Open sky fundus visualization. (a) Open sky view of posterior pole. (b) Visualization under the operating microscope is improved with the room lights dimmed and the light pipe placed into the anterior vitreous cavity
Intraoperative endoscopy.(a) Endoscopy performed at the time of consideration of Boston KPro placement. (b) Vitreous debris and hemorrhage were initially encountered, and a vitrectomy was performed to improve visualization of the posterior pole and optic nerve to determine suitability for permanent Boston KPro placement. (c) After removal of vitreous debris, the status of the retina and optic nerve was deemed amenable for permanent KPro placement
12.3 Intraoperative Considerations
12.3.1 General Principles
Among centers that offer Boston KPro surgery, one major divide in the approach to initial KPro surgery is the decision to perform KPro placement alone or, alternatively, in combination with other vitreoretinal or glaucoma procedures. Our center favors combined surgical procedures for several reasons: (1) removal of the native vitreous may decrease postoperative inflammation and inflammatory membrane formation; (2) placement of an aphakic Boston KPro with the removal of the crystalline lens and capsular bag may ultimately result in fewer long-term complications and facilitate any future retinal surgery; and (3) placement of a glaucoma drainage implant may be necessary given the high pre- and postoperative prevalence of glaucoma in many eyes that have come to require a KPro.
At the time of initial KPro placement, if the native corneal opacity permits a view, a vitrectomy many be performed followed by placement of the KPro by the corneal surgeon. If the patient’s cornea is too opaque to permit a view, a temporary KPro may be placed, at the cost of adding time to the procedure. Perhaps the most appropriate option is to perform vitreoretinal surgery after the permanent Boston KPro has been placed – this is the most common approach taken at our center.
12.3.2 Vitreoretinal Surgical Pearls in the Setting of Keratoprosthesis
Though KPro eyes can be challenging, pars plana vitrectomy through a Boston KPro is safe without deleterious effects on the device. A two- or three-port vitrectomy may be utilized. A larger-gauge vitrectomy (20 Ga) has been reported in many published series ; however, microincisional vitrectomy surgery (MIVS) is also being increasingly utilized [4, 10]. Instrumentation size may dictate whether suturing sclerotomies are necessary and sutureless procedures may be performed .
Sclerotomies are placed 1–3.5 mm posterior to the limbus. If the limbus is not well defined, calipers may be used to measure 9 mm spanning the center of the Boston KPro optic in order to facilitate placement of trocars . For the Boston type II (through-the-lid model), sclerotomies can be made directly through the eyelid (transcutaneous approach) with a microvitreoretinal (MVR) blade .
The role of scleral buckling in the management of keratoprosthesis remains limited, but may be a useful adjunct in select cases if used with caution given compromise of the anterior segment and scleral tissue secondary to KPro placement.
12.3.3 Indications for Vitrectomy
Studies report the incidence of vitrectomy after KPro placement at approximately 7 % . Indications for vitrectomy in permanent keratoprosthesis include exploratory endoscopy, vitreous biopsy, glaucoma shunt placement/revision, retroprosthetic surgical membranectomy, retinal detachment repair, suprachoroidal hemorrhage drainage, surgery for endophthalmitis, epiretinal membrane removal, removal of intraocular inflammatory debris including proliferative vitreoretinopathy, hypotony, and removal of vitreous opacities including vitreous hemorrhage [3, 4, 9]. Management of some of these postoperative complications is discussed in further detail below.
12.3.4 Postoperative Management
Placement of permanent keratoprosthesis commits the patient and ophthalmology team to lifelong follow-up. This can be particularly challenging for patients who plan to travel to their native countries for months on end, especially to areas where ophthalmologists familiar with permanent keratoprostheses are not readily available. This further emphasizes a key metric of success in these devices is proper patient selection given the need for continued, close monitoring after device placement. Glaucoma and endophthalmitis are among the most common and dreaded complications following Boston KPro surgery highlighting the need for multidisciplinary follow-up with a cornea, glaucoma, and vitreoretinal subspecialists.
Visualization of the fundus is possible with ordinary slit lamp biomicroscopy as well as binocular indirect ophthalmoscopy on a small pupil setting through the 2.8 mm diameter optic of the Boston type I KPro . Visualization of the peripheral fundus can be challenging and is enhanced with ancillary testing including B-scan ultrasonography and ultra-widefield imaging. The Optos ® ultra-widefield scanning laser ophthalmoscope is one of the most reliable and widely used imaging modalities given its ability to image through the small-diameter optical column of the Boston KPro and quick acquisition time which is particularly helpful in low-vision patients even when nystagmus is present .
12.3.5 Postoperative Complications
The most common postoperative posterior segment complications after KPro surgery are retroprosthetic membrane, infectious endophthalmitis, sterile vitritis, and retinal detachment occurring on average 7.6 months after initial surgery . Early identification and management of these complications minimizes visual morbidity.
12.3.6 Retroprosthetic Membrane (RPM)
Retroprosthetic membrane or RPM formation is one of the most commonly reported postoperative complications with an incidence between 14 and 77 % [14–20]. Risk factors for RPM formation include postoperative inflammation [14, 15], multiple additional surgical procedures at initial KPro surgery or postoperatively [15, 21], retinal detachment , infectious keratitis , and aniridia . Histologically, RPM is composed of fibrovascular tissue  believed to originate from the host’s corneal stroma, which grows through gaps in Descemet’s membrane . RPM may form as early as 1 week after KPro placement or many years after initial surgery [15, 21, 25].
