Boston Type 2 Keratoprosthesis
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Background
A keratoprosthesis or artificial cornea is indicated in cases of severe corneal blindness where either traditional corneal transplantation (full thickness or partial thickness corneal transplant) has failed or run a high risk for failure.1 The most commonly utilized keratoprosthesis is the Boston type 1 keratoprosthesis (BK1). However, there are situations where BK1 surgery is not appropriate for treating severe bilateral ocular surface disease, specifically in patients with absent tear film, ocular surface keratinization, and symblepharon formation.2,3 The Boston keratoprosthesis type II (BK2) has emerged as a pivotal intervention for these patients and may be the last resort for vision restoration. Over the past three decades, substantial advances in device design and postoperative management have been observed, enhancing the potential for significant vision recovery in this challenging patient cohort. This article endeavors to provide an overview of BK2, encompassing its background, device evolution, patient selection criteria, surgical methodology, clinical outcomes, and potential complications.
Device Design
The BK2 consists of a PMMA front plate and optic and, a titanium back plate, similar to the BK1 however, the optical stem is longer in the BK2 to allow implantation through surgically closed eyelids.4 The design of the Boston Keratoprosthesis has undergone multiple iterations in the past three decades with changes implemented to modify the ways the front plate is locked to the back plate. The original “screw-in” design was FDA approved in 1992, where the front plate was screwed into the polymethyl methacrylate (PMMA) back plate with the corneal graft in between them.5 In 2003, the “screw-in” design was modified to a “snap-on” design by addition of a c-shaped titanium locking ring behind the back plate to assemble the device in a way that minimized damage to the donor cornea.6 A titanium back plate additionally became available in 2005, replacing the PMMA back plate.7 The “snap-on” design was modified to “click-on” design by adding a laser-cut slit to the titanium back plate that enables locking of the device in place and was FDA approved in 2009.3 Currently the BK2 is offered as a "click-on" design with titanium backplate, and has been in use since 2009.3 The BK2 is available in a pseudophakic version and an aphakic version designed for eyes with a range of axial lengths.
Indications and Patient Selection
Traditional corneal transplantation in patients with severe ocular surface disease is associated with a poor prognosis and various complications including persistent epithelial defects, stromal ulceration, corneal melting, and graft failure.8,9 The BK1 is used in patients who fail traditional corneal grafts such as patients with repeated corneal transplant failure or extensive neovascularization, however implantation of a BK1 device requires a well lubricated ocular surface and normal lid function.10 The BK2 is uniquely indicated for patients with advanced end-stage ocular surface disease, characterized by an absent tear film, keratinization, symblepharon formation, and forniceal foreshortening.3,4,8,10 This sets it apart from its counterpart, the BK1. By far the most common indication for BK2 implantation are Mucous membrane pemphigoid (MMP) and Stevens-Johnsons syndrome/toxic epidermal necrolysis (SJS/TEN).3,4,10 Other indications include severe ocular cicatricial disease secondary to chemical injury, Sjogren’s syndrome, severe neurotrophic keratopathy and Kearns-Sayre syndrome.3,4 Prior to BK2 implantation, it is important to carefully estimate visual potential. Advanced glaucoma and retinal pathology should be excluded prior to the surgery.1 In general, the Boston keratoprosthesis is offered to patients with visual acuity worse than 20/400, and in cases where the fellow eye has a visual acuity of 20/40 or worse.8 In addition, it is our approach to offer a BK2 to only one eye at a time, with the fellow eye kept in reserve and treated for any other pathology including glaucoma to preserve visual potential.1 In addition to explaining the risks and benefits of the surgery, it is also important to counsel the patient regarding the need for rigorous postoperative management, frequent close follow-up and other life changes that they may encounter after BK2 implantation. Finally, a multidisciplinary team of corneal, vitreoretinal, glaucoma surgeons, rheumatologists, and anesthesiologists is essential for successful outcomes.
