This study investigated the associations between psychological parameters and physical outcome assessed by two PRO instruments, the WOMAC score and the FJS-12. We found high correlations between disease-specific outcome measures and several of the assessed psychological domains. Multivariate regression showed that catastrophising, psychological distress and somatisation explained almost 60% variance of the WOMAC score beyond the known covariates of sex, implant location and education. We found the same predictor set for the FJS-12, however, psychological parameters accounted only for half the variance seen in the WOMAC score.
Our findings indicate a significant lack of divergent validity of the WOMAC score and, to a lesser extent, of the FJS-12. The variance proportions estimated with help of the regression model suggest a substantial overlap between the orthopaedic and psychological scales. The lack of divergent validity becomes even more evident when opposing the high correlations between the WOMAC subscales themselves (above 0.80) and the correlations of the WOMAC total score with the psychological scores (up to 0.79).
This significant overlap with psychological status is not reflected in the WOMAC scales’ names (pain, stiffness, function) which somewhat misleadingly suggest to just measure physical, joint-related characteristics. This is also true for the FJS-12 which refers to joint awareness. However, the term joint awareness seems more closely related to psychological aspects.
We also found that location of joint arthroplasty (hip or knee) explained less than 5% of variance of both FJS-12 and the WOMAC score. This is interesting as it is well accepted that outcome differs between total hip and total knee arthroplasty populations [26, 27]. In contrast, the psychological scales exceeded these proportions by a factor of 10 (for both FJS-12 and WOMAC). Thus, our data indicate a stronger association between psychological factors and joint-related outcomes than that between outcome and the type of joint replaced.
Our findings compare well to other results from literature. Escobar et al.  investigated the association between WOMAC scores and the different subscales of the SF-36. They showed that both psycho-social and physical SF-36 scales correlated to the WOMAC score in a similar way. The WOMAC function subscale demonstrated the same correlation with both SF-36 social and physical function scores. WOMAC stiffness was equally correlated with SF-36 role-physical function score and mental health score. Similarly Wolfe  highlighted that divergent validity of the WOMAC may be compromised by factors such as fatigue, symptom counts, depression, and low back pain.
The strong correlation between physical and psychological scales found here and in other studies [28–30] may partially be explained by causal interdepencies that have been suggested by several longitudinal studies.
Sharma et al.  demonstrated that mental health measured with the SF-36 predicted subsequent improvement in physical function in TKA, results in line with Brander et al. , who showed that preoperative depression substantially influences Knee Society Rating Scale function scores five years post-operatively. In contrast, Lingard et al.  found (in a large prospective observational study) that although psychological distress decreased post-operatively, pre-operative levels of distress were not related to post-operative improvement (change in pain and function).
Lopez-Olivo et al.  found a strong correlation between pre-operative psychological status and post-operative physical function at 6 months. Education, coping style and locus of control over health at baseline explained 22% of variance in WOMAC pain at follow-up. A similar predictor-set explained 19% of the WOMAC function scale and 36% of the total score of the Knee Society Rating Scale.
Our study was based on a cross-sectional design which is reasonable for the investigation of divergent validity. However, it does not allow for causal interpretation of the associations between orthopaedic outcomes and psychological variables. A limitation is the limited number of predictors in our model that left a large proportion of unexplained variance. Further interesting predictors that may be of future research interest include patient activity level, social support, cognitive function, range of motion and joint stability.
A particular strength of this study is the use of a comprehensive and detailed assessment of psychological status (BSI and the Catastrophising Scale from the Coping Strategies Questionnaire). These scales are more differentiated and comprehensive than other tools such as the SF-36 which has previously been employed to assess psychosocial characteristics of arthroplasty populations.