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Synthetic keratophakia finds its place in surgical treatment of presbyopia
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Synthetic keratophakia is the broad category that defines the placement of a plastic lens in the cornea to correct a refractive error. The first of these procedures was attempted by Barraquer in Bogotá, Colombia, in 1949 using flint glass intracorneal lenses for the treatment of extreme myopia and aphakia. They failed from interface opacity, vascularization, and aseptic necrosis and melting of the cornea anterior to the implant. Those that did not spontaneously extrude were explanted. This led Barraquer to redirect his attention to human tissue keratophakia and keratomileusis, pioneering and developing the classic principles that later evolved into epikeratophakia, PRK and LASIK.
Following Barraquer’s failure, Choyce in the 1960s and 1970s attempted to treat high myopia with PMMA and polysulfone implants implanted deeply in a pocket in the corneal stroma. Some of these implants survived for years but, again, most developed interface haze, vascularization and corneal keratolysis anterior to the implant. Again, most failed spontaneously or were removed, and many patients required penetrating keratoplasty to regain functional vision.
As knowledge of the corneal physiology advanced, it was learned that the epithelium receives its energy supply from glucose diffusing through the cornea from the aqueous. Any impermeable membrane that blocks the flow of glucose to the anterior cornea and epithelium was therefore destined to fail. Solutions included the use of permeable lenses made of hydrogel materials, micro-fenestration and the use of small implants placed deeply enough in the cornea to allow glucose to diffuse around them. In addition, deeper implantation allowed normal reinnervation of the anterior cornea, avoiding a neurotrophic keratopathy.
With PRK, LASIK and phakic IOLs working well for myopia and astigmatism and IOLs for aphakia, presbyopia remained the last refractive error that might respond to the modern application of an intracorneal lens.
Today, after more than 60 years of basic and clinical research, small-diameter, extremely thin lenses placed deeply in the cornea are showing enough promise to be commercialized in many parts of the world.
As disclosure, I have had a career-long interest in synthetic keratophakia and have worked at one time or another with several companies involved in this area, including the three discussed below. The optical principles used differ between the three options currently in various stages of commercial launch and/or pre-approval regulatory clinical trial, depending on the country.
The corneal inlay with the greatest clinical experience is the AcuFocus Kamra, which uses small-diameter aperture optics to increase the depth of focus in one eye only. This implant is attractive because it works across a broad range of presbyopia and can be implanted either under a LASIK flap or in a pocket. It can also be combined with the correction of myopia, hyperopia and astigmatism in a single procedure or consecutive procedures. Most patients lose only a line or less at distance while gaining on average four lines at near. The implant can be removed in the dissatisfied patient with an office procedure. Several prominent ophthalmic surgeons have had the Kamra inlay placed in their eye and are strong advocates of the technology.
The second approach, offered by Presbia, utilizes a small-diameter hydrogel implant with a higher refractive index than the cornea to create a bifocal optical system. The performance of this optical system is well known to all ophthalmologists from their experience with bifocal and multifocal contact lenses or IOLs. Good near vision is possible, but some loss of uncorrected distance vision and contrast sensitivity as well as potential night vision symptoms are typical. The outcome is similar to modified monovision in contact lenses, in which a multifocal contact lens is fitted on the nondominant eye with a single-vision contact lens on the dominant eye. Many contact lens wearers find this an excellent option and prefer it to standard monovision.
Finally, ReVision Optics utilizes a small-diameter hydrogel inlay that has the same refractive index as the cornea. It is placed under a LASIK flap and creates central steepening to again create a center-surround multifocal optic in the cornea.
Each of these optical systems is generating highly satisfied patients in a diverse group of surgeons’ practices. The issues with interface opacity, vascularization and extrusion are responding to modern materials and design and proper medical therapy. Centration remains critical, and adjunctive devices appear capable of helping the surgeon center the device on the visual axis. A postsurgical dry eye, much like that experienced with LASIK, is possible but usually responds to aggressive medical management. In the rare case in which outcomes do not generate a satisfied patient, the inlays can be removed.
I am becoming convinced that after 60 years of research and development, synthetic keratophakia will find its place in modern ophthalmology for the surgical treatment of presbyopia. The number of patients affected by presbyopia globally exceeds the number affected by myopia, hyperopia and astigmatism, so a safe and effective surgical approach for those seeking an alternative to spectacles or contact lenses will be a welcome addition to every refractive surgeon’s practice.