Total knee arthroplasty (TKA) and unicompartmental knee arthroplasty (UKA) are 2 common and successful treatments for knee osteoarthritis1-4. Revision rates for TKA have fallen to approximately 5% at 10 years, with good functional outcomes5,6, but 17% of patients remain dissatisfied with the results of the procedure7. Furthermore, in as many as 47% of patients, osteoarthritis is restricted to 1 tibiofemoral compartment8. UKA is an alternative to TKA in patients who retain cruciate ligaments and preserved compartments and is more prevalent in a younger population compared with TKA6,9.
Outcomes following UKA and TKA are well documented in multiple randomized controlled trials and international registries3,6,9. UKA is associated with lower complication rates but higher revision rates4-6,10. Functional and patient-reported outcomes appear to favor UKA, particularly in data from the National Joint Registry for England, Wales, Northern Ireland and the Isle of Man4,9,11,12.
Because patients value specific outcomes, complications, and downstream events of surgical interventions differently, those values or preferences may impact the ideal treatment choice13. Stated-preference data are essential to delivering patient-centered care and establishing evidence regarding the value that patients with knee osteoarthritis place on processes and outcomes from arthroplasty14. The purpose of this study was to evaluate patient preferences for UKA and TKA by quantifying utility weights and the relative importance of treatment outcomes. Stated choice of TKA or UKA was examined to determine patient willingness to undergo either procedure.
Materials and Methods
A discrete-choice experiment was developed following good-practice standards for discrete-choice research by stated-preference researchers with >70 years of combined expertise13,15-20. Discrete-choice experiment are one of several experimental options used to elicit patient preferences for features of treatment21. This methodology was selected to quantify benefit-risk tradeoffs between cohorts for given clinical alternatives consisting of attributes and levels.
Respondents reviewed arthroplasty information and answered demographic questions. The Oxford Knee Score was used to evaluate baseline knee function before respondents completed the discrete-choice experiment22. The Oxford Knee Score is a licensed, validated 12-item patient-reported outcome measure scored from 0 to 48, with 0 representing the worst outcome and 48 representing the best. The Oxford Knee Score test was administered electronically, and patients were categorized according to functional status on the basis of the Kalairajah redefined scale: excellent (>41), good (34 to 41), fair (27 to 33), or poor (<27)22-24.
Respondents with excellent functional scores were directed to the end of the survey as unlikely surgical candidates based on surgeon recommendation25,26. Remaining respondents with good Oxford Knee Scores were directed to a discrete-choice experiment that evaluated preferences for improvements in knee function beyond good function (good-function cohort), and those with fair or poor scores were grouped together to receive a discrete-choice experiment evaluating preferences for improvements beyond fair function (fair/poor-function cohort). The only variance between surveys was the inclusion of 1 additional functional attribute level in the fair/poor-function cohort.
The basis of a discrete-choice experiment is selection of attributes that represent general categories under which treatments can be evaluated and of levels that convey how a treatment satisfies an attribute27. Attributes and levels are used to generate experimentally designed profiles that require tradeoffs27. The rate at which patients state they are willing to accept such tradeoffs indicates the relative importance of the attributes.
Attributes and levels (Table I) were selected through an extensive, iterative process involving literature review3-6,8,9,11,12,24,28-75, review from stated-preference experts, and input from fellowship-trained orthopaedic surgeons with varying arthroplasty views—ranging from strong support of TKA to strong support of UKA—to ensure consideration of all factors. To further assure objectivity, clinical information was compiled by an independent reviewer. Expert physicians (n = 5), including an independent orthopaedic surgeon not involved in the treatment of the study population, reviewed all clinical information for appropriateness and objectivity. Face-to-face interviews were conducted with 7 patients with knee osteoarthritis following a think-aloud protocol. Survey length, relevant attribute levels, and understanding of attribute definitions were iteratively evaluated. Additional testing occurred via an online survey marketplace to ensure appropriate attribute graphics and phrasing. Stakeholders evaluated survey length, validated experimental design efficiency, modified information delivery, and improved study design. Attributes most strongly supported by the evidence were selected: (1) chance of complications within 3 months of the surgical procedure, (2) functional ability following recovery, (3) awareness of the knee implant following recovery, and (4) chance of needing another operation within 10 years to replace the implant. Attribute levels were constructed to account for the range of variation within the knee-osteoarthritis population, from healthy individuals to those with comorbidities.
