being “within” the cornea itself, ensures that the surfaces are not directly exposed to storage media. In theory, this reduces the risk of stromal swelling and any accompanying changes in Descemet’s Membrane and endothelial cells. Bladeless cutting also eliminates the risk of epithelial cells being introduced into the cut surface and thus reduces the risk of epithelial in-growth. Corneal storage and tissue evaluation may be performed in the same way as for non-cut tissue. However, the possibility that corneal wound healing during subsequent organ culture storage could “repair” the cut needs further study.

Cheng et al. [26] showed the feasibility of the procedure for organ culture, and laboratory-based studies with hypothermically [27] and normothermically [28] stored corneas have been performed. In these studies, dissection was not always complete and required surgical intervention or a double-pass ablation to complete and allow for peeling apart of the tissue along the cutting plane. The thickness and smoothness of the surface of the lamellae was also inconsistent and was thought to be related to variations in the hydration of the deep stromal layers of the cornea. These technical difficulties are still to be overcome and may be possible, with the introduction of lasers operating at a higher Hertz and the refinement and customisation of nomograms in respect to corneal applanation and repetition rate of firing.

Stripping of Descemet’s Membrane with Endothelium

Descemet’s Membrane EK (DMEK) is the latest EK procedure whereby only Descemet’s membrane and attached endothelium is transplanted with no attached stromal tissue. The ultrathin transplant promises accelerated and optimal visual recovery and good results have been reported by Melles and co-workers [23], and others [21]. However, the technical difficulties involved in both donor tissue preparation and delivering the donor endothelium to the participating clinic presents significant challenges with reports of an 8% increase in donor tissue wastage [23]. A specific storage and delivery device for the very thin lamellae, which by nature roll up with the endothelial layer at the outside, has been developed to provide the necessary delicate handling of the tissue [29]. Interestingly, with increasing age of the donor, the thickness of Descemet’s membrane increases and the elasticity of the corneal tissue decreases. Therefore, for the preparation and handling of this kind of posterior lamellae, a higher age of the donor is preferred.

More recently, a hybrid technique of donor preparation has been described which involves removal of the Descemet’s membrane endothelial lamella pneumatically from a thick posterior corneoscleral lamella previously prepared with the microkeratome termed Descemet’s membrane automated EK (DMAEK). The periphery of the prepared endothelial graft remains attached to the stroma and the entire dissected corneoscleral disc is stored [30]. Complete detachment was achieved in 95% of cases, the average diameter of the bubble was 8 mm and the average loss in endothelial cell density after 7 days of storage was 4%. A small case series of ten patients has demonstrated that this preparation facilitates surgery, reduces complications and maintains good DMEK outcomes.

Donor Considerations for EK

As for any corneal transplant procedure, EK success is dependent upon the functional capacity and viability of the endothelial cells on the posterior lamella. Primary graft failure is much more common in EK than in PK and this has in some quarters been attributed to insufficient quality of the donor tissue provided for the procedure. In turn, this has led to surgeons requesting donor tissue that meets criteria in excess to that provided for PK surgery, i.e., younger donors, less storage time and higher endothelial cell densities than for PK corneas. However, over a number of publications, Terry’s group has shown that there is no correlation between storage time and endothelial cell survival following DSAEK surgery, and higher donor endothelial cells counts make no difference to outcome [22]. They have identified that most primary graft failures for EK are almost certainly iatrogenic and are because of surgical trauma to the endothelium during tissue preparation, insertion and subsequent positioning.

In summary, the success of pre-cut tissue is dependent on a number of codependent factors that need to be balanced for each particular transplant:

(i)The thickness of the lamellae.

The thinner the lamellae the earlier is the visual recovery and higher percentage of patients reaching optimal visual acuity. However, these benefits come with the risk of increased manipulation of donor tissue during preparation and surgery, iatrogenic damage to the endothelium, and thus increased primary graft failures, increased reoperation rates and increased wastage of valuable donor tissue during preparation.

(ii)A regularly produced smooth cutting surface.

A smoother interface surface (e.g. DSAEK vs. DSEK) can provide better optics for vision. However, smoother surfaces also result in difficulties in getting the lamellae to adhere and lead to greater dislocation rates.

Summary for the Clinician

•The choice of a particular procedure for the retrieval of donor tissue, in situ corneoscleral excision or and enucleation, has to be considered within the context of an eye bank’s local conditions to ensure an efficient, safe and quality service.

•Comparison of the different corneal storage techniques of hypothermic and normothermic organ culture, in regards to their technical, microbiological and tissue evaluation aspects shows that each technique will ensure an efficient, safe and quality service and preference is dependent on the local eye bank’s conditions.

•Most experience with pre-cut tissue for lamellar grafting has been with hypothermically stored microkeratome cut corneas and with manually prepared organ-cultured corneas. This experience has helped define the necessary corneal parameters required for a good outcome. The newer techniques of femtosecond laser predissection and Descemet’s membrane -endothe- lium complex endothelium preparations also offer great promise. Pre-cut preparation presents challenges to the eye bank, especially in regard to handling and iatrogenic damage introduced during processing.