Synthetic Inlays and Allogenic Inlays
Michael Endl, MD; Soosan Jacob, MS, FRCS, DNB; and Amar Agarwal, MS, FRCS, FRCOphth
Increased demand for visual independence in presbyopes has triggered medical innovations to attempt to reduce the need for spectacles or contact lenses. Shape-changing inlays are implants that alter the shape of the cornea, making it more prolate and inducing reading ability. This can be done with a Raindrop synthetic inlay (ReVision Optics) or an allogenic corneal graft.
RAINDROP NEAR VISION INLAY
The Raindrop Near Vision Inlay was approved by the Food and Drug Administration (FDA) to improve near vision in emmetropic (-0.50 to +1.00 manifest refraction spherical equivalent [MRSE]) presbyopes; this transparent inlay changes the shape of the anterior part of the cornea by creating a hyperprolate shape via biomechanical remodeling of the corneal stroma and epithelium when implanted under a femtosecond flap. By reshaping the anterior curvature of the cornea, the inlay creates an area of increased power in the center of the pupil, which gradually decreases towards the mid-periphery. This zone of power (or profocal cornea) generates a gradient of power, which enhances near vision, while leaving the peripheral cornea for distance vision. The smooth transition created by the corneal epithelium after a Raindrop inlay has been implanted also accounts for low levels of induced visual symptoms, a big drawback of multifocal intraocular lenses (IOLs) used for the same purpose (presbyopia correction) when patients develop cataracts. The safety of the inlay is proven and, if necessary, can easily be removed. This chapter aims to summarize the history of hydrogel inlay technology, the design and mechanism of action, and the peer-reviewed studies on the safety and efficacy of the Raindrop inlay.
Corneal inlays are a modern modality to provide improved near vision to patients with presbyopia and reduce their need for reading glasses. The idea of intracorneal inlay implantation started in the late 1940s with testing of many synthetic materials within the cornea; however, many proved unsuccessful due to lack of biocompatibility. Then in the 1950s, after extensive animal research studies, the first clinical trials were performed with a biocompatible hydrogel material.1
These studies were abandoned due to poor refractive predictability, variability in inlay parameters such as material and dimensions, and issues regarding corneal inflammatory response to the inlay. Much of this research involved implantation using a freehand corneal pocket or freehand microkeratome flap. With the advancement of femtosecond laser technology in the 1990s, the ease and predictability of creating flaps or pockets led to the development of the current inlay technologies used today.1
Figure 5-1. The size of the Raindrop Near Vision Inlay in comparison to an eye of a needle. (Reprinted with permission from ReVision Optics.)
Figure 5-2. Schematic showing the Raindrop Near Vision Inlay dimensions and properties. (Reprinted with permission from ReVision Optics.)
Figure 5-3. Slit lamp image of the Raindrop Near Vision Inlay placed at the center of the pupil in the nondominant eye. (Reprinted with permission from Nathan Rock, OD, FAAO.)
The Raindrop Near Vision Inlay, developed in 2007, is made up of a proprietary biocompatible hydrogel composed of 80% water that has a refractive index similar to corneal tissue. The material was fabricated to facilitate adequate nutrient flow while maintaining a barrier against tissue ingrowth from one side of the implant to another. This feature leads to trans-implant tissue viability and allows the implant to be removable or exchanged. Glucose flux across the Raindrop inlay is almost 10 times higher than the flux across the lenses used in the previous studies.2 The Raindrop inlay is substantially smaller and thinner than the previously developed inlays (Figure 5-1), at approximately 30 μm central thickness, decreasing to about 10 μm thickness at the periphery, and 2.0 mm in diameter (Figure 5-2).
