© Springer-Verlag Berlin Heidelberg 2015M. Soledad Cortina and Jose de la Cruz (eds.)Keratoprostheses and Artificial Corneas10.1007/978-3-642-55179-6_20
20. Future Directions for the Boston Keratoprosthesis
Department of Ophthalmology, Massachusetts Eye and Ear Infirmary/Boston Children’s Hospital, Harvard Medical School, 243 Charles Street, Boston, MA 02114, USA
20.3 Infectious Keratitis
20.4 Corneal Melts
20.7.1 Pediatric Keratoprosthesis
The type I Boston keratoprosthesis (KPro) is the most commonly used artificial cornea. Through the end of 2012, over 8,000 of these devices had been placed by almost 400 corneal surgeons throughout the world. Although the first Boston KPro was placed by Claes Dohlman over 40 years ago, acceptance of the device outside Boston was slow until approximately 2005 due to multiple complications following surgery, including endophthalmitis and corneal melting. Since then, there has been an explosion in the use of the Boston KPro, including adoption of this device as the initial corneal surgery in some patient populations. In 2012, over 1,000 of these devices were placed worldwide. In 2010–2011, 50 % of these devices were placed for the treatment of graft failure, while the remaining were used for other diagnoses such as limbal stem cell failure and corneal vascularization that typically have a poor prognosis for traditional penetrating keratoplasty .
What underlies this dramatic increase in usage and the expansion in indications for the type I Boston KPro? The short answer is that modifications to both the device itself and to the postoperative management have reduced complications and improved outcomes to the point that this device now enjoys unparalleled acceptance among corneal surgeons worldwide. Despite our successes so far, the Boston KPro is not yet a perfect device. Complications do still occur and there are certain patients, such as those with autoimmune diseases and children, for whom the Boston KPro remains problematic. This chapter will review current strategies to solve the remaining complications with the aim of further improving patient outcomes in future years.
Endophthalmitis remains the most feared complication following keratoprosthesis surgery and can result in loss of light perception and even loss of the eye. Published series of the Boston KPro have reported endophthalmitis rates that vary from 0 to 12.5 % [2–5]. A recent review suggests that the overall rate is 5.4 % . Lack of bio-integration of the KPro is at the root of the risk for endophthalmitis. The plastic cylinder of the Boston KPro stem cannot integrate with the carrier corneal graft, thus leaving a potential connection between the environment and the inside of the eye. Efforts to reduce melting of the corneal carrier graft, described below, are an essential step in eliminating this pathway for pathogen ingress into the anterior chamber.
Endophthalmitis occurrence can be essentially eliminated by the use of lifelong prophylactic topical antibiotics following surgery , but this requires ongoing patient compliance, which can be difficult, especially in elderly patients on multiple other medications. Drug-eluting contact lenses are a potential solution to this issue. Antibiotic-impregnated contact lenses have been shown to release drug at a sustained rate in vitro , although use of these lenses has not been translated to patients yet. Similarly, laboratory studies have suggested that covalent attachment of certain chemicals to KPro components may inhibit biofilm formation and may represent another approach to reduce the risk of infection .
The ideal postoperative antibiotic regimen has not been proven, and there is significant variability from surgeon to surgeon, even within a single institution. Most patients are treated with a broad-spectrum fluoroquinolone or combination antibiotic (such as polymyxin B/trimethoprim) one to three times daily. Some surgeons rotate which prophylactic antibiotic is used. Since most causative pathogens are gram-positive, topical vancomycin can be considered for high-risk groups including monocular patients or those whose diagnoses put them at increased risk of endophthalmitis (such as Stevens-Johnson syndrome (SJS) or mucus membrane pemphigoid (MMP)). However, this medication is not commercially available and must be compounded, adding expense and inconvenience.
At present, patient should be counseled about the importance of compliance with postoperative topical antibiotics before KPro surgery, and the potential consequences of noncompliance should be clearly stated. This message should be reinforced at every follow-up visit. Patients and co-managing ophthalmologists should also be educated about the signs and symptoms of endophthalmitis. Any patient suspected of having endophthalmitis needs to be managed expeditiously by an experienced team. Typically, the management involves an intravitreal tap (with culture of the aspirated fluid) and injection of antimicrobial agents. Topical or systemic antimicrobials may be useful in certain situations.
