6 Storage of Donor Cornea for Penetrating and Lamellar Transplantation |
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The reported incidence of primary graft failure of corneas stored by organ culture is low at 0–0.3% [EEBA Directory].
Organ culture is a method best suited to an established eye bank with skilled technical staff, and where donor rates are variable, or where distribution is required over wide geographical area. In addition, Europe has preferred the technique because the increased storage time allows the ability to provide ABOor HLA-matched corneas for patients with high risk of rejection, and for optimising allocation of particular corneas to specific patients.
Pre-cutting of Corneal Tissue for Endothelial Keratoplasty (EK)
Over the past decade, endothelial transplant procedures have evolved to a point where they have become a preferred method of surgical treatment for endothelial dysfunction. There have been a number of variations in the procedure, and thus variants in donor tissue preparation, over this time. Currently, the primary preparations are Descemet’s Stripping EK (DSEK) which involves manual surgical dissection and preparation of the donor lenticule, and its subtype Descemet’s Stripping Automated EK (DSAEK) which introduces microkeratome preparation of the donor lenticule [21, 22]. More recently, Descemet’s Membrane EK (DMEK), where the endothelium with Descemet’s membrane in the absence of any stroma is transplanted, has been introduced and it presents its own unique set of difficulties in donor tissue preparation [23].
The preparation of donor lenticules by the eye banks (pre-cut tissue) introduced a number of major advantages over surgeon preparation of tissue [22]). Importantly, the eye bank is able to evaluate the tissue quality of the donor posterior lenticule by (specular) microscopy, slit lamp biomicroscopy and (more recently) by optical coherence tomography [24] immediately post-resection. This allows for the exclusion of any donor lenticules that may have been compromised by the cutting or dissection procedure. It also reduces the time spent by the surgeon in preparing the tissue and saves operating room time. Indeed, eye bank staff can also become extremely proficient at the procedure due to high volumes of procedures which results in more consistent resection outcomes. Although the use of eye bank-pre- pared donor tissue simplifies EK, the ophthalmic surgeon must once again rely on the eye bank personnel to reliably perform a critical portion of the procedure. In addition, there is concern that adverse tissue changes may occur between the extended time when the tissue is pre-cut and the time the tissue is transplanted, including swelling of the lenticule due to exposure of stromal tissue to storage media and possible loss of endothelial cells [25]. However, these concerns about eye bank pre-cut tissue appear to be unfounded with reported transplant outcomes similar to that of surgeon-prepared tissue [22].
Both manual dissection and microkeratome cutting are used by eye banks in providing pre-cut tissue, and femtosecond laser cutting of EK lenticules is currently under development. However, all of these procedures present many significant financial, logistic and technical challenges for eye banks to undertake.
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E. Pels and G. Pollock |
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Microkeratome Cutting
The microkeratome (for DSAEK preparation) is the most common procedure employed by eye banks providing pre-cut tissue. This activity is growing rapidly – in the United States for the year 2009, 20% of EKs were performed with eye bank pre-cut tissue, whereas in 2008 this figure was less than 1% [22]. European data suggest their 2009 pre-cut use was approximately 6%. In New Zealand and parts of Australia, all DSAEK tissue is provided pre-cut by the eye bank. The slower uptake in Europe may be due to the fact that in many European countries pre-cutting is considered as engineering of the tissue by national regulatory authorities, and an upgrade to a Good Manufacturing Practice class B environment is required in order to perform the cutting.
The thickness of the lenticule produced by microkeratome cutting is dependent on the interactions between the speed of movement of the cutting head across the cornea, the head size and the degree of pressure applied within the artificial anterior chamber. Experience is required in balancing these variables to produce the required thickness. Automated microkeratomes with set drive speeds and controlled calibrated pressures reduce this flexibility in approach but in less experienced hands it may provide more consistency and predictability of the process. Microkeratome cutting produces a smoother stromal interface than manual resection, and this has been casually associated with increased dislocation rates of DSAEK compared to DSEK.
While there is now some accumulated experience and knowledge regarding DSAEK pre-cut tissue with hypothermically stored corneas, there is limited experience in using organ-cultured stored corneas. The dynamics of the swelling of the cornea during the storage phase and the reversal of the swelling in the transfer/ transport phase of the technique complicate the timing and approach to a DSAEK cut. In addition, at the physiological temperatures of organ culture, the epithelium continues to grow, and down-growth of the epithelium to the cutting plane might be a risk. The effects of cutting prior to storage or prior to transfer into the transport/ tinning medium are largely unknown. Limited experience of cutting after transfer and thinning (and subsequent transport) in 6% dextran medium is available (Venice Eye Bank, personal communication 2010). In Australia and New Zealand, organ cultured DSAEK corneas are cut 24 h after placement in a 5% dextran solution and short-term outcomes appear to be satisfactory, although obtaining optimal thickness of the posterior lenticule, at the time of cutting and after a period of transport, has been difficult (personal communication). The use of higher dextran concentrations to further dehydrate and thin the cornea allowing a deeper cut and a thinner lenticule, and the full effects of swelling, reversal of the swelling and transport after cutting requires further investigation.
Femtosecond Laser Cutting
Femtosecond laser preparation of donor tissue is an appealing concept since it provides bladeless computer-controlled precision cutting of the cornea with minimal adjacent stromal and endothelial cell morphological changes. The cutting plane,