CL Design & Materials
Managing Dryness With the Boston Keratoprosthesis
BY RONALD K. WATANABE, OD, FAAO, & DAVID MURAKAMI, OD, MPH
The Boston Type I Keratoprosthesis, or KPro, is an innovation in corneal transplantation that may prevent patients who have debilitating corneal conditions from losing vision in the affected eye. First introduced in 1992, the KPro has treated a range of complications or conditions including aniridia, corneal scars, ocular cicatricial pemphigoid, neurotrophic keratitis, chemical burns, and graft failures (Figure 1).
The device is comprised of a front plate, stem, and back plate made of polymethylmethacrylate; a titanium C-ring secures the device in vivo. A donor cornea is sandwiched between the front and back plates, and the entire assembly is sutured in place consistent with penetrating keratoplasty. Several international studies show that the KPro improved visual acuity for over 80 percent of patients (Kim et al, 2013).
The Need for Soft Lenses
Despite improved visual outcomes, the postoperative corneal surface is prone to greater evaporative forces and desiccation, causing severe epithelial defects, stromal thinning, dellen formation, and a possible melt. Soft contact lenses fitted over the KPro help prevent these devastating complications. Various soft lens designs and materials have been developed for this purpose. The Kontur Precision Sphere (Kontur Kontact Lens Co.) made of methafilcon A is often used due to its overall durability, large diameter options, and adequate thickness to protect the eye while maintaining moisture. Localized drying around the KPro device can result in compromised epithelial healing, leading to potential ulceration of tissue. The soft contact lens maintains a thin meniscus of tears around the edge of the front plate, decreasing surface evaporation.
Figure 1. Patient with KPro in situ.
The soft lens may require unusual curves and diameters depending on the new ocular surface topography. Kontur contact lenses offer diameters ranging from 12mm to 24mm and base curves ranging from 6.8mm to 9.8mm to accommodate these needs. A standard initial lens applied after surgery has a 16mm diameter with a 9.8mm base curve, but is often refitted once the patient has stabilized. In addition, the lenses can include a black occluder tint and 4mm central clear pupil to improve cosmesis or relieve photophobia.
Beyond the Fitting
Soft lenses over the KPro are worn on an extended wear basis, with monthly to biannual replacement. Significant lens deposits from prolonged wearing can decrease vision and comfort in about 5 percent of patients. Reduced or poor blinking can lead to inadequate replacement of tears around the lens. Recommend an extra strength daily cleanser for minor deposits or more frequent lens replacement if deposits are more significant.
Postoperative dryness in a KPro eye has significant implications that must be addressed. Soft lenses help maintain vision and support the ocular system. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #212.
Dr. Watanabe is an associate professor of optometry at the New England College of Optometry. He is a Diplomate in the American Academy of Optometry’s Section on Cornea and Contact Lenses and Refractive Technologies and is in private practice in Andover, Mass. You can reach him at firstname.lastname@example.org. Dr. Murakami is a resident in Cornea and Contact Lenses at the New England College of Optometry.