Surgical Technique
Unless contraindicated due to medical comorbidities all BK2 surgeries are performed under general anesthesia. We recommend performing a pars plana viterectomy and implanting of a glaucoma drainage device (if one is already not in place) at the time of BK2 surgery.3 The recipient globe is prepared by removing all ocular surface epithelium, including that of the bulbar, forniceal, and tarsal conjunctiva, and the corneal limbus and corneal periphery, by sharp dissection.3,8,10 Existing scar tissue is dissected and excised until the bare sclera is exposed and the globe can be freely rotated. If a glaucoma drainage device (usually an Ahmed valve) is to be implanted, the plate is secured to the sclera.
To assemble the BK2 device, the donor cornea is double trephined with a 3 mm skin biopsy punch and then an 8.5 mm corneal donor punch to create a "donut" carrier graft.8 The donor cornea and the back plate are slid down over the optical stem and secured in place with gentle pressure using the provided assembly tool.3 The device is inspected under the operating microscope and placed in corneal preservation media in a closed container until implantation.
The host cornea is trephined at 8 mm to match the diameter of the temporary keratoprosthesis. A total iridectomy is performed if aphakia is present or desired. This is followed by lensectomy or intraocular lens (IOL) removal (if a lens is present). An existing posterior chamber intraocular lens (PCIOL) can be left in place however aphakia is preferred. If PCIOL is left in place, a pupilloplasty is performed at a diameter sufficient to keep the iris from obscuring the visual axis. Subsequently, a temporary keratoprosthesis is placed to permit wide-field pars plana vitrectomy. Depending on the lens status, specific BK2 device is chosen based on the axial length of the eye.3 After pars plana vitrectomy, if an Ahmed valve tube is to be implanted, it can be placed through the inferotemporal vitrectomy port.3 Subsequently, the temporary keratoprosthesis is removed and the BK2 is secured with 12 interrupted 9-0 nylon sutures followed by peribulbar injections of vancomycin 25 mg, ceftazidime 100 mg, and triamcinolone 20 mg.3,8 Finally, eyelid closure is performed. The anterior eyelid margins are excised to remove all eyelash follicles, and a notch is created in the upper lid. The keratoprosthesis stem is subsequently locked in between the upper and lower tarsal plates on either side using interrupted 6-0 vicryl sutures with the optic positioned anterior to the tarsal plates. Finally, a complete tarsorrhaphy is performed and the skin closed with running 8-0 nylon sutures.3,10
Outcomes
Overall 37.5%-83.9% patients achieve a best corrected visual acuity (BCVA) of 20/200 or better, depending on the follow-up evaluated by authors.2–4,8,10 Including patients with SJS who were implanted with the previously used “screw-in” BK2 and BK1 devices from Jan 2000-Dec 2005, Sayegh et al reported that 75% of the eyes achieved BCVA of 20/200 or better after surgery.8 Around 50% of eyes maintained BCVA of 20/200 or better, 3 years postoperatively.8 Pujari and colleagues reported outcomes for patients implanted with BK2 (screw-in design) from Jan 2000-Dec 2009 at a single tertiary care center. Overall, 79.3% of patients with SJS, MMP or other severe ocular surface diseases who received the with BK2 achieved BCVA of 20/200 or better at some point in the post operative period.10 Lee et al reported that 95.8% patients implanted with BK2 from Jan 1992-Apr 2015 (including patients with both screw-in and click-on design), had a postoperative visual acuity of 20/200 or worse after surgery and 37.5% had a BCVA of 20/200 or better at the last follow-up (mean follow-up 70.2 months).4 Iyer and colleagues reported that 90% of patients (9 of 10 eyes) implanted with BK2 achieved BCVA of 20/200 or better postoperatively and 80% achieved BCVA of 20/30 or better.2 Furthermore, at a mean follow up of 2.75 years 70% maintained a BCVA of 20/200 or better.2 Saini et al included all BK2 patients at a