Jennifer Y. Li, Mark A. Greiner and Mark J. Mannis

Introduction

The Boston Keratoprosthesis™ (Boston KPro) was developed at the Massachusetts Eye and Ear Infirmary for the treatment of corneal blindness in patients who otherwise have a poor prognosis for a standard penetrating keratoplasty (PKP). Since its approval by the FDA in 1992, the Boston KPro has become the most commonly used keratoprosthesis device in the United States. Over 3500 Boston keratoprosthesis devices have been implanted worldwide, with the vast majority having been implanted since 2006.¹

The Boston type I KPro is the more commonly used model and has been studied more extensively. Several groups in the United States have evaluated outcomes of the Boston type I keratoprosthesis, and we now have both short-term and long-term outcome data for these patients. We review these results and discuss outcomes for specific groups of patients undergoing keratoprosthesis surgery.

Advances in the Boston Keratoprosthesis to Improve Outcomes

Several advances in both the design of the Boston type I keratoprosthesis and in our understanding of the postoperative management of these patients have led to improved outcomes and contributed to the popularity of the surgery.¹ Major postoperative complications, such as corneal melting, keratoprosthesis extrusion, and endophthalmitis can lead to removal of the keratoprosthesis and significant vision loss. However, even retroprosthetic membranes, which are the most common postoperative complications after keratoprosthesis surgery, may lead to vision decline.

Current design features of the Boston keratoprosthesis that have helped to improve retention of the KPro include holes in the back plate of the KPro, a threadless design, and a titanium locking ring.2–4 The holes in the back plate of the KPro allow nutrients, such as glucose to pass from the aqueous to the corneal carrier tissue.² An early study showed a statistically significant decrease in tissue necrosis or melt around a keratoprosthesis from 51% of eyes with a solid PMMA back plate to 10% of eyes of eyes with a back plate with holes.² The threadless design of the stem of the KPro prevents misthreading or inadequate threading of the back plate, a design that previously led to gaps between the front plate and the cornea carrier tissue and resulting anterior protrusion of the front.³ Anterior protrusion of the KPro was associated with tissue melting around the stem.³ The titanium locking ring helps prevent intraocular loosening of the KPro postoperatively.⁴

Postoperatively, the use of a bandage contact lens and long-term vancomycin drops has been instrumental to reducing vision-threatening complications of keratoprosthesis surgery. Bandage contact lenses improve outcomes by decreasing the risk of corneal melting from desiccation of the ocular surface.⁵

Long-term topical vancomycin eye drops (14 mg/mL) help decrease the risk of bacterial endophthalmitis.⁶ Bacterial endophthalmitis is a devastating complication associated with a very poor visual outcome.⁶ In their retrospective review of bacterial endophthalmitis in KPro eyes between 1990 and 2006, Durand and Dohlman. found that only two of 18 eyes regained useful vision.⁶ Eight of 18 eyes in their review lost light perception vision entirely, with five eyes retaining only hand motions or light reception vision.⁶ Fortunately, they also found a statistically significant lower incidence of bacterial endophthalmitis in patients on topical vancomycin prophylaxis, compared with those on a single commercially available antibiotic agent alone (0.35% versus 4.13% per patient-year, p = 0.001).⁶ Keratoprosthesis surgeons now routinely use topical vancomycin in combination with another commercially available broad-spectrum antibiotic agent, although the precise regimen varies among surgeons.¹

Finally, decreasing the frequency of retroprosthetic membrane formation may also help improve long-term visual outcomes after KPro surgery. Although visually significant retroprosthetic membranes can typically be treated with a simple YAG membranotomy, some membranes may be refractory to laser treatment, causing a decline in vision and necessitating more aggressive surgical removal.7–9 In some instances, the retroprosthetic membrane may be recalcitrant even to surgical removal.⁹ A titanium back plate for the Boston keratoprosthesis is currently being evaluated at the Massachusetts Eye and Ear Infirmary. The preliminary findings are encouraging, with a trend towards decreased postoperative inflammation and less frequent formation of retroprosthetic membranes in patients with a titanium back plate.¹⁰

