PDEK surgery aided by use of S-stamp
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A pre-Descemet’s endothelial keratoplasty graft has numerous advantages over a Descemet’s membrane endothelial keratoplasty graft.
The extra 10 µm to 15 µm of tissue that the pre-Descemet’s layer is composed of, while not significantly increasing the thickness as compared with a DMEK graft, gives the graft significant handling advantages by acting as a splint, making it a tougher tissue and also allowing the graft to be harvested from younger donor cornea.
A commonly used technique for identification of graft orientation in endothelial keratoplasty grafts such as DMEK, Descemet’s stripping automated endothelial keratoplasty and ultra-thin DSAEK grafts is an irreversible letter stamp. Some examples of irreversible letters that can be used are S, F and P. An S-stamp can also be marked on the PDEK graft, thereby making it easier for the beginner surgeon to adapt and transition to PDEK.
We present below Jacob’s preferred technique for preparation of a PDEK graft.
Preventing a type 2 big bubble
The corneoscleral rim is placed endothelial side up on a Teflon block (Figure 1). Jacob uses a modification of Soper’s technique of scoring the graft by simply making multiple needle punctures on the far periphery of the donor cornea just within the limbus on the Descemet’s membrane. This prevents any accidental type 2 big bubble formed in the periphery from expanding further by acting as a vent for the air. The needle punctures avoid accidental peeling/shearing off of the Descemet’s membrane, which may happen with scoring. A Dua’s clamp has also been described to prevent a type 2 bubble from being formed.
Creating a type 1 big bubble
A type 1 big bubble or a PDEK bubble is then created by injecting a small volume of air into the periphery through a 30-gauge needle on a 5 mL syringe that is inserted through the sclera and advanced in a superficial plane into the peripheral cornea. Air is injected rapidly but without trying to push the piston of the syringe excessively to initiate the PDEK bubble. The bubble is then expanded further until the required size is obtained (Figure 1). Jacob prefers to enlarge the bubble using a very dilute solution of trypan blue to allow a more controlled expansion while at the same time allowing the margins of the expanding type 1 big bubble to be clearly seen. The bubble is slowly expanded to the desired size as measured roughly with a caliper while keeping in mind that the dome-shaped graft when lying flat after cutting yields a graft larger than the size measured before deflating the big bubble. For instance, a 7.5-mm dome-shaped bubble generally gives an 8-mm graft. The standard PDEK graft size that we are able to harvest varies from 8 mm upward to even 9 mm to 9.5 mm as desired.
Cutting the type 1 big bubble
The corneoscleral rim is next placed on the concave side of the Teflon block. The big bubble is entered into at the edge with a 15° side-port blade, and the adjacent sclera is then given a nick with the blade to help easily identify the location of entry into the big bubble later. The air in the big bubble is withdrawn and replaced with 0.06% trypan blue for about 10 seconds to quickly stain the graft. The trypan blue is then flushed out from within the bubble using balanced salt solution. The graft is then again inflated with air and cut circumferentially at the edges using Vannas scissors, leaving only a small hinge so that the graft remains attached to the underlying stroma at the hinge. In case the air bubble escapes before fully cutting, a few drops of balanced salt solution allow the graft to float up so that Vannas scissors can be used to continue cutting out this “flapped” graft.
Alternately trephining the graft
As an alternative to using Vannas scissors for cutting the graft, it may also be trephined to the exact desired size. If not marking the S-stamp (for instance, when using the endoilluminator-assisted DMEK/PDEK technique also described by Jacob for determining graft orientation), a trephine is gently placed well centered on the deflated bubble and pressed down to cut through superficial stroma. The cut edges of the type 1 bubble are gently freed, and any uncut areas, if present, are cut with Vannas scissors. If planning the S-stamp, a small, localized part of the trephine blade is blunted by rubbing a steel instrument on the localized area a few times. This allows an uncut hinge following which the graft can be stamped as described below. Special trephines with a small notch in the blade would also allow a flap to be created for stamping. It should be noted that residual fluid or air within the big bubble while trephining can give a slightly larger graft than the trephine size, and therefore, fluid/air should be aspirated out before trephination.
Replacing the graft flap
The cut or trephined “flapped” graft is folded to one side and excess fluid absorbed using a cellulose spear such as Weck-Cel. A 3-mm skin punch is then used to create a full-thickness punch in the host stroma that was underlying the big bubble. The circular punched tissue is kept aside safely. The flap is made to float again such that it covers the punched hole. This may be done by either using a cellulose spear to draw the flap into position or pouring a few drops of balanced salt solution over the flap such that it opens up and covers the trephined hole.
