Progress with the KPro (2024)

In our last column, we left off with our patient being diagnosed with an advanced Pseudomonas infection, prompting the implantation of a Boston KPro Type 1. The use of the Boston KPro is rare and reserved for when a corneal transplant has a guarded prognosis or when all other options have failed.

The major components of the KPro device include a front plate composed of polymethylmethacrylate (PMMA), a donor corneal graft button and a back plate composed of PMMA or titanium.¹ The corneal tissue health is of significant concern, as it is particularly vulnerable to evaporative dry eye complications, among many others.¹ These complications include epithelial dysfunction, stromal thinning, corneal melt, dellen formation, perforation, aqueous leakage and infection.¹ When the KPro was originally introduced, the conjunctival flaps were used to protect this vital donor tissue.¹ Although still used, conjunctival flaps have largely been replaced with long-term bandage contact lens wear due to conjunctival retraction.¹

Bandage Contact Lenses

This tool has long-established therapeutic benefits in wound healing, pain management and ocular surface protection. First approved for therapeutic usage in the 1970s, indications for bandage contact lenses have blossomed over time to include, but are not limited to, corneal abrasion, corneal erosion, dry eye, graft-vs.-host disease, post-procedurally including the KPro and more.^2,3 As of the writing of this article, there are currently three FDA-approved silicone hydrogel lens materials approved for bandage contact lens usage: balafilcon A, lotrafilcon A and senofilcon A.³ The higher oxygen transmissibility of silicone hydrogels has been of particular benefit in diseased ocular surfaces, resulting in reduced corneal edema and hypoxia-related complications with extended-wear contact lens use.³

A bandage contact lens or other lens type, like sclerals, must be used to protect the donor tissue.Photo: Brandon Ayres, MD.Click image to enlarge.

When managing your KPro patients, a proper bandage contact lens fitting is essential to maintain corneal tissue health and longevity.⁴ Before surgery is performed, patients are informed and educated on proper contact lens wear. Patients must understand they will need lifelong bandage lens wear with prophylactic antibiotic treatment to help prevent infection and various other complications for which they are at higher risk.⁴ Lenses are placed on the ocular surface by the surgeon following surgery completion. Most providers recommend removal and replacement every one to three months, which should only be performed by the physician, due to the relatively fragile nature of the donor tissue.⁴

Contact Lenses for KPros

The most commonly used bandage contact lens for KPro patients is the Kontur soft lens (Kontur Kontact Lenses). Kontur contact lenses are comprised of 55% water, use methafilcon A hydrogel material and have an oxygen permeability (Dk) of 18.8. They are available in a wide variety of parameters, with base curve options ranging from 6.8mm to 9.8mm and available diameters ranging from 12mm to 24mm, allowing the practitioner ample options to achieve a successful lens fitting. Powers range from +10.00D to -20.00D with astigmatic powers of -0.75D to -5.00D in all axis locations. This broad range allows the patient the ability to achieve their best possible vision. After the completion of surgery, a plano Kontur lens with a 16mm diameter and 9.8 base curve, which is provided by the manufacturer, is placed on the eye.

The extended-wear nature of lenses in KPro patients presents with its own difficulties. Although methafilcon A has a Dk of 18.8, the low oxygen transmissibility is not of significant concern, due to the donor tissue being considered a carrier.⁴ Even though methafilcon A is considered a group IV hydrogel material (water content greater than 50% and ionic material), it is particularly good at resisting external lipid and protein deposition, which is important for lens comfort and vision in extended-wear KPro patients.^4,5 Group IV hydrogel materials typically accumulate deposits deep within the material matrix instead of the lens surface and plateaus in total deposition after one to seven days of wear.⁵ Compare this to group II hydrogel lenses (water content greater than 50% and non-ionic material), which cumulatively deposits on the lens surface without plateauing.⁵

Other soft contact lenses have been used successfully with KPros as well. Custom soft lenses dominate the literature, with Alden Optical and X-Cel Specialty Contacts being preferred.⁴ Some popular options are the HP 49 (hioxifilicon B, 49% water, Dk 15) from Alden Optical and X-Cel Specialty Contacts’ multiple Flexlens designs (atypical refractive correction, spherical, tri-curve keratoconus and post-refractive lenses, 49% water and large diameter) available in many material options (polymacon, 38% water; hefilcon A, 45% water; hioxifilicon B, 49% water; methafilcon A, 55% water; acofilcon A, 58% water; hioxifilcon A, 59% water; efrofilcon A, 74% water).⁴

FDA-approved soft bandage contact lenses have been used with KPros, too.⁴ However, case reports have shown that these lenses require more lens trials to achieve an acceptable fit in KPro patients, and in some cases, were unable to reach an acceptable fit.⁴ The lack of available lens options (i.e., base curve and diameter) make the larger soft lenses preferable. Even though silicone hydrogels allow for greater oxygen transmissibility to the donor tissue, they are prone to increased lens deposits and discomfort.⁵

If an adequate fit with soft lenses cannot be achieved for reasons such as poor comfort and vision, scleral and hybrid lenses have been shown to be an additional option. Scleral lenses provide support for the ocular surface by providing a reservoir of sterile saline that bathes the eye throughout the day. Special considerations need to be taken into account with sclerals when glaucoma drainage devices are involved, though, as glaucoma is highly common in KPro patients. There is no established wear schedule with scleral lenses, with the following guides being used currently: a 12/12 schedule with around the clock wear but switched with a second set every 12 hours; wear during awake hours and soft lens nightly wear while sleeping; continuous 24-hour wear; and wear while awake.⁶

While hybrid lenses are mentioned in the literature as an option for KPro patients, there is minimal information about fitting them.⁴ Use your trial set to get the hybrid lens to stabilize, then modify from there. The lenses need to be exchanged around every three months, depending on the patient. Keep in mind that this may not be the most cost-effective option for your patient.⁴

As with all lens-wearing patients, a proper contact lens fit is paramount—especially with KPro patients. A proper fit will show good coverage of the tissue with acceptable movement. Additionally, the lens should be large enough to provide centration.⁴ As a similar consideration like with scleral lenses, the diameter of the soft lens needs to account for the likely presence of glaucoma drainage devices; this is because erosion has been shown to occur at the edge of the lens.⁴ Large conjunctival elevations may also allow for pockets of air to develop under the contact; this could lead to focal drying. A properly fit lens should show no edge fluting (flat fit), air bubbles (steep fit) or vascular attention (tight fit).⁴

Alens that fits poorly over the eye can subsequently lead to infection. It is also documented that patients with the Boston KPro are prone to contracting infections, and this is why this patient subset is put on lifelong prophylactic antibiotics. The use of a contact lens increases the risk of infections, and a poorly fit contact lens will raise the chances of infection and complications. Our next column will address the most common complications of the Boston KPro.