15 The Femtosecond Laser in the Surgical Treatment of Presbyopia in the Cornea: Options and Limitations
15.1 Summary
Corneal inlays may be an effective solution for a growing population of presbyopic patients who desire good uncorrected vision at all distances.
The advantages of corneal inlays relative to other solutions for presbyopia include the following:
A favorable risk profile compared with intraocular surgery.
The ability to remove the inlay if needed.
Preservation of the patient’s ability to undergo further procedures and ophthalmic imaging in the future.
There are at least four inlays at various stages of commercial development and release that rely on a variety of design principles, including altering the index of refraction with a bifocal optic, altering the corneal curvature, or increasing depth of focus with small-aperture optics.
The Intracor procedure is based on the central corneal steepening created by the purely intrastromal application of five or six concentric rings with a femtosecond laser.
The Intracor procedure is not reversible.
The combination of LASIK (laser-assisted in situ keratomileusis) and Intracor is not recommended, given that postoperative corneal ectasia might occur.
Keywords: presbyopia, corneal inlay, KAMRA inlay, Flexivue Microlens, Raindrop Near Vision Inlay, Icolens, Intracor
15.2 Introduction
The wealth of clinical experience and published data on small-aperture inlays suggests that this design can provide excellent near and intermediate vision without significant loss of distance vision, contrast sensitivity, or stereopsis. Currently, there are more than 140 million people over the age of 40 years in the United States alone and it is expected that by 2020, there will be 2.1 billion presbyopes worldwide. With these demographic trends comes a continuing interest in the development of refractive surgical procedures to improve near vision for presbyopic patients. Current surgical interventions for presbyopia include corneal refractive surgery with a monovision or blended vision target; a number of surgeons have also investigated multifocal ablations. These options, like their contact lens counterparts, may reduce distance acuity, stereopsis, contrast sensitivity, or quality of vision. With refractive lens exchange, the patient may have monovision, accommodating or multifocal intraocular lenses (IOLs) implanted prior to the development of clinically significant cataract. However, many consider clear lens surgery too invasive, particularly in the early stages of presbyopia.
15.3 Corneal Inlays for the Surgical Compensation of Presbyopia
For all the above-mentioned reasons, there is significant interest in corneal inlays to compensate for presbyopia. The advantages of corneal inlays include the fact that they are additive and do not remove tissue, they preserve future options for presbyopic correction, some of them may be used in the setting of pseudophakia and/or combined with laser refractive surgery, and they are all removable and only implanted within the nondominant eye. 1 At present, there are three different types of corneal inlays commercially available. Some corneal inlays are designed to change the eye’s refractive index (refractive optic inlays). Similar to some multifocal contact lens or IOL designs, these microlenses provide distance vision through a plano central zone that is surrounded by one or more rings of varying add power for near vision. A second type of presbyopia-compensating inlay is intended to reshape the anterior curvature of the cornea to enhance near and intermediate vision via a multifocal effect (corneal reshaping inlays). The third type of corneal inlays relies on the principle of pinhole optics to increase the eye’s depth of focus by blocking unfocused light (small-aperture inlays).
Femtosecond lasers are known to provide more predictable flap thickness, lower incidence of laser-assisted in situ keratomileusis (LASIK) induced dry eye, quicker visual recovery, and better uncorrected distance visual acuity (UDVA) results than mechanical mikrokeratomes. 2, 3, 4 Creating a pocket interface by femtosecond laser minimizes the impact on the corneal nerves compared to a femtosecond laser flap, in which more nerve-fiber bundles are cut; as a result, the risk of dry eye disease is higher and this might affect outcomes.
15.3.1 Corneal Reshaping Inlays
Raindrop Near Vision Inlay
The Raindrop Near Vision Inlay (Revision Optics, Inc.) consists of a clear, permeable hydrogel material that has approximately the same refractive index as the cornea. 5 The inlay has a positive meniscus shape, a diameter of 2.0 mm, a center thickness of approximately 34 μm, and an edge thickness of approximately 14 μm. The inlay is centered on the light-constricted pupil of the nondominant eye beneath a flap created using a femtosecond laser. The inlay reshapes the central pupillary region of the cornea to provide additional optical power relative to the unchanged peripheral region (▶ Fig. 15.1).
(This image is provided courtesy of Revision Optics.)
Recently Garza et al 6 reported the 12-month results of 30 presbyopic patients after inlay implantation in the nondominant eye combined with myopic LASIK. Mean binocular UDVA, uncorrected intermediate visual acuity (UIVA), and uncorrected near visual acuity (UNVA) were better than 0.1 logMAR with 93% of patients having binocular visual acuities better than 0.1 logMAR across all visual ranges. About 10% (3/30) had trace haze across the inlay and 10% showed transient IOP elevation > 10 mmHg. According to patient questionnaires, 1 year after surgery, visual symptoms were at preoperative levels, 98% of all visual tasks could be easily performed without correction, and 90% of patients were satisfied or very satisfied with their overall vision.
