Eyeworld

EW CORNEA 114 February 2018 by Michelle Stephenson EyeWorld Contributing Writer What's new with the Boston Keratoprosthesis? An update on this device's two formats T he Boston Keratoprosthesis (KPro), the most widely used artificial cornea, is a treatment option for pa- tients with corneal disease that cannot be treated with standard corneal transplantation. Advances in design and improved outcomes with the device have increased its use in recent years. It is a collar button design keratoprosthesis that consists of three components: a front plate with optical stem, a back plate, and a titanium locking c-ring. During as- sembly, the front and back plates are snapped together with corneal tissue sandwiched between the plates. The donor cornea is then sutured to the host cornea, as with standard penetrating keratoplasty surgery. It is currently available in two formats: type I and type II. Type II is reserved for severe end-stage ocular surface disease desiccation. It is similar to the type I device, but requires a near complete permanent tarsorrhaphy to be performed, with the optic of the keratoprosthesis projecting through the small central opening in the tarsorrhaphy. "The type I is by far the more commonly implanted of the two devices," said Anthony Aldave, MD, Los Angeles. "It is used for patients who have adequate tear function and lid function. The type II is much less commonly implanted and is used for patients who do not have adequate lid closure and/or have a very dry ocular surface." Implanting the device According to Dr. Aldave, the pro- cedure for implanting both types is similar. "However, the type I is placed in a donor cornea and is implanted into the eye, similar to a penetrating keratoplasty, so the eyelids are open and move during a blink as they do in a normal eye. In the case of the type II, the recipi- ent eyes are typically very dry, so after implantation of the KPro, the eyelids are sewn together around the optic, resulting in complete coverage of the ocular surface," he explained. According to Albert Cheung, MD, Norfolk, Virginia, ideal patients for the type I KPro are those who are older and have failed multiple transplants. Patients with severe cic- atrizing or inflammatory stem cell deficiency, such as Stevens-Johnson syndrome and mucous membrane pemphigoid, are poor candidates, as are those with severe dry eye. The Boston KPro has shown promising results in this patient population. A study conducted by Dr. Aldave and colleagues 1 found that the KPro provides significant visual improvement in most eyes, with more than 50% of eyes in his study regaining and maintaining 20/200 corrected distance visual acuity each year through 8 years of follow-up. Additionally, the study found that the incidence of each postoperative complication de- creased significantly over the first 10 years after surgery. Although approximately 25% of implanted KPros were removed, more than 90% of eyes reaching 5 years of fol- low-up retained a keratoprosthesis at final follow-up. This study included 74 KPro procedures performed in 58 eyes (55 patients) with a mean duration of follow-up of 82.8 ± 20.5 months (range: 57–145 months). The most common indication for KPro im- plantation was corneal transplant failure (50%). More than half of eyes (51.7%) experienced postop- erative retroprosthetic membrane formation. Complications after implantation According to Dr. Cheung, glaucoma should be well managed before KPro surgery. "If you think the patient may need a tube, that should be placed before KPro surgery because once the keratoprosthesis is im- planted, glaucoma and intraocular pressure monitoring are more diffi- cult. We will mostly follow glauco- ma by optic nerve evaluation and tactile pressure," he said. As seen in Dr. Aldave's study, the most common complication or adverse effect following a KPro is a retroprosthetic membrane that develops over top of the optic or in the back of the optic. "They can be visually significant," Dr. Cheung said. "Often we can remove them with a YAG laser, but sometimes the membrane may be too thick for a la- ser membranotomy, and we need to surgically perform a membranecto- my to remove it. Additionally, these patients will always be at risk for infections and retinal issues. Once you implant a KPro, these patients become your patients for life, and they will require a good multidisci- plinary subspecialty team to follow the retinal and glaucoma issues." According to Dr. Aldave, "the second most common complication in our series was persistent corneal epithelial defect, which is a concern as it is associated with a significantly increased risk of infectious keratitis and corneal stromal necrosis. This can be managed in several ways, but it is commonly managed by per- forming a lateral tarsorrhaphy." There are some steps that can be taken prophylactically to de- crease a patient's chance of having microbial keratitis. "We keep these patients in bandage contact lens- es to try to prevent desiccation, epithelial breakdown, and melting Device focus Infectious keratitis following Boston KPro type I implantation typically appears as an opacification of the donor cornea under the front plate, adjacent to the optic. Source: Anthony Aldave, MD continued on page 116

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