Presbyopia Correction With a Small-Aperture Inlay
Jay S. Pepose, MD, PhD and Mujtaba A. Qazi, MD
In this chapter, we discuss the clinical indications for the small-aperture corneal inlay and provide guidelines for state-of-the-art application of this technology in clinical practice, including postoperative management. Outcomes data from the US investigational device exemption (IDE) trial, as well as other recently published reports, are reviewed. The KAMRA small-aperture corneal inlay (CorneaGen) is a valuable addition to the practice of a comprehensive refractive surgeon. It addresses a key need for patients who desire spectacle independence as they become presbyopic but have not yet developed cataracts. Outcomes have been shown to be reliable and repeatable. The KAMRA inlay has been more widely studied than any other corneal inlay technology, with nearly 50 reports in the peer-reviewed literature.1
The KAMRA inlay design has evolved considerably since the earliest clinical trials and even early commercial versions of the device. The inlay that was studied in the US IDE trial and introduced commercially in the United States reflects those learnings. It is made of polyvinylidene fluoride and nanoparticles of carbon and is 6.0 μm thick. The device has an overall diameter of 3.8 mm, with a 1.6-mm central aperture surrounded by an opaque annulus (Figures 4-1 and 4-2).
The annulus contains 8400 laser-etched microperforations ranging in size from 6 to 12 μm to allow water, CO2/O2 diffusion, and nutrient flow. The microperforations also permit 5% light transmission through the inlay. The pores are arranged in a pseudorandom pattern in order to minimize diffraction issues at night and maximize diffusion through the bulk of the inlay annulus, sparing the edges.
Figure 4-1. The KAMRA inlay has a 1.6-mm central aperture surrounded by an opaque annulus containing 8400 laser-etched microperforations. (Reprinted with permission from CorneaGen.)
Figure 4-2. The overall diameter of the inlay is 3.8 mm. Here it is shown next to a contact lens, for comparison. (Reprinted with permission from CorneaGen.) The KAMRA inlay extends depth of focus by blocking unfocused peripheral light rays, while isolating the more focused central and paracentral rays through its central 1.6-mm aperture, thereby narrowing the blur circle. A comparison of the patient defocus curves in an untreated presbyopic eye vs a KAMRA inlay-implanted eye reveals an extended range of vision in the inlay eye (IE), representing an average 2.5 diopters (D) of functional range of vision from near to far. The inlay is intended for implantation in the nondominant eye only. While this approach may be similar in application to monovision, the unique small-aperture design overcomes the limitations of monovision by preserving monocular distance acuity and stereopsis.2 PATIENT SELECTION The ideal KAMRA patient is between 45 to 60 years old, requires near correction of +1.0 D to +2.5 D, and is motivated by lifestyle or other factors to be less dependent on reading glasses. The IE should have at least 500 μm of central corneal thickness and a stable refraction. The ideal KAMRA candidate should have a preoperative distance refraction that is slightly myopic in the nondominant IE and plano in the dominant fellow eye. If laser vision correction is required to achieve these target refractions, there should be sufficient corneal thickness to accommodate both the excimer laser ablation and positioning of the inlay. In the United States, the inlay is indicated for patients with a mesopic pupil of less than or equal to 6.0 mm. However, one study has shown no correlation between pre- or postoperative pupil size and visual acuity after KAMRA inlay implantation.3 It is important to screen candidates for ocular surface disease. The central cornea should be free of punctate epithelial keratopathy. Lid or tear film problems should be treated and resolved prior to performing any surgical procedure. The AcuTarget HD Analyzer (Visiometrics) diagnostic instrument can be used to identify both tear film instability and lenticular changes. Presbyopes with early lens opacities, which can be graded by the Ocular Scatter Index of the HD Analyzer, are more likely to be dissatisfied following KAMRA inlay surgery and should be steered toward lens exchange. Finally, patients with any ocular or systemic disease that is a contraindication for corneal refractive procedures should be excluded, as should patients with unrealistic postoperative expectations. In addition, patients with topographic or tomographic patterns