Anterior Lamellar Versus Penetrating Keratoplasty

Leopoldo Spadea, MD and Vittoria De Rosa, MD

The purpose of corneal transplantation^1–59 is to replace the pathological cornea of the eye with a normal clear cornea. In penetrating keratoplasty (PK) the full thickness of the cornea is replaced, while in anterior lamellar keratoplasty (ALK) the corneal stroma is replaced down to the Descemet’s membrane (DM), so that the healthy endothelium of the recipient is left intact. Although the desired therapeutic outcomes of the 2 procedures are identical with respect to restoring vision and biomechanical efficiency, the benefits and the risk profile may be different. Replacing fewer layers of the cornea may reduce the likelihood of rejection and subsequent failure because the endothelium is retained.

Contemporary corneal transplantation techniques are the result of years of ideas, experimentations, and perseverance over centuries. PK, a procedure involving the replacement of a full-thickness portion of the cornea, has been the dominant surgical approach for most causes of corneal blindness for more than half a century. The first successful human corneal graft dates back to 1905 when the Austrian Eduard Zirm transplanted a full-thickness human cornea from an 11-year-old boy onto a woman’s eye, restoring her vision.¹ Starting from the Zirm’s success, with the development of corticosteroids, antibiotics, surgical microscopes, and improved trephines, several physicians have contributed to the refinement of corneal transplantation, making PK the treatment of choice for corneal diseases.² In recent years, however, the concept of lamellar keratoplasty (LK) has emerged, leading to a fundamental change in the history of corneal transplantation. This technique aims to selectively replace the diseased corneal stroma without removing the DM and endothelium so as to minimize unnecessary replacement of the unaffected healthy layer. As mentioned earlier, by retaining the patient’s own endothelium, the risk of endothelial rejection, a major cause of graft failure in PK, is almost eliminated and endothelial cell density is preserved. Consequently, there is no need for long-term immunosuppressive therapy with corticosteroids, decreasing the risk of cataract, glaucoma, and infections.3

With the aid of new surgical devices, such as advanced microkeratome instrumentation, excimer laser, and femtosecond laser, several authors have ventured into LK surgery utilizing different techniques. All their efforts are aimed at creating a smooth interface between host and donor tissues, thus reducing refractive irregularities.

In this regard, the field of the ALK can be divided into 2 main categories: ALK with augmented thickness and deep anterior lamellar keratoplasty (DALK). The first one includes all those techniques aimed at achieving both a thickening and a reshaping of the cornea by using a thick donor lamella of normal curvature^4–8; the second one consists of techniques in which the dissection of the host tissue is close to the DM, so that a smooth and transparent recipient bed is achieved.^9,10

Some surgeons believe that leaving in place a small portion of posterior stroma, without completely baring the DM, prevents microperforations from becoming macroperforations, thus providing better results. This is why they prefer to PD-DALK when LK enables the removal of three-quarters or more of stroma to the deeper layers¹¹ (Figures 2-1 through 2-3). On the other hand, there are those who are convinced that the surgery outcome depends on the creation of a cleavage plane as deep as possible, so that in the last decade several techniques of maximum depth dissection have been proposed^12,13 (Figure 2-4). Among these, the most successful is certainly the big bubble technique, first introduced by Anwar in 2002.¹⁴ Several variations of this technique have been proposed over the years, including hydrodissection, viscodissection, and manual dissection,^15,16 that are performed by worldwide experts with encouraging results.^17–21

The basic surgical technique for PK involves first marking the visual axis of the host cornea, which is then trephined and excised. The cutting of recipient cornea is achieved by rotation of the trephine blade until it partially enters the anterior chamber. The incision is completed with right-and-left cutting scissors. In its place, a full-thickness corneal button, 0.25 to 0.50 mm oversized, punched from the endothelial side of the donor tissue, is transplanted and sutured into place with either interrupted or continuous sutures. The use of a corneal button 0.25 to 0.50 mm larger than the diameter of the host corneal opening is recommended as it can help to reduce excessive postoperative corneal flattening and the risk of secondary glaucoma while enhancing wound closure. After suturing, the transplant is checked to ensure a tight wound seal between the donor and recipient tissue. Postoperative treatment includes a combination of topical antibiotics and steroids for 1 year postoperatively.22

