COPYRIGHT © BY THE JOURNAL OF BONE AND JOINT SURGERY, INCORPORATED
BARLOW ET AL.
THE COST-EFFECTIVENESS OF DUAL MOBILITY IMPLANTS FOR PRIMARY TOTAL HIP ARTHROPLASTY: A COMPUTER-BASED COSTUTILITY 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 indication70,71. 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
procedure72,73, 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 dislocation11,1618,36,48-53
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 arthroplasty74-76.
High and low probabilities for deterministic sensitivity analyses were derived from the studies by
Ong et al.75 and Springer et al.76, respectively.
COPYRIGHT © BY THE JOURNAL OF BONE AND JOINT SURGERY, INCORPORATED
BARLOW ET AL.
THE COST-EFFECTIVENESS OF DUAL MOBILITY IMPLANTS FOR PRIMARY TOTAL HIP ARTHROPLASTY: A COMPUTER-BASED COSTUTILITY MODEL
http://dx.doi.org/10.2106/JBJS.16.00109
Page 2
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. Halfcycle 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 literature72,80, 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.
COPYRIGHT © BY THE JOURNAL OF BONE AND JOINT SURGERY, INCORPORATED
BARLOW ET AL.
THE COST-EFFECTIVENESS OF DUAL MOBILITY IMPLANTS FOR PRIMARY TOTAL HIP ARTHROPLASTY: A COMPUTER-BASED COSTUTILITY MODEL
http://dx.doi.org/10.2106/JBJS.16.00109
Page 3
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
treatment58-63 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 arthroplasty59-61,63 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 analyses59-61,63.
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 costeffectiveness 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
COPYRIGHT © BY THE JOURNAL OF BONE AND JOINT SURGERY, INCORPORATED
BARLOW ET AL.
THE COST-EFFECTIVENESS OF DUAL MOBILITY IMPLANTS FOR PRIMARY TOTAL HIP ARTHROPLASTY: A COMPUTER-BASED COSTUTILITY MODEL
http://dx.doi.org/10.2106/JBJS.16.00109
Page 4
distributions were used for probabilities and utilities, except for the dislocation multiplier, which
was assumed to be normal.