(a, b) Slit-lamp image and anterior segment optical coherence tomography image 1 week after Descemet stripping endothelial keratoplasty (DSEK). (c, d) Slit-lamp image and anterior segment optical coherence tomography image 1 day after Descemet membrane endothelial keratoplasty (DMEK)
In DMEK, healthy donor endothelium and Descemet membrane are implanted without the additional posterior donor stroma that is included with DSEK .
11.1.1 Advantages of DMEK Compared with DSEK or PK
Nearly exact anatomic replacement of dysfunctional host endothelium.
Better and faster visual recovery and more predictable refractive outcomes [5, 6, 13, 14, 18, 24–27, 34, 41, 43, 45, 46, 49, 53].
Reduced risk of immunologic rejection, allowing less use of prophylactic topical corticosteroids and reduced risk of steroid-associated side effects [5, 6, 18, 26, 51, 52].
No need for expensive instrumentation, such as the microkeratome typically used in DSEK tissue preparation, so it is readily accessible to corneal surgeons in any surgical setting.
11.2 Step-by-Step Approach for Success with DMEK
DMEK is generally considered more challenging than DSEK or PK. A structured approach with careful attention to the key elements discussed below will help ensure success.
11.2.1 Preparing DMEK Donor Tissue
The donor tissue can be prepared by the surgeon or by the eye bank . Preparing the tissue ahead of time and preserving it in corneal storage medium saves time and reduces stress on the day of surgery [19, 21, 37].
The DMEK graft is quite thin and delicate, because it consists of DM and the endothelial layer without any stroma and the endothelium is on the outer or exposed surface, so proper handling is important to minimize endothelial cell loss and avoid inadvertent tears.
11.2.2 DMEK Donor Criteria
The tissue from older donors (over 40 or 50 years old) is easier to handle with DMEK. The thickness of DM is about 3 microns at birth increasing to about 20 microns in adulthood [29, 31]. In children, DM is more fragile and more tightly bound to the stroma. Also, the tissue harvested from young donors curls up tightly and can be difficult to unfold inside the eye .
The tissue from diabetic donors can be more prone to develop tears during preparation [23, 60]. However, since the prevalence of diabetes is so high among tissue donors (30 % or higher), diabetic tissue cannot be completely excluded for DMEK. More careful characterization of donor diabetes is needed to determine whether certain thresholds of diabetes duration or degree of glycemic control should be exclusionary. Diabetic tissue may present more challenges because of glycation products from chronic hyperglycemia, which deposit between the posterior stroma and DM creating stronger adhesion, and the wide-spaced collagen present in the DM of diabetic corneas, which may have a deleterious effect on the tensile strength [23, 60].
11.2.3 Sizing the Graft
Measure the white-to-white diameter of the host cornea before choosing your donor punch. A small size graft in bigger host cornea may not be able to replace the required amount of endothelial cells especially in non-Fuchs endothelial decompensation when the entire cornea has decompensated.
A large donor graft might get stuck in the angle in a very small eye, and it might be hard to see the edges behind a very dense arcus. It is also important to consider that the paracentesis wound does not get covered up with the graft so that the irrigating cannula used to manipulate the graft with fluid has room to get into the anterior chamber without encountering the graft itself. A graft diameter of 8.0–9.0 mm is suitable in most cases.
11.3 Harvesting the Graft
Two general approaches have been used to isolate donor DM and endothelium: direct peeling and pneumatic separation. Direct peeling (itunes.apple.com/us/book/digital-manual-ophthalmic/id803683426?mt=13) is used more widely because many have achieved a higher success rate with less endothelial cell loss with this approach. Many variations have been developed within each general approach, and the exact techniques used by the Melles group [40–42], the Kruse group , the Ignatio group , and our group [57, 58] have been detailed in the literature.
We prefer the modified SCUBA (submerged cornea using backgrounds away) technique. This technique suspends the tissue with fluid above and below making it easy to visualize and handle during preparation [22, 57, 58].
Here we will discuss every step of DMEK graft harvesting, which starts with scoring of DM and ends with trephination and final peeling.
