EFFECTIVE TOOL

Fourier domain OCT (FD-OCT) is an effective tool for obtaining corneal tomography with pachymetry even in highly pathologic corneas with thick fibrovascular panni, according to a study presented at the 2013 Congress of the Society of European Ophthalmology (SOE).

Discussing the applications of FD-OCT in cases of epithelial stem cell transplantation, Prof Marie-Jose Tassignon said that the latest imaging techniques are very helpful in assessing the success or otherwise of such transplants.

“Imaging is necessary in order to help us in the definition of success or failure in these transplanted eyes, as well as allowing researchers to compare outcomes of different clinical trials and help in the prevention of side effects,” she told delegates.

Among the advantages of cultivated limbal epithelial stem cell transplantation, Prof Tassignon cited the fact that there is no induction of limbal stem cell deficiency in the donor eye and that the procedure can be repeated if required.

It also represents an improvement in current safety standards by introducing a staged approach for pannus dissection, interspersed with intraoperative OCT imaging to allow complete pannus removal without development of untoward side effects, she said.

“FD-OCT imaging and simultaneous pachymetry mapping can be carried out prior to limbal stem cell transplantation, as well as intraoperatively to verify fibrovascular tissue removal, and postoperatively to detect graft survival and corneal thickness evolution,” said Prof Tassignon.

Explaining the concept, Prof Tassignon said that the limbo-amnion composite graft is generated by cultivating limbal epithelial stem cells on a standardised amniotic membrane, stretched within an interlockable amnion ring. The cells are cultured in CnT-20 medium with the addition of one per cent human AB serum for a period of two weeks, she said.

“The idea is to have an interlockable ring on which or into which the amniotic membrane will be stabilised. The limbal stem cells can be positioned on it and put in a stable culture medium where the cells cultivate in a reliable and reproducible way over a two-week period. Our experience has shown that two weeks is really the ideal time period after which the limbal stem cells have sufficient size in order to be transplanted safely,” she said.

To place the amniotic membrane graft, the surgeon first performs a 360-degree conjunctival periotomy and carefully prepares the recipient bed, removing all proliferative tissue, said Prof Tassignon.

Fibrin glue is applied to the surgically prepared recipient’s cornea and in one fluid motion the composite graft within the amnion ring construct is transferred from culture and positioned onto the graft bed. The required size is cut out at the level of the limbus by means of a trephine or microsurgical scissors. A protective layer of Healon and a second amniotic membrane patch is then positioned in order to protect the primary graft. To finish the procedure, the second membrane is tucked under the conjunctiva, sutured at four cardinal points and then covered with a bandage contact lens.

For intraoperative use, Prof Tassignon uses the iVue handheld SD-OCT device (Optovue Inc.) to monitor the dissection of fibrovascular tissue prior to placement of the amniotic membrane sheet containing the limbal epithelial cells.

“It is important to perform your surgery very accurately and it is important to be sure to remove all of the fibrovascular tissue and this is the reason why the handheld OCT has been very beneficial to our surgery. It gives us a good idea of how deep the fibrovascular tissue is present in the cornea so that we can be sure that we have actually removed everything,” she said. Once the surgery has been completed, OCT imaging is also useful in monitoring the postoperative evolution of the graft, said Prof Tassignon.

“We can monitor the evolution of the amniotic membrane as it becomes integrated in the corneal surface and epithelial cells regenerate on top of it. We can also use OCT to measure the thickness of the pannus and to measure the thickness of the endothelial cell layer as the cells reproduce over time,” she said.

Current clinical indications for the transplantation of limbal stem cells include multiple: infection, chemical burns, iatrogenic thermal burn and trauma, said Prof Tassignon.

“The results so far in a small series of patients have been quite good and some of them managed to avoid a secondary corneal transplantation which is quite positive,” she said. Prof Tassignon said that further studies with larger sample sizes are needed to substantiate the Antwerp team’s initial findings and to determine the limit for

minimal central corneal thickness post dissection in limbal stem cell deficiency. “We are aware that this has only been done in a limited number of patients, but based on the early results it seems to be a good idea to expand the study and to try to conduct a multicentre study in other European centres,” she said.