6.1 Introduction

Endothelial keratoplasty (EK) comprises Descemet’s membrane endothelial keratoplasty (DMEK) and Descemet’s stripping endothelial keratoplasty (DSEK) as the major variants, whereas many surgeons have also employed their automated version of DMAEK and DSAEK across the globe. In terms of visual output, DMEK as an EK procedure has always scored in spite of it being technically challenging as compared to other subtypes. Melles et al described DMEK that represents the perfect anatomic replacement of the diseased Descemet’s membrane (DM)–endothelium complex with a healthy donor DM–endothelium. ^{1 , 2}

Pre-Descemet’s endothelial keratoplasty (PDEK), ³ being a new variant in the field of EK, further extends the lexicon of EK that mainly comprises the separation of pre-Descemet’s membrane (PDL) along with DM–endothelium complex from the residual donor stroma by the formation of a type 1 bubble. ⁴ The feasibility of the PDEK procedure with both an adult and infant donor tissue ^{3 , 5} makes it highly acceptable in the era of shortage of donor tissue.

6.2 Importance of Air Dissection and Types of Bubbles

Air dissection is a well-established entity that relegates the use of microforceps or a microkeratome for dissection of the corneal stroma. An air-filled syringe is used to inject air into the stroma with the endothelial side up and is advanced under direct observation at a required depth beneath the endothelium. The advantage with air dissection is that it is cost effective as it is done manually by the surgeon and the major drawback with it is that it requires a certain amount of surgical skill set on behalf of the surgeon as the needle that is employed to inject air must be introduced at the correct depth below the DM–endothelium complex (▶Video 6.1).

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  Type 1 bubble: This is the kind of bubble that is essential to obtain for performing a PDEK procedure. This bubble typically spreads from center to periphery and is dome-shaped. The diameter of the bubble usually varies from 7.5 to 8.5 mm; this bubble never extends to extreme periphery due to adhesions between the PDL and the residual stroma. The injection of air leads to separation of the PDL–DM–endothelium complex in toto from the residual stromal bed (▶Fig. 6.1a–d).

  
  
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  Type 2 bubble: This type of bubble is typically formed when the air enters the plane between the PDL and the DM–endothelium complex. Type 2 bubble typically spreads from periphery to center and is around 10 to 11 mm in diameter. It extends up to extreme periphery as there are no adhesions between the PDL and the DM (▶Fig. 6.2a–d). With the formation of this bubble, it becomes essential to perform a DMEK instead of a PDEK procedure. Immense care should be taken when a type 2 bubble is formed as it has a thin wall and if it is subjected to excessive air push, then the bubble can rupture, leading to perforation of graft and eventually transcending into donor tissue wastage.

  
  
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  Mixed bubble: When both type 1 and type 2 bubbles are formed and they coexist, mixed bubble is said to have been achieved. These types of bubbles pose a surgical challenge to the surgeon as it requires delicate handling and manipulation to avoid the rupture of the bubble.

During the process of bubble formation, the tip of the needle may rupture the endothelium (▶Fig. 6.3a, b), which is completely an undesirable event as it may often lead to a situation where the corneal tissue can no longer be used and needs to be discarded as neither a type 1 nor a type 2 bubble can be created either for a PDEK or a DMEK procedure, respectively. However, if the DM perforation occurs and if it is recognized, the surgeon can withhold the injection of air. The surgeon can again withdraw and re-enter at a deeper plane as compared to the previous one and again attempt at making a bubble. Alternatively, a cannula attached to a syringe filled with viscoelastic can also be taken and can be reintroduced to make a bubble, although the use of viscoelastic is not recommended. The advantage with the use of viscoelastic is that it seems to plug the small hole in the DM and facilitates the creation of the type 1 bubble (▶Fig. 6.3a–d). Eventually, if the hole in the DM is big enough, then it cannot be sealed with the help of viscoelastic, and sequentially despite various efforts, only a type 2 bubble will be formed that necessitates the conversion to a DMEK procedure.

Occasionally, a very small bubble is formed after injection of air. In such a scenario, viscoelastic can come to the rescue of the surgeon as the injection of viscoelastic facilitates the breakdown of superficial adhesions between the PDL and the stromal layer, as it tends to overcome resistance offered by the collagenous layer better than air.

Despite all the surgical challenges that a surgeon comes across in the creation of a bubble, the surgical competence and the correct art of handling the corneal tissue effectively saves the loss of corneal tissue, renders economic practicality and enhances the productivity by retaining the clinical merits of the PDEK procedure.

6.3 Surgical Technique

6.3.1 Donor Graft Preparation Bubble Creation

The donor button with the corneoscleral rim is dissected from the whole cornea and is placed with the endothelial side up. For the process of bubble creation, a 30-G needle is used that is attached to an air-filled 5-mL syringe. The needle is introduced with a bevel-up position from the periphery up to the midperipheral area at a considerable depth from the DM so as to create a plane of separation between the PDL and the residual stroma (▶Fig. 6.4a). Air is injected and a type 1 bubble is formed that is dome-shaped and is around 8 mm in diameter.

Graft Staining

Staining of the graft is done with trypan blue that allows a considerable clear visualization of the graft. The bubble is penetrated with a side-port blade at the extreme periphery (▶Fig. 6.4b) and trypan blue is injected inside (▶Fig. 6.4c). The bubble is then cut all across the periphery with a corneoscleral scissors (▶Fig. 6.4d) and is placed in the storage media.

6.3.2 Recipient Bed Preparation

The procedure is performed under local anesthesia; supplemental anesthesia is administered as necessary.

In cases of bullous keratopathy, the initial step comprises scrapping and debridement of epithelium. This facilitates enhanced intraoperative view during the surgical procedure. An anterior chamber maintainer (ACM) or a Trocar ACM ⁶ is introduced into the eye that is connected to the air pump. This helps to maintain adequate depth of anterior chamber (AC) at all times and it also ensures appropriate shift between air and fluid infusion as and when required (▶Fig. 6.5a–f). A 2.8-mm corneal tunnel is made and two side-port incisions are framed. With the AC completely inflated with air, DM is scored and stripped using a reverse Sinskey hook (▶Fig. 6.6a–d). Inferior iridectomy is performed with a vitrectomy probe introduced from the corneal incision (▶Fig. 6.6e). This maneuver ensures prevention of pupillary block at a later stage.

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