Ординатура / Офтальмология / Английские материалы / Corneal Endothelial Transplant (DSAEK, DMEK & DLEK)_John_2010

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Figures 35-3A and B: (A) Prior partial thickness trephination, viscoelastic material is placed on the surface of the endothelium to protect the cells (arrow). (B) Trephine is placed on the endothelial surface to mark the area to be dissected for the Descemet transplant.

endothelium). This maneuver allows the margins of the trephination to become clearly visible to facilitate easy separation of the trephined Descemet’s membrane (Figure 35-4). A 27-gauge needle with a bent tip may be used to separate the edges of the Descemet’s membrane at the level of the trephination. Using a cyclodialysis spatula or a Sinskey hook, the Descemet membrane is then carefully separated from the posterior stroma, thus providing a donor disk devoid of the accompanying donor stroma (Figures 35-5A and B). The donor disk is left on the cornea for later transferal to the recipient stromal disk. The average endothelial cell loss in the six samples analyzed was 8.46% just after the harvesting procedure.

Transfer of Descemet’s Membrane to Posterior Stromal Disk

The recipient procedure is then begun in the modified MAPK fashion to retrieve a posterior lamellar recipient disk.70 Once the disk is removed from the recipient’s cornea, it is carefully marked to maintain the original orientation

Figure 35-4: Margins of the Descemet’s membrane (arrows) are peeled off with the bent tip of a 27-gauge needle.

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Figures 35-5A and B: A Sinskey hook is used in this case to detach the Descemet’s membrane. See the tension lines of the membrane (arrows) representing the instrument in the correct dissection plane. No resistance should be noted during this step when inserting the instrument, any resistance is a sign of a stroma-stroma dissection (incorrect plane).

in the recipient bed at the time of reinsertion in recipients bed. The recipient stromal disk will carry the endothelial cell layer and Descemet’s membrane from the donor, as an autologous carrier. The posterior lamellar disk is placed endothelial side up on a trephination punch block and

through Descemetorhexis the diseased endothelium is removed. Once the whole recipient Descemet’s membrane is removed, it is replaced by the one we previously dissected from the donor cornea, carefully sliding the membrane over the recipient stromal disk (Figures 35-6A and B). The membrane is allowed time to adhere so that the disk is now composed of recipient posterior stroma and donor Descemet’s membrane/endothelium (Figures 35-7A and B) (will be called a compound disk hereafter). After adherence is assured, the compound disk is then transplanted into the recipient’s stromal bed with special attention to preserve the original orientation of the disk through the marks previously performed, and allowed to self-adhere. The flap will then be closed and sutured securely.

Although a stromal lamellar interface does occur in this procedure, it is foreseen that its distortion effect will be negligible because the interface will be recipient-recipient in nature rather than donor-recipient, and the stromal disk orientation is preserved. It is believed that each person’s

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Figures 35-6A and B: (A) Transfer of the Descemet’s membrane to the recipient disk by sliding the disk underneath the membrane.

(B) Histology of the dissection plane showing an almost complete Descemet’s detachment with no stroma attached (PAS 200X).

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Figures 35-7A and B: (A) Descemet’s membrane completely transferred to the recipient disk before insertion in recipient stroma.

(B) Microscopic view on endothelial cells showing few damaged cells (trypan blue stain) after the procedure.

stroma has a unique micro-orientation of collagen fibrils in such a way that when a donor-recipient interface is created, there is distortion created by the differing orientation. With this procedure, this is not expected to occur.

There are several potential benefits to this approach:

1.In preparing the donor button, the endothelial side facing upwards during the whole harvesting process makes the procedure easy and protective to the donor endothelium.

2.The “Descemetorhexis” of both donor and recipient will likely decrease any optical interface distortion. As it was previously shown that the recipient’s posterior stromal surface is smooth after Descemetorhexis, combining it with donor Descemetorhexis should further eliminate any possible distortions caused by stromal collagen remnants. We expect to obtain 20/20 or better BCVA (if permitted by retinal status) in most cases, which would be a considerable improvement over more recent techniques.

3.Technical ease is afforded to the recipient procedure by use of a modified microkeratome-assisted corneal flap technique.

4.Fast recovery is warranted by current MAPK technique.

The possible challenges of this novel procedure are:

1.Descemet’s membrane folds.

2.Presence of minor postoperative astigmatism as is usual in modified MAPK.

3.Flap complications as indicated previously with the MAPK procedure.

4.Larger opening affecting the corneal stability compared to small incision approaches.

