9 Femtosecond Laser–Assisted Keratoplasty: Lamellar Anterior and Posterior

Soosan Jacob and Amar Agarwal

Keywords: Femtosecond assisted keratoplasty, Optical coherence tomography, DALK, deep anterior lamellar keratoplasty, EK, Endothelial keratoplasty, anterior lamellar keratoplasty, posterior lamellar keratoplasty

9.1 Introduction

The femtosecond laser is a near-infrared laser of 1,053 nm wavelength that uses the principle of photodisruption. It has a femtosecond pulse duration (10^–15 seconds) with the focused pulse of laser energy generating plasma that contains free electrons and ionized molecules which expand and create a shock wave. This cavitation creates a gas bubble which expands before collapsing again and it is this mechanism that is taken advantage of to produce tissue cleavage. Though it works similar to the nanosecond range Nd-YAG laser, the ultra-short pulse duration of the femtosecond laser limits the collateral damage to surrounding tissues by decreasing the required energy for a given effect. The femtosecond laser has been reported for corneal application in 1989. It has been used in both corneal refractive and lens-based surgery due to the high level of accuracy that can be obtained. In combination with real-time spectral domain anterior segment optical coherence tomography (ASOCT) or Scheimpflug imaging, the femtosecond laser can be used to create different cut patterns in the human eye by accurately focusing on the laser at different depths within the optically clear ocular tissues. The precision, accuracy, and predictability with which this can be done make the femtosecond laser a powerful tool in ophthalmology.

9.1.1 Lamellar Keratoplasty

Anterior or posterior corneal tissue may be selectively transplanted in corneal disease depending on the location of pathology. Though this preserves the undiseased precious host tissue, it entails more difficult surgery, requiring greater skill. The interface that is created can also result in visual disturbances, more so if dissection is uneven and irregular. Lamellar keratoplasty can be classified into two broad categories:

1. 
   

   Anterior lamellar keratoplasty: Anterior lamellar layers are transplanted. Depending on depth of transplantation, these can be further classified into the following:

   
   
   
   
   1. 
      

      Superficial anterior lamellar keratoplasty (SALK): Only a part of anterior stroma (<one-third or 160 µm) is replaced together with corneal epithelium and basement membrane. ¹

      
      
   2. 
      

      Deep anterior lamellar keratoplasty (DALK): All layers up to the deeper stroma or up to the pre-Descemet’s layer are replaced.

   
   
2. 
   

   Posterior lamellar keratoplasty: The posterior layers of the cornea together with Descemet’s membrane and endothelium are transplanted.

   
   
   
   
   1. 
      

      Deep endothelial lamellar keratoplasty (DLEK): Deep layers of the host corneal stroma together with Descemet’s membrane and endothelium are replaced with graft consisting of Descemet’s membrane, endothelium, and a part of deeper stroma.

      
      
   2. 
      

      Descemet stripping endothelial keratoplasty (DSEK): Manually prepared graft consisting of Descemet’s membrane, endothelium, and a part of deeper stroma are transplanted.

      
      
   3. 
      

      Descemet’s stripping automated endothelial keratoplasty (DSAEK): A microkeratome is used for automated preparation of a graft similar to DSEK.

      
      
   4. 
      

      Descemet membrane endothelial keratoplasty (DMEK): Descemet’s membrane and endothelium are transplanted.

      
      
   5. 
      

      Pre-Descemet’s Endothelial Keratoplasty: The pre-Descemet’s layer, Descemet’s membrane, and endothelium are transplanted.

      
      
   6. 
      

      Assisting techniques: E-DMEK/E-PDEK and air-pump–assisted PDEK techniques described by one of the authors help in performing DMEK and PDEK faster, more easily and with greater chances of success. (S.J.) (▶ Fig. 9.1).

      
      
      
      

      

      
      

      Fig. 9.1 E-PDEK or endoilluminator-assisted pre-Descemet’s endothelial keratoplasty (PDEK). (a) A PDEK graft as seen with microscope illumination. (b) Enhanced visualization and three-dimensional depth perception with the E-PDEK technique.

9.1.2 Femtosecond Laser Keratoplasty

Precisely controlled side cuts with exact diameter and depth programming can be done with the femtosecond laser. In comparison with vacuum trephine-assisted DALK, this may provide better healing of epithelium, better wound apposition, and earlier improvement in visual acuity. ² DALK may be performed by the big bubble technique after creating only the side cuts with the femtosecond laser (▶ Fig. 9.2). Precise identification of tissue depth for air injection can facilitate big bubble formation. ³ The IntraBubble technique creates a channel in the posterior stroma about 50 μm above the endothelium, through which a cannula for air injection is introduced, leading to cleavage of the corneal tissue. ⁴ Diakonis et al also used the femtosecond laser for pretreatment to create an intrastromal tunnel and a side cut to insert a DALK cannula in order to perform pneumodissection and obtain a big bubble. ⁵ Another commonly employed option is to create side cuts as well as a lamellar cut. Patterned cuts in the form of mushroom cut or zig-zag cuts are more useful with penetrating ⁶ keratoplasty. However, they are also possible in anterior lamellar keratoplasty ⁷ and may decrease the number of sutures required because of interlocking cuts created between donor and host cornea. Decagonal trephination patterns have also been tried. ⁸ Graft size can be tailored to match the host size very closely, even up to 0.1 mm as opposed to trephines where the smallest difference between trephines is 0.25 mm. Also, vertical side cuts are achievable with femtosecond laser unlike the sloping cuts that are achieved on trephination in very steep corneas in keratoconus. The smooth lamellar dissection by the femtosecond laser does away with the difficulties that are inherent in the manual big bubble dissection technique of DALK such as micro- and macro-perforations and double anterior chamber. It facilitates easy, rapid, and predictable surgery. However, residual pathology may be left behind in the layers of host stroma posterior to the lamellar cut which may be responsible for a slightly decreased BCVA, especially in case of dystrophies. Mild interface haze may also contribute to slightly reduced vision than obtained after big bubble DALK. Shaped patterns can increase wound stability, allow earlier suture removal, and decrease postoperative astigmatism. Care must be taken, however, to verify that there is a safe limit of host tissue posterior to the lamellar cut throughout the area of the cut and that the lamellar cut does not penetrate through the endothelium at any site. Preoperative programming is therefore very crucial in femtosecond-assisted lamellar keratoplasties. Descemet’s membrane perforation intraoperatively has been reported in femtosecond-assisted DALK. ⁹ In this case, it is still possible to convert to penetrating keratoplasty while still retaining the shaped patterns of the recipient and donor corneas.

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