11.3.1 Scoring DM
Scoring is the process of bluntly marking the periphery of the posterior cornea. It can be done with a blunt tying forceps or a Y-hook with the donor corneoscleral rim placed on a cutting block or submerged in a donor tissue-viewing chamber (Krolman viewing chamber, K55-57007-23, Bausch & Lomb, Rochester, NY) [22, 58] (Fig. 11.2). The donor is fixated with a toothed forceps, while a peripheral break or “score” in the DM is created. A blunt Y-hook (9217E, Ambler Surgical, Exton, PA, 17157 Moria, Antony, France) can be used instead of tying forceps to create the initial score in the DM, because it produces a smoother break with fewer tears. The score line should be positioned just inside the trabecular meshwork.
(a) Krolman viewing chamber. (b) Cutting block
The Y-hook should be pressed firmly against the submerged corneoscleral rim to penetrate the DM without tearing the underlying stroma and can be lightly dragged along the periphery to propagate the score. Caution should be exercised not to press too hard or impale the delicate posterior stromal fibers. A blade should not be used to create this score to avoid cutting into the stroma, which would make it difficult to grasp or lift the DM without also grabbing the edge of the connected stroma. If counterpressure is needed to penetrate the DM, the centrally protruding viewing chamber pillar (Krolman viewing chamber) can be used with the tissue submerged, or the donor can be placed on a cutting block [57, 58].
11.3.2 Improving the View of the Scored Edge
To allow visualization of the scored edge, staining should be performed for 10 s using a 0.06 % trypan blue solution (VisionBlue; DORC, Zuidland, the Netherlands) after scoring is completed and before elevating the edge of DM. Trypan blue does not stain endothelial cells in their native healthy state when applied for a short time. During a 10-s exposure, only cells that are damaged by the scoring and exposed areas of DM itself will stain [32, 58].
VisionBlue improves the view of DM breaks and DM scarring (cataract wounds). “Snail tracks” in the endothelium from the donor harvest also stain, as do insufficient scoring strokes that only denuded endothelial cells. Such unscored zones can cause radial tears during the next step of lifting the edge of the DM.
11.3.3 Lifting the Edge of DM
After scoring and staining, the donor corneoscleral rim is returned to the storage chamber for visualization under the microscope. The scored edge of the donor DM is lifted 360° with a small blunt instrument such as a microfinger (Moria, Antony, France) . The tip of the microfinger is inserted 0.5 mm under the edge of DM and held against the stroma (Fig. 11.3a). The microfinger is glided circumferentially while maintaining the depth of the penetration to lift a consistent width of the DM. It should be glided from the area of minimum tension (loose DM attachment) toward the area of maximum tension (firm DM attachment). If a radial tear develops, it should first have the loose edges torn off leaving a new continuous torn edge. This converts the tear into a “crater” in the scored diameter. Then gently glide the microfinger back and forth along the free edges to lift the peripheral DM down to the circumference of the crater. The crater is lifted last. In lifting the crater, always carefully watch for further tearing. Each crater can limit the final diameter of the donor graft depending on its size and the diameter of the cornea with which you started.
(a) Using a microfinger to lift the scored edge of the graft. (b) Peeling off the donor Descemet membrane and endothelium while the tissue is submerged in storage medium. (c) Cutting block with trephine. (d) Donor Descemet membrane roll
11.3.4 Peeling DM
Before final trephination, 360° partial peeling of the DM is performed. This allows any zones of strong adhesions (which can develop tears) to be discovered. Such areas can be excluded from the final trephination depending on where they are located. The limbus is fixated with a toothed forceps, and the DM edge is grabbed with a Tubingen forceps (2457E, Ambler Surgical) or other large, smooth tiers, exactly central to fixation . To avoid tears, the Tubingen forceps should not be twisted or elevated away from the cornea, because twisting can break DM just as forceps can do with suture. Each pull should be straight across . A prominent stress line will appear if the pull deviates to the side.