Future

With the many possible options for corneal transplantation, its future seems promising. The advent of adhesives and advances in materials science will continue to foreward the various corneal transplantation techniques. In particular, the femtosecond laser provides promise because it creates smooth stromal cuts with consistent interfaces. Studies are underway using this technology. In addition, the possibility of culturing endothelial cells and reinserting them in patients in an autologous fashion may be feasible soon. We are exploring new methods of culturing these cells pointing in this direction for the near future.

For all of the newer procedures, long-term follow-up data is needed to determine whether these techniques will become safe and effective alternatives to penetrating keratoplasty.

References

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Introduction

The goal of most transplant surgery is the specific replacement of the diseased structure, with as little disruption to neighboring structures as possible. Corneal surgery is no exception. In the rapidly evolving field of posterior lamellar keratoplasty (PLK) the progression from penetrating keratoplasty (PK) to PLK is an excellent example. Now the thin layers of stroma carrying endothelial cells in many variations of PLK first described by Melles1 and further developed by Terry and Oulsey2 and Price3 -5 go a long way to satisfying this aim. However, the goal of specific endothelial cell transplantation6, 7 is not fulfilled. In this chapter a technique enabling specific endothelial cell transplantation with no corneal stroma is described. Although in its infancy, in comparison to other techniques of PLK, true endothelial cell (TEnCell) transplantation, transplants only donor’s Descemet’s membrane (DM) as a carrier for the endothelial cell layer.8

Indications

•Fuchs’ endothelial dystrophy

•Pseudophakic bullous keratopathy.

Aim of Surgery

•Pain relief

•Visual recovery.

Developing the Endothelial Cell

Harvesting Technique

The essential and challenging step of the procedure is a reliable method of successful harvesting of the endothelial cell sheet on the DM.

The initial technique was developed during the preparation of the donor material for a deep anterior lamellar keratoplasty (DALK). The DM and endothelial cell layer of the donor tissue is usually removed prior to anterior lamellar corneal grafting. This is often done by scrapping it or removing it in pieces. Rather than destroying the endothelial tissue, there is opportunity to remove DM and endothelial cells in one piece.

Instrumentation

i.Trephine block

ii.Two pairs of plain microforceps

iii.Handheld, long handled 7.5 – 7.75 mm trephine

iv.Pollock forceps

v.Modified Sinskey hook/Custom bent needle for DMrhexis

vi.Tappin’s endothelial cannula (ALTOMED®)

vii.Insulin syringe.

Practicing Harvesting

The surgeon can practice and master the technique of DM harvesting in a single piece, when preparing the DALK tissue, without compromising its quality.

Harvesting Technique

The donor button is positioned epithelial side down in a block. The DM peel is initiated at Schwalbe’s line using plain microforceps. In this case, iris root remains attached (Figure 36-1). A circumferential arc of about four clock hours of Schwalbe’s line is dissected. Usually this is found after removing iris root (Figure 36-2). The microforceps are used to grip the free edge of DM and this is extended for three millimetres towards the center of the cornea (Figure 36-3). This loose flap is then placed back on the corneal stroma (Figure 36-4). The hand held trephine is used to cut the donor button (Figure 36-5). The free edge of DM is picked up at the edge of the trephined button using the same two pairs of plain microforceps (Figure 36-6) and the disk of DM is peeled as a single sheet (Figures 36-7 and 36-8).

Histological Analysis, Vital Staining and Viability

Histological analysis of the harvested tissue demonstrates that this technique provides a viable endothelial cell layer supported only by DM. There is no attached stroma.

Figure 36-1: Initiation of DM peel. The peel begins by picking up Schwalbe’s line. In this example some of the iris root remains attached. Schwalbe’s line can be seen as the white line grasped by a pair of nontoothed forceps.

Figure 36-2: Dissection of an arc of four clock hours.

Figure 36-3: Extension of free edge towards the center of the cornea.

Figure 36-5: Manual trephination.

Figure 36-6: Free edge of Descemet’s membrane picked up with microforceps.

Figure 36-8: Completion of Descemet’s membrane peel.

Figure 36-11: Vital staining of the harvested tissue. The non-viable cells have taken up staining. The cell walls of the viable cells can faintly be seen.

Figure 36-9: Cross section of endothelial cells on Descemet’s membrane.

Figure 36-10: Methylene blue staining of intact endothelial cell sheet.

Harvested tissue is stained with haematoxylin and eosin (Figure 36-9) and methylene blue (Figure 36-10).

