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Advanced surgical techniques enhance use of keratoprosthesis
Boston K-Pro device has proven clinical track record and is gaining in popularity among corneal surgeons.
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Corneas with a high risk of corneal graft failure after a penetrating keratoplasty are often good candidates for a keratoprosthesis, also known as an artificial cornea. Over the past several decades, various designs and models of keratoprostheses have come to the marketplace. Most of these keratoprostheses had limitations, and their clinical applications were often relatively short-lived. The Boston K-Pro, previously called Dohlman K-Pro, has a proven clinical track record to date, with a relatively good success rate and a higher retention rate compared with most of the previous models of artificial corneas. Thus, the Boston K-Pro has continued to gain in popularity among corneal surgeons in the United States and worldwide.
 Thomas John |
The device consists of “sandwiching” a donor corneal disc in between PMMA plates, namely, a front plate and a back plate. The diameter of the front plate is 6 mm, and the diameter of the back plate is 8.5 mm. This allows a margin of error of about 1.25 mm. Hence, one can easily decenter the plates, and such a decentered unit will be an oval, instead of a circular unit. Placing such an oval unit into a circular recipient corneal opening will result in unwanted distortions and cause difficulties in applying uniform sutures to the donor corneal rim. Because the donor corneal rim will be fully covered by the decentered plates, this will prevent suture placement in the areas of decentration. Hence, the use of a John K-Pro Centration Ring (ASICO) will allow proper centration of the K-Pro unit and result in a circular, well- assembled unit that will fit into the recipient corneal opening, prevent globe distortions and permit uniform suturing of the donor to the recipient corneal rim.
This article presents the advanced surgical techniques in the use of the Boston K-Pro.
Surgical technique
General anesthesia is preferred. First, the diameter of the trephine to be used on the recipient cornea is decided. The donor corneal disc is oversized by 0.5 mm. Trephination is carried out from the epithelial side on the patient’s cornea and the donor cornea to facilitate proper corneal fit.
Donor corneal preparation: The donor cornea is mounted within an artificial anterior chamber (Moria) (Figure 1, upper left) that was coated with Viscoat (chondroitin sulfate, sodium hyaluronate, Alcon), and the chamber is pressurized. Trephination is carried out from the epithelial surface using a Hanna trephine (Moria) (Figure 1, upper right). The excised corneal disc (Figure 1, lower row) is then placed on a Teflon block with the epithelial side down. Trypan blue (VisionBlue, Dutch Ophthalmic USA) is used to stain the donor corneal disc (Figure 2). The John K-Pro Centration Ring is placed on the donor corneal disc (Figure 2, bottom right, and Figures 3 and 4) so that a uniform rim of the trypan blue-stained donor corneal disc is visible. A 3-mm punch is passed through the central opening in the centration ring and trephination is carried out (Figures 3 and 4). This results in a well-centered 3-mm opening within the donor corneal disc (Figure 5, upperleft). The newly designed front plate of the Boston K-Pro has a threadless stem (Figure 5, upper right). This front plate is attached to a circular adhesive disc (Figure 5, lower left). The donor corneal disc is placed over the back surface of the front plate such that the central stem passes through the donor corneal opening (Figure 5, lower right). Next, the newly designed back plate with an increased number of holes (Figure 6, upper left) is placed over the endothelial surface of the donor cornea, such that the central stem of the front plate passes through the back plate. Finally, the titanium snap-ring is placed over the back plate and pressed in position so that it locks into the groove in the stem of the front plate (Figure 6, upper right and lower row). This completes the Boston K-Pro-donor-corneal assembly into a single unit (Figure 6, lower right).
