Femtosecond Laser in Corneal Transplantation
Michael D. Greenwood
Nicholas C. Risbrudt
The femtosecond laser has played an integral role in laser vision correction as well as cataract surgery. In recent years, ophthalmology has been utilizing this technology in corneal transplantation with femtosecond laser-assisted keratoplasty (FLAK). Corneal disease ranks as the fifth leading cause of blindness in the world with the cornea being the most commonly transplanted human tissue worldwide.1 Since the first successful full-thickness corneal transplant performed by Eduard Zirm in 1905,2 corneal transplantation techniques have continued to evolve as improvements in medicine and technology advance.
Conventional corneal transplantation, in the procedure of penetrating keratoplasty (PKP), removes the diseased host corneal tissue and involves replacing it with the full-thickness donor tissue. Since the cornea is a multilayered structure, many pathologies only affect individual layers of the cornea, hence selective lamellar replacement of the diseased layers only while retaining the unaffected layers represents a new paradigm shift in the field.3 For corneal pathologies in which the corneal endothelium is spared, Deep Anterior Lamellar Keratoplasty (DALK) may be the preferred approach. DALK is a partial-thickness corneal transplant where the host endothelial cell layer and Descemet’s membrane (DM) complex are retained and the layers of the cornea anterior to that are replaced. Descemet’s Membrane Endothelial Keratoplasty (DMEK) is a single-layer corneal transplant where the host endothelial layer and DM are replaced but all other corneal layers are retained. This is becoming the preferred approach for many surgeon’s when dealing with pathologies affecting only the endothelium such as Fuchs corneal dystrophy. Descemet’s stripping endothelial keratoplasty (DSEK) is similar to DMEK in that it is a partial-thickness corneal transplant. However, DSEK involves transplantation of donor endothelium, DM, and also some stromal tissue. Indications for DMEK and DSEK are similar, however, more surgeon’s are preferring DMEK to approach pathologies isolated to the endothelial layer as it has shown to provide more rapid patient recovery.4,5
Advancements in surgical techniques and technology are allowing surgeon’s to customize their approach while performing different forms of manual, microkeratome, and femtosecond laser-assisted corneal transplantation. The femtosecond laser has the ability to create accurate vertical, horizontal, and oblique incisions in
the cornea with minimum collateral damage to adjacent corneal tissues and is now being used to do precise corneal trephination for donor and recipients.6,7 FLAK can currently be used for PKP and DALK.
the cornea with minimum collateral damage to adjacent corneal tissues and is now being used to do precise corneal trephination for donor and recipients.6,7 FLAK can currently be used for PKP and DALK.
INDICATIONS
Key Indications
Keratoconus
Corneal scarring or opacification
Previously failed PKP
Pellucid marginal degeneration
Corneal ectasia
Various corneal dystrophies
Chemical and mechanical trauma
The most common indication for PKP varies by region and is highly dependent on socioeconomic and geographical location. The leading etiologies of corneal blindness worldwide are primarily due to anterior corneal pathology with a normal endothelium, with the highest prevalence being in developing countries.8
Generally, indications for DALK can be considered for all corneal pathologies other than those affecting the corneal endothelium. The most common indication for DALK is likely keratoconus as these patients may benefit the most by retaining their own endothelium.9 Thankfully, corneal collagen cross-linking has greatly limited the need for cornea transplantation due to keratoconus.
CONTRAINDICATIONS FOR PENETRATING KERATOPLASTY
Severe ocular surface disease with limbal stem cell deficiency is relative contraindications for a full-thickness corneal transplant as they may preclude to poor healing.10 Examples include the following:
Stevens-Johnson syndrome
Advanced aniridia
Ocular cicatricial pemphigoid
Neurotrophic keratitis
Chemical and thermal burns
The National Health Services Blood and Transplant Agency in the United Kingdom has the following absolute contraindications: transplant unlikely to restore corneal function or integrity or remove tissue that would otherwise have led to further damage to the eye.11
CONTRAINDICATIONS FOR DEEP ANTERIOR LAMELLAR KERATOPLASTY
Contraindications for DALK include all corneal pathologies in which the patient’s corneal endothelium and/or DM are damaged or compromised. Bullous keratopathy is the major contraindication of DALK, which requires deeper corneal transplantation techniques.10 Also, keratoconus patients with scars due to acute hydrops may not be good candidates for DALK since air may escape through the break in DM and prevent complete dissection.8
PREOPERATIVE PREPARATION FOR CORNEAL TRANSPLANT
A full ocular examination, complete with medical history, including current medication and allergies, is performed prior to the procedure. Since many of the patients undergoing corneal transplantation are older with other existing conditions, guidance should be given to them regarding their current medications before and following surgery. Health care providers may have protocols regarding different classes of medications and stopping/starting schedules for patients to follow.
