CHAPTER 4

Onlays and Inlays

Refractive error, including presbyopia, may be corrected by placing preformed tissue or synthetic material onto or into the cornea. This treatment alters the optical power of the cornea either by changing the shape of the anterior corneal surface or by creating a lens with a higher index of refraction than that of the corneal stroma. Tissue addition procedures, such as epikeratoplasty, have fallen out of favor because of the poor predictability of the refractive and visual results, loss of corrected distance visual acuity (CDVA; also called best-corrected visual acuity, BCVA), and difficulty of obtaining donor tissue. Compared with donor tissue, synthetic material can be shaped more precisely, and it can be mass-produced. Because of problems with re-epithelialization when synthetic material is placed on top of the cornea, synthetic material generally has to be placed within the corneal stroma. This placement requires a partial or complete lamellar dissection using specialized instruments. Early work using lenticules made of glass and plastic resulted in necrosis of the overlying stroma because glass and plastic are impermeable to water, oxygen, and nutrients. Current techniques use lenticule inlays made of more permeable substances such as hydrogel, with or without microperforations in the lenticule, to increase the transmission of nutrients. Another type of inlay indirectly alters the shape of the central cornea by using midperipheral corneal ring segments made of polymethylmethacrylate (PMMA). Because the ring segments are narrow, the overlying stroma can receive nutrients from surrounding tissue.

Keratophakia

In keratophakia, a plus-powered lens is placed intrastromally to increase the curvature of the anterior cornea to correct hyperopia and presbyopia. After a central lamellar keratectomy is performed with a microkeratome or femtosecond laser, the flap is lifted, the lenticule is placed onto the host bed, and the flap is replaced and adheres without sutures. Lenticules can be prepared from either donor cornea or synthetic material; these types are referred to as homoplastic and alloplastic lenticules, respectively.

Homoplastic Corneal Inlays

A homoplastic inlay is created from a donor cornea by a lamellar keratectomy after removal of the epithelium and Bowman layer. The lenticule (fresh or frozen) is then shaped into a lens using an automated lathe. The lens can be preserved fresh in refrigerated tissue-culture medium, frozen at subzero temperatures, or freeze-dried.

Keratophakia has been used to correct aphakia and hyperopia of up to 20.00 D, but few studies on this procedure have been published. Troutman and colleagues reported on 32 eyes treated with homoplastic keratophakia, 29 of which also underwent cataract extraction. Even for procedures done by experienced surgeons, refractive predictability was still low: the eyes of 25% of patients were