Surgical Technique for Descemet Membrane Endothelial Keratoplasty

Introduction

Descemet membrane endothelial keratoplasty (DMEK) provides a true anatomic exchange of diseased endothelium and Descemet membrane (DM) with healthy donor tissue. There are several advantages of DMEK compared with Descemet stripping endothelial keratoplasty (DSEK), including faster visual recovery, lower rejection rate, and higher quality of vision. However, graft preparation, iatrogenic graft failure, and previously reported high rates of graft detachment are obstacles that have prevented many corneal surgeons from adopting this surgery. With eye banks currently preparing preloaded DMEK tissue, more corneal surgeons are beginning to adopt DMEK because the risk of preoperative tissue loss has been eliminated.

Similar to DSEK, all corneal surgeons will experience a learning curve with DMEK and should select their first patients keeping this in mind. This chapter discusses the authors’ experiences performing DMEK and provides a literature-based review of surgical techniques that have been implemented with DMEK. Although the techniques necessary to perform DMEK are different than DSEK, successful outcomes can be consistently achieved. The chapter outlines a sequential approach to this technique and highlights several vital intraoperative techniques that can make DMEK a reproducible, predictable, and successful procedure.

Injectors

One of the key steps in DMEK surgery is the safe and controlled delivery of the fragile DMEK scroll into the anterior chamber. The ideal DMEK injector should have multiple features including:

• A closed system that prevents backflow from the anterior chamber and is capable of maintaining anterior chamber volume

• A material that is safe for the endothelium

• No need for viscoelastic

• Easy loading with minimal handling of the graft

• An adequate caliber to avoid compression of the graft

• A tapered tip to seal the surgical incision well.

Currently there are no US Food and Drug Administration (FDA)-approved injectors specifically for DMEK. American surgeons have found many creative off-label methods to overcome this obstacle. Many surgeons use modified intraocular lens injection cartridges for this purpose. The Staar microinjector (Staar Surgical Company, Monrovia, CA) was one of the most common inserters initially used for DMEK in the United States ( Fig. 135.1B ). The Staar injector is an open system with a foam plunger and requires a viscoelastic agent to help deliver the tissue. This technique risks the introduction of viscoelastic into the anterior chamber during tissue injection, which can impede graft attachment. Another popular IOL cartridge–based system is the Viscoject intraocular lens (IOL) injector (Bausch & Lomb, Aliso Viejo, CA; see Fig. 135.1A ). The Price group reported improved rebubble rates when transitioning from the Staar injector with the use of viscoelastic to the Viscoject, which does not require viscoelastic and is a small closed-system inserter.

A similar closed-system injector can be created with the AMO Emerald One Series IOL cartridge (Abbot Laboratories Inc., Abbot Park, IL), with a few key differences compared with other IOL cartridge injectors (see Fig. 135.1E ). Assembled with a standard Luer-Lok syringe filled with balanced salt solution (BSS) and 14-French gauge nasogastric tubing, the Emerald One Series IOL cartridge is large enough that it can be used to aspirate the graft into the injector and small enough to be used through a 2.75-mm clear corneal incision. Similar to other injectors made of clear or translucent materials, graft orientation can often be maintained during injection. The authors suggest using a 1- or 3-mL Luer-Lok syringe with this injector to further enhance controlled delivery into the anterior chamber. An optional three-way stopcock can also be added to the injector for safety to remove air bubbles and refill the syringe without disturbing the loaded graft (see Fig. 135.1D ).

Glass injectors have been promoted in the European literature. Several European techniques use glass injectors such as the curved glass pipette made by the Dutch Ophthalmic Research Center (DMEK surgical disposable set; DORC, Zuidland, The Netherlands) or the Geuder glass injector (Geuder AG, Heidelberg, Germany, CorneaGen, Seattle WA; see Fig. 135.1F ). Glass potentially offers a smoother surface than plastic and may cause less endothelial cell damage. Surgeons have also adopted off-label use of a glass Jones tube (Straiko Modified Jones tube for DMEK #80000-DMEK, Gunther Weiss Scientific, Portland, OR). Jones tubes are FDA approved for lacrimal surgery. Similar to the modified AMO injector, the Jones tube can be used to aspirate the graft into the inserter, reducing forceps contact with the graft and the risk of endothelial cell trauma. Coupled with 14-French gauge nasogastric tubing, the Jones tube is attached to a 3- or 5-mL syringe full of BSS, creating a closed system with a fluid reservoir for tissue injection and chamber maintenance (see Fig. 135.1C ). The technique also allows slow, controlled delivery of tissue with optimal anterior chamber maintenance. An additional feature of the Straiko modified Jones tube is a central dilation, which results in decreased fluid velocity in the central portion of the injector relative to the tip. This differential in velocity helps to prevent excessive graft aspiration and maximize control of graft delivery.

