Centration ring, trypan blue staining facilitate keratoprosthesis surgery

New threadless design makes it easier for the surgeon to assemble the unit.

Thomas John

Visual rehabilitation in cases of high-risk corneal transplantation with normal to adequate tear film and good ocular surface wetting includes keratoprosthesis.

These keratoprostheses include corneal-sandwich, full-thickness keratoprosthesis, such as the Boston keratoprosthesis, and the intracorneal, lamellar-type of keratoprosthesis, such as the AlphaCor artificial cornea. Yet another type of keratoprosthesis currently in the developmental stage is a foldable, intrastromal, lamellar keratoprosthesis.

Among the various currently available artificial corneas, the Boston keratoprosthesis has been gaining in popularity among corneal surgeons in the United States and worldwide. In this article, I describe the benefits of using trypan blue (Vision Blue, DORC International) and the John K-Pro Centration Ring (ASICO) to facilitate the Boston K-Pro surgical procedure.

– Thomas John, MD
OSN Surgical Maneuvers Editor

Introduction

The Boston type 1 keratoprosthesis, commonly known as Boston K-Pro, is configured like a collar button, with a front and back plate that is connected by an intervening stem. This device is manufactured from a medical-grade PMMA material.

The Boston K-Pro received 510(k) marketing approval from the U.S. Food and Drug Administration in 1992. The new threadless design of the Boston keratoprosthesis has advantages over the older design, which was a screwed-on type of device. The threadless device makes it easier for the surgeon to assemble the unit.

Additionally, the back plate does not cause any shredding damage to the corneal tissues, as can happen with the older screwed-on device. Also, manufacturing lends itself to molding technique for the new threadless design, rather than machining methods for the older screwed-on type of K-Pro.

Figure 1. John K-Pro Centration Ring (ASICO) has a highly polished, smooth, doughnut configuration in the bottom half of the instrument (top row); The K-Pro Centration Ring helps center the punch and results in a well-centered central corneal opening (bottom row).

Figure 2. Donor cornea with no trypan blue staining (left column); Donor cornea with trypan blue staining (right column). Images: John T

Boston keratoprosthesis involves a sandwich technique in which the cornea is sandwiched between the front and the back plates of the keratoprosthesis. The central stem connects the two plates and forms a single transplantable unit. For ideal implantation, the donor and the recipient wound architecture should be circular. Decentration of the Boston K-Pro will result in an oval-shaped K-Pro unit being implanted into a circular opening in the recipient cornea with resultant corneal distortion and eccentricity of the central region of the keratoprosthesis as it relates to the patient’s cornea.

The use of the John K-Pro Centration Ring (Figure 1) prevents such decentration during Boston K-Pro surgery. Additionally, the use of trypan blue highlights the cornea dark blue and helps during the suture placement and alignment of the Boston K-Pro to the recipient cornea.

Surgical techniques

The donor cornea is encased within an artificial chamber (Moria), and trephination is carried out from the epithelial surface using a Hanna trephine (Moria). The donor corneal disc is then immersed in trypan blue, and the excess stain is removed using a Weck-Cel spear (Figure 2).

A John K-Pro Centration Ring is placed on the endothelial surface of the donor cornea, such that the rim of the centration ring is concentric to the cut edge of the donor corneal disc (Figure 1). This ensures proper centration for the subsequent trephination. A 3-mm punch is used for the central trephination, passing the circular blade through the central opening in the centration ring (Figures 1 and 2). This step ensures proper centration of the trephine on the donor cornea (Figure 2).

Figure 3. Intraoperative photograph showing a failed corneal graft with 360° neovascularization, lipid keratopathy and multiple areas of corneal stromal scarring (upper left); Intraoperative biomicroscopy reveals significant corneal stromal edema (insert); Recipient corneal trephination using the Hanna trephine (middle insert); Diffuse bleeding at the trephination site due to the corneal neovascularization (upper right); A bipolar cautery is used at the trephination site to obtain hemostasis (lower left insert); No active bleeding is seen at the trephination site (lower left); Recipient cornea has been excised, revealing the iris with a previous iridectomy (lower right).

Figure 4. Assembled Boston K-Pro with no trypan blue staining (left column); Assembled Boston K-Pro with trypan blue staining (right column).

The recipient cornea is trephined and prepared as shown in Figure 3. Complete hemostasis is essential. The assembled Boston K-Pro with the trypan blue staining highlights the cornea and the corneal margin (Figures 4 and 5). This blue corneal disc contrasts with the unstained recipient corneal rim and helps clearly demarcate the donor-recipient corneal margins. Proper orientation and suture placement is facilitated by the use of trypan blue stain (Figure 5).

Figure 5. Completed intraoperative view of the Boston K-Pro with no trypan blue staining (left column); Completed intraoperative view of the Boston K-Pro with trypan blue staining (right column); Intraoperative, slit-lamp view of the Boston K-Pro (upper right); Boston K-Pro centered on the recipient cornea (insert).

The combination of the John K-Pro Centration Ring and trypan blue staining helps facilitate optimal centration of Boston K-Pro in the recipient eye and may be considered by all surgeons doing K-Pro surgery.

References:

• Akpek EK, Harissi-Dagher M, Petrarca R, et al. Outcomes of Boston keratoprosthesis in aniridia: a retrospective multicenter study. Am J Ophthalmol. 2007;144(2):227-231.
• Aldave AJ, Kamal KM, Vo RC, Yu F. The Boston type I keratoprosthesis: improving outcomes and expanding indications. Ophthalmology. 2009;116(4):640-651.
• John T. Artificial cornea: surgical use of Boston keratoprosthesis. Ann Ophthalmol. 2008;40(1):2-7.
• Zerbe BL, Belin MW, Ciolino JB; Boston Type 1 Keratoprosthesis Study Group. Results from the multicenter Boston Type 1 Keratoprosthesis Study. Ophthalmology. 2006;113(10):1779.

• Thomas John, MD, is a clinical associate professor at Loyola University at Chicago and is in private practice in Tinley Park and Oak Lawn, Ill. He can be reached at 708-429-2223; fax: 708-429-2226; e-mail: tjcornea@gmail.com. Dr. John receives a small royalty from ASICO for surgical instruments.