We created a hypothetical 60-year-old cohort with end-stage ankle arthritis for whom nonoperative management had failed. The cohort age and demographics reflect the mean age and demographics among participants in the largest TAA and ankle fusion studies [20
]. The TAA study had approximately 50% females and 50% males [20
]. The clinical pathway modeled began with patients undergoing a TAA or ankle fusion. Then a Markov cost-effectiveness decision model was created to analyze health outcomes and direct medical costs. Patients who survived surgery were followed annually, transitioning between health states until they died or reached 100 years of age [11
] (Fig. ). Each health state was assigned a utility [9
] value to represent quality of life in QALY and a cost measured in 2007 US dollars. The model was constructed using decision-analysis software (TreeAge Pro 2009; TreeAge Software, Inc; Williamstown, MA, USA). When the model was run, TAA and ankle fusion arms of the tree were compared based on the number of years patients spent in each health state with the associated QALY and costs. Costs and utilities were discounted at a rate of 3% per year (range, 0%–5% per year in sensitivity analyses) [11
Fig. 1 Individuals initially undergo either TAA or ankle fusion. As the model progresses patients enter different health states, represented by the ovals in the diagram, based on reported rates in the literature. A certain percentage of patients will die after (more ...)
Several assumptions were made in model construction: (1) age-specific mortality rates for patients who survive surgery follow 2004 US life table norms [2
]; (2) perioperative mortality after any ankle surgery was estimated at 0.02% [28
]; (3) patients who need a revision TAA will undergo one revision surgery and then have conversion surgery to an ankle fusion or have below-knee amputation (BKA), based on the prevalence of TAAs that progress to BKA as reported in the literature [13
]; (4) patients who have ankle fusion will undergo one revision and possibly one fusion/BKA [13
], then have no additional surgeries until death, however, patients who have ankle fusion still could have progressive ipsilateral arthritis; and (5) if patients undergo fusion after TAA or revision fusion, all ipsilateral arthritic joints will be fused simultaneously, leading to improved QALY for individuals in the fusion compared with the arthritis states.
Model parameters for revision and complication rates were obtained from published studies (Table ). Ankle fusion revision rates were estimated from data reported by SooHoo et al. [25
]. The Scandinavian Total Ankle Replacement (STAR; Small Bone Innovations, Morrisville, PA, USA) prosthesis, a mobile-bearing design, was used for modeling TAA survivorship. Three different fixed-bearing implants and one newer mobile-bearing implant are approved for use in the United States. Some evidence suggests greater longevity in STAR mobile-bearing prostheses over standard fixed-bearing designs [16
]. Furthermore, the study used for the reference case has the largest reported series of TAAs (200 patients), with some of the longest followup data published (range, 5–13 years), and greater than 80% survivorship [28
]. This study was used to estimate annual revision rates for Years 1 to 6 [28
]. For Year 7 and onward, revision rates were estimated at 0.021 based on the average observed revision rates from Years 6 to 11 (Table ) [28
]. There are currently no 15- to 20-year followup studies. We tested TAA yearly survival rates from 2% to 20% in a sensitivity analysis using published rates [1
Model parameters from literature review and pooled results
Annual probability of total ankle arthroplasty
Data on the occurrence and rate of short-term complications were obtained from SooHoo et al. [25
] for ankle fusions and Saltzman et al. [20
] for the STAR TAA based on 2 years of followup. The complications included were: postoperative infections, major wound problems, nontraumatic bone fractures, implant problems, or osteolysis [20
]. Other reports of complication rates between 5% and 25% for TAA [1
] and 0.5% to 10% for ankle fusion [13
] were used to establish ranges for sensitivity analyses.
Coester et al. reported the rate of radiographic and clinical arthritis progressing in the ipsilateral hindfoot and forefoot after ankle fusion is 100% at 22 years of followup [5
]. That calculates to an average progression of arthritis of 4.5% per year, which is consistent with 2.8% of ankle fusion patients underwent subtalar fusion within 5 years [25
]. The rate of progressing ipsilateral arthritis after TAA (based on various implants) is 0.7% to 3% per year [25
]. In theory, this is attributable to increased motion of the ankle leading to decreased stress being placed on surrounding foot joints. Young patients after ankle fusion or TAA will experience consequences of lower utilities from progressive ipsilateral foot arthritis. The progressive arthritis rate is not well defined in the literature; and therefore is an important variable for sensitivity analyses.
The costs of TAA, ankle fusion, and ankle revision surgeries were estimated from Medicare 2007 payment schedules (Table ). The total reimbursement was the sum of the DRG code payment, professional charges based on 2007 CPT codes, and implant costs. The latter were estimated at $200 for the two screws needed for an ankle fusion and $9000 for TAA implants [7
]. The surgeon fees, anesthesiologist fees, hospital charges, and physical therapy charges are assumed to be equivalent as operative time and length of hospital stay are similar between the two procedures, and therefore do not need to be added to the total costs in this model. A yearly TAA followup office charge was added to the TAA costs but not to ankle fusion costs as life-long followup is not necessary for patients who have fusion. The costs of short-term complications for TAA and ankle fusion were estimated at $2000, considering rehospitalization and treatment costs.
Variables used in the model with associated utilities
All health states were assigned utilities ranging from 0.0 (representing death) to 1.0 (representing perfect health) based on reported quality of well-being index scores (Table ) [9
]. Utilities estimated by SooHoo and Kominski [24
] were used (Table ). Arthritis of the lower extremity was assigned a utility near 0.7 from SF-36 surveys, similar to values of other lower extremity impairments [10
]. TAA was given a utility of 0.9 [24
], which is higher than that of ankle fusion (0.8), as studies suggest improved functional outcomes after TAA, including gait and ROM [6
]. A recent study reported equivalent utilities of 0.73 one year postoperatively for patients who had TAA and ankle fusion [23
]. These values were used in a sensitivity analysis, but not in the primary analysis because the study was not a randomized controlled trial. The study included an older TAA group compared with the ankle fusion group, and had a 50% attrition rate [23
]. No differences in utilities based on race or gender have been established; therefore, the utilities were not stratified by gender or race.
Short-term negative postoperative experiences (eg, pain, limited mobility) were accounted for with acute procedure tools (ie, ‘disutilities’). The disutility of having a TAA was −0.125 owing to patients wearing a cast for 6 weeks (1.5 months divided by 12 months). The disutility of ankle fusion or BKA was −0.25 owing to the 3 months of nonweightbearing and limited mobility (3 months divided by 12 months). Revision arthroplasty and revision fusion also were assigned disutilities of −0.125 and −0.25, respectively, as the healing time for revision surgery is similar to that for primary procedures (Table ).
To determine if TAA is cost-effective compared with ankle fusions, we calculated the ICER by dividing the difference in costs by the difference in utilities (QALY) between TAA and ankle fusion. This ratio reflects how much the improved quality of life of TAA compares with the less expensive ankle fusion. This ICER calculation then can be used to compare TAA with different medial therapies across all specialties. One-way sensitivity analyses were performed by changing the range of values of one variable while keeping all other variables static. These analyses determine when the cost per QALY gained for TAA relative to ankle fusion surgery exceeded the willingness-to-pay threshold of $100,000/QALY [18
]. From these analyses, the importance of the uncertainty present in the literature and which variables will change the conclusions of the model can be determined. Sensitivity analyses were performed on the revision surgery rate, risk of perioperative complications or risk of future ipsilateral arthritis for each procedure, procedure costs, patient age, and health utilities for each procedure.