Until recently, adoption of DMEK has required that a surgeon be proficient in preparing and loading donor tissue into the selected injector system. Thus as with previous advances in endothelial keratoplasty (EK) surgery, the eye bank community has responded to the increasing popularity of DMEK by offering highly processed donor tissue to the surgeon. Tissue-processing procedures for DMEK surgery that have been validated by eye bank surgery include prestripping, prestaining, prestamping with an orientation mark, and punching the graft to size. Furthermore, these preclinical studies have determined that prestaining and preloading the grafts causes no additional endothelial cell loss (ECL) and that prestained tissue retains color saturation over a 3-day shipping period to a degree that would not make surgical use of this tissue more burdensome to an experienced surgeon. ^,

Clinical studies performed by Newman et al. in which 111 eyes with endothelial failure underwent DMEK using donor tissue that was prestripped, prestained, S-stamped, preloaded into a Straiko modified Jones tube, and delivered in an Optisol-filled viewing chamber 1–2 days later determined that tissue characteristics and handling of the tissue were no different from those of surgeon-loaded tissue. Scroll tightness, time to unscroll and center the tissue, postoperative rebubble rate, and graft failure rate using prestained, preloaded tissue all were similar to data previously published by this group using surgeon-loaded tissue. In addition, ECL measured 3 and 6 months postoperatively in eyes that underwent surgery with preloaded DMEK tissue was 26.7% ( n = 63 eyes) and 30.9% ( n = 67 eyes), respectively, rates that are comparable with postoperative data from eyes that underwent surgery with surgeon-loaded tissue.

Thus the use of prestained, preloaded tissue may be considered a good alternative method of delivering DMEK tissue that avoids the complicating factors of staining, punching, and loading the graft intraoperatively. Preloaded tissue also has the additional benefit of postprocessing tissue evaluation prior to releasing the prepared graft for use in the operating room (OR).

An international group has similarly reported success with preloaded DMEK tissue in cell culture medium. The unique feature of their study is that the tissue is folded and loaded with the endothelium facing inward. This conformation may allow easier tissue orientation intraoperatively. Initially, a proof of concept study put forth by Parekh et al. determined that the manipulations of punching, stripping, trifolding tissue with the endothelial side inward, and loading the tissue into an IOL cartridge resulted in metabolically active tissue with limited damage to the endothelium, suggesting that the use of processed donor tissue for DMEK had potential to reduce surgical time, cost, and tissue waste. This lab study was followed by a clinical study using preloaded endothelium-in DMEK tissue. The clinical study consisted of 46 consecutive DMEK surgeries. The surgeries were successful, with an average vision recovery of 20/25, average cell loss of 29% at 3 months, a rebubble rate of 19.6%, and no primary graft failures. Preloaded grafts with the endothelium facing inward provide an additional eye bank processed option for surgeons.

With all closed-system DMEK injectors, care must be taken to avoid overpressurization of the anterior chamber. Injection of excess BSS creates an unfavorable pressure gradient that can result in ejection or incarceration of the graft when the injector tip is withdrawn. To avoid this situation, releasing fluid from a paracentesis is recommended prior to withdrawing the injector or at any time the surgeon feels the eye is becoming overpressurized. In addition, shallowing the anterior chamber after graft insertion and prior to injector removal can maintain graft orientation and simultaneously begin the unfolding process. Another technique for reducing graft ejection is by pressing a second instrument (e.g., 30-G cannula) atop the main incision while withdrawing the injector, essentially creating a “trap door” at the wound. It is also helpful in many situations to position the graft near the opening of the injector so that it is introduced into the eye with minimal influx of BSS ( Video 135.1 ).

Video 135.1 Descemet Membrane Endothelial Keratoplasty Injectors. Michael D. Straiko.

