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Surgical techniques vary for adult and pediatric Boston KPro
A surgeon details the steps for placement of a type 1 and type 2 Boston keratoprosthesis and explains variations in pediatric surgery.
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When conventional full-thickness penetrating keratoplasty or corneal transplantation has a high risk for graft failure, an artificial cornea may be the alternative choice in an attempt to visually rehabilitate the patient. An artificial cornea may be considered in patients with one or more failed penetrating keratoplasties. Other considerations include vision worse than 20/200 in the involved eye with contralateral compromised vision without end-stage glaucoma or retinal detachment. Based on the WHO’s 2010 global blindness data, it has been estimated that bilateral corneal blindness affects 4.9 million individuals, or 12% of the total 39 million blind worldwide.
While the original concept of an artificial cornea dates back to 1789 to Guillaume Pellier de Quengsy, the first human artificial cornea using a quartz crystal implant took place in 1855. Over a long period of time, artificial cornea designs and materials evolved to improve the relative acceptance of an artificial cornea to the human eye after removal of the diseased, opaque or cloudy recipient cornea. Currently, the most popular form of keratoprosthesis in clinical use is the Boston keratoprosthesis, also known as the Dohlman-Doane keratoprosthesis, which was developed at Massachusetts Eye and Ear Infirmary in Boston in the 1960s and gained FDA approval in the U.S. in 1992. The other FDA-approved artificial cornea is the AlphaCor device. Some of the other globally available artificial corneas, not approved by the FDA, include osteo-odonto keratoprosthesis, LVP KPro and KeraKlear (KeraMed).
There are two types of Boston KPros, type 1 and type 2, the former being the most widely used in the U.S. and mostly in eyes with relatively good tear function, while the latter is limited to eyes with absent or very poor tear function. While designs and choices in materials for the Boston KPro have expanded over the last several decades for both adult and pediatric age groups, continued challenges over time after such a surgical procedure include, but are not limited to, retroprosthetic membranes, corneal melt with or without device extrusion, glaucoma and inflammation.
In this column, Dr. Utine describes her Boston KPro surgical techniques in a stepwise fashion in adult and pediatric age groups.
Thomas “TJ” John, MDOSN Surgical Maneuvers Editor
Initially described by Dohlman in the mid-1960s, the Boston keratoprosthesis surgical technique is unique but relatively easy compared with other artificial corneal surgeries. The KPro has a collar button design with a front plate and a back plate that sandwich a fresh donor corneal graft as a carrier.
Type 1 KPro
A type 1 KPro is used in non-cicatrizing diseases of the cornea and has a front plate made of PMMA that is 5 mm in diameter. Its optical surface has a diameter of 3.5 mm, which can have plano refractive power or correct the aphakic refractive error based on axial length of the eye. The back plate has a diameter of 8.5 mm and eight holes to 16 holes on it to allow the carrier cornea to stay in contact with the aqueous, promoting its nourishment and reducing the incidence of melt. Additionally, fibrous ingrowth into these holes helps to anchor the KPro complex in the eye. A titanium back plate (Figure 1) is thinner than a PMMA one, thus causing less anterior chamber crowding and closed-angle glaucoma. Its biocompatibility leads to less retroprosthetic membrane formation. The initial design, with threads for the back plate, had been changed to a “snap-on mechanism” with an incomplete circle of a titanium locking ring.
Type 1 KPro surgery has the following steps:
1. KPro complex should be prepared and assembled before recipient corneal trephination. Donor cornea is trephined 0.25 mm to 0.5 mm larger than the back plate.
Source: Canan Asli Utine, MD, MSc, FEBO, FICO
2. A central 3-mm opening is punched out using a disposable dermatological trephine (Figure 2).
3. A doughnut-shaped corneal tissue is placed back to front plate on the adhesive tape, with the optical cylinder passing through the central opening (Figure 3).
4. The fenestrated back plate is placed on top of the donor tissue posteriorly (Figure 4).
5. A titanium locking ring is snapped around the stem portion of the front plate, which protrudes posteriorly through the cornea and the back plate, and the assembly is locked (Figure 5). The recently designed incomplete circle of back plate firmly keeps the KPro assembly secure, without the need for a titanium ring (Figure 6).
6. The recipient bed should be prepared with a 0.5-mm to 1-mm smaller trephine.
7. The KPro assembly is then sutured in the recipient bed using 16 interrupted 10-0 nylon sutures, as in penetrating keratoplasty, after any indicated anterior segment surgical maneuvers, such as cataract extraction, are performed (Figures 7 and 8).
8. A hydrophilic bandage contact lens is placed to keep a certain amount of tear film intact on the KPro and prevent dellen formation.
9. If indicated, vitreoretinal surgery can be performed immediately after KPro implantation (Figure 9). Aqueous shunt devices can be implanted before or after KPro surgery, or concurrently (Figure 10).
