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

Introduction

Many of the complications associated with the Boston type 1 and type 2 keratoprosthesis (KPro) have been addressed successfully through design alterations, surgical technique modifications, and medical therapy. Despite this, KPro implantation remains a surgery of last resort, in part because the complications can result in permanent vision loss (Table 51.1). Although the use of a donor cornea improves the biocompatibility and ease of implantation, complications related to implant integration after Boston KPro surgery remain of significant concern, including implant extrusion, periprosthetic tissue necrosis, and infection. The ocular surface is the gateway to many of these complications. In this chapter, the authors describe the complications associated with the Boston keratoprosthesis, with special emphasis on the role of the ocular surface in the pathology.

Epithelial Defects and Contact Lens Related Complications

Epithelial defects on the donor or recipient cornea can be problematic after Boston KPro implantation and, as expected, occur more frequently in dry eye, inflammatory, and stem cell deficiency states. Given the elevated profile of the KPro’s anterior plate relative to the donor cornea, the surrounding ocular surface is more vulnerable to evaporative forces and dellen formation,¹ and the area adjacent to the anterior plate in particular has been associated with persistent epithelial defects.² In addition to this mechanical disadvantage, chronic ocular surface dryness and an inhospitable ocular surface related to the preoperative diagnosis can also lead to epithelial defects. Frequently, one or more of the common mechanisms contributing to KPro related epithelial defects – diminished aqueous production from conjunctival scarring, evaporative dysfunction from meibomian gland scarring, exposure keratopathy from lagophthalmos, and trichiasis from cicatricial entropion – is associated with the pathology prompting KPro surgery. Occult microbial keratitis without corneal infiltrate can prevent epithelialization, and positive fungal and bacterial cultures of persistent epithelial defects with resolution after appropriate antimicrobial treatment have been documented.² Regardless of the cause, epithelial defects place the donor cornea at risk for further tissue destruction from collagenases, as well as other enzymes released from polymorphonuclear leukocytes in the tear film.¹ These defects can ultimately lead to keratolysis, implant extrusion, microbial keratitis, and endophthalmitis.

Treatment of a persistent epithelial defect after KPro implantation is approached as one would after penetrating keratoplasty (PK), with copious lubrication, emollient ointments or consideration of bandage soft contact lens (SCL) placement and lateral tarsorrhaphy. Medial tarsorrhaphy may be necessary if lateral tarsorrhaphy does not cover the area of concern sufficiently. Procedures to provide additional coverage of the defective area with a conjunctival flap or mucosal grafts may be required and performed at the time of tarsorrhaphy. Additionally, a lamellar patch graft using donor corneal tissue may be needed with or without conjunctival, mucosal or amniotic membrane graft coverage. Attention should also be paid to the toxicity profile of topical eye drops required for treatment, with an attempt to minimize toxic preservative-containing agents when possible.

The routine use of a wide-diameter contact lens, typically a Kontur™ hydrogel SCL of 16-mm diameter or larger (Kontur Kontact Lens Co., Inc., Hercules, CA), helps prevent and treat epithelial defects and reduces the risk of corneal infection and melting. A suitable contact lens fit maintains a thin tear meniscus, especially around the anterior plate, and diminishes evaporative forces to achieve better hydration of the corneal surface. In the presence of an epithelial defect, the SCL promotes epithelialization by acting as a mechanical barrier protecting the vulnerable corneal surface from the shear forces of blinking and may limit access of polymorphonuclear leukocytes to the underlying tissue, thereby retarding stromal keratolysis.¹ Postoperative use of a SCL may also provide improved comfort for patients bothered by the anterior plate edge, correction of spherical refractive error, and in the case of tinted lenses, diminished glare and photophobia with improved cosmesis.¹ Typically, KPro surgeons replace the SCL on a periodic basis and/or whenever there is debris build-up on the lens surface, since the deposits can adversely affect vision and represent a nidus for attachment of microbes (Fig. 51.1).

SCL use after Boston KPro surgery has improved outcomes, but bandage contact lens use per se is not free of complications. Difficulty retaining the SCL can be problematic, and patients with multiple contact lens dislocations require particular attention. Replacement requires additional office visits and contributes significantly to the real cost of the prosthesis. As with any SCL, infection can occur and has been reported with the Kontur™ SCL used in conjunction with KPro surgery, although the majority of reported Kontur™ SCL-related infections have occurred in patients with a history of glaucoma surgery.1,3,4 Kontur™ SCLs have been associated, in particular, with glaucoma drainage tube erosion (Fig. 51.2).^5,6 In the series reported by Li et al., one-third of patients with glaucoma drainage device (GDD) erosion incurred conjunctival breakdown at the edge of contact lens. In that series, the erosion event was implicated in the development of endophthalmitis and periprosthetic infection leading to extrusion in two eyes, despite daily prophylaxis with vancomycin. Presumably, the focal mechanical trauma of the contact lens edge on top of the GDD, in combination with a dysfunctional ocular surface, contributes to the erosion of these devices. Although an SCL is needed to protect the ocular surface, careful attention must be paid at each follow-up visit to the status of all hardware in KPro eyes and, in particular, to glaucoma surgical sites to detect erosion and infection.

The initial strategy to promote retention of the SCL is lateral tarsorrhaphy, although some patients will experience recurrent contact lens dislocation despite this and may require medial tarsorrhaphy or further lid surgery to address any underlying lid malposition. A promising approach to the problem of SCL retention, especially in KPro patients with a dry ocular surface, is refitting with a scleral rigid gas-permeable (RGP) contact lens. Scleral RGP lenses are an underappreciated option in the treatment of patients with ectasia, irregular astigmatism, and ocular surface disease. Design features of modern scleral RGP lenses allow for successful fitting of eyes with challenging contours, including GDDs and foreshortened fornices. Modern fluid-ventilated scleral RGP lenses create an expanded pre-corneal tear film over the ocular surface, and some designs do not contact the ocular surface at all. Although expensive, this device appears to be well tolerated and may prove to be useful in ocular surface management after KPro surgery.

Corneal Infiltrates

Infiltrates can occur on the donor or recipient cornea, and although the location of the infiltrate relative to the implant is important for structural integrity, an etiology may not be presumed based on location alone. Donor corneal infiltrates may be inflammatory and sterile, but with bacterial and fungal isolates recovered in all of the major series in the KPro literature,2,3,6–8 the suspicion for microbial keratitis should be very high (Fig. 51.3). Risk factors for bacterial keratitis after KPro surgery include significant blepharitis, noncompliance with the postoperative chronic antibacterial eye drop regimen, lid abnormalities, and the presence of other surgical hardware in the eye, including GDDs.⁵ Risk factors for fungal keratitis after KPro surgery include use of a contact lens and topical vancomycin,⁹ and similar to PK, topical steroid administration. Corneal infiltrates may be diagnosed clinically as infectious, but a corneal scraping for Gram stain and bacterial and fungal cultures typically is performed. Persistent epithelial defects may also be considered for culture, given the possibility of occult microbial keratitis.²

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