The probability of mortality following revision total hip arthroplasty was assumed to be the same regardless of its indication70,71

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THE COST-EFFECTIVENESS OF DUAL MOBILITY IMPLANTS FOR PRIMARY TOTAL HIP ARTHROPLASTY:ACOMPUTER-BASED COST- UTILITY MODEL

http://dx.doi.org/10.2106/JBJS.16.00109 Page 1

Appendix Technical Supplement

Probabilities

It was assumed that the probability of mortality was the same following conventional and DM primary total hip arthroplasty, and this probability was derived from the 90-day mortality rate of 0.65% following elective total hip arthroplasty that Berstock et al.⁶⁹ reported in a recent systematic review. The probability of mortality following revision total hip arthroplasty was assumed to be the same regardless of its indication^70,71. The base-case estimate (2.15%) was derived from the 90-day mortality rate for elective total hip arthroplasty revision published in a systematic review by Singh et al.⁷⁰. The probabilities of discharge to different postacute-care destinations after primary total hip arthroplasty were based on patient age at the time of the procedure^72,73, and they were assumed to be similar after revision total hip arthroplasty on the basis of discharge destination data reported via the NIS⁴⁵.

For probabilities of dislocation following DM arthroplasties, included studies had a total of 7,131 patients with mean a follow-up of 11.1 ± 6.1 years for intraprosthetic dislocation^11,16-

18,36,48-53 and a total of 5,171 patients with a mean follow-up of 11.3 ± 6.3 years for large articulation dislocation11,16-18,48-53. Event probabilities were calculated for each study by first determining an annual rate from the event frequency and the follow-up period. Rates were converted to probabilities with use of standard mathematical methods:

Probability = 1 – e-(rate * time)

Weighted average probabilities were determined on the basis of the sample size from each study.

The probability of repeat dislocation for conventional total hip arthroplasty was derived from the study by Kotwal et al.⁶⁶, who found that approximately 60% of patients who had a first dislocation after the primary total hip arthroplasty went on to have a second dislocation and 51%

of all of the patients experiencing even 1 dislocation eventually needed revision for instability.

The mean time to a second dislocation was 247 days, and the mean time to revision total hip arthroplasty was 4.6 years⁶⁶. These frequencies and follow-up times were converted to rates and subsequently to annual probabilities, as described above. Those authors further characterized the extent of the revisions needed, with 61.4% of the patients needing acetabular revision alone, 11.4% needing head and liner exchange, 2.3% needing femoral revision alone, and 25.0%

needing revision of both components⁶⁶. These data were also used to determine the weighted average cost of revision total hip arthroplasty performed for instability. The probability of recurrent dislocation after large articulation DM instability was assumed to be equivalent to the probability of recurrent dislocation for conventional bearing couples, the derivation of which is described above⁶⁶.

Revision probabilities for reasons other than instability were derived from 13-year follow-up data from the Australian Orthopaedic Association National Joint Replacement Registry⁷⁴. The base-case estimate for the probability of repeat revision for any cause was also derived from this registry, and it was assumed that the probability of a third revision (i.e., chronic failure in the model) was equal to the probability of a repeat revision total hip arthroplasty^74-76. High and low probabilities for deterministic sensitivity analyses were derived from the studies by Ong et al.⁷⁵ and Springer et al.⁷⁶, respectively.

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The probability of follow-up outpatient care was determined by recommendations from Teeny et al.⁷⁷ and by Losina et al.⁷⁸, who assumed that patients had, on average, 2 follow-up appointments every 3 years. Employment multipliers were created for patients less than 65 years old and those 65 years or older in order to adjust expected productivity losses. For patients less than 65 years old, Losina et al. published overall employment multipliers for men (0.71) and women (0.61). A weighted average multiplier was created by multiplying these numbers by the percentage of men (44.1%) and women (55.9%) receiving total hip arthroplasty on the basis of 2012 NIS estimates⁴⁵. The employment multiplier was equal for men and women over 65 years old (0.21)⁷⁸. Both multipliers were varied by 10% for deterministic sensitivity analyses.

Quality of Life

For successful primary total hip arthroplasty, it was assumed that no difference in utility scores existed between conventional and DM total hip arthroplasties⁵⁴. Bozic et al.⁸ used

rigorous time-trade-off methodology and examined multiple health states (e.g., revision total hip arthroplasty). Both the disutility for primary total hip arthroplasty dislocation and the disutility for undergoing revision total hip arthroplasty were assumed to be the difference between the utility of primary total hip arthroplasty (0.96) and the utility of failed primary total hip

arthroplasty (0.59)⁸. It was assumed that the utility of a repeat revision total hip arthroplasty was 10% less than that of a revision total hip arthroplasty, and the disutility associated with repeat revision total hip arthroplasty was assumed to be the difference between the utility of revision total hip arthroplasty (0.84) and the utility of failed revision total hip arthroplasty (0.57)⁸. Half- cycle corrections were applied to disutility weights to account for midcycle transitions. The utility of chronically failed revision total hip arthroplasty was set equal to the published utility of chronically infected total hip arthroplasty (Table IV)⁸.

