refractive index of the cornea. The Presbia Flexivue Microlens (Presbia, Irvine, Calif.) is a 3-mm wide, 15- to 20-micron thick transparent hydrogel implant. (See Figure 4.) It is placed in a stromal pocket at a depth of approximately three-fifths of the corneal thickness. The central plano zone allows light rays from distance to pass unobstructed and focus on the retina. The surrounding rings of progressively increasing powers from +1.50 to +3.50 D focus light rays from near and intermediate objects.
Figure 4: The Presbia Flexivue Microlens.
Pocket vs. flap placement
Inlays may be placed in a stromal pocket or under a lamellar flap. The pocket technique provides a number of potential advantages. First, the majority of peripheral corneal nerves are preserved, which helps to maintain corneal sensitivity and provide potentially quicker visual recovery. Pocket procedures also preserve the peripheral biomechanical properties of the cornea, which like the anterior lamellae, provide a majority of the biomechanical corneal stability.
On the other hand, creation of a lamellar flap can be an attractive alternative as it offers access to a stromal bed for excimer ablation, allowing for full control of the refractive target and the ability to treat ametropia.4 In addition, a lamellar flap allows easy access to the inlay in the event repositioning or removal is warranted.
Emerging techniques with dual interfaces are becoming more popular, providing the best of both worlds. Advances in image-guided surgery will aid dual-interface procedures. Cataract surgery can be performed without difficulty, as inlays minimally obstruct the surgeon’s view.11
An increasing body of evidence supporting the safety and efficacy of presbyopic corneal inlays is building. The majority of published literature demonstrates a marked improvement in near and intermediate vision, with minimal disruption to distance vision.5,6,7,8,9,10 Furthermore, inlays are removable, can be repositioned and have added benefits when compared to other corneal presbyopic correction options.
The ability to combine presbyopia-correcting inlays with other refractive procedures makes this technology an attractive option for the surgical management of presbyopia. Different inlay designs with varying mechanisms of action are in development. Corneal inlays are becoming a viable surgical option for the rapidly developing area of the surgical correction of presbyopia.
- Barraquer JI. Modification of refraction by means of intracorneal inclusions. Int Ophthalmol Clin. 1966;6(1):53-78.
- Lane SL, Lindstrom RL, Cameron JD, et al. Polysulfone corneal lenses. J Cataract Refract Surg. 1986;12(1):50-60.
- McDonald MB,McCarey BE,Storie B,et al. Assessment of the long-term corneal response to hydrogel intrastromal lenses implanted in monkey eyes for up to five years. J Cataract Refract Surg. 1993;19(3):213-222.
- Tomita M, Kanamori T, Waring GO 4th, et al. Simultaneous corneal inlay implantation and laser in situ keratomileusis for presbyopia in patients with hyperopia, myopia or emmetropia: Six-month results. J Cataract Refract Surg. 2012;38(3):495-506.
- Waring GO 4th, Klyce SD. Corneal inlays for the treatment of presbyopia. Int Ophthalmol Clin. 2011;51(2):51-62.
- Yilmaz OF, Alagoz N, Pekel G, et al. Intracorneal inlay to correct presbyopia: Long-term results. J Cataract Refract Surg. 2011;37(7):1275-1281.
- Pallikaris IO, Slade S, Vukich J, Bouzoukis D. Corneal inlays for presbyopia. Paper presented at: XXIX Congress of the ESCRS; September 19, 2011; Vienna, Austria.
- Seyeddain O, Hohensinn M, Riha W, et al. Small-aperture corneal inlay for the correction of presbyopia: 3-year follow-up. J Cataract Refract Surg. 2012;38(1):35-45.
- Pallikaris IG. Intracorneal refractive inlays for the treatment of presbyopia: visual outcomes and safety. Paper presented at: American Academy of Ophthalmology annual meeting; Oct 23, 2009; San Francisco, Calif.
- Waring GO 4th. Correction of presbyopia with a small aperture corneal inlay. J Refract Surg. 2011;Nov;27(11):842-845.
- Tan TE, Mehta JS. Cataract surgery following KAMRA presbyopic implant. Clin Ophthalmol. 2013;7:1899-1903.
Financial Disclosure: Dr. Waring IV is on the Medical Advisory Board for AcuFocus, Inc. Dr. Sane has no financial interest in the material presented.
The authors acknowledge the support of an unrestricted grant from Research to Prevent Blindness to the Department of Ophthalmology at the Medical University of South Carolina in Charleston, where Dr. Waring is employed.