20.3 Infectious Keratitis
The use of a bandage contact lens in the setting of chronic topical steroids and low-dose antibiotics increases the risk of both bacterial and fungal keratitis . Prompt recognition and management of infectious keratitis reduces the likelihood of progression to endophthalmitis. The bandage lens and corneal graft should be carefully inspected at each visit. Deposits on the bandage lens may represent fungus. If deposits are present, the bandage lens should be changed and the patient followed closely to ensure that no corneal infiltrates develop. Topical 5 % povidone-iodine (after topical anesthesia) may be used to irrigate the eye prior to placement of a clean bandage lens.
A whitish infiltrate in the corneal carrier graft, especially near the KPro stem, may represent an early infection and should not be overlooked to reduce the chance of graft melting and progression to endophthalmitis. Standard corneal cultures should be taken, including cultures for fungus, and frequency of antibiotic drops increased until the cultured results are known. Changing the bandage lens is prudent in this situation. Culture of the bandage lens can be considered. Fungal infections, typically caused by Candida species, can present in this way, and consideration should be given to initiation of topical antifungal drops while waiting for the culture results.
Ongoing research is evaluating whether antifungal prophylaxis is effective in reducing fungal keratitis after KPro. For now, prompt recognition and appropriate management is needed to prevent devastating complications resulting from infectious keratitis.
20.4 Corneal Melts
The placement of holes in the backplate of the Boston KPro improves access of the corneal carrier tissue to nutrients within the aqueous humor and was a major advancement in prevention of corneal melts. Similarly, the routine use of a bandage contact lens, which diffuses evaporative forces thereby reducing desiccation of the carrier graft, dramatically decreased the risk of corneal melts in KPros placed in patients without underlying autoimmune diseases (such as SJS and MMP).
However, corneal melts do still occur, especially in the autoimmune diseases, and can lead to exposure of the KPro backplate (and subsequent endophthalmitis or KPro extrusion). Risk factors include persistent epithelial defects, chronic exposure either from a poorly fitted bandage contact lens or from an inability to retain the bandage lens, and retroprosthetic membranes (see below).
The corneal carrier graft should be inspected for thinning at each follow-up visit. Anterior segment OCT can be helpful to monitor subtle thinning of the graft . Once thinning is recognized, steps should be taken to correct inciting factors. A different bandage contact lens may be needed if the cause is poor lens fit or lens retention. For patients with chronic exposure who are not able to retain a bandage lens, permanent tarsorrhaphy may be an option. Prevention of retroprosthetic membrane formation (see below) is likely to reduce the development of corneal melts.
Melting of the corneal carrier graft remains a significant issue in patients with underlying autoimmune diseases. Patients with SJS or MMP (and to a lesser extent those with a history of serious chemical injury) should be counseled about the ongoing risk of corneal melting, and this should be carefully considered prior to embarking upon keratoprosthesis surgery. Especially close follow-up of these patients is essential. Ongoing research is exploring the efficacy of systemic immunomodulatory agents such as infliximab in reducing melting in these high-risk populations . In vitro modification of the corneal carrier graft using collagen cross-linking with riboflavin and ultraviolet light is also under investigation as a possible solution to this ongoing problem.
20.5 Retroprosthetic Membranes
Retroprosthetic membranes (RPM) are the most common complication after Boston KPro, occurring in anywhere from 31 to 65 % of cases [2, 13]. While these membranes were previously thought to be a minor inconvenience for the patient and the KPro surgeon, readily treatable by Nd-YAG membranotomy, or less commonly by surgical excision, more recent work suggests that membranes visible posterior to the KPro optic are commonly associated with more extensive fibrosis behind the KPro backplate that is typically not visible except within the holes of the backplate and, in fact, may underlie more serious complications like corneal melting, KPro extrusion, and progressive angle closure. Use of backplates made of titanium appears to reduce the incidence of RPM formation.