Dry Eye Dx and Tx
Keratoprosthesis: Rescue of Last Resort
By Timothy T. McMahon, OD, FAAO, & Jose De La Cruz, MD, MS
Hereditary conditions, ocular surface disease, chemical injury, trauma, repeated corneal transplant failures, and uncontrolled inflammatory disease can destroy the optical and anatomical integrity of the cornea. The end result was once blindness. But through the ingenuity and, to some degree, the courage of a few brave individuals around the world, a device known as a keratoprosthesis was developed.
Claes Dohlman, MD, PhD, developed the Boston Keratoprosthesis (KPro) Types I and II. The type II device is meant for extremely severe conditions in which any ocular exposure is problematic. The need for the Type II device is quite rare—the Type I KPro (Figure 1) has much wider use.
The KPro is composed of two polymethylmethacrylate (PMMA) parts: a front plate with a threadless stem that carries the optical properties of the device, and a back plate with a threadless central opening to accommodate the front plate stem and corneal graft (donor tissue). The device looks like a mini “hubcap” when assembled, with the front plate on top and the back plate below sandwiching the corneal graft (Doane et al, 1996; Aldave et al, 2009). In addition, a titanium ring is locked in place under the back plate for added security. The corneal graft then serves as a scaffold where the assembled KPro is sutured to the host cornea with 10-0 nylon sutures. If the natural lens is removed (as generally recommended), a KPro model with aphakic correction is used. If the eye is left pseudophakic, a plano KPro model is put in place.
Figure 1. Boston KPro, Type I.
The use of a contact lens over the KPro is recommended to limit desiccation of the ocular surface and therefore minimize the possibility of corneal thinning and stromal melting (Dohlman et al, 2002). These advances are not without other complexities that also need to be addressed. Because most of these eyes have undergone multiple corneal grafts prior to this procedure, they are at greater risk of developing glaucoma as the angle structures scar and deteriorate. Frequently a glaucoma tube shunt is implanted at the time of surgery when topical therapy has been maximized or the long-term risk of progression is present.
The best way to manage the risk of infection is with chronic antibiotic coverage. What drugs, how many, and how often is still being sorted out, but prudence suggests that proper gram positive organism coverage is warranted.
The general health of the retina is also a factor to consider after KPro implantation. The incidence of CME, specifically after combined procedures, is one that can slow down the process of visual rehabilitation.
As this procedure involves different management modalities and subspecialties, we recommend a team approach to maximize success in visual rehabilitation as well as to be able to recognize and timely treat any possible complications. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #181.
Dr. McMahon is a professor of ophthalmology at the University of Illinois at Chicago in the Department of Ophthalmology and Visual Sciences. Recently he accepted the position of professor and associate director for Clinical Affairs and Optometrist-in-Chief at the University of Waterloo in Ontario, Canada, where he now resides. Dr. De La Cruz is attending surgeon and assistant professor on the Cornea Service at the University of Illinois Eye and Ear Infirmary in Chicago. He also heads the Artificial Cornea Program at this institution.