TABLE I -
Attributes and Levels for the Discrete-Choice Experiment Trade-Off Questions
|Chance of complication
||1 out of 100 people (1%) will have a serious complication within 3 months of operation
|4 out of 100 people (4%) will have a serious complication within 3 months of operation
|11 out of 100 people (11%) will have a serious complication within 3 months of operation
||You are able to lead an active lifestyle after recovery (Super Excellent)
|You are able to do all daily activities after recovery (Excellent)
|You are able to do daily activities with some limitations after recovery (Good)
|Normality: awareness of knee implant
||You are never aware of your knee implant after recovery
|You are seldom aware of your knee implant after recovery
|You are mostly aware of your knee implant after recovery
|Chance of needing another operation
||3 out of 100 people (3%) will need another operation within 10 years to replace their implant
|12 out of 100 people (12%) will need another operation within 10 years to replace their implant
|19 out of 100 people (19%) will need another operation within 10 years to replace their implant
*Respondents who scored Fair/Poor function on the Oxford Knee Score were shown the Good level of the functional ability attribute. Respondents who scored Good function on the Oxford Knee Score were filtered to a separate study design with 2 functional attribute levels that did not include Good function.
Respondents in the good-function cohort received a survey with 2 functional levels indicative of “Super Excellent” and “Excellent” function. “Super Excellent” was created to gauge patient interest in the creation of highly durable implants that would allow participation in high-impact sports, an activity currently not recommended with marketplace implants. “Excellent” was created according to the functional abilities of someone scoring excellent on the Oxford Knee Score, as verified through patient interviews and physician panel discussions. Respondents in the fair/poor-function cohort received a survey with 3 functional levels. The third level corresponded to good function according to the Oxford Knee Score. Level selection assumed that surgical intervention would result in some functional improvement.
Respondents completed a fixed-attention check, 8 random discrete-choice experiment trade-off questions, and 2 fixed final trade-off questions. Each random trade-off was followed by an opt-out in which respondents could choose to remain in their current functional state (Fig. 1). Lighthouse Studio (version 9.5.3; Sawtooth Software) generated 300 questionnaires that were randomly assigned to ensure adequate combinations and variations. The orthogonal discrete-choice experiment featured connectivity and positional balance to reduce order bias and did not include attribute randomization, concept sorting, or attribute prohibitions.
Sample and Recruitment
The instrument was released at a large academic medical center to orthopaedic patients 40 to 80 years old with a knee osteoarthritis diagnosis verified through International Classification of Diseases, 10th Revision (ICD-10) coding (M17.9) and supported by corresponding Oxford Knee Scores76. Enrollment occurred through clinic visits and emailed survey invitations. The study received institutional approval and was registered with ClinicalTrials.gov (NCT03058380).
Response quality was addressed by identifying respondents who straightlined or always selected Operation A or always selected Operation B in the discrete-choice experiment because such responses are considered a strong indicator of a lack of respondent attention77. Statistical comparison of respondent characteristics in the cohorts was conducted. P values were computed with use of the Fisher exact test and Student t test (2-tailed p values). Chi-square 2-way contingency tables tested for independence of variables. Attribute dominance—a classification of respondents unwilling to trade off among attributes, and thereby choosing a specific attribute with the better level every time it is displayed within a choice question—was examined between cohorts. Individuals with extended (i.e., >1 hour) survey completion times were considered outliers and removed only from the time analysis.
