Alloplastic Corneal Inlays
Alloplastic inlays offer several potential advantages over homoplastic inlays, such as the ability to be accurately mass-produced in a wide range of sizes and powers. Synthetic material may have optical properties that are superior to those of tissue lenses.
For insertion of the inlay, a laser in situ keratomileusis (LASIK)–type flap or a stromal pocket dissection can be performed; such procedures are technically easier than a complete lamellar keratectomy. Experiments performed in the early 1980s resulted in corneal opacities, nonhealing epithelial erosions, and diurnal fluctuation in vision because fluid and nutrients were blocked from reaching the anterior cornea. Thus, to allow for the transfer of fluid and nutrients to the anterior cornea, either permeable materials were used or microperforations were incorporated into the inlays. Because of work performed by Knowles and others, most subsequent studies used water-permeable hydrogel implants. Hydrogel lenses have an index of refraction similar to that of the corneal stroma, so these lenses have little intrinsic optical power when implanted. To be effective, hydrogel inlays must change the curvature of the anterior cornea. Other mechanisms of action include inlays with refractive power and small aperture inlays. Of the 3 presbyopic inlays that have been developed in the United States, only 1 is currently available.
In 2015, the US Food and Drug Administration (FDA) approved the KAMRA corneal inlay (AcuFocus Inc, Irvine, CA). This small-aperture inlay is indicated for the improvement of near vision in presbyopic patients who require near correction. This device is an ultrathin (5-μm), biocompatible polymer that is microperforated to allow improved nutrient flow. The 3.8-mm-diameter inlay has a central aperture of 1.6 mm and is generally implanted in the nondominant eye (Fig 4-1). The surgeon places the inlay into an intrastromal pocket created by femtosecond laser, using a spot and line separation of 6 × 6 μm or less. The inlay should be placed at a depth equal to or greater than 200 μm, centered on the patient-fixated, coaxially sighted corneal light reflex. Although the inlay has no refractive power, the central aperture functions as a pinhole to increase depth of focus and improve near vision without changing distance vision. (See Corneal Inlays in Chapter 9.)
In the FDA study, an average gain of 3 lines of uncorrected near vision in the implanted eye was observed at 12 months. With a 6 × 6 spot and line separation in the FDA study, 95% of eyes achieved the primary efficacy endpoint of 20/40 or better uncorrected near acuity, and a primary safety endpoint of 0.0% eyes having greater than or equal to 2 lines of persistent loss of BCVA. Rare but reported complications include refractive instability, decentration, and haze. In the FDA study with a 6 × 6 spot and line separation, 2.9% of inlays were removed, and all eyes with removals returned to their preoperative BCVA.
Ismail MM. Correction of hyperopia with intracorneal implants. J Cataract Refract Surg. 2002; 28(3):527–530.
Knowles WF. Effect of intralamellar plastic membranes on corneal physiology. Am J Ophthalmol. 1961;51:1146–1156.
Waring GO IV. Correction of presbyopia with a small aperture corneal inlay. J Refract Surg. 2011;27(11):842–845.
Whitman J, Dougherty PJ, Parkhurst GD, et al. Treatment of presbyopia in emmetropes using a shape-changing corneal inlay: 1-year clinical outcomes. Ophthalmology. 2016;123(3): 466–475.
Figure 4-1 Small-aperture corneal inlay for the surgical treatment of presbyopia. A, The inlay is surgically implanted into a stromal pocket in the nondominant eye. B, Slit-lamp photograph of a small-aperture inlay.
(Courtesy of George O. Waring IV, MD.)
Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.