The Boston KPro Restores Vision

A highly useful option for heavily damaged corneas.

By Salim Butrus, MD

Since Dr. Eduard Zirm performed the first successful human allograft corneal transplantation in 1905 without the help of sutures, keratoplasty procedures have gone through tremendous changes and evolutions. In the past 50 years, lamellar and penetrating keratoplasties have become popular worldwide, especially with the development of surgical microscopes, fine sutures, modern eye banking and anti-inflammatory and immunosuppressive agents.

Indications for corneal transplantations include keratoconus and other ectatic conditions such as pellucid marginal degeneration and keratoglobus, corneal scarring, adherent leukomas, Fuchs' and other corneal dystrophies, aphakic and pseudophakic bullous keratopathies, previous failed corneal grafts, congenital corneal clouding, chemical or thermal burns and autoimmune conditions such as Stevens-Johnson syndrome (SJS), ocular cicatrical pemphigoid (OCP) and others.

Although studies have shown that the five-year survival rates of corneal grafts have been high in keratoconus (95%) and corneal dystrophies, it is much lower (20%) in patients with previous corneal grafts and vascularized corneas, as seen in Stevens-Johnson syndrome and other corneal diseases. Based on this, patients with failed grafts do not have a high probability of success if transplanted again despite advancements in surgical techniques and anti-rejection agents.

Slit-lamp photo of left eye of a 37-year-old man, following alkali injury and a failed corneal transplant. This eye had only hand-motion vision. The other eye had no light perception.

Here, I will discuss the use of the Boston KPro, an artificial cornea that allows ophthalmic surgeons to restore useful levels of vision to patients when corneal grafts have failed. For patients with the most severe cornea damage, the Boston KPro can be a lifeline that permits a return to a normal or near-normal lifestyle. Some of my most rewarding cases have been in seeing the transformative outcomes that we can achieve with the KPro.

KPro for Severely Damaged Corneas

Keratoprosthesis, a procedure that today uses the Boston “KPro” or another artificial cornea, was first performed in 1789 by Dr. Pelliere De Quengsy, a French ophthalmologist. The materials used for the procedure at that time were glass, plastics and tooth dentin. The Boston KPro, which is today the most widely used artificial cornea, was designed by Dr. Claes Dohlman at Massachusetts Eye & Ear Infirmary and approved by the FDA in 1992. Two other artificial corneas currently in use are the Alphacor (Addition Technology) and osteo-odonto Kpro (OOKP) by Strampelli and modified by Falcinelli.

For the past 40 years, Dr. Dohlman has been developing and improving the Boston KPro procedure, in which the cornea sits as a skirt to hold a central PMMA prosthesis and keep the central visual axis clear and useful for vision. The early versions of the Boston KPro were not very successful due to frequent rejections, necrosis and extrusion with secondary retinal detachment. Recent versions, however, overcame many initial problems and current outcomes are much more promising.

Appearance of same eye six months after implantation of the Boston KPro. The visual acuity was 20/25 uncorrected. PHOTOS COURTESY OF JAMES CHODOSH, MD

Since 2002, 6,000 KPros have been implanted worldwide. It is estimated that 45 million people worldwide are bilaterally blind, eight million of whom are believed to be afflicted with a form of corneal disease (1.5 million children). In Africa, trachoma is the leading cause of blindness. In the US, causes include infection, trauma, herpes simplex, dystrophies, autoimmune diseases and graft failure.

The Type I KPro is the most commonly used; however, there is also the Type II KPro designed with anterior extension to allow implantation through surgically closed lids. It is generally reserved for patients with severe symblepheron or ankyloblepheron, or severe ocular surface keratinization like SJS and OCP.

The KPro is machined as a front part and a separate backplate (with holes), which is threaded and locked to the stem clamping a corneal graft. The corneal tissue serves as a skirt holding the central KPro to keep the vision clear. The front plate is 7.0 mm in diameter with a stem diameter of 7.33 mm. The backplate is titanium 8.5 mm, 7.0 mm (pediatric) in diameter with 16 holes for nutrition from the aqueous humor to the corneal stroma and its keratolytes. The KPro is assembled in the operating room just before transplantation of the recipient cornea is made and sutured to the host cornea with 16 interrupted 10-0 nylon sutures. Then, a large flat bandage soft contact lens is fitted for hydration purposes.