Early Postoperative Outcomes of the Boston Type 1 Keratoprosthesis

Early postoperative outcomes for the Boston type 1 keratoprosthesis are generally excellent. Table 52.1 highlights studies looking at shorter-term outcomes (average follow-up period < 12 months). Within the first few months after surgery, visual improvement is often drastic. Dunlap et al. reported short-term visual outcomes of the Boston type I keratoprosthesis and found that in their series of 126 eyes, 82.5% (104 eyes) had improved visual acuity within the first 6 months.¹¹ Of those 104 eyes, 31 eyes (30%) achieved best-corrected vision within 1 week of surgery.¹¹ By postoperative month 1, 59 eyes (54%) had achieved their best-corrected visual acuity.¹¹ The 22 eyes which did not have an improvement in visual acuity postoperatively had co-morbid conditions limiting visual potential.¹¹

Robert and Harissi-Dagher reported similar outcomes in their patients.¹² In a group of 47 eyes, they observed that 84% had an improvement of best-corrected visual acuity, with the median visual acuity improving from hand motions preoperatively to 20/150 by postoperative month 3.¹² There was 100% anatomic retention of the keratoprosthesis in these patients.¹²

Zerbe et al. also reported positive outcomes in the United States multicenter study.⁷ Postoperatively, 57% of eyes had a best-corrected vision of 20/200 or better, compared to just 3.6% preoperatively.⁷ There was a 95% rate of KPro retention in this study.⁷

However, when evaluating outcomes for the Boston type I keratoprosthesis, it is important to consider more than just the best-corrected visual acuity postoperatively. Short-term improvement in visual acuity is only one measure of success. Postoperative complications related to the KPro, such as recalcitrant retroprosthetic membranes, endophthalmitis, device extrusion, and glaucoma often lead to vision loss over time, especially as the cumulative risk of developing a vision-threatening complication increases. Therefore, a second outcome measure to consider in these patients is long-term retention of postoperative visual acuity improvements.

Interestingly, even in their relatively short follow-up period (average, 10 months; range, 3–18.5 months), Robert and Harissi-Dagher had four patients (8%) who had progressively worsening vision.¹² One patient lost vision from phthisis; three patients lost vision from progression of glaucoma.¹² Similarly, Zerbe et al. reported that in 62 eyes with at least 1 year follow-up (average, 14.5 months; range, 12–27 months), 12.9% (eight eyes) had decreased vision at the final follow-up visit, compared to 1 month postoperatively.⁷ The reasons cited for vision loss included end-stage glaucoma, retinal detachment, and age-related macular degeneration.⁷

Long-Term Outcomes of the Boston Type 1 Keratoprosthesis

An analysis of the long-term outcomes of the Boston KPro suggests the need for continued research and development in this field of keratoprosthesis surgery. Table 52.2 summarizes the KPro outcomes from the published literature.

For the most part, outcomes remain positive for the first 2 to 3 years after keratoprosthesis surgery.13–15 Aldave et al. reviewed a series of 57 keratoprosthesis surgeries that were performed in 50 eyes. Seventy-nine percent (38/48 eyes) had improved vision at the final postoperative visit, compared with preoperative vision.¹⁴ They found it encouraging that the percentage of eyes with a visual acuity of 20/200 or better increased over time.¹⁴ Of the eight eyes with at least 3 year follow-up data, all eight eyes (100%) had a visual acuity of 20/200 or better.¹⁴ The authors speculate that many of the complications of keratoprosthesis surgery which affect vision, such as retroprosthetic membrane formation, cystoid macular edema and posterior capsular opacification, appear and are managed in the first postoperative year, allowing for better vision later in the postoperative course.¹⁴ These are by far the best results reported in the literature to date. However, despite these encouraging results, long-term retention of the keratoprosthesis shows a downward trend with a 90% retention rate (35/39) at 1 year decreasing to 61% (14/23) at 2 years.¹⁴