Chayet et al 7 reported the 12-month results of presbyopic patients after inlay implantation in the nondominant eye combined with hyperopic LASIK. The mean UNVA in the surgical eye (n = 16) improved from 0.8 logMAR preoperatively to 0.0 logMAR postoperatively, with patients having a mean gain of greater than seven lines of UNVA. The mean binocular gain in UNVA was also seven lines (preop: 0.7 logMAR; postop: 0.0 logMAR). The UDVA in the surgical eye significantly increased from 0.5 logMAR preoperatively to 0.2 logMAR postoperatively and was even better binocularly (0.4 logMAR preop; 0.0 logMAR postop). CDVA remained stable during the follow-up. One patient had recurrent haze with corneal inlay removal after 9 months with UDVA improvement from 0.4 to 0.0 logMAR 1 week after explantation. The only statistically significant change in any visual symptom between preoperatively and postoperatively was in halos at the 1-month visit. At 12 months, no patient reported moderate, marked, or severe visual symptoms in any category.
Parkhurst et al 8 described two cases of successful femtosecond laser–assisted cataract surgery (FLACS) in patients with the inlay in place. They found that the Raindrop inlay did not interfere with the visualization of intraocular structures and thus cataract surgery was not technically more difficult, nor did it require additional ocular rotations. Additionally, when performing FLACS, the transparent inlay allowed effective delivery of the femtosecond laser energy, making complete cuts without tags. However, the fact that none of these cases ended within 0.5 D of plano shows that IOL power calculation is an area of improvement for future cases.
As Pinsky stated within a recent publication, 5 an important concern is the effect of the inlay on the long-term health of the cornea due to disturbances in the concentration profiles of metabolic species. A hydrogel inlay placed within the stroma could impede the flow of glucose and lactic acid as they move across the cornea following their concentration gradients. The flux of metabolic species is modified by an inlay, depending on the inlay relative diffusivity. For the Raindrop hydrogel material with a relative inlay diffusivity of 43.5%, maximum glucose depletion and lactate ion accumulation occur anterior to the inlay and both are less than 3%. However, there exists the theoretical possibility that the pores of the hydrogel material may become obstructed in time with a concomitant reduction in diffusivity. Below 20% relative diffusivity, glucose depletion and lactate ion accumulation increase exponentially. In general, glucose depletion and lactate ion accumulation are highly sensitive to inlay diffusivity and somewhat insensitive to inlay depth. Glucose depletion increases slightly with increasing depth of inlay placement.
15.3.2 Refractive Optics Inlay
Flexivue Microlens
The Flexivue Microlens (Presbia, Los Angeles, CA), based on a precursor known as InVue lens, is a transparent hydrophilic disk with a 3.0-mm diameter and an edge thickness of approximately 15 μm. The central 1.6-mm diameter of the disk is plano, while the peripheral zone provides near addition power. The base powers available range from +1.50 to +3.50 D in 0.25-D increments. The lens material refractive index is 1.4583. At the center of the disk is a 0.15-mm hole for the transfer of oxygen and nutrients to the cornea. 9 The pocket is created using standard femtosecond laser parameters (temporal incision; channel width: 4.20 mm; channel depth: 280–300 μm) associated with traditional LASIK in conjunction with a mask that is used to create a pocket to facilitate inlay implantation without affecting the rest of the cornea. The inlay is loaded in the inserter and afterward implanted, centering the lens in the pocket based on the line of sight.
Three papers within the last years reported data of emmetropic presbyopic patients after monocular implantation in a corneal pocket created with a femtosecond laser. 9, 10, 11 Although Bouzoukis et al 10 described only the feasibility of creating this pocket in the nondominant eye of a 56-year-old woman, Limnopoulou et al 11 reported (n = 47) a significant increase in monocular (pre-op: 0.68 logMAR; 12 months post-op: 0.14 logMAR) and binocular UNVA (pre-op: 0.53 logMAR; 12-months post-op: 0.13 logMAR). However, UDVA in operated eyes significantly worsened from 0.06 logMAR preoperatively to 0.38 logMAR postoperatively, maintaining stable binocular values. The mean SE (spherical equivalent) changed from 0.66 to –1.95 D. Also, CDVA significantly worsened from 0.00 to 0.10 logMAR (17/47 lost one line). Within this trial, no complications, no removal, and no replacement occurred, and also intraocular pressure, endothelial cell count, and central corneal thickness remained stable. Limnopoulou also reported no tissue alterations in confocal microscopy.