Though RPM may form and remain visually insignificant, they may progress to visual significance or even contribute to KPro failure and chronic hypotony if progressive and exert traction over the ciliary body . RPM has also been cited as a risk factor for retinal detachment .
RPMs that are not visually significant are best treated with observation, and those that progress should be treated promptly with a single-session YAG membranotomy . Even with increased energy settings which can damage the KPro optic, YAG membranotomy may fail and necessitate surgical membranectomy (Fig. 12.3) . From a practical standpoint, early management with a noninvasive YAG membranotomy is preferred to the more invasive intraocular surgery, which may be avoided if YAG membranotomy is performed before the membrane becomes too thick.
Surgical membranectomy for retroprosthetic membrane. (a) Microincisional vitrectomy was performed for a dense retroprosthetic membrane that was not amenable to YAG membranotomy. (b, c) The vitreous cutter is used to remove the RPM from the backplate of the KPro and clear the visual axis (c). Intravitreal preservative-free triamcinolone was injected into the vitreous cavity after removal of the RPM
12.3.7 Infectious Endophthalmitis
Infectious endophthalmitis (IE) is the most dreaded KPro complication and often results in vision loss or loss of the eye. Given the Boston KPro never fully integrates with the host, there is a persistent communication between the nonsterile ocular surface and the sterile anterior chamber, resulting in a conduit for microbial invasion . This complication may present weeks, months, and even years after placement of a KPro [29, 30]
Topical antibiotic prophylaxis is the single most important intervention in preventing endophthalmitis. With proper topical prophylaxis, including vancomycin, the incidence of bacterial endophthalmitis was 0.35 % per patient-year compared to 4.13 % in patients not receiving vancomycin . Another large series with patients maintained on both vancomycin and a fourth-generation fluoroquinolone reported an incidence of 2.4 % or 0.014 infections per patient-year . Other series using up-to-date prophylactic topical antimicrobial agents have reported cumulative endophthalmitis rates of 6.7 %  and 7.1 % .
There is no universally agreed upon antimicrobial regimen at this time. Most KPro surgeons recommend lifelong topical vancomycin along with a fourth-generation fluoroquinolone for high-risk groups (e.g., autoimmune patients with ocular cicatricial pemphigoid or Stevens-Johnson syndrome), whereas non-autoimmune patients in favorable prognostic categories may be initially managed with a single-agent topical fluoroquinolone in the immediate postoperative period and then transitioned to polymyxin B/trimethoprim, which is inexpensive and provides broad-spectrum (including gram-positive) coverage.
Traditionally, the most common microbial organisms responsible for infectious endophthalmitis (IE) included gram-positive bacteria and less commonly gram-negative infectious. However, with the introduction of vancomycin prophylaxis, there has been a decrease in gram-positive infections and an apparent increase in fungal infections (Fig. 12.4) . Causative microbial organisms are related to geographic location and climate, and, worldwide, infectious endophthalmitis is more common outside the United States .
Fungal endophthalmitis. (a) A three-port 25-gauge vitrectomy was performed for fungal endophthalmitis prior to removal of Boston KPro device and placement of cornea patch graft. (b, c) During surgery, numerous inflammatory vitreoretinal bands (red arrow) consistent with Aspergillus fungal endophthalmitis were identified and removed with vitrectomy
Clinically IE presents with a sudden onset of decreased vision, eye redness, eye pain, and significant inflammation. On exam, fibrin and vitritis are often present; donor corneal edema and inflammatory retinal membranes or precipitates may also be seen .
The most important diagnostic evaluation of IE includes corneal culture and vitreous biopsy for identification of causative microbe. The mainstay of treatment is prompt injection of broad-spectrum intravitreal antimicrobials including vancomycin, amphotericin, and ceftazidime. Use of subconjunctival antibiotics is less common . The timing and use of intravitreal corticosteroids remains controversial, particularly with presumed fungal endophthalmitis , but may play an important role in decreasing ocular inflammation. Infectious disease specialists at our institution have often advocated for the use of intraocular steroids in the presence of fungal endophthalmitis as the use of amphotericin is not only fungistatic but also fungicidal, and when administered at appropriately high concentration, its mechanism of action is not impeded by concomitant intravitreal steroid use. Visual prognosis after IE is guarded and depends on the type of pathogen. One series reported 80 % of infected eyes lost useful vision, even after prompt treatment .
12.3.8 Sterile Vitritis
Sterile vitritis is an autoimmune phenomenon (to the device itself or corneal antigens released during tissue necrosis) and is best thought of as a type of uveitis . Incidence ranges from 0 to 14.3 %, with many studies reporting rates less than 5 % [13, 15, 16, 18, 21, 22, 25, 34, 36–40]. In contrast to IE, patients present with painless vision loss without the characteristic injection and tenderness of IE [40, 41]. On examination, the eye typically appears quiet anteriorly with a dense vitreous cellular reaction often described as a “snowflake” pattern . Onset is variable and may be months to years after surgery [13, 25, 41].