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Images: John T |
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Recipient cornea: The opaque cornea that is completely vascularized is seen in Figure 7. After making radial markings on the recipient corneal surface (Figure 7, upper right and lower left), trephination is carried out using a Hanna trephine set at 400 µm (Figure 7, lower right). Complete hemostasis is essential (Figure 8, upper row) before using a 15° super blade to make an entry into the anterior chamber (Figure 8 , lower left). Viscoat (chondroitin sulfate, sodium hyaluronate, Alcon) is injected into the anterior chamber (Figure 8, lower right). The recipient cornea is excised using corneal microscissors (Figure 9, upper and lower rows). Automated, bimanual, open-sky, anterior vitrectomy is performed to remove the vitreous from the anterior chamber (Figure 10, upper left).
Attachment of artificial cornea to recipient corneal rim: A well-centered Boston K-Pro (Figure 10) unit, coated with chondroitin sulfate, sodium hyaluronate within the openings (Figure 10, upper right) is then placed into the central opening in the recipient cornea without any undue globe distortion. The donor corneal rim is attached to the recipient corneal rim using multiple interrupted 10-0 nylon sutures (Figure 10, lower row), and a large diameter contact lens (Kontour Kontact Lens) is placed over the artificial cornea (Figure 10, lower right), completing this surgical procedure.
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Surgical pearls and tips
- To perform artificial corneal surgery, the surgeon should have prior experience in PK.
- A preoperative intravenous osmotic agent may be used as necessary.
- Complete hemostasis is essential before entering the anterior chamber with a super blade. Any blood within the vitreous cavity will delay postoperative visual recovery, and in the anterior chamber, it can have deleterious effects with regard to slow visual recovery, inflammation, synechiae and possible membrane formation.
- Trephination from the epithelial side on the donor and recipient corneas will allow proper corneal fit.
- An artificial anterior chamber is necessary to trephine the donor cornea from the epithelial side.
- A Hanna trephine allows precise depth setting in microns for the trephination.
- Use of trypan blue is helpful in good visualization of the donor corneal disc. This is helpful in seeing the disc edges while centering the John K-Pro Centration Ring. It is also helpful in visualization of the donor corneal margins while suturing the assembled Boston K-Pro unit to the recipient corneal rim.
- Avoid decentration in assembling the Boston K-Pro unit by using the John K-Pro Centration Ring. Decentration will result in an oval K-Pro unit and in a mismatch when trying to insert the assembled unit into the recipient corneal opening.
- Use of a large diameter contact lens is recommended at the completion of the surgery.
Treatment
Preoperatively, a prophylactic antibiotic, either Iquix (levofloxacin 1.5%, Vistakon), Vigamox (moxifloxacin 0.5%, Alcon) or Zymar (gatifloxacin 0.3%, Allergan) is used. Postoperatively, a topical steroid, Pred Forte 1% (prednisolone acetate 1%, Allergan), and an antibiotic levofloxacin 1.5%, moxifloxacin 0.5%, or gatifloxacin 0.3% is used four times daily. In addition, vancomycin 14 mg/cc ophthalmic solution (prepared by an outpatient licensed pharmacy) is used four times daily. For globe protection, the patient is asked to wear glasses or an eye shield during the day and a shield at night for the operative eye. Postoperative activity limitation is the same as for a PK.
For more information:
- Thomas John, MD, is a clinical associate professor at Loyola University at Chicago and 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 has no direct financial interest in the products discussed in this article, nor is he a paid consultant for any companies mentioned.
References:
- Aquavella JV, Qian Y, McCormick GJ, Palakuru JR. Keratoprosthesis: Current techniques. Cornea. 2006; 25:656-662.
- Kim MK, Lee SM, et al. Long-term outcome in ocular intractable surface disease with Seoul-type keratoprosthesis. Cornea. 2007; 26:546-551.
- Tay AB, Tan DT, et al. Osteo-odonto-keratoprosthesis surgery: a combined ocular-oral procedure for ocular blindness. Int J Oral Maxillofac Surg. 2007; 36:807-813.
- Zerbe BL, Belin MW, Ciolino JB: Boston type 1 keratoprosthesis study group. Results from the multicenter Boston Type 1 Keratoprosthesis Study. Ophthalmology. 2006; 113:1779.e1-7.