The DM is peeled by quadrants (“corridor method”, Fig. 11.3b). This technique minimizes tension by minimizing the width of the peel zone [57, 58]. The more centrally a peel is taken, the wider it gets, with increasing tension on the DM. The peel edge of the first quadrant is taken about halfway to the center of the cornea. Peel 2 should be started 180° away and also is taken halfway to the center. Peel 3 is started 90° to the left or the right of peel 2. A narrow corridor of unpeeled DM remains at this point. Because the peel zone now can get no wider than this corridor, the tension is low, making it safe to peel quadrant 3 close to the center of the cornea. Peel 4 (180° away from peel 3) is taken halfway to the center. Efforts should be made to grab a “peak,” not a “crater” along the perimeter as the starting point of each pull. The meridian of peel 4 is marked as the intended start site for the final peel after trephination. Because peel 3 is taken close to the middle, the donor would fully separate from the stroma before the forceps advances over the opposing sclera (thus reducing the risk of endothelial touch).
Alternatively, one can peel DM by sequentially rotating the donor tissue 30–60° at a time and stripping partway across . This technique is especially useful when encountering separation resistance. This can be felt as tension or seen as smile configuration or stress line as the tissue is being peeled. When the leading edge of the peel has minimal resistance, it forms a straight line. When the right and left edges of the peel lag behind the central portion (forming a “smile”), the resistance is high. In such circumstances, peel at 45° to both sides of this site, then the peel can be continued from the original site. This technique lessens tension by limiting the width of each peel.
11.3.5 Handling Horseshoe Tears
Continuing to pull after a horseshoe tear (HST) develops can tear the donor in half. Interestingly, “descemetoschisis,” or “lamellar splitting” of the DM, can precede most HSTs as a tongue or triangular attachment of the anterior layers of the DM on the stroma. If an HST is suspected but hard to see as the tissue is being peeled, stop and apply focal VisionBlue staining. To complete the preparation in the presence of an HST, one strategy is to stop pulling at that site, partially peel all other quadrants, trephine, and then start the final donor peel 180° away from the HST [57, 58]. If two HSTs develop 180° apart, the tongue of one has to be lifted so that the peel can be continued through that HST. The centrally protruding Krolman viewing chamber pillar can be used for counterpressure to lift the tongue [57, 58]. The tissue with descemetoschisis or partial tears can still be used safely for DMEK .
11.3.6 Trephining DM
The trephine size should be selected based on the size of cornea and type of pathology. For FECD, an 8-mm graft diameter is sufficient, because FECD patients have healthy peripheral corneal endothelium. However, with pseudophakic bullous keratopathy or failed endothelial keratoplasty, a 9-mm graft will provide more healthy endothelial cells.
To trephine, the corneoscleral rim is placed on the cutting block and aligned with any imperfections outside the trephine zone. A cutting block with a marked central 8-mm area, such as the Hanna punch with a Price-cutting block (17169 & 630/660–7540, Moria), helps to get perfect alignment especially if the punch needs to be decentered [57, 58]. Suction is activated on the cutting block. The DM is checked to ensure that it is not curled. If it is curled, the storage solution is wicked from under the DM with a cellulose sponge so that the DM follows the fluid peripherally to lay flat. More fluid can be dripped on and soaked away as necessary. The guide cylinder is attached. The trephine punch cylinder is advanced into the guide until the blade makes contact with the cornea without rotating (Fig. 11.3c). The corneal thickness holds the punch cylinder up so that a small gap remains present between the guide and punch rims. The thumb and index finger of one hand is placed in this gap to hold the circumference of both rims to prevent rotation, while the other hand is used to lightly tap the punch in all four quadrants. When the gap is lessened, tapping is stopped so that the cornea is only penetrated but not perforated [57, 58].
11.3.7 Final Peel of the Donor
The donor is briefly restrained with VisionBlue, the sclera is grasped at the meridian of peel 4, submerged, and the peripheral rim of the DM beyond the trephine marks is removed with caution to separate sites with adhesions last. The final peel should be done at the same meridian where peel 4 is made, so the donor gets separated when the peel reaches the middle. When submerged, the donor forms a scroll with the endothelium on the outside [19, 40]. The scroll can be secured in the corneoscleral rim filled with the storage solution if it is to be used at that time (Fig. 11.3d). It should be placed in a glass vial with smooth edges filled with the storage solution if it is intended to be used another day [57, 58].
 described making three semicircular orientation marks along the edge of the graft to help in later determining the anterior-posterior orientation of the donor graft while positioning it in the anterior chamber (AC).  described making an S-stamp on the donor tissue during preparation to confirm orientation after insertion into the eye.