Random trade-off question data were analyzed following good-practice standards78. We used a random-parameters logit model in StataSE (version 15; StataCorp) to separate population-level preference estimates for both cohorts. Respondent treatment choices were estimated as a function of attribute levels. All attributes were effect-coded. With effect coding, 0 indicates the mean effect of all attributes as opposed to the effect of all omitted levels in dummy coding, and the coefficient for the omitted level is the negative sum of coefficients for all other levels within an attribute79. Discrete-choice experiment results are preference weights or log-odds indicating average preference for levels in each attribute. These weights are not directly interpretable. Only differences between preference weights in an attribute are meaningful. Attribute importance weights were calculated as the greatest change in preference weights that can be achieved with levels for each attribute.
We normalized preference weight estimates to facilitate comparisons of preferences across cohorts. The difference between the largest and smallest coefficients for the chance of serious complications was calculated. Attribute importance was normalized to 10. Remaining preference weights were rescaled to preserve relative attribute importance within cohorts. Rescaled preference weights represented respondent preferences for each attribute level as an equivalence to the chance of serious complications and can be compared across cohorts.
Confidence intervals (CIs) around rescaled preference and importance weights were estimated with use of the Krinsky and Robb procedure with 10,000 draws and rescaled accordingly80. If CIs between levels of a single attribute do not overlap, preference weights are statistically different from each other at better than the 5% level. Similarly, if CIs between attribute importance measures do not overlap, importance weights are statistically different from each other at better than the 5% level.
Two final-choice questions—reflecting actual attribute rates for UKA and TKA—examined respondent choices between operations (see Appendix). The first question displayed attributes and levels corresponding with the discrete-choice experiment design. The second included a modified functional attribute presented as a probabilistic outcome aligning with values in the literature and using person diagrams with rates for the ability to perform daily activities following recovery. Respondents were blinded to intervention type. Analysis of the final choice was completed with use of stated-choice probabilities and coefficients from random-parameters logit models to calculate preference weights for probabilistic function levels (TKA, 36%; UKA, 46%). We leveraged a probability-weighting function81 proposed by cognitive psychologists and empirically validated as a way to uncover the impact of uncertain outcomes. The function included 2 parameters ([δ],[γ]) that defined risk attitudes (i.e., patients are risk-averse, risk-neutral, or-risk tolerant) and discriminability (i.e., whether patients are able to discriminate differences in risk). Based on utility scores and the 2 final questions, predicted-choice probabilities for UKA and TKA were calculated for both cohorts.
Three hundred and forty-eight respondents met the inclusion criteria and were enrolled, of whom 300 completed the survey. In addition to the 48 participants who did not complete at least 1 discrete-choice experiment question, 42 participants were excluded from analysis for scoring “Excellent” on the Oxford Knee Score (n = 32) or for always choosing Operation A or B on the random trade-off questions (n = 10). The remaining 258 surveys were analyzed (Fig. 2). No exclusions were made on the basis of attribute dominance because no differences were found between cohorts for any attribute.
In total, a pivot design was utilized to filter 72 respondents into the good-function cohort and 186 into the fair/poor-function cohort. Respondents in the good-function cohort were significantly more likely to be married (p < 0.001) and have a postgraduate degree (p < 0.001) compared with those in the fair/poor-function cohort (Table II). These respondents were also more likely to report a desire to return to impact sports (p = 0.004), whereas those in the fair/poor-function cohort reported being more focused on activities of daily living (p = 0.004).
TABLE II -
Patient Characteristics and Information on Knee Status*
||Whole Sample (N = 258)
||Good-Function Cohort (N = 72)
||Fair/Poor-Function Cohort (N = 186)
||65.2 ± 8.4
||64.3 ± 7.6
||65.4 ± 8.7
| Black or African American
| White or Caucasian
| Single, never married
| <4 years of college
| Bachelor’s degree
| Postgraduate degree
| Full time (≥30 hours)
| Part time (<30 hours)
| Not employed
|Functional goals for treatment‡
| High-impact sports
| Low-impact sports
| Activities of daily living
|Length of time with knee pain‡
| <1 year
| 1-3 years
| 3-5 years
| 5-10 years
| >10 years
*Data are missing for some categories.
