Article

Vitreoretinal Surgical Pearls in Cases With the Boston Keratoprosthesis Type 1

Strategies to ensure successful outcomes.

Related

The Boston keratoprosthesis type 1 (B-KPRO; Massachusetts Eye and Ear Infirmary) is the most common keratoprosthesis in use worldwide. The B-KPRO received 510(k) clearance by the US Food and Drug Administration (FDA) in 1992, and the basic design consists of three principal components forming a collar button design: a front plate with an embedded optical stem, a back plate with eight or 16 holes, and a titanium locking ring.1 Newer designs have a built-in locking ring. Donor corneal tissue is placed between the front and back plates, which snap together. The implant is then sutured to the host eye, restoring optical clarity (Figure 1).

Figure 1. Boston keratoprosthesis type 1 implant in a patient with aniridia.
IMAGE COURTESY OF KATHRYN COLBY, MD, PHD, LOUIS BLOCK PROFESSOR AND CHAIR, DEPARTMENT OF OPHTHALMOLOGY AND VISUAL SCIENCE, THE UNIVERSITY OF CHICAGO SCHOOL OF MEDICINE AND BIOLOGICAL SCIENCES.

The popularity of B-KPROs has surged in the past two decades from fewer than 50 surgeries reported in the United States in 2002 to 1,161 cases performed in 2009. Currently, more than 9,000 total procedures have been carried out worldwide.2-4

Historically, implanting a keratoprosthesis was considered a procedure of last resort after several failed cadaveric corneal transplants. More recently, indications for the B-KPRO have expanded and include prior graft failure from recurrent herpetic disease; ocular trauma; limbal stem cell deficiency; aniridia; autoimmune disease, including Stevens-Johnsons syndrome and ocular cicatricial pemphigoid; acid, alkali, or thermal burns; and silicone oil keratopathy.

In children, the B-KPRO has been implanted for congenital pediatric corneal conditions, including congenital glaucoma, aniridia, Peters anomaly, and cases of inborn anterior segment dysgenesis, but with poor results.5 Although the type 1 B-KPRO is the most common type in use, a type II version is also available for cases with advanced keratinization or severe fornix injury requiring permanent tarsorrhaphy. Due to the limited scope of this article, the type II B-KPRO and other designs will not be discussed further.

As the prevalence of successful B-KPRO implants increases, it is important that the vitreoretinal surgeon become comfortable operating in the context of a keratoprosthesis. The advent of newer minimally invasive vitreoretinal surgery instruments, including small-gauge 23- to 27-gauge vitrectomy instruments, ultrawide angle imaging, and small-gauge vitreoretinal endoscopes, has significantly enhanced the management of both emergent and nonemergent conditions affecting this unique patient population.6-8

This article will briefly review the history of the B-KPRO, followed by a discussion focused on increasing familiarity with basic surgical techniques and potential vitreoretinal conditions that the vitreoretinal surgeon will most commonly encounter in patients with the B-KPRO.

HISTORY

Although the earliest attempts at keratoplasty date to the 19th century, Dohlman et al published the first report of a polymethylmethacrylate (PMMA) B-KPRO transplant in 1974.9,10 The FDA subsequently approved the B-KPRO device in 1992 and several improvements to the original design have occurred since this time.11,12

In 1996, holes were placed in the backplate that improved aqueous contact with the graft, reducing the risk of corneal necrosis. In 2004, a titanium locking ring was added, which prevented intraocular disassembly over time. This ring has been replaced in newer models with a built-in “click on” locking mechanism that reduces the essential components from three to two.12 Additional improvements in design include a threadless configuration from an older screw-type design that lessens the risk of endothelial damage during assembly and the use of a titanium backplate instead of PMMA, which is thinner and stronger and has superior biocompatibility.13-15

Infection risk has been significantly reduced after the institution of lifelong topical antibiotic therapy.16 Pseudophakic or aphakic status is required for the B-KPRO to be implanted, and separate configurations are available for each lens condition. For pseudophakic eyes, a plano power optical stem is used, and for aphakic eyes, a customized power can be added based on axial length measurements.4 Both adult-sized (8.5-mm diameter) and pediatric-sized (7.0-mm diameter) back plates are also available.4

INDICATIONS

The most common indication for B-KPRO is in patients with a failed penetrating keratoplasty (PKP). A 2016 meta-analysis reported better long-term visual outcomes and reduced postoperative glaucoma risk in patients with prior failed PKP who underwent B-KPRO in lieu of an additional PKP.17 Additional indications include recurrent infections, particularly from herpetic disease, and cicatricial conditions, including Stevens-Johnsons and ocular cicatricial pemphigoid.