11.4 Recipient Preparation
11.4.1 Preoperative Preparation
We prescribe topical ketorolac tromethamine 4 times a day for 2–3 days prior to surgery. This is especially useful if DMEK surgery is done under topical anesthesia, because it decreases the baseline level of prostaglandins in the eye and conjunctiva, which in turn reduces pain and discomfort during surgery .
Anesthesia considerations include:
As in any ocular surgery, the patient’s head and eye should remain still.
The eye should be soft with minimal back pressure.
Surgery may take longer for those new to the procedure.
We prefer to perform DMEK with topical anesthesia supplemented by intravenous sedation and intracameral lidocaine (non-preserved) to eliminate the risks associated with general anesthesia or a local block. We place scleral traction sutures with 4–0 silk at 6 and 12 o’clock to help prevent the patient from squeezing or moving the eye and facilitate eye positioning by the surgeon . For those who prefer to use a peribulbar or retrobulbar block, it is necessary to achieve a soft eye by applying a Honan balloon or similar device for 30 min to avoid posterior (vitreous) pressure during the surgery.
11.4.3 DMEK Wound Construction
Create one self-sealing clear corneal incision (main incision) for donor insertion and two self-sealing paracentesis wounds for DM removal and donor manipulation. The corneal wound can be superior or temporal, and the paracentesis wounds should be 45° away from the main wound. The corneal incision can be created with 2.8 mm or smaller keratome knife and the paracentesis wounds with 75° super sharp paracentesis blades. It is helpful to mark the non-cutting edge of the blade with gentian violet prior to making incisions, because it allows easier identification of the incisions during the rest of the surgery. This is crucial in cases with cloudy corneas or loose epithelium. The paracentesis wounds should be self-sealing, but not so long as to be covered by the donor graft, to allow better manipulation of the graft in the eye with irrigating cannulas.
11.4.4 Removal of DM
Unlike DSEK, in DMEK the donor tissue does not stick easily with retained DM or endothelium , so thorough removal of host DM is essential. Before punching the donor, we use the trephine to indent the epithelium of the recipient to verify if the planned donor size is appropriate and to serve as a reference mark for DM removal. Next, we outline the area of DM to be removed (scoring) and strip DM from within that area using an inverted hook (Price Endothelial Keratoplasty Hook; Moria SA or reversed Sinskey hook, D.O.R.C. International, Zuidland, the Netherlands) introduced via paracentesis (Fig. 11.4a). Scoring and stripping can be done under viscoelastic, balanced salt solution (BSS) infusion, or air infusion. Air or viscoelastic provides better visualization than BSS and prevents increasing corneal edema during the stripping process. Viscoelastic provides a more rigid and stable chamber but is more expensive than BSS or air, and it needs to be completely removed before graft insertion, because residual viscoelastic in the interface prevents donor attachment and may lead to interface opacity [5, 6].
(a) Air in the anterior chamber provides good contrast while removing the host Descemet membrane from the planned area where the graft will be placed. (b) After host Descemet membrane removal, the host cornea is stained with VisionBlue to help visualize any loose tags of host Descemet membrane or stroma. (c) A peripheral iridotomy is created with intraocular scissors; note that it is done inferiorly. (d) DMEK donor tissue being loaded into an intraocular lens cartridge. (e) Injector loaded with DMEK donor tissue
Air infusion is the most cost-effective method and provides the best contrast to ensure complete DM removal from within the planned graft area. The AC can be intermittently filled with air through a paracentesis or AC maintainer with an assistant injecting air as needed to keep the AC inflated.
After removing the host DM, it is advisable to spread it out on the surface of the cornea to make sure that it has actually all been retrieved. This is especially important for novice surgeons. Any retained DM can cause poor donor adhesion as well as visual aberration .
