The next wave in refractive surgery in the United States will be the correction of presbyopia with corneal inlays. Inlays are an additive, removable technology. Implanted monocularly, they offer better binocularity and distance acuity than monovision1,2 and may carry less risk than intraocular surgery.3 For these reasons, corneal inlays are an exciting option for presbyopes who do not yet have lens changes.
Important material and design innovations over the past decade have made the implantation of an optical device in the intrastromal space a viable option. There are now at least three different inlays that have either completed or are undergoing US clinical trials, two of which are designed for implantation in a deep stromal pocket. (A third type is designed to reshape the cornea and is placed under a flap.) Globally, more than 20,000 inlays have been implanted commercially. Just as important to the success of these tiny devices has been the evolution of surgical technique made possible by advances in femtosecond laser technology.
The most important change to femtosecond lasers has been their speed. The first femtosecond laser on the market operated at just 15 kHz. Since then, the speed of corneal femtosecond lasers has increased tenfold, enabling smoother beds and a nearly infinite variety of cuts. For example, in addition to LASIK flaps, the iFS laser (Abbott Medical Optics) has been used to create advanced arcuate incisions, to customize corneal trephination patterns that have dramatically improved visual acuity after penetrating keratoplasty and made big-bubble deep anterior lamellar keratoplasty easier to perform, and form a variety of corneal channels and pockets.
Early innovators used a mask or shield to block some of the laser pulses to create pockets for corneal inlays. Users of the iFS platform can now take advantage of specialized pocket software that was recently cleared by the FDA for marketing in the United States; this software was already in use internationally. The pocket software provides a level of control and precision that further enhances the surgeon’s ability to create the pocket exactly how and where desired. For example, surgeons will be able to customize many parameters, including the channel’s width and depth, sidecut radius and angles, offset from the center of the surgical field, and raster parameters (Table). The pocket can be designed, viewed, and easily adjusted on a touchscreen, the same as for a LASIK flap (Figure).
Customizability of the pockets is key. The Kamra inlay (AcuFocus) is 3.8 mm in diameter and 5 μm thick. The Flexivue Microlens (Presbia) is slightly smaller but thicker, at 3.2 mm in diameter and 15 to 20 μm in thickness. Surgeons will need to create pockets specifically for each inlay. As we ophthalmologists continue to learn more about the optimal placement for each of these devices, the ability to place the pocket precisely will be very important to the adoption of and success with corneal inlays when one or more of these devices become available in the United States.
POCKET BENEFITS AND LESSONS
One of the first lessons learned from early inlay experience is that, for several of the designs, deeper implantation is better.3,4 Current recommendations suggest a depth of at least 200 μm for the Kamra inlay and 280 to 300 μm for the Presbia lens.1,3 I initially implanted the Kamra inlay under a flap, but I found that the thick flap required to position the inlay at an appropriate depth was not desirable. The shift to corneal pockets—now recommended even when LASIK will be performed simultaneously—brought immediate benefits. For example, pocket implantation reduced the incidence of dry eye, improved refractive outcomes and visual recovery, made it easier to center the inlays, and reduced the removal rate (Kamra Global Registry, data on file with AcuFocus).
Another important lesson from clinical trials and international experience is that a tighter spot/line separation (at least 6 x 6 or the equivalent) results in a smoother pocket, with better results.5 Adjusting the side-cut angle is also an important technical modification, because it inhibits epithelial incursion into the pocket.
In short, many of the key pearls from corneal inlay experience to date reinforce the idea that accurate, customizable channels are critical to optimizing results. New software that makes pocket creation easy and repeatable with the proven iFS femtosecond laser platform is a welcome step toward incorporating presbyopia-correcting inlay technology into practice.
1. Konstantopoulos A, Mehta J. Surgical compensation of presbyopia with corneal inlays. Expert Rev Med Devices. 2015;5:1-12.
2. Fernandez EJ, Schwarz C, Preito PM, et al. Impact on stereo-acuity of two presbyopia correction approaches: monovision and small aperture inlay. Biomed Opt Express. 2013;4(6):822-830.
3. Lindstrom RL, MacRae SM, Pepose JS, Hoopes PC Sr. Corneal inlays for presbyopia correction. Curr Opin Ophthalmol. 2013;24(4):281-287.
4. Ismail MM. Correction of hyperopia by intracorneal lenses; two-year follow-up. J Cataract Refract Surg. 2006;32:1657-1660.
5. Ophthalmic Devices Panel Executive Summary. Kamra Inlay PMA# P120023. http://www.fda.gov/downloads/ AdvisoryCommittees/CommitteesMeetingMaterials/MedicalDevices/MedicalDevicesAdvisoryCommittee/OphthalmicDevicesPanel/ UCM400434.pdf. Accessed March 16, 2015.