†Values are given as the mean and standard deviation.
‡Values are given as the percentage of patients, with the count in parentheses.
Estimated preference weights aligned with the natural ordering of the levels for each attribute; that is, better clinical outcomes had larger preference weights, excluding the super-excellent functional level, in the fair/poor-function cohort (Fig. 3). Within the good-function cohort, there were significant differences between preference weights for several attributes, including complications and revision. Within complication rates, there was a difference between the 4% and 11% levels (p = 0.05). Significant differences were found between all levels of revision rates. Within the fair/poor-function cohort, differences were found between preference weights for all 4 attributes. All levels of the complication and revision attributes showed significant differences. Differences were present between excellent and good (p < 0.001) and super-excellent and good (p < 0.001) functional levels and between “seldom aware” and “mostly aware” (p < 0.001) and “never aware” and “mostly aware” (p < 0.001) levels of the normality attribute. Comparison of preference weights between cohorts revealed no differences for any attribute.
Overall Attribute Importance
Overall attribute importance indicated that adverse events were most important for both cohorts (Fig. 4). Respondents with good, poor, or fair function valued serious complications and revision rates equally. Respondents in the good-function cohort valued the functional attribute the least (attribute importance, 6.7; 95% CI, 0.6 to 26.9), whereas those with worse function in the fair/poor-function cohort were least concerned with normality (attribute importance, 18.1; 95% CI, 13.9 to 21.9). Within the fair/poor-function cohort, there were differences in attribute importance for the following comparisons: (1) complications (32.6; 95% CI, 28.1 to 37.2) and function (21.2; 95% CI, 17.3 to 25.4; p = 0.001), (2) complications (32.6; 95% CI, 28.1 to 37.2) and normality (18.1; 95% CI, 13.9 to 21.9; p < 0.001), (3) function (21.2; 95% CI, 17.3 to 25.4) and revision (28.1; 95% CI, 24.4 to 31.7; p = 0.018), and (4) normality (18.1; 95% CI, 13.9 to 21.9) and revision (28.1; 95% CI, 24.4 to 31.7; p = 0.001).
Within the good-function cohort, there were differences in attribute importance for the following comparisons: (1) complications (34.4; 95% CI, 13.2 to 40.7) and function (6.7; 95% CI, 0.6 to 26.9; p = 0.003), (2) function (6.7; 95% CI, 0.6 to 26.9) and normality (24.6; 95% CI, 8.6 to 29.9; p = 0.032), and (3) function (6.7; 95% CI, 0.6 to 26.9) and revision (34.3; 95% CI, 24.2 to 57.0; p = 0.013).
Attribute importance comparisons between cohorts revealed that the importance of function was significantly higher for the fair/poor-function cohort compared with the good-function cohort (p = 0.03).
The 2 final-choice questions yielded slightly different results (Table III). Given the assumed functional form for the value of outcomes under uncertainty and known preferences for outcomes elicited through the main discrete-choice experiment, preference weights were 0.23 for 36% and 0.54 for 46% in the good-function cohort and 0.47 for 36% and 0.59 for 46% in the fair/poor-function cohort. Based on these utility scores and combined final 2 choice questions, predicted-choice probabilities for TKA and UKA were 42% and 58%, respectively, in the good-function cohort and 54% and 46%, respectively, in the fair/poor-function cohort.
TABLE III -
Results from Final-Choice Questions and Combined Results of the Final Treatment Choice Question Between TKA and UKA*
|Final Treatment Choice
||Good-Function Cohort (N = 70)
||Fair/Poor-Function Cohort (N = 182)
|Final-choice question #1
|Final-choice question #2
|Final predicted-choice probabilities
*Values are given as the number of patients, with the percentage in parentheses.