We recommend using VisionBlue after scoring and stripping of the DM to make sure there are no loose tags of DM or stroma or areas of attached DM (Fig. 11.4b). After removal of DM, fill the AC with BSS (first remove air or viscoelastic) and then inject VisionBlue. Let it stain for about a minute, and then irrigate the AC until it is clear. Areas of exposed DM will stain dense blue, those with exposed stroma will stain light blue, and those with attached DM will have minimal or no staining. Loose DM tags can be removed with irrigation and aspiration (I & A) or intraocular forceps after staining with trypan blue. Scissors can be used to cut off loose pieces of stroma. We create an inferior peripheral iridotomy (PI) with intraocular scissors or a vitrector in I & A/cutting mode and use bimanual I & A to aspirate the posterior pigment layer and ensure the PI is patent (Fig. 11.4c).
11.4.5 Inserting the Donor Tissue
Staining of donor tissue: After isolating the DM roll from the donor cornea, stain it with VisionBlue for about 60 s to make it more clearly visible during the process of insertion and orientation inside the AC.
Loading the tissue: The DMEK graft can be inserted into the eye with a glass pipette, intraocular lens (IOL) cartridge, or custom injector [19, 32, 45, 46]. An IOL cartridge provides a more controlled delivery than a glass pipette , but it is important to use a cartridge that seals against the back pressure in the eye, because using viscoelastic to create a seal in the cartridge impairs graft attachment [45, 46]. Various IOL injectors can be used (VISCOJECT, Medicel AG, Switzerland; Carl Zeiss Meditec, Jena, Germany; Softec; Lenstec Inc., St. Petersburg, FL). The IOL injector needs to be modified by removing the plunger and spring and reattaching the plunger without spring. This prevents back suction and inadvertent damage to the donor graft. Fill the cartridge with BSS and keep it in a well or container filled with BSS. The cartridge will float in a container filled with BSS, so it needs to be stabilized or held with forceps. With the other hand, hold the edge of the stained donor tissue with a small tying forceps. Direct the donor tissue into the cartridge filled with BSS until it is halfway up the barrel tip (Fig. 11.4d). Assemble the cartridge to the injector. Make sure the donor tissue moves without getting stuck in the plunger, and move the plunger forward until it seals its tip with the sides of the cartridge (Fig. 11.4e).
Tissue insertion: Place the cartridge into the main wound with the bevel facing downward; insert the tube all the way into the AC and insert the donor (Fig. 11.5a). Wound-assisted insertion of the donor DM roll should be avoided because it can damage the donor endothelial cells. Before removing the cartridge, decompress the AC by venting a paracentesis wound with an instrument and push on the cornea just central to the wound to prevent donor ejection from the AC.
(a) DMEK graft being inserted into the anterior chamber; note that the tip of the cartridge is well inside the anterior chamber. (b) Partially unwrapped graft with small air bubbles underneath; note that the anterior chamber is partially filled, and the wound has been sutured with 10–0 nylon. (c) Gentle strokes on the surface of the cornea with a hydrodissection cannula help center the DMEK graft. (d) The anterior chamber is filled with air, the DMEK graft is well centered, and the wound was sutured with 10–0 nylon
Suture wound: After insertion of donor tissue, the main wound may immediately be sutured with 10–0 nylon to prevent the graft from being expulsed from the AC.
11.4.6 Graft Orientation and Positioning
After the graft has been inserted, it may be loosely or tightly curled. Small intermittent bursts of BSS from a 10-ml syringe help in moving and rotating the donor tissue. With careful indirect manipulation of tissue with air and fluid, the graft can be oriented endothelial side down [19, 36, 41, 42]. The fluid can be directed to uncurl the donor. Before positioning the graft against the host cornea, it is important to confirm the orientation of the donor endothelium. The tissue can be pre-marked with an S-stamp or with asymmetrical orientation marks along the edge [32, 61]. Orientation can also be assessed by positioning the tip of a 30-gauge cannula on top of the membrane and underneath one of the peripheral curls . We prefer to use a hand-held slit beam or intraoperative OCT to confirm which way the graft is curled inside the eye before fully unfolding it, knowing that the endothelium always faces outward [10, 54].