Marking the S-stamp
The corneoscleral button is then flipped over, and the trephined-out hole is seen from the epithelial side. The PDEK graft is seen at the base of the trephination with the pre-Descemet’s layer side now facing the surgeon. All fluid is gently dried from the pre-Descemet’s layer side using a cellulose spear, and an S-stamp or any other irreversible letter stamp is used to apply the mark (Figure 2). The stamp is applied after allowing the alcohol in the ink to evaporate for 10 seconds to decrease the risk for any localized endothelial cell loss. It is likely, though, that such loss may be less than with DMEK because of the protective effect of the pre-Descemet’s layer.
Completing the graft cut
The punched-out small circular cap is then replaced, and the graft is turned endothelial side facing up again. The remaining small hinge is cut, and the detached marked and stained graft is ready to be loaded into a glass inserter or any other cartridge of choice and transported to the surgeon for direct injection into the patient’s eye (Figures 3 to 4).
Discussion
The S-stamp is a commonly used technique that is useful to identify a flipped graft and rectify its position before floating it up. It has long been used for DMEK grafts in a similar manner as described earlier. Premarked and preloaded DMEK and DSAEK grafts are supplied by eye banks to increase the ease and speed of surgery and decrease operating room time. This also removes the stress of graft loss during preparation, allows better patient logistics and removes the possibility of surgery having to be postponed in case of graft loss. Graft loss during preparation can result in a significant financial loss as well if the graft is being prepared by the surgeon. For all these reasons, most surgeons prefer to have premarked and often also precut and preloaded grafts ready to be inserted.
PDEK grafts can also easily be prepared and supplied by eye banks as described above. Having similar premarked, prestained, precut and preloaded PDEK grafts readily available for use will help PDEK to be rapidly adopted by surgeons. Because PDEK surgery is easier to perform than DMEK even in complex eyes, it will allow the advantage of these grafts for a much larger pool of patients.
This is important because PDEK shares many surgical advantages of a DSAEK graft and many visual advantages of a DMEK graft, thus giving it unique advantages over both. A PDEK graft can be easily centered even after floating up when used in combination with the air pump-assisted technique as described by Jacob. The combination of the air pump-assisted technique together with the resilient and sturdy nature of the PDEK graft allows not only easy centration but also uncreasing graft edge unfolding and potentially may also allow quicker intraoperative graft attachment and a lower risk for graft detachment than DMEK. The graft can therefore quickly be floated up even in complex eyes, and time-consuming maneuvers that are required to perfectly center and completely open a DMEK graft can be avoided, especially in complex eyes and also for beginners. These advantages make PDEK far easier than DMEK even in complex cases such as vitrectomized eyes, failed penetrating keratoplasty grafts and aniridic eyes while still retaining the advantages of a DMEK graft, such as thin nature, less hyperopic shift, less effect on posterior corneal curvature and decreased rate of rejection as compared with DSAEK. In addition, PDEK offers the big advantage of being able to use young donor tissue with ease, thereby allowing a much larger endothelial cell pool to be transplanted.
Conclusion
Wider availability of prestamped, prestained, precut PDEK grafts supplied by eye banks will allow surgeons to rapidly adopt PDEK and will also allow easier transition from DSAEK by surgeons desiring to do so, even in complex eyes. Until eye bank supply is freely available, surgeons may also prepare the graft themselves as described above.
- References:
- Agarwal A, et al. Br J Ophthalmol. 2014;doi:10.1136/bjophthalmol-2013-304639.
- Agarwal A, et al. Cornea. 2015;doi:10.1097/ICO.0000000000000486.
- Jacob S. Open Ophthalmol J. 2018;doi:10.2174/1874364101812010175.
- Jacob S, et al. Cornea. 2017;doi:10.1097/ICO.0000000000001225.
- For more information:
- Amar Agarwal, MS, FRCS, FRCOphth, director of Dr. Agarwal’s Eye Hospital and Eye Research Centre, is the author of several books published by SLACK Books, sister company of Healio publisher 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 aehl19c@gmail.com; website: www.dragarwal.com.
- Soosan Jacob, MS, FRCS, DNB, of Dr. Agarwal’s Eye Hospital in Chennai, India, can be reached at dr_soosanj@hotmail.com.
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