The Boston type artificial cornea (Boston Keratoprosthesis: KPro) was developed through years of research by Dr. Claes Dohlman and received FDA approval in 1992. It is currently the most widely used artificial cornea in the world 4), with over 4,500 surgeries performed to date. The concept of an artificial cornea was first described in 1789 by Frenchman Guillaume Pellier de Quengsy.
The device has a “collar-button” design consisting of three parts: a front plate (integrated with the optical stem), a back plate, and a titanium locking C-ring. It is manufactured by machining medical-grade PMMA, and a donor cornea is sandwiched and fixed between the front and back plates. There are Type 1 and Type 2; Type 2 is for severe end-stage ocular surface diseases and requires permanent eyelid closure. Type 1 is more widely used.
Design evolution
Sixteen holes are provided in the back plate to allow corneal nutrition supply through aqueous humor diffusion. In 2004, a titanium C-ring was introduced to prevent device disassembly, and in 2007, a threadless design was adopted to simplify assembly. The latest Lucia design increases contact between the cornea and aqueous humor by changing the shape of the titanium back plate.
Device specifications
There are plano power for pseudophakic eyes and custom power for aphakic eyes. Back plates are available for adults (8.5 mm diameter) and children (7.0 mm diameter). The device cost is approximately $5,000, but it is provided on a sliding scale for impoverished countries.
The target is severe corneal diseases for which full-thickness penetrating keratoplasty (PKP) is not indicated or has repeatedly failed. Indications include multiple failed transplants, Stevens-Johnson syndrome, ocular cicatricial pemphigoid (OCP), chemical trauma, aniridia, herpetic keratitis, limbal stem cell deficiency, neurotrophic keratopathy, and extensive corneal neovascularization 4). It may be a first-line choice in cases of repeated graft failure or aniridia.
In end-stage ocular surface disease associated with Stevens-Johnson syndrome, the prognosis for corneal surgery is extremely poor. One report indicated that the 1-year and 2-year survival rates for PKP were 20% and 0%, respectively 1). Even after KPro, high complication rates have been reported: corneal stromal necrosis 59%, corneal infiltration 30%, persistent epithelial defect 59%, and device extrusion 52% 1).
Typical findings in KPro candidates include severe corneal opacity, corneal neovascularization, conjunctival keratinization, limbal stem cell deficiency, and markedly decreased Schirmer test values. In cases of chemical injury, complete eyelid loss and severe exposure keratopathy may be present 3). In phthisis bulbi, axial shortening, scleral thickening, choroidal thickening, and retinal detachment may be observed 2).
| Prognostic Category | Characteristics | Retention Trend |
|---|---|---|
| Non-scarring disease | Repeated graft failure | Best |
| Chemical injury | Acid or alkali injury | Intermediate |
| Autoimmune disease | Stevens-Johnson syndrome, ocular cicatricial pemphigoid | Poor (37.5-55%) |
In autoimmune diseases, the frequency of reoperation is high and final visual acuity is poor 4). The degree and duration of past inflammation affect the prognosis.
The success of KPro1 largely depends on the preoperative ocular surface condition. In end-stage Stevens-Johnson syndrome patients, conjunctival keratinization is severe, so KPro1 has a high risk of failure if performed directly 1). A stepwise approach has been reported: first, control ocular surface inflammation with systemic immunosuppression (methotrexate); then, perform minor salivary gland transplantation (MSGT) and oral mucosal membrane grafting (MMG) to lubricate the ocular surface and reduce keratinization; finally, implant KPro1 1).
With this approach, Schirmer values improved from 0 mm to 3 mm at 2 weeks after MSGT, and visual acuity improved from light perception to 20/60 after KPro1 implantation at 11 months, maintained for over 24 months 1).
Risk factors include severe dry eye, conjunctival keratinization, history of corneal infection, multiple failed PKP, and active autoimmune disease. Use in children is controversial due to difficulty in postoperative management and risk of complications, and is not recommended as a primary treatment 4).
In end-stage ocular surface diseases such as Stevens-Johnson syndrome and ocular cicatricial pemphigoid, conjunctival keratinization and severe dry eye are profound. Implanting KPro directly carries extremely high risks of infection and device extrusion. By improving lubrication with minor salivary gland transplantation and reducing keratinization with mucosal grafting before KPro implantation, long-term device retention becomes possible.