Direct Costs

Base-case estimates for hospital costs for primary and revision total hip arthroplasty were derived from mean 2011 NIS data⁴⁵, surgeon fees were derived from mean 2011 CMS data⁷⁹, and anesthesiologist fees were derived from Losina et al.⁷⁸. The high and low estimates for hospital costs and surgeon fees were based on the upper and lower percentiles reported in the 2011 NIS⁴⁵ and 2011 CMS data⁷⁹; the base-case estimate for anesthesiologist fees was varied by

±25%⁷⁸.

Base-case estimates and ranges for deterministic sensitivity analyses for hospital and surgeon costs related to revision total hip arthroplasty for reasons other than instability were weighted averages based on the frequencies of different revision procedures (e.g., acetabular revision) reported in the 2014 Australian Orthopaedic Association National Joint Replacement Registry for revisions of primary total hip arthroplasty⁷⁴. The estimates for repeat revision total hip arthroplasty were assumed to be 10% more than revision total hip arthroplasty.

Direct costs related to closed reductions were derived from Sanchez-Sotelo et al.⁶⁷ and were varied by ±50% for deterministic sensitivity analyses. Costs for various post-discharge destinations were likewise derived from prior published literature^72,80, as were direct costs for ongoing long-term care, including annual follow-up visits⁷⁸, radiographs⁷⁸, and the cost of a chronic failed revision arthroplasty⁵⁸. The latter costs were varied by ±25% for deterministic sensitivity analyses.

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Indirect Costs

Nwachukwu et al.⁶⁵ and Bedair et al.⁶² previously described a method for determining indirect medical costs related to lower-extremity arthritis, based on lost-wage estimates, derived from absenteeism attributable to lower-extremity impairment from arthritis⁵⁷ and/or its

treatment^58-63 and the weighted median daily income for U.S. workers 45 to 65 years of age⁶⁴. It was assumed that there are 240 total working days in 1 calendar year. The weighted median annual U.S. wage for workers 45 to 65 years of age in 2010 was $59,702⁶⁴. This value was adjusted to $63,782 2013 U.S. dollars using the Consumer Price Index⁵⁵. Therefore, the median daily wage was determined to be $266. The product of the total number of days absent from work in 1 year and this daily wage defined productivity losses.

From Li et al.⁵⁷ it was determined that workers with lower-extremity arthritis miss approximately 40 work days per year because of absenteeism related to their arthritis. For deterministic sensitivity analyses, missed work days were varied from 0 to 240 days. In the model, it was assumed that patients needing revision total hip arthroplasty would experience similar absenteeism. Nunley et al.⁶¹ found that primary total hip arthroplasty resulted in a mean of 6.9 weeks (35 days) out of work, and for deterministic sensitivity analyses absenteeism related to primary total hip arthroplasty was varied from 5 to 240 days. It was assumed that patients suitable to return to work after total hip arthroplasty would have only minor disability and thus minimal productivity loss after recovering from surgery. Therefore, it was further assumed that absenteeism after total hip arthroplasty for patients returning to work was 10% of that for a patient with end-stage osteoarthritis (4 days)⁶². The same percentage was assumed for patients returning to work after revision total hip arthroplasty⁶². Absenteeism associated with a primary total hip arthroplasty dislocation was estimated to be 10 days, on the basis of expert opinion, and was varied from 5 to 20 days for deterministic analyses. It was estimated that patients

undergoing revision total hip arthroplasty would require 3 months to recover⁶², during which they would miss 60 work days. It was assumed that repeat revision total hip arthroplasty would result in 50% greater absenteeism (90 days) compared with the initial revision.

Productivity losses were further adjusted on the basis of the probability of continued employment after primary total hip arthroplasty^59-61,63 and revision total hip arthroplasty⁶³. The base-case estimate for the probability of continued employment after primary total hip

arthroplasty was 0.904, and this was varied from 0.66 to 0.99 for deterministic analyses^59-61,63. Espehaug et al.⁶³ found that 53% of revision total hip arthroplasty patients who were employed prior to revision surgery returned to work afterward. In the absence of other evidence, it was assumed that the same probability would apply to repeat revision patients who were employed prior to their second revision total hip arthroplasty. These probabilities were varied ±50% for deterministic analyses. It was assumed that patients with a chronic failed total hip arthroplasty would be completely disabled and unable to work.

Probabilistic Sensitivity Analysis

For this analysis, 10,000 Monte Carlo simulations were used to repeat the cost-

effectiveness analysis. During each simulation, values for each model parameter were selected from specified distributions around the base-case estimates. Distribution ranges varied according to the level of uncertainty of each parameter. Gamma distributions were used for costs, and beta

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distributions were used for probabilities and utilities, except for the dislocation multiplier, which was assumed to be normal.