Type 2 KPro
A type 2 KPro is designed for severe dry eyes and cicatrizing diseases, as desiccating inflammation may lead to melting of the carrier cornea, consequent leakage, endophthalmitis or KPro extrusion. It has an extended 3.5-mm nub protruding from the front plate and through the lids after a complete permanent tarsorrhaphy, performed to prevent evaporation of tears and to delay or prevent melts.
Type 2 KPro surgery involves the following steps:
1. Surgery is performed under general anesthesia.
2. Before trephination of the host cornea, extensive dissection and removal of all ocular surface epithelium is necessary.
3. Additional procedures such as pars plana vitrectomy (with an Eckhardt temporary keratoprosthesis) or implantation of a glaucoma drainage device are performed.
4. Total iridectomy and lensectomy are performed. Unless a stable IOL is already present, the patient is left aphakic.
5. Type 2 KPro assembly is secured with interrupted 10-0 nylon sutures. Knots are rotated toward the host cornea but do not need to be buried.
6. Peribulbar vancomycin (25 mg/0.5 mL), ceftazidime (100 mg/0.5 mL) and triamcinolone (20 mg/0.5 mL) are administered.
7. The upper and lower tarsi are approximated with two or three interrupted 6-0 Vicryl sutures on each side of the KPro. Eyelid margins are closed with 8-0 nylon mattress sutures over plastic bolsters. A notch is created in the upper lid with Vannas scissors to allow the nub to protrude.
An alternative to type 2 KPro surgery is the modified type 1 KPro surgery, in which the KPro is totally covered by tarsal conjunctiva or buccal mucosa. A central opening of 3.5 mm is created at least 3 months postoperatively (Figure 11).
Pediatric KPro
Surgery has several advantages over allograft surgery, such as PMMA being immunologically inert, not inducing regular or irregular astigmatism, and eliminating the need for IOL implant or visual rehabilitation challenges in aphakia. KPro is retained without extrusion or rejection and is appropriate for the management of pediatric corneal opacity.
Pediatric KPro surgery has the following steps:
1. General anesthesia is mandatory in this age group. If indicated, retina and glaucoma specialists should also participate for a joint management approach.
2. A scleral fixation ring should be used due to low scleral rigidity in pediatric eyes.
3. A conjunctival peritomy helps with a more accurate centration and diameter selection.
4. Depending on the size of the globe and the cornea, a smaller back plate of 7 mm diameter with a single row of eight holes or a larger plate of 8.5 mm diameter with 16 holes might be chosen.
5. The KPro complex should be prepared and assembled before recipient corneal trephination. Donor cornea is oversized by approximately 1 mm larger than the back plate, particularly in cases of aphakia.
6. The recipient bed should be trephined using a 0.5 mm to 1 mm smaller trephine than that for the donor cornea to facilitate suturing.
7. In case of anterior segment dysgenesis, take care to trephine the recipient cornea because vascularized iris adhesions or lens located immediately behind the cornea might be present.
8. A peripheral iridotomy is mandatory in intact irises to prevent pupillary block.
9. Lensectomy is recommended in all pediatric cases, as an aphakic KPro without a lens implant is preferred. A round posterior capsulotomy and anterior vitrectomy are also necessary.
- References:
- Aquavella JV, et al. Am J Ophthalmol. 2005;doi:10.1016/j.ajo.2005.06.041.
- Aquavella JV, et al. Ophthalmology. 2007;doi:10.1016/j.ophtha.2007.01.027.
- Dohlman C. Keratoprosthesis. In: Krachmer J, Mannis M, Holland E, eds. Cornea. Mosby-Yearbook Inc.; 1997:1855-1872.
- Eghrari AO, et al. Ocul Immunol Inflamm. 2016;doi:10.3109/09273948.2015.1037458.
- Han SB, et al. Boston KPro type 1 in the pediatric age group. In: Cortina M, de la Cruz J, eds. Keratoprostheses and Artificial Corneas. Springer; 2015:145-152.
- Utine CA, et al. Ocul Immunol Inflamm. 2011;doi:10.3109/09273948.2011.621580.
- Utine CA, et al. J Cataract Refract Surg. 2011;doi:10.1016/j.jcrs.2011.03.045.
- For more information:
- Canan Asli Utine, MD, MSc, FEBO, FICO, can be reached at Dokuz Eylul Universitesi Tp Fakultesi, Goz Hastaliklari Anabilim Dali/Dokuz Eylul University, Izmir International Biomedicine and Genome Institute, Mithatpasa cad. No:1606 nciralt Yerleskesi 35340 Balçova, zmir, Turkey; email: cananutine@gmail.com.
- Edited by Thomas “TJ” John, MD, a clinical associate professor at Loyola University at Chicago and in private practice in Oak Brook, Tinley Park and Oak Lawn, Illinois. He can be reached at email: tjcornea@gmail.com.
Disclosures: Utine and John report no relevant financial disclosures.