In prosthokeratoplasty, a donor cornea is assembled between the front and back plates and sutured using a technique similar to PKP. Surgeons experienced in PKP can perform this procedure. Currently, lens extraction is recommended during KPro surgery, and it is often performed simultaneously with glaucoma surgery (aqueous drainage device implantation), iridoplasty, and vitrectomy.
In cases of phthisis bulbi, simultaneous KPro-I with vitrectomy and silicone oil injection has been reported 2). By selecting the KPro optical power based on axial length shortening (18 mm) and combining 360° retinectomy with perfluorocarbon use, improvement from light perception to 20/250 was maintained for 3 years 2).
Infection Prevention
Lifelong topical antibiotic use is essential 4). The standard regimen is daily polymyxin B/trimethoprim, or a fourth-generation fluoroquinolone (moxifloxacin/gatifloxacin) ± vancomycin 4). The incidence of infectious endophthalmitis is 2.7% per year, which is very high among ophthalmic surgeries.
Glaucoma Management
Preoperatively, 60–76% of patients have glaucoma, and postoperatively, 14–39.3% develop new or progressive intraocular pressure elevation 4). With the device in place, conventional tonometry is not possible, so digital palpation of the sclera is used. When combined with tube shunt surgery, the rate of glaucoma progression in poorly controlled cases has been reported to decrease to 2% 4).
Bandage Contact Lens and Eye Drops
Continuous wear of a BCL maintains ocular surface moisture and prevents stromal melting. Steroid eye drops (prednisolone) must also be continued lifelong. In cases of chemical injury, a scleral contact lens (18 mm) was effective in preventing melting around the KPro and improving visual acuity 3).
Lifelong use of antibiotics (fluoroquinolone ± vancomycin) for infection prevention, steroid eye drops (prednisolone) for inflammation control, and glaucoma medications is necessary. In some cases, cyclosporine eye drops or PRGF (plasma rich in growth factors) eye drops are also added.
Visual improvement rates (achieving 20/200 or better) are 50–65% at 3 years, and device retention rates are reported as 80–87.8% 4). Retention rates are 90–92% at 1 year, 80–88% at 2 years, and 74% at 5 years 4). In cases without prior corneal surgery, retention rates of 100% have been reported, suggesting that early implantation may lead to favorable outcomes. In autoimmune diseases such as Stevens-Johnson syndrome and ocular cicatricial pemphigoid, retention rates decrease to 37.5–55% 4).
In a staged approach (MSGT+MMG → KPro1) for end-stage Stevens-Johnson syndrome, visual acuity of 20/60 was maintained for over 24 months 1). In combined surgery (KPro-I + vitrectomy + SO) for Grade III phthisis bulbi, ambulatory vision of 20/250 was maintained for 3 years 2). In a case of severe acid injury with complete eyelid loss, KPro1 combined with a scleral CL improved vision to 20/100 3).
| Complication | Incidence |
|---|---|
| Glaucoma (preexisting) | 72.0–86.0% |
| RPM formation | 25.0-55.0% |
| Vitreoretinal complications | 14-29.5% |
Retroprosthetic membrane (RPM) occurs in 25-65% of cases and is thought to be primarily caused by inflammation 4). Most cases can be managed with YAG laser membranotomy.
A design limitation of the device is that the optical portion does not integrate with living tissue 4). The constant interface between biological tissue and artificial material can lead to persistent epithelial defects, sterile necrosis, and perforation 4). This structural vulnerability increases susceptibility to endophthalmitis, so lifelong prophylactic antibiotic use is recommended 4).
Infectious endophthalmitis presents with sudden eye pain, redness, and vision loss, requiring urgent treatment with intravitreal injections or vitrectomy. The incidence of postoperative complications tends to decrease significantly in the first 10 years after KPro implantation 4).
Glaucoma in KPro eyes is the leading cause of vision loss, and a comprehensive management strategy is essential. Pre-existing glaucoma is present in 36-76% of KPro recipients, and postoperatively, 14-28% experience elevated intraocular pressure, with 2-28% developing new glaucoma. The rate of cup-to-disc ratio progression is 0.075 per year, approximately seven times that of primary open-angle glaucoma, necessitating early and aggressive intervention.