Malandrini et al 9 evaluated the biocompatibility of this inlay based on healing of corneal wounds and analysis of corneal structural features using in vivo confocal microscopy (IVCM) and anterior segment optical coherence tomography (OCT) in 52 patients. Postoperative slit lamp examinations showed clear corneas without evidence of thinning, scarring, or vascularization and well-centered inlays at all time points in all eyes. In the early postoperative period, IVCM showed intense cellular activity in the stroma around the inlay, edema, inflammation, and degenerative material deposition, but normal regularity after 12 months. Anterior segment OCT showed a regular planar shape of the corneal pocket in all eyes. At 1 month, hyper-reflective areas beneath the inlay and microfolds were observed in 21 of the 52 eyes. After 12 months, the anterior segment profile was regular and interface pocket reflectivity decreased over time. Three inlays were explanted before the 6-month visit as the patient reported significant discomfort caused by a reduction in distance vision and the presence of significant halos and glare; three additional explantations occurred before the 12-month visit. After removal, IVCM and anterior segment OCT showed clear corneas without signs of irregularity. The mean preoperative CDVA of patients having explantation was –0.06 logMAR and the mean postexplantation CDVA was 0.00 logMAR.
Icolens
The Icolens system (Neoptics AG, Huenenberg, Switzerland) comprises a microlens with a positive refractive power, a femtosecond laser (Femto LDV, Ziemer Ophthalmic Systems AG) with a pocket-cutting algorithm, a preloaded deployment device, and purpose-designed positioning instruments. The 3.0-mm Icolens has a bifocal design with a central zone for distance and a peripheral positive refractive zone for near. The central zone has a diameter of 1.8 mm, an edge thickness of 15 μm, and a 0.15-mm central hole to facilitate nutrient flow. It is manufactured using a copolymer of 2-hydroxyethyl methacrylate and methyl methacrylate, both of which have hydrogel properties. The material has a refractive index of 1.460 in hydrated conditions. The treatment range is currently available from 1.5 to 3.0 D (for Presbyopia) and from –1.0 to +1.5 D (for Ametropia).
Baily et al 12 reported in a recent publication the 12-month results after monocular implantation of this refractive optics inlay in emmetropic patients. The Femto LDV femtosecond laser was used for pocket creation (temporal pocket incision; diameter: 3.6 mm; depth: 290 μm). After pocket creation, the preloaded device was inserted into the corneal pocket until the hole located on the leaves was centric to the pupil. The mean UNVA in the surgical eye (n = 52) improved from 0.78 logMAR preoperatively to 0.44 logMAR postoperatively, with patients having a mean gain of 3.48 lines of UNVA. The UDVA in the surgical eye significantly worsened from 0.05 logMAR preoperatively to 0.22 logMAR postoperatively (mean loss of 1.67 lines), but binocularly, UDVA could be maintained (mean gain: 0.48 lines). Also a mean loss of CDVA postoperatively was evident (–1.78 lines). There was no significant change in corneal topography or endothelial cell count. On the satisfaction survey (n = 40), 90% of patients reported being satisfied with the overall procedure in general. In all cases in which the inlays were explanted (11/52), the reason was poor refractive outcomes rather than adverse events (7/11: inadequate centration; 3/11: ambiguous ocular dominance; 1/11: unrealistic patient expectations). Explantation was uneventful, with no adverse events or complications occurring and all patients returning to the baseline refraction.
15.3.3 Small-Aperture Inlays
The KAMRA Inlay
The KAMRA inlay (AcuFocus Inc, Irvine, CA) is the inlay that has been studied the most among its class. It is approved in 50 countries outside the United States with more than 20,000 inlays implanted today worldwide and also received Food and Drug Administration (FDA) approval in April 2015. The current generation of the inlay (model ACI7000PDT) is a 5-μm-thin microperforated artificial aperture, with a total diameter of 3.8 mm and a central aperture of 1.6 mm made of polyvinylidene fluoride with incorporated nanoparticles of carbon (▶ Fig. 15.2). The opaque permeable material has a light transmission of 6.7%; it further features a pseudorandom microperforation pattern consisting of 8,400 holes ranging in size from 5 to 11 μm in diameter to allow water and nutrition flow in order to prevent corneal thinning and epithelial decompensation. Inlay implantation is performed in a femtosecond laser–created lamellar pocket that is 220 μm or deeper. 13 If the procedure is combined with LASIK, a dual interface technique is used. First, the excimer laser correction is performed under a thin flap; second, the inlay is implanted at least 100 μm below in a pocket interface. The inlay is usually inserted directly in the line of sight.
Fig. 15.2 KAMRA Inlay: the Kamra inlay is a small-aperture, microperforated, opaque inlay made of polyvinylidene fluoride. It relies on the principle of pinhole optics to increase the eye’s depth of focus by blocking unfocused light.
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