Patient demand for spectacle independence is growing. The advances in laser and non-laser technology have allowed ophthalmologists to offer their patients the freedom to choose between depending on their glasses, or to go spectacle free. Presbyopia, defined as the age-related loss of the ability to clearly accommodate onto near objects, has become the last frontier for refractive vision correction.
The complexity and illusiveness of presbyopia necessitates the ophthalmologist to tackle its pathology in different ways. Corneal, lenticular and even scleral approaches have been previously explored in an attempt to reverse this age-related phenomenon. Corneal monovision and presbyopic laser-assisted in situ keratomileusis (presbyLASIK) procedures give conflicting results. Monovision gives the patient near vision on the expense of far and binocular vision; while presbyLASIK requires further development in its nomogram in order to achieve consistent postoperative results.1–3 Conductive keratoplasty, a technique in which the cornea is moulded into a new shape using radiofrequency waves, has also been studied as an option for presbyopia treatment, with patients usually experiencing a large overcorrection followed by a significant regression of their refractive outcome overtime.4
Other procedures described in the management of presbyopia include scleral expansion and anterior sclerotomy techniques.5,6 Lens surgery with implantation of multifocal or accommodating intraoperative lenses (IOLs) have also shown to reduce dependence on reading glasses;7 however, risks associated with intraocular surgery, difficulty in lens exchange, biometric errors, patient dissatisfaction and loss in contrast sensitivity and photopic phenomena make them far from perfect.8 Also, there is an age bracket (40–50 years old) where patients might be experiencing presbyopia symptoms but still had not developed cataract, and in these patients lens surgery is probably less than ideal. Pseudophakic patients with monofocal IOLs are also not eligible for secondary lens implantation, and in these patients an intracorneal inlay might be a good option.
One of the earliest proposed methods for presbyopia correction is additive refractive keratoplasty. This term refers to procedures in which a foreign material is added to the corneal tissue to modify the refractive condition of the eye. Albeit this method is not new, recent advances in technology, its potential for reversibility and ease of application has made it a subject of great interest to ophthalmologists in recent years.9
As mentioned, intracorneal inlays are far from being a novel idea. José I. Barraquer performed experiments with corneal implants as early as 1949, although with unsatisfactory results.10 Since then, this refractive technology has undergone a series of improvements.
Early inlays were composed of flint glass and plexiglass for the correction of aphakia and high myopia. Earlier trials attempted to use poly(methyl methacrylate) (PMMA) and polysulfone inlays to treat high myopia.11 High index polymers were an optically attractive material choice; however, poor permeability limited their use. As a result, Barraquer developed human donor stromal lenticules for inlays (keratophakia) and onlays (epikeratophakia). Claes H Dohlman was the first to describe the use of a permeable lenticule in 1967.12 Hydrogel inlays were developed so as not to impede metabolic gradients across the stroma including nutrient flow to the anterior cornea. Although semipermeable hydrogel polymers allow free nutrient flow, they have a relatively low index of refraction and are therefore limited in optical power.
Past inlay designs include the Kerato-Gel™ (Allergan, Inc., Irvine, California), which was designed for aphakia and was composed of lidofilcon A. The Chiron inlay® (Bausch + Lomb, Rochester, New York) was a meniscus hydrogel optical lens that ranged from 1.50 to 3.50 diopter (D) in add power with a diameter ranging from 1.8 to 2.2 mm. The PermaVision Intracorneal Lens® (Anamed, Lake Forest, California) was composed of a hydgrogel-based material called Nutrapore with a water content of 78 %. This lens, measuring 5.0 to 5.5 mm in diameter with a central thickness of 30 to 60 μm, intentionally altered the anterior surface curvature. This was followed by the IntraLens (now ReVision Optics®, Inc., Lake Forest, California) in the evolution of space-occupying lenticules that intentionally altered the surface curvature to create a kind of multifocal cornea for the treatment of hyperopia and presbyopia. These technologies served as the precursor for the Raindrop inlay (formerly the Vue+ and PresbyLens, ReVision Optics, Inc.). The Intracorneal Microlens™ (BioVision AG, Brüggs, Switzerland) was a 3 mm diameter, 20 μm thick hydrogel annular add inlay with a central opening free of optical power that also allowed nutrient flow to the anterior central cornea. This inlay was designed for placement in a stromal pocket with a mechanical microkeratome pocket maker. The Microlens went on to be known as the InVue™, the precursor to the Flexivue inlay™ (Presbia Coöperatief UA, Amsterdam, the Netherlands ).13