14 Descemet Membrane Endothelial Keratoplasty Modifications and Nonkeratoplasty Concepts: Hemi-DMEK and Descemet Membrane Endothelial Transfer The introduction of the Descemet membrane endothelial keratoplasty (DMEK) resulted in the improvement of postoperative clinical outcomes and a reduction of complication rates as compared to previous techniques, especially in eyes with Fuchs endothelial corneal dystrophy (FECD).1 In addition, DMEK also provided new insight into corneal endothelial cell biology and physiology.2 Clinical observations after standard (circular) DMEK suggested that corneal clearing may not only depend on complete coverage of the denuded stroma after removal of the diseased Descemet membrane and endothelium (descemetorhexis). In eyes with decentered grafts, corneal clearance was also observed in bare stromal area between the graft and the edge of the descemetorhexis.3 Furthermore, corneal clearing may also not rely on complete donor-to-host apposition, as seen in eyes with a (partially) detached DMEK graft.4,5 These observations led to further modifications of the standard DMEK technique, such as using half-moon-shaped grafts (hemi-DMEK), and nonkeratoplasty surgical approaches, such as performing a descemetorhexis only or a descemetorhexis with a free-floating graft secured in the main incision (Descemet membrane endothelial transfer [DMET]). A recent modification of DMEK, the so-called hemi-DMEK, was based on (1) the clinical observation that no complete coverage of the entire denuded recipient stroma by a graft is required in order to achieve corneal clearance, and (2) the idea to reduce endothelial tissue shortage.3,6,7,8 With the introduction of DMEK, more efficient use of corneal donor tissue became feasible because it was now possible to generate two transplants from one donor cornea. The donor cornea could be split in such a way that the posterior part of the donor cornea could be used as a DMEK graft, and the remaining anterior part could be used as a transplant for deep anterior lamellar keratoplasty (DALK).9, 10,11,12 This approach, however, could not reduce endothelial tissue shortage because only one endothelial graft could be obtained from one donor cornea. For penetrating keratoplasty (PK) and Descemet stripping (automated) endothelial keratoplasty (DSEK/DSAEK), where the graft is generally thicker at the periphery than in the center, a centrally trephined transplant is required for optical reasons. A DMEK graft, however, is evenly thin over its whole surface area because it consists only of Descemet membrane and endothelium. Therefore, for DMEK, the peripheral portions of the graft could also be used without resulting in optical impairment. Thus, in 2014 the Melles group introduced the concept of hemi-DMEK, in which a half-moon-shaped (semicircular) Descemet membrane graft is prepared from one untrephined 11.5 to 12 mm (full) diameter donor Descemet membrane sheet (► Fig. 14.1). In contrast to standard DMEK graft preparation, where only the trephined central 8.5 to 9.5 mm circular DMEK graft is used and the outer Descemet membrane rim is discarded, in hemi-DMEK, two semicircular grafts for two recipients may be used from one untrephined, full-diameter Descemet membrane sheet, potentially doubling the number of endothelial grafts harvested from the same donor pool.13 Since the total surface area of the half-moon-shaped graft is similar to the standard (circular) DMEK graft, the graft shape may be the main difference between both techniques. As a result, for the first time two endothelial transplants could be obtained from one donor corneal button. A small case series of patients with FECD showed that visual outcomes at 6 and 12 months after hemi-DMEK are comparable to those after standard DMEK.14,15,16 Despite the shape mismatch between the circular descemetorhexis and the semicircular hemi-DMEK graft, corneal clearance occurred over the entire cornea by 3 to 6 months postoperatively (► Fig. 14.2), also over initially bare stromal areas (denuded of Descemet membrane).14,15 Similar observations have been made after standard DMEK in eyes with a decentered graft or a partially detached DMEK graft, suggesting that, besides donor endothelium, host endothelial cells may also be actively involved in corneal clearance after endothelial keratoplasty.3,5 It should be noted, however, that endothelial cell density after hemi-DMEK appears to be lower for the currently available study group as compared to endothelial cell density after standard DMEK. Longer follow-up data may show whether further endothelial cell density decline is comparable for hemi-DMEK and standard DMEK. Longer follow-up data and data of larger study groups should show whether, besides visual outcomes, allograft rejection rates and graft survival in hemi-DMEK prove to be similar to standard DMEK. Hemi-DMEK could then become the next step in endothelial keratoplasty for the treatment of FECD, potentially doubling the pool of endothelial graft tissue. The observation that complete donor-to-host apposition may not (always) be required for corneal transparency and deturgescence after posterior lamellar keratoplasty in eyes with graft detachment after DSAEK17 and DMEK4,5 has questioned the concept of keratoplasty.2 In eyes with 50% graft detachment/apposition after DMEK, endothelial cells were also detected over the recipient bare stroma underlying the detachment, suggesting an active role of endothelial cells in migration and redistribution as a healing response after keratoplasty.3 These observations suggested that it is possible to reestablish corneal clearance in the presence of at least some physical contact between the donor graft and host stroma.
14.1 Hemi-Descemet Membrane Endothelial Keratoplasty (Hemi-DMEK)
14.2 Descemet Membrane Endothelial Transfer (DMET)
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