Conventional applanation tonometry cannot be used on PMMA implants. The following alternative methods are employed:
| Measurement method | Characteristics |
|---|---|
| Digital palpation | Most common. Can detect pressures above 30 mmHg within a 5 mmHg error. |
| Scleral air tonometer | Approximately 9 mmHg higher than corneal measurements. Correction formula: IOPk = 11.9 + 0.32(IOPs) − 0.05(age) |
| Schiotz tonometer | Most accurate on the temporal sclera |
Anterior segment OCT is superior to UBM for visualizing angle structures, but UBM is advantageous for evaluating glaucoma drainage device position within the ciliary sulcus. The 10-2 visual field test using target V is useful for detecting glaucoma progression, and functional assessment is considered more reliable than structural assessment.
After KPro implantation, the reduced ocular surface area decreases the absorption efficiency of topical eye drops. Additionally, due to extensive angle closure, drugs that act on trabecular outflow are less effective.
Approximately 13–42% of patients require a glaucoma drainage device after KPro surgery. The valved Ahmed glaucoma valve is the most widely used.
Timing of Glaucoma Drainage Device Placement
Before KPro (3–6 months prior): Allows sufficient time for capsule formation
Simultaneous placement with KPro: Reported to improve visual acuity maintenance and intraocular pressure at 1 year.
After KPro (delayed placement): Cup-to-disc ratio progression rate is faster than the former two.
Types and outcomes of glaucoma drainage devices
Ahmed (valved): A valved GDD widely used in KPro eyes. Designed to avoid postoperative hypotony 5).
Baerveldt (non-valved): Non-valved GDD. Selected based on scarring, conjunctival status, and hypotony risk 5).
Tube placement position: Sulcus is recommended. Reduces risk of contact with the back plate and tube exposure.
In KPro eyes, standard intraocular pressure measurement is difficult, so glaucoma progression assessment requires a combination of optic nerve findings, imaging, and visual field testing 5). When medication is insufficient, GDD or CPC should be considered, but complications such as tube obstruction or exposure must be monitored.
A treatment that uses a diode laser to ablate the ciliary processes and reduce aqueous humor production, performed transsclerally or endoscopically. Transscleral CPC has been reported to reduce intraocular pressure by approximately 60%. Advantages include no permanent implant, thus lower risk of implant exposure or endophthalmitis; however, multiple treatments may be required, and excessive ciliary body damage carries risks of hypotony and phthisis. It is a particularly useful option for type II KPro with severe conjunctival scarring or after conjunctival resection.
In KPro eyes, optic neuropathy may progress even with adequate intraocular pressure control. It has been suggested that inflammatory cytokines directly damage retinal ganglion cells and induce apoptosis, and that changes in scleral rigidity due to the implant may cause biomechanical damage to the optic nerve at the level of the lamina cribrosa. Regular monitoring of the optic disc and visual fields is essential.
The home tonometer (iCare Home) has been reported to have 91% agreement with Goldmann applanation tonometry (within 5 mmHg), but 1 in 6 patients cannot use it properly, making patient selection and education important 6). The contact lens-based IOP sensor (Triggerfish CLS) can measure 24-hour IOP patterns in an outpatient setting 6). Additionally, sustained drug delivery systems such as the bimatoprost intracameral implant (FDA approved in 2020) are being developed 6).
The three mainstays of glaucoma treatment in KPro eyes are pharmacotherapy, glaucoma drainage devices (GDD), and cyclophotocoagulation (CPC). Aqueous suppressants are preferred, and prostaglandin analogs are avoided due to the risk of macular edema. The Ahmed glaucoma valve is the most widely used GDD, and implantation before or concurrent with KPro surgery is recommended. CPC is a useful alternative when conjunctival scarring is severe.
The Boston Type 2 keratoprosthesis (BK2) is a keratoprosthesis for end-stage ocular surface disease that is difficult to manage with Type 1. Indications include severe cases with complete loss of the tear film, keratinization of the ocular surface, and symblepharon 7). While Type 1 requires a well-hydrated ocular surface and normal eyelid function, Type 2 involves permanent tarsorrhaphy, and an extended PMMA optical stem protrudes through the closed eyelids 7).
