The Boston Keratoprosthesis (Boston KPro) is a collar button design keratoprosthesis (KPro)

Keratoprosthesis

Keratoprosthesis is a surgical procedure where a severely damaged or diseased cornea is replaced with an artificial cornea. While conventional cornea transplant uses donor tissue for transplant, an artificial cornea is used in the Keratoprosthesis procedure...

 or artificial cornea. It is composed of a front plate with a stem, which houses the optical portion of the device, a back plate and a titanium locking c-ring. It is available in type I and type II formats. The type I design is used much more frequently than the type II which is reserved for severe end stage dry eye conditions and is similar to the type I except it has a 2 mm anterior nub designed to penetrate through a tarsorrhaphy. The type I format will be discussed here as it is more commonly used.

The type I Kpro is available in single standard pseudophakic power or customized aphakic optic with an 8.5 mm diameter adult size or 7.0 mm diameter pediatric size back plate. The device is currently machined from medical grade polymethylmethacrylate (PMMA) in Woburn, Massachusetts

Woburn, Massachusetts

Woburn is a city in Middlesex County, Massachusetts, USA. The population was 38,120 at the 2010 census. Woburn is located north of Boston, Massachusetts, and just south of the intersection of I-93 and I-95.- History :...

 in the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

. During implantation of the device, the device is assembled with a donor corneal graft positioned between the front and back plate which is then sutured into place in a similar fashion to penetrating keratoplasty (corneal transplantation).

Recent improvements in design

1. The addition of holes in the back plate. The device currently has 16 holes which provides a surface area of 18 mm² for diffusion of nutritious aqueous to support the donor graft stroma and keratocytes
   Corneal keratocyte
   Corneal keratocytes are specialized fibroblasts residing in the stroma. This corneal layer, representing about 85-90% of corneal thickness, is built up from highly regular collagenous lamellae and extracellular matrix components. Keratocytes play the major role in keeping it transparent, healing...
   
   .
2. In 2004, a titanium locking c-ring was added to prevent intraocular unscrewing of the device.
3. In 2007, a threadless design was introduced which simplified assembly and produced less damage to the donor graft when the device was assembled during the surgical procedure

Indications

The Boston KPro is a treatment option for corneal disorders not amenable to standard penetrating keratoplasty (corneal transplantation) or corneal transplant. The Boston KPro is a proven primary treatment option for repeat graft failure, herpetic keratitis, aniridia

Aniridia

Aniridia is the absence of the iris. Aniridia usually involves both eyes. It can be congenital or caused by a penetrant injury. Isolated aniridia is a congenital disorder which is not limited to a defect in iris development, but is a panocular condition with macular and optic nerve hypoplasia,...

  and many pediatric congenital corneal opacities including Peter's anomaly. The device is also used to treat cicatrizing

Cicatrization

Cicatrization can mean:* the process of a wound healing to produce scar tissue* scarification, a form of body modification that uses cicatrization to create patterns on the skin....

 conditions including Stevens–Johnson syndrome  and ocular cicatricial pemphigoid

Cicatricial pemphigoid

Cicatricial pemphigoid is a rare chronic autoimmune subepithelial blistering disease characterized by erosive skin lesions of the mucous membranes and skin that results in scarring of at least some sites of involvement.Cicatricial pemphigoid has been referred...

, and also ocular burns.

Postoperative management

1. Indefinite placement of a bandage contact lens is needed to maintain adequate ocular surface hydration and prevent stromal melt, dellen formation, tissue melt and necrosis.
2. Indefinite daily topical antibiotic prophylaxis.
3. Life-long topical steriods.
4. Close follow-up with an ophthalmologist to monitor for complications associated with the device. Although the surgical procedure is relatively straightforward for surgeons trained to perform corneal transplants, the follow-up required after KPro placement is life-long.

Postoperative complications

Most common postoperative complications in order of decreasing prevalence include retroprosthetic membrane (RPM), elevated intraocular pressure/glaucoma, infectious endophthalmitis, sterile vitritis, retina detachment (rare) and vitreous hemorrhage (rare).

Prognosis and outcomes

4 major studies have been completed to date showing outcomes with the type I Boston KPro:

1. Multicenter Boston KPro Study is the largest published to date with 141 Boston type I keratoprosthesis procedures from 17 surgical sites by 39 different surgeons. At an average follow-up of 8.5 months, retention rate of the device was 95%, 57% had BCVA ≥ 20/200. Postoperative complications included RPM in 25%, high IOP
   Intraocular pressure
   Intraocular pressure is the fluid pressure inside the eye. Tonometry is the method eye care professionals use to determine this. IOP is an important aspect in the evaluation of patients at risk from glaucoma...
   
   in 15%, sterile vitritis complicated the postoperative course of 5% of eyes. Importantly, no cases of infectious endophthalmitis were reported in this large series.
2. A large single surgeon series with 57 modern type I Boston KPro procedures from UCLA medical center. At an average follow-up of 17 months, retention rate of the device was 84%, 75% had BCVA ≥ 20/200. Postoperative complications included RPM in 25%, high IOP in 15%, sterile vitritis complicated the postoperative course of 5% of eyes. Postoperative complications included RPM in 44%, high IOP in 18%, sterile vitritis complicated the postoperative course of 10% of eyes. No cases of infectious enophthalmitis were reported in this series.
3. A single center study from UC Davis of 30 Boston type I KPro procedures. At an average follow-up of 19 months, retention rate of the device was 83%, 77% had BCVA ≥ 20/200. Postoperative complications included RPM in 43%, high IOP in 27%, sterile vitritis complicated the postoperative course of 3% of eyes. The rate of infectious endophthalmitis in this study was 10%.
4. A retrospective study of 36 Boston Type I KPro procedures from a single institution. At an average follow-up of 16 months, retention rate of the device was 100%, 83% had BCVA ≥ 20/200. Postoperative complications included RPM in 65% and high IOP in 38%. Infectious endophthalmitis complicated 11% of eyes during the postoperative period.

External links

• http://www.djo.harvard.edu/files/6425_1055.jpg
• http://www.djo.harvard.edu/files/6426_1055.jpg
• http://www.masseyeandear.org/specialties/ophthalmology/cornea-and-refractive-surgery/keratoprosthesis/
• http://bmctoday.net/crstoday/2009/09/article.asp?f=CRST0909_16.php
• http://eyewiki.aao.org/Boston_Keratoprosthesis_(KPro)