The Boston Keratoprosthesis Type I

The Boston Keratoprosthesis (Kpro) was developed at the Massachusetts Eye & Ear Infirmary over the last 40 years. It is a collar button–shaped polymethylmethacrylate (PMMA) device consisting of 2 curved plates sandwiched around a corneal autograft or allograft. A soft bandage contact lens is used indefinitely after surgery to prevent evaporative losses and thus minimize the risk of corneal melts. For severe dry eye and cicatricial diseases, a “Type II” Boston Keratoprosthesis can be implanted through the eyelids. Implantation of either the type I or type II device is performed in 1 stage, and is technically similar to standard corneal transplantation. The Boston Keratoprosthesis is by far the most commonly used prosthetic cornea in the world: more than 3000 devices had been implanted worldwide by December 2008 (Boston Keratoprosthesis Update Newsletter V:2008).

Patients

In a retrospective cohort study chart review, patients who underwent keratoprosthesis surgery at the Massachusetts Eye and Ear Infirmary between the years 2003 and 2008 were identified. Patients had to have a minimum of 2 years of visual acuity follow-up data recorded. Because patients with autoimmune diseases and chemical burns are known to be poor prognostic groups for keratoprosthesis surgery, they were excluded from this study. Eighty-two patients were included.

Patient characteristics with underlying diagnoses prior to keratoprostheis surgery are given in Table 1 . Infectious causes included: fungi, 3 (3.6%); idiopathic but presumed microbial ulceration, 3 (3.6%); Pseudomonas , 1 (1.2%); Herpes simplex virus, 2 (2.4%); and Varicella zoster virus, 1 (1.2%). A complete ophthalmic exam was performed prior to Kpro surgery. Median preoperative best-corrected visual acuity (BCVA) in the treated eye was logMAR 2.12 ± 0.38 (Snellen equivalent Hand Motions). Visual acuity values were normally distributed per Shapiro-Wilk testing. The IRB/Ethics Committee decided approval was not required for this study. The project does not fall under HIPAA requirements because no protected health information is recorded or linked by code to data.

Decision Analysis

In order to make cost-utility calculations, several model assumptions were made. Average cost-effectiveness was determined, comparing keratoprosthesis surgery to no further intervention (the current visual state). The typical risks of keratoprosthesis failure include infection, endophthalmitis, extrusion, and retinal detachment. Complications and additional procedures were incorporated into a weighted cost decision tree. A list of the most common postoperative complications and treatments is given in Table 2 .

In our sample, 2-year anatomical retention was 99% and 5-year anatomical retention in a sub-sample (n = 27) was 89%. Based on available literature, the multicenter retention rate at 8.5 months is purported to be 95%. In a separate sample, 2-year retention rate decreased to 61%, but this figure included patients with autoimmune diseases, chemical injury, and severe glaucoma. Furthermore, unpublished multicenter data report 2-year retention in 350 non-autoimmune patients to be as high as 94% (Ament J, Todani A, Ciolino J, Belin M. Multicenter KPro retention data. KPro Study Group, Albany, New York. Ongoing study.). Because of the variability in reported retention rates the following model assumptions were made: 1) 2-year anatomical retention was set at 94%, a value lower than reported in this sample but more representative of the larger multicenter data; 2) since 5-year survival data exist for the most updated version of the Boston Keratoprosthesis, the time frame chosen for the CUA was 5 years; 3) by French Curve extrapolation, we conservatively estimated 85% anatomical retention for our entire sample at 5 years; 4) a yearly transplant survival probability of 97% was interpolated from the extrapolation curve; 5) since all patients had a minimum 2-year follow-up, comparing best spectacle-corrected visual acuity (BSCVA) preoperatively and at 2 years was utilized to calculate incremental utilities; 6) in patients without 5-year follow-up, BSCVA was assumed to be stable throughout the remaining study period; 7) we assumed surgery in only 1 eye and the outcome was ignored in cases where the keratoprosthesis had been implanted in the second eye; 8) since the mean patient age was 65, we postulated that all patients would survive the 5 study years; and 9) follow-up for all patients was assumed to be similar.

Utility Assessment

As described, the time-tradeoff method was used for patient-based utility assessment in the CUA. For each patient, the appropriate utility value based on past studies was matched to the corresponding BCVA both preoperatively and 2 years postoperatively. The mean (±standard deviation) preoperative utility value was 0.449 (±0.113), increasing to 0.630 (±0.174) postoperatively. For each patient, the difference between preoperative and postoperative utility was calculated and the mean of these differences obtained. Decreases in vision were accounted for by incorporating negative utilities into the mean incremental utility calculation. The mean incremental utility in our study population was 0.182. The total QALY gain (comparative effectiveness) was also determined by multiplying the years of utility gain by years of benefit duration and comparing it to the preoperative utility (quality of life) state.

Identification of Costs

The costs for the Kpro, surgical procedure, and hospitalization were obtained from the Massachusetts Eye and Ear Infirmary CPT diagnosis and procedure codes. The costs associated with postoperative care were obtained from clinic billing sheets with corresponding CPT codes. Costs were initially obtained based on these codes; however, the CUA was conducted based on the current Medicare reimbursement rates in 2009. Reimbursement for keratoplasty done at the time of keratoprosthesis insertion has typically been at 50% of the allowable. There are special circumstances where the patient’s own cornea can be harvested for keratoprosthesis implantation to mitigate costs, but this is rarely done. Ophthalmologic visits were conducted every 3 months for the first year, every 6 months over the second year, and then annually. Maintenance costs included bandage contact lenses, medications, and patient travel. An overview of all costs is given in Table 3 .

The expected value decision tree is illustrated in the Figure . Future costs, such as follow-up visits and possible complications requiring procedures, are weighted based on normal clinical practice and complication rates observed in our sample (refer to Table 2 ). Total costs were calculated by the summation of initial costs with appropriately discounted future costs. Discounted future costs included inevitable costs and the average of weighted probable/possible future costs.

Expected value decision tree for The Boston Keratoprosthesis: preoperative, perioperative, and postoperative management. Decision tree is broken down into initial, inevitable, probable, and possible costs. Costs are reported based on the allowable reimbursement from Medicare. Probability of occurrence is displayed below the decision/cost item. Italics indicate costs that require appropriate discounting. Discounting can occur over some or all of the years being assessed in this study depending on need and utilization of the service. Furthermore, in some instances, such as bandage contact lens (BCL) use, the cost changes after the initial year of use. Non-italicized costs are initial, one-time expenses that do not require discounting.

Discounting

There is a consensus that both utilities and costs should be discounted in health care economics analyses. We decided that the commonly used 3% discount rate was appropriate.

Calculation of Quality-Adjusted Life Years

The quality-adjusted life years for the 5-year period was calculated using the following formula:

Calculation of Cost

The equation for the total discounted cost associated with Kpro surgery is:

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