© Springer-Verlag Berlin Heidelberg 2015
M. Soledad Cortina and Jose de la Cruz (eds.)Keratoprostheses and Artificial Corneas10.1007/978-3-642-55179-6_1717. Boston KPro Type I in the Pediatric Age Group
(1)
The Wilmer Eye Institute, The Johns Hopkins University, Baltimore, MD, USA
(2)
Department of Ophthalmology, Kangwon National University, Hyoja 3 dong 17-1, Chuncheon, Kangwon-do, 200-947, South Korea
(3)
Department of Ophthalmology, The Wilmer Eye Institute, The Johns Hopkins Hospital, Baltimore, MD, USA
(4)
Cornea and Anterior Segment Service, Wilmer Eye Institute, Johns Hopkins University School of Medicine, 600 North Wolfe Street, Woods 375, Baltimore, MD 21287, USA
17.1 Introduction
17.3 Indication
17.4.1 Device
17.4.2 Surgical Technique
17.6 Complications
17.7 Perspective
17.1 Introduction
Loss of vision from loss of corneal clarity is the third leading cause for blindness worldwide. Trachoma and vitamin A deficiency have been identified as the most prevalent causes of corneal blindness outside developed countries [1]. Based on 2005 World Health Organization statistics, eight million people have bilateral corneal blindness (less than 10/180 visual acuity in the better eye); 1.5 million of these are children. Some of the causes of corneal clouding in infants and children in developed countries include Peters’ anomaly, congenital hereditary endothelial dystrophy, sclerocornea, dermoid tumors, corneal edema secondary to congenital glaucoma, leukomas due to birth trauma, and infectious keratitis [2, 3]. Significant corneal opacities in infants can lead to profound deprivation amblyopia. For this reason, early surgical intervention is recommended [4]. Surgery usually involves a full thickness penetrating keratoplasty (PK). However, corneal surgeons are often reluctant to perform PK in pediatric patients because of the greater risk for failure compared with adult patients [5–7].
17.2 Limitations of Donor Corneal Transplantation
Although PK has thus far been the mainstay treatment of corneal opacity in children and infants, the surgery is replete with problems, including an increased risk of rejection, a high incidence of intraoperative complications due to low sclera rigidity and positive vitreous pressure, and the challenges of vision rehabilitation due to the frequent presence of aphakia and significant astigmatism in the postoperative period [8].
The prognosis of PK in pediatric patients is poorer than that in adults, and even worse in patients younger than 1 year of age. Outcomes of PK are particularly worse in children born with anterior segment dysgeneses [4, 9–11]. Intermediate-term studies showed “graft survival” rates of 44–79 % at mean follow-ups of 12–50 months [7, 12–14]. However, these studies included patients of all ages, all kinds of preoperative etiologies, and follow-up periods of all lengths. It is well known that younger children are less likely to have good success with PK. A robust inflammatory response after PK in pediatric patients often complicates the postoperative course and prolongs the period of deprivation amblyopia for several more months after surgery. Significant refractive errors often caused by surgically induced astigmatism and aphakia also play a significant role in the amblyopia [10, 15]. Thus, the visual outcome continues to be even more unfavorable than anatomical success rates (i.e., graft clarity). In 1995, Dana et al. [11] reported that only 33 % of patients attained a visual acuity greater than 20/200 despite an anatomical success of 80 %. In 2000, Aasuri et al. [7] also reported that only 34 % of their patients achieved a visual acuity of 20/400 or better, although 66.2 % of patients maintained a clear graft. Likewise, in 2007, Sharma et al. [12] demonstrated that 30.1 % of patients were able to see better than 20/200, although 77 % had a clear cornea at the last exam.
Allograft rejection resulting in failure of the graft occurs in about 40–50 % of pediatric patients after PK [7, 11, 16–18]. Regraft itself is a significant risk factor of graft failure; thus, allograft rejection can often cause multiple graft failures that inevitably lead to deprivation amblyopia [10]. In addition, systemic immunosuppression to help prevent rejection is problematic and not feasible in this age group.