Although various methodologies have been employed to elicit patient preferences regarding TKA, willingness for a surgical procedure, and costs, these studies have not specifically addressed the question of TKA versus UKA and the associated tradeoffs of each82-85. To our knowledge, the present study is the first to directly contrast these 2 options in a clinical population and evaluate preferences on the basis of functional outcome scores.
Overall, the findings of the present study indicate that patients with knee osteoarthritis place the greatest value on adverse events such as complications and revision when considering treatment. Unsurprisingly, patients with better functional status value function following the surgical procedure less than those who are functionally worse off (fair/poor-function cohort). Those with good function are also more concerned with the ability to participate in sports compared with those with poor or fair function, who are more focused on activities of daily living. These findings suggest that there is a population of individuals with good function for whom a surgical procedure is appropriate and aligned with patient preferences. There was also minimal difference in value between achieving excellent or super-excellent function, indicating that an extremely durable implant for participation in high-impact sports is not necessarily a better option compared with the current marketplace implants.
The results of this study indicated that those with good functional status tended to prefer UKA, whereas those with worse function preferred TKA. The choices between procedures, however, were split nearly evenly (good-function cohort: TKA, 42%, and UKA, 58%; fair/poor-function cohort: TKA, 54%, and UKA, 46%). These findings have important implications for policy and suggest that the trend of increased utilization of UKA is consistent with patient preferences and is patient-centered. Some may argue that the percentage of the knee osteoarthritis population who qualify for UKA is far lower than the proportion who desire the procedure86. This disagreement is based on several factors, including data discrepancies between national registries regarding revision rates, design of knee implants, and patient qualification criteria86. Historically, to qualify for UKA, ideal patients have included those <60 years old with symptomatic end-stage osteoarthritis in the medial compartment of the knee, low levels of activity, weight of <180 lb (<82 kg), intact cruciate ligaments, and certain flexion requirements86. These criteria have lessened with the development of new implants and the expanded use of UKAs87-89. A comprehensive review of reported estimates from peer-reviewed publications indicates that 6% to 50% of patients with knee osteoarthritis may qualify for UKA87,89-91. The results of the present study highlight the role of preference elicitation and application through shared decision-making because there are a larger number of patients who, if eligible for UKA, would choose this treatment over TKA. Furthermore, patient preferences may not necessarily align with physician preferences, with physicians tending to be more risk-averse. Physicians should thoroughly discuss treatments and tradeoffs with patients to promote more informed shared decision-making.
Study limitations included lower enrollment numbers in the good-function cohort than in the fair/poor-function cohort; however, the sample size for both cohorts was adequate for analysis. Following de Bekker-Grob et al., the minimum sample sizes were 61 and 24 for the good and fair/poor-function cohorts, respectively92. Also, although patients with good function may undergo arthroplasty, it is more common for patients with worse functional status to undergo a surgical procedure25,26. This discrepancy is reflected in enrollment numbers between cohorts. Additionally, all study respondents were enrolled at a single academic medical center. Although we cannot guarantee that this cohort is representative of the overall knee osteoarthritis population, it was representative of the local population. A final limitation was the respondent evaluation of hypothetical treatments in the discrete-choice experiment; actual treatment choices may vary from stated choices.
In summary, patient preferences for outcomes of knee arthroplasty vary among patients. Unsurprisingly, patients with worse baseline functional status valued functional improvements significantly more than those with good baseline function. Overall, complication and revision rates were most important. Clinicians should focus on these when discussing treatments. There were no differences among preference weights between cohorts. There appeared to be a preference for UKA that, if combined with the appropriate patient criteria, may support increased utilization of the procedure for a segment of the population.
Supporting material provided by the authors is posted with the online version of this article as a data supplement at jbjs.org (https://links.lww.com/JBJS/G142).
Note: The authors thank Diane B. Covington, PA-C, MHS, and Justin R. Lynch, PA-C, for their assistance with participant enrollment.
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