tertiary care center implanted with the most recent “click-on” device from Jun 2009-Mar 2021. They reported that 89.3% of the eyes (50 of 56 eyes) achieved postoperative BCVA of 20/200 or better at some point in the postoperative period (mean follow-up 45.8 months) and more than 50% of those patients retained that vision for more than 48 months.3 Moreover, half of the eyes that had a follow-up of more than five years (9 of 18 eyes) kept a BCVA of 20/200 or better.3 Implantation of a previous BK2 model, not receiving a concomitant glaucoma drainage device intraoperatively and not being on pre-operative immunosuppression were reported as some of the risk factors for not retaining BCVA of 20/200 or better postoperatively.3 Finally, many BK2 patients end up needing replacement surgeries due to device extrusion. Retention of BK2 without extrusion is reported to range anywhere from 50%-90%.2–4,10 The most common causes for replacement include corneal melt and local peri-stem infection. 4
Complications
Common complications after BK2 surgery include formation of retroprosthetic membrane (RPM), skin retraction, glaucoma, choroidal effusion, and retinal detachment among others.3,4,8 The incidence of RPM varies from 10%-60% in the literature.2,4 In early stages, RPM can be treated with an Nd-YAG laser and is rarely the cause of irreversible vision loss by itself. On the contrary, glaucoma and retinal detachment are the most common causes of irreversible vision loss in BK2 patients.3,10 The incidence of retinal detachment has been reported to be from 10%-28%.2,3,10 Glaucoma development or progression is another cause of irreversible vision loss in BK2 patients. Although the definition of glaucoma varies in the literature some 8.3%-41.1% BK2 patients develop glaucoma or exhibit glaucoma progression.3,4 The incidence of endophthalmitis incidence has decreased since the adaptation of long-term prophylactic antibiotic usage postoperatively.11 Nonetheless, the incidence of endophthalmitis postoperatively in BK2 patients varies from 5%-17.9%.2–4,10 Other postoperative complications after BK2 implantation include choroidal effusion (30.3%), choroidal hemorrhage (8.3%), keratitis (19.6%), vitritis (23.2%), vitreous hemorrhage (16.1%), sterile melt (10%), and wound leak (34%).2–4,10
References
1. Avadhanam V, Smith H, Liu C. Keratoprostheses for corneal blindness: a review of contemporary devices. Clinical Ophthalmology. Published online April 2015:697.
2. Iyer G, Srinivasan B, Agarwal S, et al. Boston Type 2 keratoprosthesis- mid term outcomes from a tertiary eye care centre in India. Ocular Surface. 2019;17(1):50-54.
3. Saini C, Chen TC, Young LH, et al. Restoration of Vision in Severe, Cicatricial, Ocular Surface Disease With the Boston Keratoprosthesis Type II. Am J Ophthalmol. 2022;243:42-54.
4. Lee R, Khoueir Z, Tsikata E, Chodosh J, Dohlman CH, Chen TC. Long-term Visual Outcomes and Complications of Boston Keratoprosthesis Type II Implantation. Ophthalmology. 2017;124(1):27-35.
5. Doane MG, Dohlman CH, Bearse G. Fabrication of a keratoprosthesis. Cornea. 1996;15(2):179-184.
6. Dohlman CH, Harissi-Dagher M, Khan BF, Sippel K, Aquavella J V., Graney JM. Introduction to the use of the Boston keratoprosthesis. Expert Rev Ophthalmol. 2006;1(1):41-48.
7. Todani A, Ciolino JB, Ament JD, et al. Titanium back plate for a PMMA keratoprosthesis: Clinical outcomes. Graefe’s Archive for Clinical and Experimental Ophthalmology. 2011;249(10):1515-1518.
8. Sayegh RR, Ang LPK, Foster CS, Dohlman CH. The Boston keratoprosthesis in Stevens-Johnson syndrome. Am J Ophthalmol. 2008;145(3):438-444.
9. Tugal-Tutkun I, Akova YA, Foster CS. Penetrating Keratoplasty in Cicatrizing Conjunctival Diseases. Ophthalmology. 1995;102(4):576-585.
10. Pujari S, Siddique SS, Dohlman CH, Chodosh J. The boston keratoprosthesis type II: The massachusetts eye and ear infirmary experience. Cornea. 2011;30(12):1298-1303.
11. Durand ML, Dohlman CH. Successful prevention of bacterial endophthalmitis in eyes with the boston keratoprosthesis. Cornea. 2009;28(8):896-901.
