Darren G. Gregory

Introduction

Since the late 1990s the use of amniotic membrane (AM) in ocular surface surgery has increased rapidly in both scope and volume. The benefits and limitations of AM have become clearer as experience and literature on the subject have grown. Rather than give an exhaustive listing of every described AM application or feature, the goal of this chapter is to provide a practical guide for the use of amniotic membrane, with specific examples highlighting particular points or techniques.

Amniotic membrane is the innermost layer of the fetal membranes that are derived from, and contain, the growing embryo during pregnancy. It consists of a single epithelial layer, a thick basement membrane and an avascular stroma. Although, its use in ocular surface surgery was first described in 1940,¹ there was little further mention in the literature in subsequent decades until a cryopreserved version of AM became commercially available in 1997 (Amniograft, Bio-Tissue, Miami, USA). The interesting history of the emergence of AM in ophthalmic surgery is chronicled in an extensive review by Dua et al.² Amniotic membrane is commercially available in North America in two forms, cryopreserved and freeze-dried. The freeze-dried form offers the convenience of not requiring refrigeration for storage, but unfortunately the literature examining its use is minimal. For this reason, the focus of this chapter will be on the use of the various forms of cryopreserved AM.

Cryopreserved AM comes as a sheet in a variety of sizes, with the stromal surface attached to a piece of carrier paper. Non-toothed forceps can be used to completely free the AM from its adhesion to the paper. Once fully freed from the underlying paper, it can be easily slid onto the ocular surface. The stromal surface, which was adherent to the carrier paper, is placed against the surface of the area being treated. The stromal surface can always be identified using a cellulose sponge, which will stick to the stroma, but not the epithelial surface (Fig. 37.1). Most recently, a grid pattern has been placed on the side of the paper with the AM so identification of the tissue surface is easy. The stromal surface of the AM is the side adherent to the paper grid.

Prokera^TM (Bio-Tissue, Miami, USA) is a sheet of AM stretched across the lumen of a 16-mm thermoplastic ring set and can be placed on the eye like a contact lens (Fig. 37.2). Insertion is best accomplished by placing the patient supine and inserting the Prokera^TM under the upper eyelid. The inferior edge of the ring is then tucked under the lower eyelid. The device is quite slippery, so great care must be taken during the insertion process. Prokera^TM serves a role similar to a therapeutic contact lens, but also provides added anti-inflammatory and healing benefits in eyes with a disordered ocular surface. The AM portion only covers the cornea and limbus and does not protect the fornices or tarsal conjunctiva from inflammation or epithelial defects. Those areas would need to be treated with a regular sheet of cryopreserved AM.

Both clinically and in vitro, AM has been shown to have significant anti-inflammatory, antiscarring, and antiangiogenic activity.3,4 These effects allow adjacent normal tissue, if available, to repopulate injured areas before inflammation and scar tissue take over the healing process, allowing regeneration rather than repair. The stromal component seems to provide this benefit by a variety of mechanisms.³ The basement membrane/epithelial surface of AM is an effective scaffold that promotes epithelial migration into injured areas. This feature has also allowed its use as a substrate for ex vivo expansion of ocular surface epithelial progenitor cells for grafting when an adequate supply of native normal tissue is not available.^5–7

Basic Principles

The use of AM in ocular surface surgery can be divided into two broad categories: a temporary patch (Box 37.1) or a permanent graft (Box 37.2). There is some overlap, but the categories are based on the therapeutic goals of the treatment and the ultimate fate of the membrane. As a temporary patch, amniotic membrane is used like a ‘biological bandage’ to cover and protect injured conjunctival or corneal epithelium. The tissue underneath the membrane is provided an opportunity to heal with decreased inflammation and scarring. The membrane is sloughed off, or removed, once the epithelium underneath it has healed. As a permanent graft, the goal is to get epithelium to grow over top of the AM rather than underneath it. The AM then becomes part of the subepithelial substrate.