The Boston Keratoprosthesis Type I (Boston KPro I; Massachusetts Eye and Ear Infirmary) is an artificial cornea that can be used in people with severe corneal opacity, a condition caused by scarring or clouding of the cornea that can lead to blindness.
The current model of the Boston KPro I is the 'Snap-on' model of the device (several other versions of the KPro have previously been available). The Boston KPro I consists of 3 sterilised components that have better optical properties than donor corneal grafts, for example, reducing glare from light passing through the device (Sayegh et al. 2010). The 3 components are:
a front plate, made of clear polymethyl methacrylate (PMMA) plastic, with a central stem
a titanium back plate which is 8.5 mm in diameter for adult models and 7.0 mm in diameter for paediatric models
a titanium locking ring.
An assembly tool (used to secure the locking ring onto the stem of the front plate) and an adhesive patch (used as an aid to hold the front plate steady during the assembly process) are included with the device. The Acuderm, a 3 mm disposable skin biopsy punch, is also supplied with the device.
The Boston KPro I is assembled around a donated corneal graft before insertion into the person's eye. The corneal graft is prepared and a central hole is made in it with the Acuderm punch so that it will fit over the stem. For stability, the front plate of the Boston KPro I can be placed upside down on the adhesive patch. The graft is then placed over the Boston KPro I front plate and the assembly tool is used to gently push it down over the stem. Viscoelastic material is applied to the back surface of the graft and the back plate is placed over the stem without any rotating movement.
The locking ring is pressed onto the stem with a finger and the assembly tool is used to press the locking ring firmly into the groove (usually with an audible snap). The keratoprosthesis should be inspected under the operating microscope for correct assembly.
When fully assembled, the Boston KPro I device has the shape of a collar-button and the front plate acts like a lens. The central portion of the person's opaque cornea is removed, and if the natural lens is in place, it is also removed. The prosthesis is then transferred to the person's corneal opening and secured as in standard transplantation. After the procedure, a soft contact lens is applied to the surface and worn permanently. The person needs to use eye drops throughout their life to prevent endophthalmitis.
The device is available for both pseudophakic eyes (eyes in which the natural lens is replaced with an intraocular lens) and aphakic eyes (absence of the lens due to surgical removal, a perforating wound or ulcer, or congenital anomaly).
Implantation of an artificial cornea (keratoprosthesis) is one of few treatment options available for people with severe corneal opacity in wet blinking eyes in whom standard corneal grafts have failed or are not suitable. The Boston KPro I is a 1‑step procedure unlike Alphacor, the only other CE-marked artificial cornea for wet blinking eyes that has been used on a wide scale, which is implanted in 2 stages separated by a period of 2 to 3 months.
In pseudophakic eyes, the Boston KPro I keratoprosthesis procedure can be easily repeated if the first implant fails, because it is available off the shelf and does not need to be specially made for patients (Avadhanam et al. 2015). In aphakic eyes, the Boston KPro I needs to be customised for people based on the axial length of the eye.
According to the manufacturer, the double-plated (collar-button) Boston KPro I design has advantages over designs in which the optical stem is anchored by a flexible looping extension placed within or in front of the person's cornea. It is thought that positioning the Boston KPro's back plate entirely behind the corneal tissue may give better long-term retention than other arrangements (Cruzat et al. 2013).
The Boston KPro I can restore sight for several years. The main reason for eventual vision loss after implantation is post-operative glaucoma (Ahmad et al. 2015).
Standard treatment for significant corneal opacity is a full-thickness corneal transplant, known as penetrating keratoplasty (PK). During PK, the opaque cornea is removed using a trephine (hole saw) and replaced with a donor cornea. Some people cannot have PK for reasons including: disease severity; severe involvement of the conjunctiva; a failed previous corneal transplant; or when measures needed to prevent graft rejection are contraindicated. For these patients, PK using an artificial cornea (keratoprosthesis) may be an option.
NICE interventional procedures guidance on implantation of a corneal graft–keratoprosthesis for severe corneal opacity in wet blinking eyes found that the current evidence on the efficacy of this procedure was adequate in the short to medium term. The evidence on safety showed a high incidence of significant adverse effects, but it concluded that there are few options for patients with severe corneal opacity if standard corneal grafts have failed or are not appropriate. The guidance recommended that this procedure may be used with normal arrangements for clinical governance, consent and audit.
NICE is aware of the following CE-marked devices that appear to fulfil a similar function as Boston KPro I:
AlphaCor KPro (Argus Biomedical).
KeraKlear Artificial Cornea KPro (KeraMed).
Legeais BioKPro‑III (FCI Ophthalmics).
Boston KPro I would be used in a tertiary care setting. The implantation procedure would be done in a specialist ophthalmic operating theatre on a patient who has had a general or a local anaesthetic. It is typically a day case procedure.
Boston KPro I should only be used by surgeons specialising in the implantation of corneal grafts or keratoprostheses. Based on the indications for use, the device is likely to be used in people with severe corneal opacity:
who have had at least 1 failed graft
in whom standard donor grafting is unlikely to be successful
who need a repeat PK but cannot have systemic immunosuppression
who have high-risk features such as total limbal stem cell loss, deep corneal neovascularisation, but whose blink and tear mechanisms are reasonably intact (wet blinking eyes)
whose vision in the eye being considered for grafting is poorer than 6/60 (metric) and who have reduced vision of 6/12 in the opposite eye.
The Boston KPro I is not suitable for people with retinal detachment or extreme optic nerve cupping.
The instructions for use for Boston KPro I (Massachusetts Eye and Ear Infirmary 2014) describe any additional training needed to use this technology.
The Boston KPro I costs $3,000 (USD) within the EU, about £2,904 (excluding VAT). A shipping fee is charged in addition to this.
The NHS tariff for PK is around £1,500 (NHS 2015 [see papers January 2016, 'Letter re PbR Tarriffs']). However, tariffs can underestimate the full long-term costs of a procedure.
The NICE interventional procedures guidance on implantation of a corneal graft–keratoprosthesis for severe corneal opacity in wet blinking eyes states that antibiotics and steroids can be used in conjunction with PK and keratoprosthesis. The manufacturer recommends that levofloxacin (around £7 for a 5‑ml bottle) and vancomycin (£12.50 for a 1‑g powder concentrate for solutions) are used post-operatively. Corticosteroids can cost around £30 to £80 for a pack of 30 tablets, which is about £3 to £8 per dose (British national formulary [BNF] 2016). Specialist commentators noted that bandage contact lenses are also used after the operation. One commentator indicated that they cost £9 for a pack of three, and another stated that they cost £12.25 per lens.
No other practical difficulties have been identified in using or adopting the technology.
The Boston KPro I is currently used in 2 NHS centres. Limited use in the NHS makes it difficult to estimate the costs associated with using the Boston Pro or to compare them with relevant current practice.
No published evidence on the resource consequences of adopting the Boston KPro 1 were found in the systematic review on cost effectiveness.