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Crescentic DALK combined with PK treats pellucid marginal degeneration
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Pellucid marginal degeneration is an uncommon, nonhereditary, noninflammatory, bilateral, inferior crescentic peripheral thinning and ectatic disorder that can progress slowly over several years and result in significant visual deterioration. It usually presents in the second to fifth decade of life and has no sex or racial predilection. It stands second to keratoconus in the category of noninflammatory corneal thinning disorders. The exact etiology remains elusive. However, 10% of pellucid marginal degeneration cases are associated with keratoconus and 13% with keratoglobus, so it raises the question of whether pellucid marginal degeneration, keratoconus and keratoglobus are phenotypic variations of the same disease entity or different diseases.
The affected region inferiorly usually extends 1 mm to 2 mm in height. It is 1 mm to 2 mm from the limbus and extends horizontally for about 6 mm to 8 mm. There is commonality in the histopathologic findings between keratoconus and pellucid marginal degeneration, namely, an area of stromal thinning with normal-appearing epithelium, endothelium and Descemet’s membrane with Bowman’s membrane disruptions. Differential diagnosis includes keratoconus, keratoglobus, Terrien’s marginal degeneration, furrow degeneration and peripheral corneal melts, such as in Mooren’s ulcer.
Visual rehabilitation requires surgical intervention when conservative treatments, including the use of contact lenses, have failed. While penetrating keratoplasty may be considered, it is challenging because the zone of corneal thinning and ectasia is located close to the inferior limbus. This would require a large graft of 9 mm or more that is inferiorly decentered. Such a graft would place itself in the category of increased risk of graft rejection.
Full-thickness PK requires adequate corneal thickness in the peripheral recipient corneal rim to establish proper donor-recipient approximation. In pellucid marginal degeneration, Terrien’s marginal degeneration and peripheral corneal melts, all have a thinned peripheral cornea, which can result in a mismatch of corneal thickness between the donor corneal button and the peripheral recipient corneal margins and a step-ladder edge-lift effect that should be avoided. An uneven corneal surface will often result in suboptimal tear film spread over the corneal surface. Additionally, the corneal sutures can pull through the thinned peripheral cornea and may result in corneal wound dehiscence that will often require surgical intervention.
In this column, I describe the combined technique of crescentic deep anterior lamellar keratoplasty along with full-thickness penetrating keratoplasty in the surgical management of pellucid marginal degeneration. This technique keeps the full-thickness PK in the optimal central region of the cornea, thus decreasing the increased risk of graft rejection that is associated with a decentered PK in pellucid marginal degeneration.
Surgical technique
The surgical procedure is performed under general anesthesia. Both the horizontal and vertical dimensions of the cornea with pellucid marginal degeneration are measured using Castroviejo calipers to determine the selection of the trephine diameters (Figure 1). Two Hanna trephines (Moria) with different diameters are used to create partial-thickness trephinations on the patient’s cornea (Figure 2). The inferior margin of the larger trephine reaches close to the inferior limbus, thus covering the inferior thinned part of the patient’s cornea. The second trephine has a smaller diameter, and it is centered on the patient’s cornea (Figure 2). The superior margins of these two trephinations meet to form a single superior cut on the recipient cornea. This trephination pattern results in a central circular trephination and a peripheral crescentic trephination. All trephination marks are highlighted using a sterile surgical marking pen for easy visualization during surgical dissection (Figure 2). Next, the crescentic lamellar dissection is carried out to a depth close to the recipient Descemet’s membrane, taking care not to perforate the Descemet’s membrane in the region of peripheral corneal thinning in the inferior region of the cornea (Figure 3).
Figure 1. Intraoperative photos of a cornea with pellucid marginal degeneration. Measurements of the vertical and horizontal corneal dimensions are taken using Castroviejo calipers.
Figure 2. The central circular trephination and the larger eccentric trephinations partially meet superiorly (arrows).
Images: John T
Figure 3. Intraoperative Deep near-Descemet’s membrane lamellar dissection of the peripheral corneal crescent.
Figure 4. Central region of the donor cornea is trephined (yellow star), and the corneal button is removed.
