Best practices for handling, preserving PDEK corneal tissue still being established

In PDEK, it is unknown how much endothelial cell loss occurs, but an advantage is the ability to use donors of all ages.

Issue: July 10, 2017

BySumit Garg, MD

ByElizabeth Shen, MD

Pre-Descemet’s endothelial keratoplasty, a new variant in the field of endothelial keratoplasty, mainly comprises the separation of the pre-Descemet’s membrane along with the Descemet’s membrane-endothelium complex from the residual donor stroma by the formation of a type 1 bubble. The feasibility of the PDEK procedure with adult or infant donor tissue makes it highly acceptable when there is a shortage of donor tissue. The problem is making the grafts, and today eye banks supply ready-made PDEK grafts to the surgeon.

In this column, I would like to invite Sumit “Sam” Garg, MD, to share his experiences on eye bank-prepared PDEK grafts.

Amar Agarwal, MS, FRCS, FRCOphth
OSN Complications Consult Editor

Tissue bank preparation of endothelial tissue is constantly evolving. The increase in Descemet’s membrane endothelial keratoplasty procedures has been swiftly followed by improved strategies for DMEK preparation. Pneumatic dissection of tissue, widely used for Descemet’s anterior lamellar keratoplasty, has more recently become a time-saving “no-touch” alternative to the manual dissection traditionally used for DMEK preparation. It has also become the primary technique for developing pre-Descemet’s endothelial keratoplasty grafts.

PDEK

While DMEK strips only Descemet’s membrane and endothelium from recipient stroma, the proposed advantage of PDEK lies in the preservation of the pre-Descemet’s layer. The big bubble technique has enabled the eye bank community to obtain a graft that potentially combines the advantages of younger donor tissue with the better visual outcomes of DMEK. Other potential advantages of PDEK tissue include higher endothelial cell counts (secondary to younger donors), grafts more resilient to tearing (due to the slightly thicker nature of the graft), and theoretically similar rates of rejection as compared with DMEK.

In DMEK preparation, a type 2 air bubble is injected, which extends to the corneal periphery separating pre-Descemet’s layer and Descemet’s membrane. This yields a DMEK graft approximately 15 µm thick and 10 mm in diameter. For a PDEK graft, a type 1 air bubble is injected between posterior stroma and pre-Descemet’s layer. This centralized bubble does not extend to the periphery, leaving peripheral tissue that is fused to the rest of the stroma. This yields a graft that is about 25 µm to 30 µm thick and 7 mm to 7.5 mm in diameter. The type of air bubble created depends on the cleavage plane at which air is injected and thus determines the type of graft.

PAGE BREAK

Technique

All selection criteria for DSAEK and DMEK tissue also apply to PDEK tissue selection, except for donor age. DMEK grafts require tissue to be from a donor in the fifth or sixth decade, whereas PDEK grafts can be taken from donors as young as 2 years of age.

1. The donor sclerocorneal discs are placed endothelial side up and covered with viscoelastic, balanced salt solution or Optisol. Scoring Descemet’s membrane at the limbus every few clock hours is an optional step that may help avoid formation of a type 2 bubble. A 30-gauge needle is inserted approximately 2 mm from the limbus, bevel up, into the posterior stroma (Figure 1).

2. The needle is advanced to the center, and air is injected until a big bubble is formed.

Figure 1. Insertion of 30-gauge needle into posterior stroma of donor tissue. Formation of a type 1 bubble by injecting air into the posterior stroma.

Figure 2. Graft stained with trypan blue before being cut.

Figures 3 and 4. Cutting along the border of the graft outside the diameter of the bubble. S-stamp applied to the pre-Descemet’s side of the graft.

Figure 5. The backward S-stamp when visualized with endothelium side up.

3. If the surgeon prefers to cut the graft, the bubble can be deflated and the tissue stored and delivered to the surgeon to be cut in the operating room. In this instance, trypan blue is typically applied before storage to visualize endothelial damage. Alternatively, if the surgeon prefers precut tissue, the graft with the bubble is cut with scissors by the eye bank before storage (Figure 2).

4. A hinge spanning approximately 1 clock hour is maintained to keep the graft in place (if scissors are used), and an S-stamp is applied to the pre-Descemet’s side of the graft before storage (Figures 3 and 4). Note that the graft must be gently folded to expose the pre-Descemet’s side and then folded back (Figure 5). The graft, stained with trypan blue, can then be grossly assessed for endothelial loss.

Endothelial cell loss

The primary disadvantage of a pneumatic dissection is the inability to check endothelial cell count with specular microscopy after the bubble is formed. Studies of eye bank-prepared DMEK tissues report an estimated endothelial cell loss of between 5% and 15%. The few studies in the literature on PDEK suggest that endothelial cell loss in PDEK tissue preparation is no worse than what has been observed in DMEK preparation.

Because PDEK preparation is still in its incipient stages, it is unknown exactly how much endothelial cell loss occurs. However, the ability to use donors of all ages is a significant advantage over DMEK preparation because tissue from younger donors has higher endothelial counts than older donors. Furthermore, grafts that are precut could in theory be used for dual purposes: a PDEK graft and a DALK graft. Determining the best practices for handling and preserving precious corneal tissue is of utmost importance. The emergence of PDEK is yet another stride toward improving endothelial keratoplasty.

For more information:
Amar Agarwal, MS, FRCS, FRCOphth, is director of Dr. Agarwal’s Eye Hospital and Eye Research Centre. Agarwal is the author of several books published by SLACK Incorporated, publisher of Ocular Surgery News, including Phaco Nightmares: Conquering Cataract Catastrophes, Bimanual Phaco: Mastering the Phakonit/MICS Technique, Dry Eye: A Practical Guide to Ocular Surface Disorders and Stem Cell Surgery and Presbyopia: A Surgical Textbook. He can be reached at 19 Cathedral Road, Chennai 600 086, India; email: dragarwal@vsnl.com; website: www.dragarwal.com.
Sumit “Sam” Garg, MD, is vice chair of clinical ophthalmology and medical director at the Gavin Herbert Eye Institute, University of California, Irvine. He can be reached at Gavin Herbert Eye Institute, University of California, 850 Health Sciences Road, Irvine, CA 92697-4375; email: gargs@uci.edu.

Disclosures: Agarwal reports no relevant financial disclosures. Garg reports he is the co-medical director of SightLife Surgical.