Microkeratome-Assisted Lamellar Keratoplasty

Microkeratome-assisted lamellar keratoplasty (MALK) involves first using the microkeratome to perform an automated anterior lamellar corneal dissection of the recipient cornea. With the aid of an artificial chamber maintainer, the same microkeratome is used to cut the donor lamella in way to match the host stromal bed. The donor lenticule is placed over the host bed and sutured in place under tension. The automated lamellar dissection results in optimally smooth lamellar dissection but has its disadvantages in cases of topographical corneal irregularity or differential corneal thinning, because any irregular corneal surface is replicated in the microkeratome cut. Bu sin et al,⁶ trying to achieve a final corneal shape as similar as possible to the physiologic curvature of the donor cornea, introduced a modification of the MALK technique, including a full-thickness trephination of the residual bed before suturing the donor graft in place. They postulated that the recipient’s residual stroma can preserve a keratoconus memory, so through the disruption of the recipient’s architecture, they achieved a better postoperative refractive error and spectacle-corrected visual acuity.

Excimer Laser Lamellar Keratoplasty

Excimer laser lamellar keratoplasty is a procedure in which a deep plano excimer laser ablation is done on the host cornea and a donor lamellar button, with or without an excimer laser refractive ablation on the posterior surface, is sutured into the recipient bed. It starts with a mechanical deep epithelialization followed by a photo-therapeutic keratectomy ablation performed by the excimer laser. A 7.0-mm round stainless-steel mask is placed on the cornea to create a vertical and regular edge of the ablation. The ablation depth setting ranges from 110 to 200 μm, always with the goal of a minimum estimated residual corneal bed of 200 μm. A 2.5-mm stromal pocket is created around the circumference of the ablation floor with a circular movement using a disc knife. The donor lamellae are obtained from a cornea mounted on an artificial anterior chamber, cut with a microkeratome and then dehydrated in a silicon gel. After being rehydrated in balanced salt solution for 10 minutes before surgery, the donor lamella is secured in the recipient bed with four 10-0 nylon cardinal sutures at the 6, 12, 9, and 3 o’clock positions. After the introduction of the wing of the donor lamella in the stromal pocket, 16 interrupted 10-0 nylon sutures were placed. Finally, the knots are buried and intraoperative suture adjustment is performed. The postoperative therapy consists of topical antibiotics until complete reepithelialization. Then topical steroids are administered for at least 1 month. Within 3 months after surgery, all medications are stopped23 (Figures 2-5 and 2-6).

In recent years, the development of new custom ablation algorithms has improved results and expanded treatment indications for excimer laser technology. The concept of custom ablation relies on the integration of the ablation profile map with the pachymetric data in order to perform a surgery specific to each patient’s needs. In this technique, named custom lamellar ablation for transplantation (CLAT), the donor ablation is performed by positioning the lamella on a concave support with the endothelial side exposed to the laser so that the corneal thickness is uniformly reduced with the excimer laser. Then the graft is inverted on a convex support with the epithelial side exposed for the excimer laser ablation. A wing of the lamella is obtained on the perimeter by positioning on the center a 7.5-mm diameter mask and performing a 250-μm depth ablation. At the end, the donor cornea is punched using a suction punch. The receiving bed is created using a 3-dimensional pachymetry map obtained from the tomographer and calculating the intersection of an ideal surface referenced from the posterior surface of the cornea. The irregular volume above this ideal surface is ablated with excimer laser. Using a circular movement with a disc knife a 2.0-mm stromal pocket is obtained around the circumference of the ablation floor. The donor lamella is then secured into the recipient bed with 4 cardinal sutures and the wing of the donor lamella is introduced into the stromal pocket and 16 interrupted 10-0 nylon sutures are finally placed24 (Figures 2-7 through 2-10 and Video 2-1).