Endothelial keratoplasty is rapidly replacing PK in corneal endothelial diseases in adult patients because of its advantages over PK, including rapid visual recovery, reduction in postoperative astigmatism, a minimal risk for suture-related complications, a stable wound, and a reduced rate of endothelial rejection [19, 20]. This technique may also be of use in selected pediatric patients [21–24]. However, considering that the main indication for pediatric keratoplasty is stromal opacity, the number of patients who can benefit from endothelial keratoplasty would be limited [25]. Prior studies showed that only 5–21 % of pediatric PKs were performed for endothelial disorders, such as congenital hereditary endothelial dystrophy or posterior polymorphous dystrophy [5, 6, 13]. Moreover, endothelial keratoplasty in pediatric patients has its own drawbacks. Manipulation of the donor disc (with a smaller diameter than adult donor lenticule and a lower total cell count) within the small anterior chamber of a pediatric patient increases the risk of endothelial cell loss that can lead to graft failure. The requirement of prone positioning in the early postoperative period to facilitate the attachment of donor graft is often impossible in young children.
Boston type 1 keratoprosthesis (KPro) has several advantages over donor keratoplasty. For this reason, it can be considered an alternative approach, particularly in patients who are thought to have an increased risk for graft failure after PK [10, 15]. First, the clear optical cylinder made of polymethylmethacrylate, which is an immunologically inert material, eliminates the risk of allograft rejection and ensures prompt recovery of a clear visual axis [4, 15]. Second, KPro does not induce regular or irregular astigmatism as it maintains a spherical anterior plane and can actually correct refractive errors related to axial length, as refraction can be built into the anterior plate [4]. Third, KPro implantation eliminates the need for intraocular lens implantation or visual rehabilitation challenges with aphakia.
17.3 Indication
In addition to multiple donor graft failures, studies have demonstrated favorable outcomes of KPro implantation in adults as the primary procedure for the treatment of corneal opacities caused by various diseases, such as ocular trauma [26], herpetic keratitis [27, 28], aniridia [29], limbal stem cell deficiency [30], autoimmune ocular surface disorders including Stevens-Johnson syndrome [31], and congenital corneal opacities [3, 4, 15].
Unlike in adults, the use of an artificial cornea is still limited in children and infants. Thus far, more than 6,000 KPros have been implanted in the USA and overseas, but only a small portion of these surgeries were performed in the pediatric age group [32]. The first report of its use in this age group was published in 2006 by surgeons at The Wilmer Eye Institute [4] in Baltimore, Maryland, USA, and included two patients. Aquavella et al. [15] subsequently published in 2007 a larger case series with longer follow-up, which included 22 eyes in 17 patients. In 2010, Nallasamy and Colby [10] detailed their experience of using the KPro in a 6.5-month-old girl for the treatment of large congenital lacrimal gland choristoma that invaded the visual axis.
The results of studies in adults also suggest that the Boston KPro can be an option for the management of limbal cell deficiencies, including aniridic keratopathy and Stevens-Johnson syndrome, where PK carries an extremely high risk of graft failure [29–31]. Although there has been no report of the use of the KPro in children with limbal stem cell deficiencies, whether acquired or congenital, the device would conceivably be helpful in children who suffer from those diseases. The most remarkable merit would be that KPro implantation does not necessarily require intensive systemic immunosuppression that is indispensable after keratolimbal autograft for limbal stem cell replenishment. Early treatment of those diseases, even from infancy, is expected to result in significant visual recovery, particularly in those with aniridia, where aniridic keratopathy frequently manifests from the first decade of life in 90 % of the affected patients and is a major cause of visual loss [29–31].
17.4 Device Construction and Surgical Technique
17.4.1 Device
Depending on the size of the globe as well as the cornea diameters, a smaller back plate (7 mm in diameter) with a single row of 8 holes or a larger plate (8.5 mm in diameter) with 16 holes might be used. The standard anterior plate measuring 5 mm in diameter is used in both pediatric and adult cases.