Prior to corneal transplantation, more conservative treatments for vision rehabilitation should be exhausted, such as a change in glasses prescription, specialty contact lenses, and when indicated corneal cross-linking. The success rates for PKP and DALK are high compared to other organ transplants because of the low incidence of immunologic rejection.7 Unfortunately, when rejection does occur, secondary procedures are inevitably less successful.7 When comparing rejection rates of PKP and DALK, DALK has a lower rejection rate due to the host retaining the endothelial layer, which is a common site of rejection to occur.
Risks and benefits of the operation must be discussed in depth with each patient, as well as quality of life after surgery. Corneal transplantation requires lifelong follow-up appointments between the doctor and the patient, and success rates are highly dependent on the diligence of the patient to adhere to postoperative instructions given by the doctor.
If the indication for corneal transplantation is due to scarring from since fungi are microbes, could it be stated as bacterial, fungal, protozoan, or inflammatory etiologies time is needed to achieve a quiet eye prior to surgery. Often times, six months or up to a year may be needed to accomplish this. In patients with corneal neovascularization, time is also needed to quiet the eye as corneal vasculature drastically increases the levels of graft failure and rejection. If corneal transplantation is initiated too soon after suffering any of the etiologies stated previously, graft rejection and failure rates significantly increase. Often times, a long course of pharmaceutical therapies such as corticosteroids, antibiotics and immunomodulators are needed to prepare an eye for surgery.
It is of utmost importance to appropriately set patient expectations prior to corneal transplantation. Refractive error including astigmatism, myopia, and hyperopia following transplantation can at times be unpredictable as outcomes are dependent on individual patient healing. In the far majority of cases, after a successful corneal transplant, the patient will require vision correction in the form of glasses and/or specialty rigid contact lenses to achieve highest vision potential postoperatively.
PROCEDURE
The procedures for femtosecond laser PKP and DALK are similar except that the PKP is full thickness while the DALK spares the host DM and endothelium.
The first step for a PKP is preparing the donor graft with an artificial anterior chamber. Once that is completed, the donor cornea is placed underneath the femtosecond laser and the appropriate graft is cut (Figure 9.1). The donor graft is then dissected from the donor tissue and is set aside (Figure 9.2). If the procedure is a DALK, the donor endothelium is removed at this time. The host tissue is then cut by the femtosecond laser using a matching pattern (Figures 9.3 and 9.4). Some surgeon’s may choose to have the donor tissue be 0.25 mm larger in diameter. The host tissue is then removed (Figures 9.5 and 9.6), and the donor tissue is put in its place (Figure 9.7). Sutures are then placed and the case is completed (Figures 9.8 and 9.9). The femtosecond laser allows the surgeon to create different trephination patterns in the host and donor corneal tissue. Traditionally, the most common pattern for manual PKP is a “top-hat.” This pattern provides increased biomechanical stability and a strong resistance to leakage. With the femtosecond laser, more customizable trephination patterns can be achieved to create more surface area at the graft-host junction, thus, increasing the tensile strength and resulting in a faster recovery. Zigzag and Christmas tree shapes are examples of trephination patterns that allow for more surface area between the graft and the host, providing excellent safety, and may allow for earlier suture removal.
 FIGURE 9-1 Preparing to place the femtosecond laser on the donor cornea in a DALK case. Image courtesy of Arturo S. Chayet and Denisse Pinkus. |
 FIGURE 9-2 Dissection of the donor cornea. Image courtesy of Arturo S. Chayet and Denisse Pinkus. |
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