It has been claimed that the reproducibility of the lamellar cuts in the donor and recipient corneas provided by the femtosecond laser may decrease the necessity of corneal suturing, thus allowing for an early visual rehabilitation.25

DALK

Among DALK techniques, the most performed is certainly the big bubble technique. It consists of a partial trephination of the cornea, performed with a calibrated guided trephine, to a depth between 60% and 80%. A 27- or 30-gauge needle attached to an air-filled syringe is inserted into the deep stroma through the bottom of the trephination groove and a forceful injection of air causes the separation of the DM from the corneal stroma. The bubble appears as a circular area with a dense white border corresponding to the trephination groove. A keratectomy, anterior to the big bubble, is carefully performed so as not to accidentally breech the bubble. Then the bubble is pierced near the center of the cornea and an opening in the anterior wall of the air-pocket is formed. Viscoelastic is injected and scissors are used to divide the anterior stroma into 4 sections, which are then removed exposing the DM, by cutting each quadrant at the edge of the trephination. The detection of a sixth layer of the cornea, the Dua’s layer, beyond the last row of kerotocytes, led the surgeon to assume that the big bubble cleaves off a distinct layer at the posterior surface of the corneal stroma, which is not residual stroma but a well-defined, acellular strong layer.26 For the preparation of the donor cornea the lamella is punched out from the endothelium side and the DM with endothelium is gently stripped off with a dry sponge. Then the button is sutured in place using two continuous, 12-bite, 10-0 nylon sutures or 24 interrupted sutures (Figures 2-11 and 2-12 and Video 2-2).

Sugita and Kondo²⁷ described the hydrodelamination technique, a procedure combining air and fluid injection. A saline solution is injected in a small depression, which is created in the deeper stroma after a partial trephination. This procedure helps to achieve a cleavage plane over DM so that the loosened tissue can be removed in thin layers with forceps and scissors. The DM is recognized from its shiny and smooth appearance.

Farid and Steinert²⁸ introduced a new approach combining big bubble DALK with the femtosecond laser zigzag incision in which the matching and interlocking donor-host wound increases surface area and create a customized donor-host match. In a zigzag incision, the laser creates an angled posterior cut, a lamellar ring cut in mid-stroma, and an angled anterior cut, each of which intersect. According to Farid and Steinert,²⁸ this type of wound allows rapid healing and good biomechanical stability. Furthermore, the laser cut allows an exact cut depth within 70 μm of the DM, allowing more precise placement of the air needle.

Yao¹⁵, in an attempt to achieve a thorough removal of the stroma, developed the hooking detaching technique. It consists of creating a pocket in the recipient bed by means of a three-quarter trephination, followed by a peeling of the remnant stroma to approach DM along the trephined margin in the area between 11 and 1 o’clock by the aid of a golf-shaped knife. In this area the residual stromal fibers are hooked and lifted by forceps with a tip of concaved teeth for the consequent exposure of the DM. A 27-gauge cannula connected to a syringe containing viscoelastic is inserted in the pocket between the stroma and the DM and the injection of viscoelastic material allows the detachment process.¹⁵

A research line other than the techniques described up to now was disclosed by Tsubota et al in 1998.²⁹ Tsubota’s divide-and-conquer technique differs from the others because the DM deep dissection is not performed by injecting any substances and it is practiced completely manually. It originally derives from the application of cataract phacoemulsification technique to the deep lamellar keratoplasty. The corneal stroma is divided into 4 quadrants to facilitate lamellar dissection at approximately 70% deep, until the DM is exposed in the central area. In this way it increases the repeatability and the standardization of the procedure.

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