17.4.2 Surgical Technique
17.4.2.1 Preoperative Evaluations
Preoperative measurement of axial length should be performed to determine the optical power of aphakic prosthesis. Ultrasonography can also be helpful for the evaluation of retina, as fundus examination is difficult due to corneal opacity in most cases. Intraocular pressure should be recorded. In infants or young children, examination under anesthesia should be conducted for these detailed evaluations [15]. If indicated, retina and glaucoma specialists should also participate in the examination under anesthesia for an estimation of visual potential and, if indicated, for a joint approach to address accompanying retinal disorders or glaucoma [15].
17.4.2.2 Intraoperative Technique
In general, the standard technique for the Boston KPro implantation in adults is used as follows [33, 34]:
1.
General anesthesia is mandatory for pediatric patients.
2.
Conjunctival peritomy should be performed for more accurate centration and diameter selection.
3.
Suturing a scleral fixation ring is preferred due to low rigidity of sclera and cornea in pediatric patients.
4.
For prompt placement of the KPro, the device should be prepared and assembled with the donor tissue prior to the recipient cornea trephination.
First, donor cornea is trephined measuring usually 0.25–0.5 mm larger than the back plate.
Then, a central 3 mm opening is punched with the disposable skin trephine.
The corneal tissue is then inserted between the front plate of the device (with the optical cylinder passing through the central opening), and the fenestrated back plate is placed on top of the donor tissue.
The unit is then secured by snapping the locking titanium ring around the stem, on top of the back plate.
5.
Preparation of the recipient bed should then be done. The recipient should be trephined using a 0.5–1 mm smaller trephine to facilitate the suturing. Assuming a 7 mm back plate is being used, the recipient cornea can be trephined at 7–8 mm. The donor tissue should be oversized, by about 1 mm in pediatric cases, particularly in the presence of aphakia.
Great care should be taken when excising the cornea because there are often vascularized iris adhesions or the lens might be up against the cornea with or without adhesions.
If the iris is intact, a peripheral iridectomy is mandatory to prevent papillary block.
As an aphakic KPro without the use of the intraocular lens is preferred in pediatric patients, a lensectomy is recommended in all cases. A round posterior capsulotomy and possible anterior vitrectomy are also necessary due to the high incidence of subsequent opacification in younger children.
6.
The device is then secured in the recipient bed and sutured in a standard fashion using 16 interrupted 10-0 nylon sutures.
7.
In cases where vitreoretinal surgery is needed, the retinal procedures can be performed immediately after KPro implantation.
In some cases, a temporary prosthesis (Eckardt, DORC, Kingston, New Hampshire, USA) might need to be used to facilitate vitreoretinal surgery. The prosthesis is exchanged for the KPro after completion of the retinal procedures.
8.
When surgical control of intraocular pressure is required, aqueous shunts can be implanted either before or after KPro insertion, or concurrently.
9.
Immediately after the conclusion of the surgery, a hydrophilic bandage lens should be applied. A 16 mm lens is included with the commercially available Boston KPro type I set.
10.
A light patch with antibiotic drops is applied and maintained for 24 h.
17.4.2.3 Postoperative Management
Long-term use of topical antibiotics and steroids is recommended, as in adults [3, 4, 15]. Aggressive use of topical steroids is required because of the active immune response and propensity for inflammation in pediatric patients [15]. As the crystalline lens is removed in virtually all cases, development of cataracts is not a concern [10, 15]. Indefinite use of a bandage contact lens is also required for maintenance of a comfortable ocular surface and prevention of complications including dellen, tissue melt, and necrosis [10, 15, 35]. Moist chamber goggles can be used for maintenance of hydration of the lens, and even partial tarsorrhaphy can be considered in cases where maintenance of a bandage lens is difficult. A supply of spare lenses and parental education should be done so that reinsertion can be accomplished locally in case of loss [15].
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