In general, patients with noninflammatory conditions have the best outcomes, while cicatricial or infectious conditions perform the worst. A multicenter trial from the Boston Type 1 Keratoprosthesis Study Group reported the best outcomes in patients with chemical burns.1 Implantation in children is not recommended at the current time and is associated with a higher rate of complications, higher chance of device failure, and worse visual outcomes than observed in adults.5,18

PREOPERATIVE VITREORETINAL CONSIDERATIONS

The vitreoretinal surgeon is an important partner in the successful long-term care of patients with the B-KPRO from the preoperative assessment period through the life of the transplant.19 In the setting of unknown visual potential from corneal scarring, which limits indirect ophthalmoscopy, B-scan ultrasonography can assess the posterior segment for signs of retinal detachment, vitreous hemorrhage, tractional preretinal membranes, or advanced glaucomatous cupping. If B-scan ultrasound shows equivocal findings, a small-gauge vitreoretinal endoscopic procedure can assess visual potential prior to implantation of the B-KPRO device.20

In some cases, endoscopic surgery can be used to perform a complete pars plana vitrectomy (PPV) and address posterior-segment pathology simultaneously, including retinal detachment, without the use of a binocular indirect viewing system, macular contact lens, or temporary keratoprosthesis.21

PPV is recommended by many surgeons, but not required, at the time of B-KPRO placement.19 In the largest prospective series of B-KPRO outcomes, the Boston Type 1 Keratoprosthesis Study Group reported that only 24.5% of 300 eyes underwent PPV at the time of B-KPRO placement.1 They reported no significant improvement in visual acuity in eyes that underwent PPV at the time of B-KPRO surgery vs those that did not up to 48 months after the initial surgery.1 Separate reports have confirmed this finding.22,23

Some experts recommend PPV at the time of B-KPRO implantation because it may reduce the need for additional procedures in the future. However, it does not prevent retroprosthetic membrane (RPM) formation, retinal detachment, chronic hypotony, cystoid macular edema formation, epiretinal membrane (ERM) formation, endophthalmitis, or corneal melt up to one year after the initial surgery.23 Ultimately, in the long term, patients undergoing concurrent PPV and B-KPRO implantation may suffer fewer postoperative complications or require fewer additional procedures.22,23

We generally recommend combining PPV with B-KPRO placement in eyes with significant media opacity including inflammatory debris or vitreous hemorrhage, which may limit visual acuity gains, or in cases with vitreoretinal pathology present at the time of implantation, such as ERM, macular hole, or retinal detachment.

VITREORETINAL COMPLICATIONS

Vitreoretinal complications are numerous following B-KPRO placement. The most common conditions that will prompt referral to a vitreoretinal surgeon include a dense RPM not amenable to YAG laser membranotomy, vitreous hemorrhage, ERMs causing visual distortion, macular holes, retinal detachment, chronic hypotony with or without choroidal detachment, and infectious or sterile endophthalmitis.24

GENERAL PRINCIPLES

Prior to undertaking vitreoretinal surgery, a careful preoperative assessment should take place to assess for any significant media opacity, RPM formation, conjunctival scarring, fornix foreshortening, and the presence of glaucoma tube shunt devices that may be present, sometimes in multiple locations. The intraocular lens status should be noted as it will affect access to any RPM.

If RPM prevents visualization of the posterior segment, B-scan ultrasonography can be applied in a trans-scleral manner to visualize the posterior segment. If chronic hypotony is present with choroidal detachments, their locations should be documented to guide placement of trocars in a safe manner.

For most cases, a standard three-port PPV can be performed with placement of trocars between 3.0 mm and 3.5 mm posterior to the limbus.

To minimize distortion of the sclera during port placement, a localized peritomy can be created with Vicryl mattress sutures preplaced and a 25 gauge microvitreoretinal blade used to initiate the sclerotomy. In most situations, direct transconjunctival placement of ports with a straight or beveled approach is safe and well tolerated, even in cases with extensive conjunctival scarring or glaucoma tube shunts in multiple places.