Attention is then directed to a single donor cornea that is used for both the central PK and the peripheral crescentic DALK. Using similar diameter trephines that were used on the patient’s cornea, the central part of the donor cornea is trephined, creating the central opening in the donor cornea. The resulting donor corneal doughnut is utilized for preparing the crescentic donor lamellar graft (Figure 4).
The donor lamellar graft is brought to the surgical field. Because the same diameter trephines were used for both the donor and recipient corneas, the donor crescentic graft matches the crescentic recipient corneal bed. The donor crescentic graft is then flipped, epithelial side down, and the stromal margins are beveled using Vannas micro scissors to facilitate a better fit of the donor graft to the recipient bed without any edge lift of the donor tissue. Additional debulking of the donor graft is carried out.
Having completed the donor graft preparation, the recipient lamellar dissection is then carried out within the crescentic outline (Figure 5). Dissection is carried out to near Descemet’s membrane and extended beyond the margins to facilitate vertical cutting of the corneal margins. The crescentic recipient corneal tissue is then excised using the John ALK scissors (Asico). These specially designed scissors have a distal circular disc that pushes the Descemet’s membrane focally as the stromal tissue is excised, thus protecting the recipient Descemet’s membrane from accidental perforation (Figure 5). Figure 6 displays the recipient crescentic Descemet’s membrane bed with few stromal fibers and vertical cut margins to facilitate proper donor tissue alignment on the recipient corneal bed. Tisseel fibrin glue (Baxter) fills the donor-recipient interface in the final resting position of the donor graft.
Figure 5. John ALK scissors are used for safe dissection of the recipient corneal crescent.
Figure 6. Intraoperative broad- and slit-beam views of the recipient crescentic Descemet’s membrane bed with few stromal fibers on the surface.
Figure 7. Fibrin glue is used on the stromal side of the donor crescentic tissue and on the recipient crescentic Descemet’s membrane bed.
Figure 8. Left: Anterior chamber is entered with a 15° superblade. Right: The central cornea is excised, full thickness, using corneal microscissors. Top: Viscoat is injected into the anterior chamber. Bottom: Miostat constricts the pupil, and the recipient eye is ready to receive the donor corneal graft.
Figure 9. Left: The full-thickness donor graft is introduced into the surgical field. Right: The donor graft is sutured in place using interrupted 10-0 nylon sutures. The crescentic DALK graft restores the peripheral thin cornea to near-normal thickness for optimal attachment of the full-thickness donor corneal graft.
Figure 10. Main figure: The completed slit view of the combined crescentic DALK with PK. Notice the uniform corneal surface with matching donor-recipient corneal margins and no step-ladder mismatch of the donor-recipient tissues. Oval inserts: Final suturing steps and the completed views of the recipient cornea. The lamellar crescentic DALK has thickened the otherwise thin peripheral cornea in pellucid marginal degeneration to restore a normal uniform corneal surface.
Tisseel contains two components: sealer protein solution (fibrinogen) and thrombin solution. Fibrinogen or thrombin is applied to the stromal side of the donor graft, and the other component is applied to the recipient Descemet’s membrane (Figure 7). The crescentic donor graft is lifted and placed on the recipient corneal bed. The John ALK compression disc (Asico) is used to facilitate uniform attachment of the donor graft to the recipient corneal bed without any Descemet’s membrane folds or the formation of a false anterior chamber. A few interrupted 10-0 nylon sutures are used to further anchor the lamellar graft. This completes the lamellar portion of the surgical procedure.
Next, full-thickness central PK is performed. The anterior chamber is entered with a 15° superblade, and the patient’s central cornea is excised after injecting Miostat (carbachol, Alcon) and Viscoat (sodium chondroitin sulfate, sodium hyaluronate, Alcon) (Figure 8). The donor graft is placed in position and sutured with interrupted 10-0 nylon sutures (Figure 9). The completed views of the combined crescentic DALK and full-thickness PK are shown in Figure 10. Notice the uniform corneal surface especially along the margins of the donor-recipient corneal tissues, without any edge lift, thus ensuring uniform tear film spread on the newly created corneal surface (Figure 10).