Harvesting DM involves tractional forces that may pose a threat to the viability of the endothelial cells and compromise the final outcome. Vital staining of the

harvested endothelial cell sheet 9-11 shows viability of the cells after harvesting (Figure 36-11). Vital staining involves staining the harvested tissue with Trypan blue and Alizarin Red-S. Trypan blue stains only the nuclei of non-viable cells and Alizarin Red-S stains the extracellular space, demonstrating the outline of all cells. Figure 36-11 shows a small number of non-viable cells. Under higher power, the cell walls of the viable cells can faintly be seen. The most important measure of final endothelial cell viability after transplantation is the postoperative endothelial cell function as evidenced by the final corneal clarity, visual acuity, central corneal thickness and endothelial cell count.

TEnCell Transplantation

Once the skill of reliably harvesting the endothelial cell layer on DM has been attained, the surgeon can proceed to performing TEnCell transplantation. The harvested sheet requires introduction into the eye, and this is achieved by placing it, endothelial side down on Tappin’s cannula (Figure 36-12) upon which is a layer of 2% hydroxypropyl methylcellulose (HPMC) (Figure 36-13). The patient’s pupil is dilated using tropicamide 1% to facilitate visualization of the host Descemet’s membrane excision (Descemetorhexis).12 An 8.0 mm partial thickness (2/3 of corneal depth) superior corneal incision or limbal incision is made with a 15 degree blade (Figure 36-14). A 27 gauge bent needle or a modified Sinskey hook is introduced into the anterior chamber (AC) in order to perform a modified Descemetorhexis which is necessary to remove the diseased endothelium (Figure 36-15).

The procedure is performed like an inverted capsulorhexis, avoiding the use of viscoelastic as this may

Figure 36-12: Tappin’s cannula (Altomed ®).

Figure 36-13: Tappin’s cannula with a layer of HPMC and the harvested endothelial sheet.

Figure 36-14: An 8.0 mm partial thickness corneal incision.

Figure 36-15: Descemetorhexis.

interfere with adherence of the transplanted endothelial cell sheet. The Descemetorhexis can be performed in a curvilinear pattern or it can be scraped off in small segments. Care should be taken not to destroy the posterior stromal surface. The procedure can be difficult as the detached DM tends to fold back to its initial position in the absence of viscoelastic. It is useful to gently apply a 7.5 mm trephine on the epithelium in order to outline the area of DM to be excised. Alternatively calipers and Jensen violet can be used. Once the Descemetorhexis is performed, a Simco cannula is introduced into the AC to remove the remains of the detached tissue (Figure 36-16). At this point, the initial 8 mm incision is made full-thickness and the AC is formed with balanced salt solution (BSS) (Figure 36-17). The edge of the cornea is lifted with a pair of microtoothed forceps and the cannula with the donor DM is introduced into the AC of the recipient eye (Figure 36-18).

Figure 36-16: Removal of detached Descemet’s membrane with Simco cannula.

Figure 36-17: Full-thickness corneal incision.

Figure 36-18: Introduction of Tappin’s cannula with endothelial sheet into the anterior chamber.

Figure 36-19: Injection of air through Tappin’s cannula.

Figure 36-20: Closure of the corneal incision.

Care should be taken so that the thin sheet of tissue is not folded during insertion. Once the cannula is in the AC, a bubble of filtered air is injected through its center, which elevates the donor tissue and apposes it to the recipient stroma (Figure 36-19). HPMC is used because the air-bubble propagates better than with other viscoelastics, ensuring that the air is evenly distributed under the donor tissue which helps lift the endothelium into place against the posterior surface of the cornea.

The cannula is withdrawn from the AC. This is a critical step and care should be taken not to drag the implanted tissue during withdrawal. This can be achieved if the surgeon, while withdrawing the cannula, continues to inject air into the anterior chamber. The corneal incision is closed using 10.0 nylon sutures (Figure 36-20). It is important to avoid the edge of the donor endothelium with

the needle as this can dislodge the edge of the grafted tissue. It is important to maintain an air bubble in the anterior chamber during suturing as this keeps the endothelium in place and allows the surgeon to know where to inflate more air at the end of the operation (Figure 36-21). This avoids inadvertently injecting air between the cornea and donor endothelium.

It is important that the air-bubble is the correct size; usually ½ to ¾ that of the AC. If the air-bubble is smaller than this, the DM may not attach successfully. If the air bubble is too big, pupil block glaucoma can be induced.

Subconjunctival antibiotic and steroid is injected and a bandage contact lens is applied. Postoperative antibiotic drops and steroids are started. [Editorial Note: Prednisolone acetate 1% (Pred Forte 1%, Allergan Inc., Irvine, CA,) six times daily and levofloxacin 1.5% (Iquix, Vistakon Pharmaceuticals,