Wide-angle viewing systems, such as the Zeiss Resight (Carl Zeiss Meditec) and the Oculus BIOM are very effective at visualizing the posterior segment through the B-KPRO optical stem, even in cases with RPM present (Figure 2). Macular contact lenses are an effective alternative. Core vitrectomy and removal of peripheral vitreous should be performed in all cases.

Figure 2. Wide-angle viewing systems allow for uninhibited visualization of the posterior segment and retinal periphery. These photos were taken during epiretinal membrane removal. From left to right, the posterior pole and holes in the backplate are visible; core vitrectomy is performed, followed by scleral depression.
IMAGE COURTESY OF JAY M. STEWART, MD, PROFESSOR OF OPHTHALMOLOGY, DEPARTMENT OF OPHTHALMOLOGY, UNIVERSITY OF CALIFORNIA SAN FRANCISCO.

More complex maneuvers, including scleral depressed exam with vitreous base shaving, are challenging with a B-KPRO present but possible to a limited degree. Reducing the infusion pressure can help soften the eye and allow for greater indentation with the scleral depressor. Chandelier illumination is an option if bimanual techniques become necessary, or a skilled assistant is not available.

It is safe to use intraocular dyes and stains to assist in visualization of the peripheral vitreous or preretinal membranes. Common agents include triamcinolone acetonide, indocyanine green, and brilliant blue.

Intraocular tamponades for retinal detachment repair are also well tolerated and include perfluorocarbon heavy liquids and silicone oil endotamponades. Intraocular gas and air may cause condensation to form on the back surface of the optical stem during fluid/air exchange, which can limit the surgeon’s view. This problem can be remedied by injecting a cohesive viscoelastic, such as Healon (Johnson & Johnson) through a trocar and onto the posterior aspect of the optical stem.

Sutureless vitrectomy is possible even in cases with prior conjunctival scarring from previous surgery or chemical burns with fornix foreshortening, although we usually recommend suturing sclerotomy sites with 8-0 or 7-0 interrupted Vicryl sutures, especially if the surgeon is repairing a retinal detachment with silicone oil.

RETROPROSTHETIC MEMBRANE

RPM is the most common postoperative complication, with an incidence between 25% and 65%.2,25 The majority of cases can be managed with office-based YAG membranotomy, although over time, the membrane may become increasingly fibrotic and vascularized, and surgical membranectomy may be required.2 The vitreous cutter can be used in some cases; however, it is often more effective to use a bent 23-gauge or 25-gauge needle to tear the membrane away from the optical stem. Intraocular forceps can then be used to finish the peel.

Care must be taken to prevent damaging the clear optical stem surface with this technique. No antifibrinolytics are currently available to prevent regrowth of RPMs after surgery, and repeat vitrectomy is sometimes required.

ENDOPHTHALMITIS

Symptoms of severe eye pain and eye redness with an acute reduction in vision should prompt an immediate evaluation for endophthalmitis in any patient with a B-KPRO. Prior to the administration of daily topical antibiotics, endophthalmitis occurred in 12% to 14% of cases.4,26-28 The most common pathogens are Gram-positive cocci, including Streptococcus pneumonia, Staphylococcus aureus, and Staphylococcus epidermidis.16,29

The high incidence of infection prompted the addition of lifelong daily topical antibiotics, which reduced the incidence of endophthalmitis by 75% or more.11,16,30 Fluoroquinolones were initially used, followed by topical vancomycin after reports of emerging resistance.27 Endophthalmitis may occur in the immediate postoperative period; however, it often occurs 10-18 months after B-KPRO placement.16,31 Therefore, close follow-up for the life of the patient is necessary.

Endophthalmitis is often a consequence of delayed infectious keratitis in patients with the B-KPRO, and it requires aggressive management. The initial regimen consists of a topical fluoroquinolone eyedrop used every one to two hours, in addition to topical fortified vancomycin four to eight times per day.

