Important patient characteristics differ prior to total knee arthroplasty and total hip arthroplasty between Switzerland and the United States

At the time of primary total knee and hip arthroplasty, clinically important differences in age, education level, obesity prevalence, medical comorbidities, preoperative pain levels and emotional health were observed between the US and Swiss cohorts. The level of functional impairment at the time of TKA and THA reflected significant, and similar, disability in both countries. Because existing research documents that implant longevity, post-operative complication rates, and improvement in pain and function after TJA vary by patient characteristics, these data suggest that future cross-national TJA outcome comparisons should address pre-TJA patient differences prior to drawing conclusions [6–14].

Obesity is a well-known and important risk factor for short- and long-term complications and a threat to prosthesis survival [8, 13]. The pattern and prevalence of population obesity differs between the US and Switzerland, especially among TKA patients, and the differences have become more pronounced during the past decade. Between 1999 to 2009, the “Bus Santé”, a large cross-sectional population-based study in Geneva, reported 35% of the population were overweight and 12% obese [28]. The prevalence did not change during the 10 year time period [29]. In contrast, the corresponding prevalences of overweight and obesity in the US were 34% and 30.5%, respectively, in 1999, and 33% and 35.7%, in 2009 [30]. The observed prevalences of obesity among our TJA patients exceeded population-based prevalences, both in Geneva and in the US. The differences were larger in TKA as compared with THA patients. This finding parallels the understanding that obesity is an important risk factor for OA, especially for knee OA [31].

Obesity is also known to be associated with younger age at the time TJA is performed [32]. We found that the US TJA patients were substantially younger in every BMI category as compared to the Geneva patients, although the difference in age was highest in the greatest BMI category. Thus, increased BMI may explain only part of the observed age difference as reported previously [33, 34]. Other reasons for the younger age of the US TJA patients may be related to cultural expectations or health care access and delivery differences. Age is an important factor influencing joint replacement outcomes. Younger age increases the risk of prosthesis failure due to the generally more active lifestyle in younger people [35]. Because of the substantially younger age in the TKA and THA US cohorts there is an expected greater proportion of patients still working and thus likely exposing their arthroplasty to higher demands.

While patients in the US had greater BMI, the mean pain score among TJA patients in the US was less than the pain score reported in Switzerland. However, prior research has documented that greater BMI is associated with greater arthritis pain [36, 37]. This relationship exists in these data as well. In both the US and Swiss TKA and THA patients, higher BMI correlated with more pain. TJA patients in both countries reported similar, and substantial, functional limitations. Consistent with previous reports, higher BMI was associated with poorer function in both countries.

Only one study evaluated the association between education and preoperative pain and function levels [15] prior to TKA using the WOMAC score. In contrast to our results they did not report an association. However, Keurentjes et al. [38] using the SF-36 found lower preoperative scores in less educated THA and TKA patients. The discrepancy in education levels between the US and Swiss cohorts may be exaggerated in this study. The well-documented disparity in the use of TJA in the US among minority patients [39–45] and those with lower income and education will skew the education level in this cohort toward a higher mean US education [46]. In contrast, the proportion of patients with tertiary education in the Swiss cohort was in accordance with the reported levels of education in 2012 in Switzerland (29% with tertiary education) based on the annual OECD survey of adults aged 55–64 years [47]. However, of importance, both cohorts illustrate a significant association between lower education level and greater pre-operative pain.

Despite the younger age in US patients, the prevalence of comorbidities (diabetes- TKA, cardiac disease-THA) was greater. Diabetes and cardiac disease are both associated with obesity and with increased short-term post-TJA complication rates [9, 11, 14]. However, researchers have reported that medical comorbidities, in contrast to other patient factors, are not key predictors of patient-reported outcomes in THA [48]. In contrast to the pattern in medical comorbidities, emotional health, as measured by the SF MCS, was substantially poorer in Switzerland than in the US. However, a number of reasons may explain this difference. First, population-based normative values for the SF-12 MCS vary between the two countries (US MCS norm 50 as compared to the French-speaking area of Switzerland MCS norm 46.3, [49] accounting for some of this difference. Second, the poorer emotional health is possibly related to the greater pain level among the Swiss patients, because higher levels of OA pain have been associated with greater disability and depressed mood [50]. And third, patients with lower socioecomonic status, who constitute a much higher proportion in the Swiss as compared to the US cohort, have been shown to report lower emotional health [51]. The MCS differences are important as it has been reported that patients with poorer pre-TJA emotional health may be at risk for suboptimal postoperative outcomes [7]. A multicomponent psychosocial support program has been suggested prior to and following surgery including consistent counselor support as well as education and coping skills training to address anxiety, pain management, depression and the role of social supports [42].

In summary, the greater prevalence of obesity and medical comorbidities, plus a younger mean age, potentially increase the risk for complications and revisions among the US patients, as compared to the Swiss. However, the Swiss reported a much higher proportion of patients with a low level of education, higher pain levels and poorer emotional health at the time of surgery. These differences should be considered in future cross-cultural comparisons of short and long-term outcomes after TJA.

Few prior studies have evaluated differences in patient risk factors across international patient groups. In 2004, Lingard and colleagues evaluated the predictors of pain relief and functional gain after TKA in the US, England, and Australia [7]. While the researchers evaluated the role of BMI, only pre-operative physical function, poor emotional health, and greater medical comorbidities were associated with poorer outcomes. In a study of 12 European countries with nearly all THA patients having advanced radiographic hip OA, the level of pre-operative pain, disability, and patient risks varied across countries [44]. Another sample of TJA patients from 10 countries found TJA patients had worse mean pain and function scores than OA patients without TJA but there was substantial overlap in symptoms between the two groups, and no consistent pain and function profile existed for TJA patients across countries [17]. Gromov and colleagues reported that US THA patients had a younger age and higher BMI in accordance with our findings, however, they found greater pain and poorer function than in the European patients [18]. Finally, Gordon and colleagues report that patient predictors of pain and function in THA performed similarly across two countries (Denmark and Sweden) [52].

Surgeons, policy makers, and implant manufacturers rely on national registry reports for comparative implant information. Registry reports as those from the Scandinavian countries, the United Kingdom, or Australia present implant survival within age and sex sub-groups, and identify differences in implant survival by these patient attributes, While the United Kingdom, Sweden, the Netherlands, and New Zealand implant registries are now collecting PROMs [49–51, 53, 54] national registries do not yet adjust implant survival comparisons by pre-operative function and/or comorbidities. Our data document important differences in patient characteristics between TKA and THA patients from different countries. Future research will examine the impact of these differences on TJA outcomes.

Limitations

While we carefully pre-defined measures to assure comparable data, there are possible measurement limitations. First, both countries are dependent upon documentation practices for medical comorbidities and it is possible that the difference between the US and Switzerland is related to differences in documentation. For example, the US database lacked documentation of secondary OA, while the Swiss registry was able to differentiate primary and secondary OA. Second, different methods were used to collect and score patient-reported outcomes (SF12 vs SF36; WOMAC vs HOOS/KOOS). However, previous psychometric research documents that the scores are comparable within SF versions and between the WOMAC and the KOOS. Finally, while this report is based on a sample of patients from the US and Switzerland, the respective registries have documented that the demographics of participating patients are comparable to the total population with health care coverage in these countries/regions and both cohorts include diverse surgeons.