Most of the tissue used for penetrating keratoplasty is issued through eye banks that store the corneoscleral button either in hypothermic storage at 2-68C or in organ culture at 31-378C.These two preservation techniques differ in technical aspects, tissue evaluation possibilities, storage time and microbiological safety. Hypothermic storage is simple and requires little expensive equipment. In general a pre-storage evaluation of the endothelium is performed by specular microscopy and storage time is usually around 7-10 days. Organ culture is a relatively complicated technique requiring more expertise and well-equipped facilities. Evaluation of the endothelium is not only performed before storage, but is routinely performed after storage through the use of light microscopy. With organ culture the allowed storage period is longer, up to four weeks. The vulnerability of organ culture to microbial contamination can be turned into an advantage because it allows the detection of residual micro-organisms on the cornea before surgery. Both preservation techniques seem to result in similar graft survival.The method of choice for preservation of the donor cornea is dictated by a number of factors mentioned in this review and this helps to explain the geographical differences in the use of the different techniques.

Corneal allograft rejection does not necessarily cause a higher than expected endothelial cell loss; almost half of the patients in this study showed a decline in ECD that is comparable to the decline in patients who undergo PKP and have an uneventful follow-up. The most important variable influencing the extent of endothelial cells loss is a delay in diagnosis and treatment.

Graft failure through endothelial cell loss is a constant threat throughout the lifetime of a corneal graft. It can occur at various time points after transplantation. Primary graft failure has nowadays become increasingly rare owing to meticulous eye banking methods and improved surgical techniques. Corneal graft rejection is always held responsible for endothelial cell loss. However a rejection episode that is promptly and adequately treated does not necessarily lead to a higher than expected cell loss. A more smouldering danger to ultimate graft survival is late endothelial failure. This gradual graft decompensation is secondary to a decrease in cell density below that necessary to maintain corneal deturgesence. The process of transplantation itself seems to set off a series of events (possibly immunological) that greatly exacerbates the endothelial cell loss compared to virgin corneas. This accelerated cell loss persists for at least 10-15 years after transplantation, after which a more-stable situation is reached and cell attrition returns to normal rates. This provides a strong rationale for setting high donor standards of minimal cell density. Newer transplantation techniques such as deep anterior lamellar keratoplasty and deep lamellar endothelial keratoplasty could provide possible solutions to prevent this late endothelial failure. However they still have to prove themselves in the long run.