It should be noted that long-term use of topical vancomycin is a risk factor for developing fungal endophthalmitis.16,32 This concern should prompt the addition of a topical antifungal agent, such as natamycin 5%, amphotericin B 0.5% (Fungizone, Thermo Fischer), or voriconazole 1% (Vfend, Pfizer), to the daily regimen between four and eight times per day. An oral antibiotic, such as levofloxacin or moxifloxacin, and an oral antifungal, such as voriconazole, can also be added to this regimen as both achieve high intraocular concentrations.33-36

A low threshold for PPV with culture and injection of antibiotics and antifungals should be considered in any case of fulminant panuveitis or any case that does not respond immediately to topical and oral antibiotic and antifungal therapy. The prognosis for patients with the B-KPRO and culture-proven endophthalmitis is poor. Roughly half of cases will respond to antibiotics with or without PPV, while the other half will require the graft to be explanted and sent for culture and a new keratoprosthesis placed.32

RETINAL DETACHMENT

The incidence of retinal detachment in patients with B-KPRO ranges from 2.8% to 19%, depending on the reported case series.4 Surgeons should recognize that the view to the posterior segment may be compromised by RPM formation, and only a limited view to the peripheral anterior retina may be available in many cases.

The incidence of proliferative vitreoretinopathy (PVR) has been reported to be higher in patients with B-KPRO, and visual outcomes are often poor following repair.7,37 A standard three-port PPV vitrectomy can be employed with a bent 25-gauge needle to remove any RPM obstructing the surgeons view, as described above. Placement of the ports varies in the literature, but we recommend between 3.0 mm and 3.5 mm posterior to the limbus.

There are no restrictions on the use of intraocular dyes, stains, or tamponades, such as perfluorocarbon, and silicone oil is well tolerated.6,7,37 Retinectomy may be required in cases of severe PVR. The use of scleral buckles has not been well discussed in the published literature but is probably reasonable to consider in certain cases in lieu of a retinectomy.

CHRONIC HYPOTONY

Glaucoma remains the single most consistent threat to long-term visual acuity in patients with a B-KPRO, and many surgeons recommend placing a glaucoma drainage implant at the time of the initial surgery. However, any subsequent chronic hypotony is often more difficult to treat.2,4,11

The incidence of glaucoma can be as high as 64%, whereas chronic hypotony may occur in up to 20% of cases.3 The pathogenesis of chronic hypotony in B-KPRO patients is multifactorial and is thought to occur initially from elevated intraocular pressure (IOP) secondary to gradual closure of the anterior chamber angle, followed by RPM formation that grows over the optical stem and nonpigmented epithelium of the pars plicata, slowly diminishing aqueous production.

Monitoring IOP after B-KPRO placement is challenging as scleral tonometry is inaccurate, and digital palpation is often difficult to interpret. A gradual decline in visual acuity with serous choroidal detachments best visualized by B-scan ultrasonography may be the most reliable sign that chronic hypotony is occurring.

Management initially requires stopping all IOP hypotensive medications. If stopping medications fails, glaucoma tube implants can be tied off surgically to decrease outflow. Definitive management includes PPV with peeling of any RPM covering the optical stem and ciliary processes followed by silicone oil injection to act as a long-term intraocular volume maintainer and tamponade.3,38 This approach is successful in the majority of cases, and visual acuity often improves following surgery.38

EPIRETINAL MEMBRANE AND MACULAR HOLE

ERM and full-thickness macular holes can occur in patients with a B-KPRO. Idiopathic formation is probably the most common cause; however, patients with chronic inflammatory conditions are particularly prone to developing preretinal membranes, with associated distortion of macular architecture.

Surgery involves standard techniques, including use of intraocular dyes and stains, followed by membrane stripping and gas tamponade in the case of macular holes.39 Rates of closure of macular holes and successful ERM peeling are probably similar to those in the general population.

SUMMARY

Between 15% and 20% of patients will suffer long-term visual decline after B-KPRO placement over the initial 40-month postoperative period.40 Complications range from easily manageable to potentially devastating. Despite progress in the management of these complex patients, studies have suggested a device failure rate in approximately 16% of eyes by 26.5 months.1,11,40

A vitreoretinal surgeon is an essential partner in the long-term care of patients with a B-KPRO, from the preoperative assessment through the life of the graft. The number of patients who receive the B-KPRO will likely continue to increase in the years ahead, and vitreoretinal surgeons will increasingly see patients with B-KPROs in their practices. Familiarity with the device and experience treating the various types of emergent and nonemergent complications will provide this unique patient population with the best possible outcomes. RP

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