Akin to Descemet stripping automated endothelial keratoplasty (DSAEK) surgery, there are multiple ways to insert DMEK graft tissue. Whether the injector material is a key component as suggested by Dapena et al. or the injector design is more important as suggested by Kim et al. remains to be seen. Interestingly, in their small series, Kim et al. found a postoperative ECL of 28 ± 16% with an Alcon B cartridge attached to a syringe of BSS. This finding compares favorably to the ECL of 12%–29% found by Ham et al. in their larger study using a closed-system glass injector. These data suggest that the design features of an injector (e.g., a closed system) may prove more important than its material properties. Further clinical trials and laboratory work with vital dye staining for endothelial damage are still necessary to better inform this important topic.

Recipient Preparation

Preparation of the recipient eye for successful DMEK surgery involves creation of surgical incisions, removal of the recipient’s DM and corneal endothelium, constriction of the pupil, and creation of a peripheral iridotomy.

Wound Creation

A temporal approach is preferred, in the widest corneal dimension, to help ensure the primary incision will not interfere with graft implantation. ^, The number of total incisions needed for DMEK surgery varies based on surgical technique. The majority of DMEK surgeons will create two to four paracentesis incisions in addition to the primary incision. Paracentesis incisions of 1 mm are created superior and inferior to the main temporal incision.

All wounds for DMEK should be self-sealing because this aids greatly in chamber stability and subsequent positioning of the graft. Making the incisions parallel to the iris plane facilitates the creation of self-sealing paracenteses. It is helpful to mark the entrance of the incisions with a surgical marking pen so that the paracenteses can be located and accessed easily during the surgical procedure. The internal opening of the paracenteses should not overlap with the area where the graft will be placed.

After the paracentesis incisions are made, the primary incision is created. The primary incision is sized to fit the injector chosen by the DMEK surgeon ( Table 135.1 ). The primary incision should allow a snug fit with the DMEK injector and prevent fluid egress and subsequent graft ejection. Although a self-sealing stable incision is ideal, the authors recommend suturing even the smallest main incisions to avoid any potential loss of the graft, air, or gas during later maneuvers.

Endothelium–Descemet Membrane Resection

After creating the surgical incisions, the diseased endothelium–Descemet membrane (EDM) complex is stripped from the host cornea. This step can be accomplished in numerous ways, but it is widely agreed that the creation of a smooth area of resection without residual DM or stromal fibrils is of utmost importance. Residual tags of DM and stromal fibrils may allow fluid to collect between the graft and the recipient stroma, thereby preventing attachment of the graft. It may also result in a suboptimal interface, which can compromise the visual quality in DMEK patients.

Kruse was the first to report a lower rate of graft separation and subsequent rebubbling when the graft does not overlap with host DM. To avoid overlap, the area of stripping on the host cornea should be slightly larger than the diameter of the planned DMEK graft ( Fig. 135.2 ). This results in a small area of bare posterior stroma devoid of any DM coverage. Initially, there may be corneal edema overlying the uncovered areas; the edema typically resolves over days to weeks, likely as donor endothelial cells migrate to cover the bare stroma. The potential increase in graft attachment may result in a loss in cell density as donor cells migrate to cover areas of bare stroma. A small decrease in endothelial cell density may be a salient trade if it is accompanied by a lower rate of graft detachment. Regardless of the planned area of resection, the authors recommend an optical zone marker or calipers be used to mark the area of resection. ^, These marks facilitate precise graft centration after unfolding.

A stable anterior chamber facilitates stripping of the EDM. This stability can be accomplished with a cohesive viscoelastic, air, or BSS using an anterior chamber maintainer. The use of viscoelastic affords the most stable anterior chamber but requires extra supplies including an irrigation and aspiration unit. Some have raised concerns that viscoelastic devices may impede donor attachment. However, a cohesive viscoelastic facilitates grasping and removing the EDM from the recipient stromal bed, can be used to prevent and control bleeding, and can also be removed completely from the anterior chamber.

Using air provides a less stable anterior chamber but improves visualization of DM as it is scored and removed. No matter what method of support is used, a blunt-tipped instrument is used to score a circle in the recipient EDM. ^, After establishing this initial break, the host EDM tissue is pulled carefully in toward the center of the eye and removed through the main incision. As mentioned previously, great care must be taken to avoid disturbing the underlying stroma, especially in cases of pseudophakic bullous keratopathy in which this maneuver is more difficult compared with Fuchs endothelial corneal dystrophy. It is of utmost importance that the cohesive viscoelastic, if used, is removed in its entirety prior to insertion of the graft.