suggestive of forme fruste keratoconus or corneal ectasia are not suitable candidates. Post–refractive surgery patients are often highly motivated to maintain (or regain) spectacle independence as they become presbyopic and can be good candidates for a KAMRA inlay. Successful KAMRA implantation has been reported in post-LASIK presbyopes4–6 and in small numbers of patients with prior radial keratotomy7 or phakic intraocular lenses (IOLs).8 ELEMENTS OF A KAMRA PROCEDURE Implantation of the small-aperture inlay is not difficult and should be well within the skill set of ophthalmic surgeons. A state-of-the-art procedure includes implantation in a carefully constructed pocket in a patient with the target refraction, appropriate centration of the inlay, and the appropriate postoperative topical therapy regimen. Each of these elements is explained in greater detail below. Pocket Construction The KAMRA inlay should be implanted in a lamellar pocket at a depth of greater than or equal to 250 μm, or greater than or equal to 40% of the total corneal depth. Implantation deep in the posterior two-thirds of the stroma preserves corneal nerves and positions the inlay in a region of the cornea with a lower number of keratocytes that can be potentially activated and transformed into myofibroblasts,9,10 thereby reducing the chance of hyperopic shift and haze. A recent study also demonstrated that eyes in which the inlay has been implanted in a deep pocket (greater than 250 μm) achieve better uncorrected near visual acuity (UNVA) than those with shallower pockets.6 Patients in the US IDE study with pockets deeper than 250 μm also had better results than those with less than 250 μm pockets (Figure 4-3).11 A femtosecond laser should be used to create the pocket, with the laser settings adjusted to achieve tight raster spacing. This ensures a smooth lamellar resection, which provides better quality optics and further reduces the wound healing response. In the US IDE trial, subjects with 6 μm x 6 μm spot-line separation or tighter on the iFS laser (Johnson & Johnson Vision) had statistically significantly better UNVA and were more likely to achieve a stable manifest refraction spherical equivalent (MRSE) at 12 months compared to those with wider spot-line separation.11 Target Refraction Experience has shown that depth of focus and patient satisfaction are greatest with a refraction of at least -0.75 D in the nondominant, IE and plano in the fellow eye. Although small-aperture inlays are quite forgiving of residual astigmatism, correcting astigmatism greater than 0.5 D would also be expected to improve performance.12 As discussed next, the inlay may be combined with laser vision correction to achieve the target refractions. Figure 4-3. Intraoperative view of the inlay. (Reprinted with permission from Tracy Schroeder Swartz, OD, MS, FAAO, Dipl ABO.) APPROPRIATE CENTRATION For optimal inlay centration, the HD Analyzer is used to identify the visual axis and accurately reference it to the entrance pupil. In most eyes, the inlay should be centered on the first Purkinje image. For patients with a large angle kappa (ie, greater than 300 μm distance between pupil center and first Purkinje), the KAMRA inlay should be centered midway between the first Purkinje and the coaxially sighted pupil center (of a constricted pupil). The small-aperture design is quite forgiving. Centration within 300 μm of the visual axis seems to be well tolerated and has little influence on postoperative visual acuity for most patients.13 However, if proper centration is not achieved during the initial surgery, the inlay may be recentered to improve visual acuity (Figure 4-4).14 Postoperative Regimen Adequate postoperative control of the healing response is essential for long-term success with the KAMRA inlay. Most surgeons begin with a potent topical corticosteroid, such as prednisolone acetate 1%, 4 times a day for the first week, then switch to loteprednol 0.5% or fluorometholone 0.1%, which are less likely to be associated with steroid-related rise in intraocular pressure on a slowly tapering schedule over 3 to 4 months. The ocular surface must also be aggressively managed, as the inlay’s small aperture amplifies the need for the tear film to function as an effective refractive surface over the central cornea. Postoperatively, patients should be encouraged to use artificial tears regularly. Many surgeons also rely on temporary punctal occlusion or prescribe a topical dry eye medication such as cyclosporine or lifitegrast, along with oral omega-3 supplementation. Figure 4-4. (A) Slit