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    • Basic and Clinical Science Course - Excerpt
  • 2020–2021 BCSC Basic and Clinical Science Course™

    Go to Academy Store Learn more and Purchase.

    13 Refractive Surgery

    Chapter 4: Onlays and Inlays

    This chapter includes a related video, which can be accessed by scanning the QR code provided in the text or going to www.aao.org/bcscvideo_section13.

    Refractive errors, including presbyopia, may be treated by placing preformed tissue or synthetic material onto or into the cornea. This approach alters the optical power of the cornea by changing the shape of the anterior corneal surface or by creating a lens with a higher index of refraction than the corneal stroma that is then implanted within the cornea. Intracorneal inlays with small apertures that use the pinhole effect to increase depth of focus have been developed. Tissue addition procedures, such as epikeratoplasty, have fallen out of favor because of the poor predictability of the refractive and visual results, loss of best-corrected visual acuity (BCVA; also called corrected distance visual acuity, CDVA), 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 reepithelialization 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 category of corneal inlays are corneal ring segments made of polymethyl methacrylate (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 more than 3.00 D from the intended correction. Complications included irregular lamellar resection, wound dehiscence, and postoperative corneal edema. Although the procedure was originally intended to be used in conjunction with cataract extraction for the correction of aphakia, the complexity of the procedure and the unpredictable refractive results could not compete—in the early 1980s—with aphakic contact lenses or the improved technology of intraocular lens (IOL) implantation. Homoplastic keratophakia using tissue from stromal lenticule extraction has been suggested, however, for treating hyperopia, presbyopia, and ectatic corneal diseases.

    • Ganesh S, Brar S, Rao PA. Cryopreservation of extracted corneal lenticules after small incision lenticule extraction for potential use in human subjects. Cornea. 2014;33(12): 1355–1362.

    • Lim CH, Riau AK, Lwin NC, Chaurasia SS, Tan DT, Mehta JS. LASIK following small incision lenticule extraction (SMILE) lenticule re-implantation: a feasibility study of a novel method for treatment of presbyopia. PLoS One. 2013;8(12):e83046. www.ncbi.nlm.nih.gov/pmc/articles/PMC3859649/. Accessed November 6, 2016.

    • Sun L, Yao P, Li M, Shen Y, Zhao J, Zhou X. The safety and predictability of implanting autologous lenticule obtained by SMILE for hyperopia. J Refract Surg. 2015;31(6):374–379.

    Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.

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