This space-occupying inlay reshapes the anterior corneal surface, creating a hyperprolate region of increased power, although the inlay itself has no intrinsic power. This allows the individual to focus on near and intermediate objects.3 The Raindrop inlay received FDA approval in June 2016. Its approved indication includes implantation at 30% of the central corneal thickness, with a minimum depth of 150 μm and a minimum residual stromal bed thickness of 300 μm. The preoperative central corneal thickness must be between 500 to 600 μm. The inlay is implanted in the nondominant eye under an 8.0-mm femtosecond flap and centered over the light-constricted pupil (Figure 5-3).4 The inlay comes in a preloaded titanium inserter. After the flap is opened completely, the inlay inserter is placed on the stromal bed and delivered with the help of a second instrument, typically the end of a round cannula. The inlay is positioned over the pupil and allowed to dry for approximately 1 minute before the flap is closed. Patients follow a benzalkonium chloride–free strong steroid taper for 1 month, followed by a mild steroid taper for the next 2 months, with preservative free artificial tears applied as needed.5
The Raindrop inlay’s mechanism of action within the cornea was studied in 30 subjects implanted with the inlay using wavefront techniques. The change in the cornea’s anterior surface elevation is shown in Figure 5-4 using an iTrace Visual Function Analyzer (Tracey Technologies). The change in epithelial thickness after inlay implantation was measured using optical coherence tomography (OCT; Optovue Inc) and displayed epithelial thinning centrally with peripheral thickening (Figure 5-5). The difference between the inlay thickness and the resulting elevation change is attributed to attenuation of the effect by the much thicker flap and epithelial remodeling. This remodeling extends the inlay effect to about twice the inlay diameter with the resulting anterior corneal height change providing about 5.0 diopters (D) of refractive add power at the center of the pupil, extending to 0.25 D at the 4.0-mm diameter (Figure 5-6). This central elevation results in improved near and intermediate vision.6
The first 20 presbyopic patients implanted with the Raindrop inlay as part of a research study experienced improved near vision by 1 week postoperative, with all patients achieving an uncorrected near visual acuity (UNVA) of 20/40 or better. At 12 months postoperative, all patients had a UNVA of 20/32 or better for the inlay eye, the mean uncorrected distance visual acuity (UDVA) for the inlay eye was better than 20/32, and the binocular distance vision was better than 20/20. There was a low incidence of ocular and visual symptoms. All subjects were “satisfied” or “very satisfied” with the overall visual outcome of the Raindrop procedure. The subjects also reported low visual symptoms; only 11% of Raindrop subjects have moderate or worse visual symptoms 12 months after implantation.7
Figure 5-4. Axial map from iTrace software (Tracey Technologies) showing an increase in D at the center of the cornea after inlay implantation. (Reprinted with permission from ReVision Optics.)
Figure 5-5. OCT before and after inlay implantation within the cornea. The inlay appears as a dark area within the stroma.
Steinert et al3 analyzed 188 patients implanted with the Raindrop inlay in the nondominant eye. The center-near power profile forms in-focus near images, with continuous annular regions providing natural intermediate to distance zones. In the inlay eye, mean UNVA was 20/25, mean uncorrected intermediate visual acuity (UIVA) was 20/25, and mean UDVA was 20/32 postoperatively. Binocularly all subjects were 20/25 or better at distance. The inlay induces a continuous center-near power profile and provides good visual acuity and performance over a 2.0 D range of preoperative refraction (ideally -0.5 D to +1.5 D preoperative refraction). Near task performance was improved significantly in good and dim lighting conditions compared to preoperative measurements.3
Figure 5-6. The mean change in anterior corneal surface height and the axial power induced by the surface change. Error bars represent one standard deviation.
In a subset of 30 patients from the pivotal US clinical trial, the mean reading add had a reduction of 1.6 D. At 1 year postoperative, the average distance-corrected near acuity improved by more than 3 lines, with patients achieving 20/40 or better of distance-corrected acuity across a 3.5 D defocus range. Ninety-seven percent of patients had a binocular uncorrected visual acuity of 20/32 at distance, intermediate, and near. There was not a significant change in binocular contrast sensitivity, and satisfaction was high.8 Another study looked at high-order aberrations after Raindrop implantation. Only spherical aberration was significantly changed after implantation, and this optical effect was quantified using the vector surgically induced refractive change calculation, showing that increased depth of focus and improvement of near vision was attributed to increased negative spherical aberration.9
Figure 5-7. Example of a patient’s refractive maps derived from corneal topography preoperatively and 3 months postexplant.
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