The basic structure of the BK2 consists of a PMMA front plate with optic, a titanium back plate (with click-on mechanism), and a donut-shaped donor cornea. The optical stem is longer than that of the BK1 and is designed to penetrate the permanent tarsorrhaphy 7). Lens powers are available for pseudophakic and aphakic eyes based on axial length.
| Period | Design Name | Main Improvements |
|---|---|---|
| 1992 | Screw-in | First-generation FDA-approved model |
| 2003 | Snap-on | Addition of C-ring to reduce damage |
| 2005 | Titanium back plate | PMMA changed to titanium |
| 2009 onward | Click-on | Laser-cut lock |
Typical indications include mucous membrane pemphigoid (MMP) and Stevens-Johnson syndrome/toxic epidermal necrolysis. Severe ocular surface scarring after chemical injury, Sjögren’s syndrome, and severe neurotrophic keratopathy are also candidates. Patient selection involves comprehensive assessment of visual impairment, presence of light perception, glaucoma or retinal disease, and the need to preserve the fellow eye 7).
Poor prognostic factors for maintaining postoperative visual acuity of 20/200 or better include use of the older model (screw-in type), lack of simultaneous glaucoma drainage device implantation, and absence of preoperative immunosuppressive therapy.
The procedure is performed under general anesthesia. Simultaneous pars plana vitrectomy (PPV) and glaucoma drainage device (GDD) implantation are recommended. The entire ocular surface epithelium, including bulbar, fornix, palpebral conjunctiva, and limbus, is removed by sharp dissection. A donor cornea is fashioned into a donut shape. After trephination of the recipient cornea, total iridectomy and lensectomy are performed. PPV is then performed under a temporary keratoprosthesis, and the BK2 is secured with twelve 9-0 nylon interrupted sutures. Finally, the anterior eyelid margin is excised to remove eyelash follicles, and a permanent tarsorrhaphy is performed.
Lifelong follow-up and prophylactic antibiotic therapy are required 7). In patients with autoimmune disease or high-risk monocular status, infection prevention must be rigorous, and long-term monitoring for glaucoma onset/progression, retinal detachment, and endophthalmitis is necessary 7).
| Complication | BK2 reported rate |
|---|---|
| Wound leak | 34% |
| Choroidal detachment | 30.3% |
| Retroprosthetic membrane (RPM) | 10–60% |
| Vitritis | 23.2% |
| Keratitis | 19.6% |
| Vitreous hemorrhage | 16.1% |
| Endophthalmitis | 5–17.9% |
| Glaucoma onset/progression | 8.3–41.1% |
Corneal melting and infection around the stem are the most common causes of BK2 reoperation. The incidence of retinal detachment is 10–28%, and along with glaucoma, it is the leading cause of irreversible visual impairment.
In a long-term series by Lee et al., most cases had preoperative visual acuity of 20/200 or worse. At the final follow-up, 37.5% of eyes improved to 20/200 or better, with retroprosthetic membrane (RPM) formation, glaucoma progression, retinal detachment, and endophthalmitis being major postoperative challenges 7).
Type 1 is used in patients whose eyelids can close normally, and the device is exposed on the corneal surface. Type 2 is used in patients with severely damaged ocular surfaces, such as Stevens-Johnson syndrome or ocular pemphigoid, and involves permanent eyelid closure with the device tip protruding between the eyelids. Type 1 is more widely used, but type 2 is an alternative for end-stage cases with severe ocular surface dryness.
The latest Lucia design features an improved titanium back plate shape that enhances biocompatibility and is expected to reduce RPM and stromal melting. Even for conditions previously considered difficult to treat, such as phthisis bulbi, visual recovery is being achieved through combined KPro-I and vitreoretinal surgery 2). In cases of short-term phthisis, surgical intervention may be feasible, and visual recovery is suggested for cases up to Yanoff-Fine classification Grade III 2).
Staged ocular surface reconstruction (MSGT+MMG+immunosuppression → KPro1) for end-stage ocular surface diseases such as Stevens-Johnson syndrome enables treatment of cases previously considered ineligible 1). Submandibular gland transplantation to moisten the ocular surface before KPro1 has also been reported 1).
Scleral contact lenses can be safely used in KPro-wearing eyes, contributing to both prevention of exposure keratopathy and visual improvement 3). For dissemination in developing countries, methods for assembling KPro using the patient’s own cornea and international pricing strategies are being developed.
