Conventional corneal transplantation, in the form of penetrating keratoplasty (PK), involves full-thickness replacement of the cornea, and is a highly successful procedure. However, the cornea is anatomically a multi-layered structure. Pathology may only affect individual layers of the cornea, hence selective lamellar surgical replacement of only the diseased corneal layers whilst retaining unaffected layers represents a new paradigm shift in the field. Recent advancements in surgical techniques and instrumentation have resulted in several forms of manual, microkeratome and femto-second laser-assisted lamellar transplantation procedures. Anterior lamellar keratoplasty (ALK) aims at replacing only diseased or scarred corneal stroma, whilst retaining the unaffected corneal endothelial layer, thus obviating the risk of endothelial allograft rejection. Posterior lamellar keratoplasty/endothelial keratoplasty (PLK/EK) involves the replacement of the dysfunctional endothelial cell layer only. Whilst significant technical and surgical challenges are involved in performing lamellar micro-dissection of a tissue which is only 0.5 mm thick, the benefits of a more controlled surgical procedure and improved graft survival rates have resulted in a shift away from conventional PK. This review details the current advances in emerging lamellar corneal surgical procedures and highlights the main advantages and disadvantages of these new lamellar corneal procedures.
The major advances in corneal transplantation over the last 50 years, have been due to the development of the microsurgical microscope, improvements in microsurgical techniques, better understanding of the fundamental basis of immunological allograft rejection and the role of the corneal endothelium in graft clarity and corneal graft rejection. All these factors have resulted in the success of penetrating keratoplasty (PK) as the predominant form of corneal transplantation, with at least 1 million transplants performed since 1961 (www.restoresight.org). Penetrating keratoplasty is full-thickness replacement of the cornea. Prior to PK surgery, in the middle of the last century, the most commonly performed corneal transplantation procedure was anterior lamellar keratoplasty (ALK). This involved only exchanging the corneal stromal and epithelial layers and avoided replacement of the deepest layer of the cornea (the endothelial layer). Avoiding the replacement of the endothelial layer circumvented the risk of endothelial immunological graft rejection, which was the most common reason for PK failure prior to our understanding of transplantation immunology and the role of the donor endothelial layer as the major cause of graft failure from allograft rejection. Subsequent development of topical steroid eye-drops (to prevent and treat corneal graft rejection) led to greatly enhanced PK graft survival rates. Hence, PK surgery became the dominant form of modern corneal transplantation in the second half of the last century, and is still the most common form of corneal transplant surgery performed worldwide today. This shift from ALK to PK occurred because the surgical technique used for ALK resulted in an irregular interface with sub-optimal visual results.
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