Aphakic patients get an aphakic KPro, whose power is determined by axial length measurement. Pseudophakic patients receive a standard pseudophakic KPro, while phakic patients get simultaneous lens extraction and KPro in the same setting.

KPro Postop Care

Although preoperative evaluation of the patient is addressed in Table 1, postop care involves keeping the soft contact lens in place all the time, use of long-term topical steroids, fluoroquinolones for one year, then followed by Polytrim (polymyxin B sulfate and trimethoprim sulfate) drops, vancomycin 12.5 mg/ml mixed with 0.005% benzalkonium chloride. Some surgeons in humid areas treat with antifungal agents like Amphoteracin B 0.15% drops once a week every month. Patients with absent lids are usually not good candidates for the KPro, as well as those with severe chronic inflammation like SJS, OCP and others. Patients who had previous grafts are the best candidates for KPro. The KPro is reported to be successful in aniridia patients and remains challenging and controversial in pediatric cases (Table 2).

My Experience With the KPro

I have been performing Boston Keratoprosthesis implantation for about 10 years and have done about 60 procedures, most commonly on patients with failed corneal grafts, who postoperatively have the best clinical outcome. I have also done a couple of cases with Stevens-Johnsons Syndrome, one patient with thermal burn, a victim of the 9/11 attack on the Pentagon and two patients with ocular cicatricial pemphigoid.

I started doing them under general anesthesia, but now I perform the surgery under monitored anesthesia care (MAC) anesthesia with retrobulbar injection of Lidocaine and Marcaine. General anesthesia is used on young and pediatric patients.

I start the surgery with assembling the keratoprosthesis under the microscope. Then, the recipient cornea is trephined, usually with an 8.5-mm trephine. In those patients who have had multiple eye surgeries, resulting in an opaque cornea peripheral anterior synechiae, and distorted anterior segment anatomy, I perform partial trephination and then meticulous and careful dissection to release the adhesions between the cornea and at the iris. Anterior segment reconstruction is important in these complex cases. In phakic patients, I do a lensectomy with IOL insertion and then suture the assembled KPro to the host cornea with 16 interrupted 10-09 sutures. At the conclusion of the procedure, I insert a large-diameter flat contact lens.

I have two interesting cases that I would like to discuss. The first patient is a 35-year-old female who presented with complete vascularization and conjuctivalization of her corneas in both eyes. Visual acuity was light perception in each eye. After retina consultation, I performed KPro on her right eye with 20/40 vision postop. She developed a retro KPro membrane that had to be removed twice by a pars plana approach and now she's enjoying 20/70 vision in that eye six years after the initial procedure.

The second case is a 45-year-old gentleman with severe keratoglobus in the right eye and best-corrected vision 20/400. The left eye had a corneal transplant that failed and he was left with hand-motion vision. The patient declined surgery until he later had a ruptured cornea in the right eye with endophthalmitis and NLP vision. I did a keratoprosthesis in the left eye a year ago and he is still enjoying 20/40 vision.

KPro Outcomes Continue to Improve

The KPro has come a long way and is now truly a valuable and successful option for most patients with severe ocular surface disease or high-risk keratoplasties. Postoperative rehabilitation is quick — even quicker than a regular cornea transplant. Postop visual acuity improves within a month, sometimes as soon as after one week. Table 3 addresses some of the postop complications that can occur after a KPro procedure.

Thanks to the regular use of topical antibiotics and/or antifungals in connection with the KPro procedure, the incidence of infections is very rare. Glaucoma patients have to be treated aggressively with anti-glaucoma medications or surgical intervention with shunts weeks before the KPro procedure is performed. Postop intraocular pressure is measured by digital palpation.

Future usage of intra-camera sensors being tried by Dr. Samir Melki in Boston could be helpful in accurate measurement of intraocular pressures. Other complications like retro KPro membrane formation can be treated with YAG laser membranectomy or a surgical approach through the pars plana, usually performed by a retina specialist. Titanium backplates are more inert than those made of PMMA and are supposed to decrease the incidence of retro KPro membrane. In my experience, KPro membranes hinder the vision after KPro implantation. They are usually dense, hard and very difficult to YAG and have a high recurrence rate after surgical removal. OM

Salim Butrus, MD, is in solo private practice at the Eye Center on Capitol Hill in Washington, DC, and has an affiliation with Georgetown University Hospital, where he teaches, does research and performs corneal transplant and other complex procedures.