lamp image of the inlay. (Reprinted with permission from Nathan Rock, OD, FAAO.) (B) Optical coherence tomography image of the inlay showing the masking effect outside of the clear 1.6-mm central aperture. OUTCOMES WITH THE KAMRA INLAY The US IDE trial was the pivotal trial that led to the approval of the KAMRA inlay in the United States. It was initiated in 2009 and conducted at 24 centers across the United States, Europe, and Asia, enrolling 508 emmetropic presbyopes who underwent monocular implantation of the inlay. To be included in the study, subjects had to be 45 to 60 years old, with MRSE between +0.5 D and -0.75 D, UNVA worse than 20/40 and better than 20/100, and with best-corrected distance visual acuity (BCDVA) of 20/20 or better in both eyes. KAMRA inlays were implanted at a minimum attempted depth of 200 μm into a femtosecond laser–created pocket or under a 200-μm flap. In the course of conducting the IDE trial, it became clear that the best results could be achieved with a tight spot-line separation (setting of less than or equal to 6 μm x 6 μm), as described previously. The following IDE efficacy results are for the subset (n = 166) of patients who underwent pocket lamellar resection with tight spot-line separation (less than or equal to 6 μm x 6 μm, using the iFS femtosecond laser during the study, although equivalent settings are now possible on other lasers as well). EFFICACY AND VISUAL FUNCTION Visual Acuity In the IDE trial, there was an average gain of 3 lines of UNVA 12 months after KAMRA inlay implantation. This gain remained stable through 60 months postoperatively. The average change in uncorrected distance visual acuity (UDVA) was a half-line decrease at 12 months, which also remained stable for 60 months (Figure 4-5). At 3 years, mean monocular UNVA was J2 (Jaeger; 20/25) and mean monocular UDVA was 20/25. Binocularly, the mean UNVA and UDVA were J2 (20/25) and 20/16. Ninety percent of subjects achieved J4 (20/32) or better binocular UNVA and 99% achieved 20/32 or better UDVA at 12 months. Figure 4-5. Visual performance data from the IDE study is available out to 5 years. UNVA, both binocular (OU) and in the IE, improved postoperatively and remained stable through 5 years. Binocular UDVA was unchanged compared to preoperatively. (Reprinted with permission from CorneaGen.) Figure 4-6. When paired with a small amount of myopia (-0.75 D), patients can achieve up to 2.75 D of functional depth of focus, with minimal compromise to distance vision in the IE. (Reprinted with permission from CorneaGen.) Depth of Focus and Refraction The extent to which the small-aperture inlay extends depth of focus is influenced by the refraction in the implanted eye. Previous experience has shown that eyes with a -0.75 D refraction can achieve up to 3.0 D of continuous functional range of vision. The range is reduced in emmetropic to slightly hyperopic eyes. When the refraction in the IE is +0.5 D prior to implantation, for example, patients achieve approximately 1.5 D of continuous functional vision (Figure 4-6). The depth of focus achieved in the IDE trial closely mirrors prior experience as well as the predictions of theoretical models for depth of focus with a small-aperture inlay.15 In the trial, 91.9% of patients with preoperative myopic refractions achieved J5 (20/40) or better UNVA vs 76.2% with preoperative hyperopic refractions. Contrast Sensitivity Contrast sensitivity testing was performed on a predetermined subset of 327 IDE trial subjects. There was some decrease in monocular photopic contrast sensitivity postoperatively and a slightly greater decrease in monocular mesopic contrast sensitivity. However, the mean contrast sensitivity in the IEs remained well within normal limits through 36 months postoperatively and binocular contrast remained essentially unchanged from preoperative levels. Contrast sensitivity in the IDE study was compared to the performance of 78 subjects randomized to bilateral implantation of 1 of 3 leading presbyopia-correcting IOLs: the Crystalens AO (Bausch + Lomb), ReSTOR +3.0 D multifocal (Alcon Laboratories, Inc), and TECNIS +4.0 D multifocal (Johnson & Johnson Vision). KAMRA inlay subjects had better monocular contrast sensitivity than the multifocal IOL patients and better binocular contrast sensitivity than all 3 IOL groups (Figure 4-7).16 Figure 4-7. KAMRA inlay subjects had better binocular mesopic contrast sensitivity than subjects implanted with accommodating or multifocal IOLs. (Reprinted with permission from CorneaGen.)
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