Outcomes of Boston Keratoprosthesis Type 1

Introduction

The Boston keratoprosthesis (KPro) was approved by the US Food and Drug Administration in 1992 as an alternative to penetrating keratoplasty (PK) in patients having previously failed PK. It has become a viable therapeutic option for a wide range of conditions, including chemical and thermal injury, congenital aniridia, bullous keratopathy, infectious keratitis, Fuchs dystrophy, keratoconus, Stevens-Johnson syndrome (SJS), keratoconjunctivitis sicca, and mucous membrane pemphigoid (MMP) after previous graft failure.

Since its inception, more than 12,000 Boston KPros have been implanted worldwide. With its increasing popularity, the outcomes of its implantation have also been extensively studied, especially in the past decade. This chapter will focus on clinical outcomes of the Boston KPro, including visual acuity, prosthesis retention, and postoperative complications.

Visual Acuity and Prosthesis Retention

In general, the Boston KPro has been shown to significantly improve visual acuity in most patients. Preoperative best corrected visual acuities (BCVA) vary depending on the diagnosis, but are generally very poor, with most studies reporting more than 80% of patients having a preoperative BCVA of worse than 20/200. Postoperative BCVAs are generally significantly improved in the short term, around 6 months to 1 year. Most studies report postoperative BCVA of 20/200 or better in at least 40% of patients, with a range from 43% to 89% in larger studies (56% in the multicenter study). A subsequent follow-up study of the multicenter cohort reported that, once achieved, BCVA of 20/200 or better was retained for an average of 47.8 months. Studies evaluating outcomes of keratoprosthesis reimplantation have shown that repeat operations did not result in a higher risk for failure compared to patients receiving their first keratoprosthesis, ^, although vigilance must be maintained for comorbidities such as glaucoma.

In a multicenter study with 7 years of follow-up, about half of patients who achieved a BCVA of 20/200 or better maintained this level of visual acuity. Advancing glaucoma and age-related macular degeneration were commonly cited reasons for deterioration of visual acuity after initial improvement postoperatively. A small percentage of these patients also suffered visual loss from postoperative complications, such as retinal detachment, infection, corneal melting, and extrusion of the prosthesis.

Most studies report prosthesis retention rates of 80% to 95%, ^, depending on the duration of follow-up (95% in the initial multicenter study). A subsequent multicenter follow-up study of 300 eyes reported probabilities of retention at 1 year and 2 years of 94% and 89%, respectively. In a longer-term multicenter study, the retention rate at 7 years was estimated at 67%. The most common cause of prosthesis failure was reported to be corneal melting leading to prosthesis extrusion. Other causes of prosthesis failure included infectious keratitis and endophthalmitis, as well as severe retroprosthetic membranes (RPM). The presence of severe ocular surface disease, such as autoimmune disease, including SJS; toxic epidermal necrolysis (TEN) syndrome; keratoconjunctivitis sicca; and MMP and chemical injuries has been reported to be a significant risk factor for failure of the prosthesis. ^{, ,} Additional risk factors included the number of prior failed grafts and tarsorrhaphy, although this latter risk factor was likely not due to the procedure itself but rather because those undergoing tarsorrhaphy were at higher risk for exposure-related complications.

Patients with MMP have been shown to have worse visual outcomes with the type 1 KPro. In fact, in this study, despite initial gains in visual acuity, all patients had visual acuities worse than 20/200 at final follow-up. Prosthesis retention rate was low at 37.5%. As previously mentioned, multiple groups have reported that autoimmune conditions are associated with worse outcomes, possibly related to ongoing inflammation and tissue damage. ^{, ,}

Postoperative Complications

Common postoperative complications that have been reported include the development of RPM, progression of preexisting glaucoma, development of new glaucoma, retinal detachment, sterile vitritis, choroidal detachment, cystoid macular edema, and vitreous hemorrhage. Other less common but potentially devastating complications include infectious keratitis, endophthalmitis, corneal melt, and extrusion of the prosthesis.