This study revealed that FJS-12 and KSS-2011 had a strong correlation in patients who underwent TKA, confirming the validation of the two most recent scoring system with each other. Although noise was hypothesized to have an effect on joint awareness, only a weak correlation was found with FJS-12 and KSS-2011. In fact, BCS had the worst score in noise, but the best score in KSS-2011, and the greatest ROM. Noise is suggested to have a limited effect on joint awareness and clinical outcomes. Residual symptoms and joint functions rather than noise perception might be important for joint awareness after TKA.
Of the KSS-2011 subscores, correlation coefficients of approximately 0.60 were found for “symptoms,” “patient satisfaction,” and “functional activities” for FJS-12. It became clear that joint awareness was correlated with postoperative pain and activities of daily living. Although FJS-12 does not directly question patient satisfaction, it also can evaluate patient satisfaction because it also is associated with patient satisfaction from KSS-2011, which directly asks for satisfaction. In addition, as the functional activities of KSS-2011 are divided into subcategories (“walking and standing,” “standard activities,” “advanced activities,” and “discretional activities”), our study indicated a significant correlation between FJS-12 and standard activities. Therefore, joint awareness was influenced by the difficulty level of routine activities, such as walking on an uneven surface, turning or pivoting, climbing up or down a stairs, rising from a low chair, and stepping to the side. FJS-12 is a simple questionnaire with only 12 items and has few ceiling and floor effects. Therefore, FJS-12 is a useful tool to evaluate residual symptoms, daily activity performance, and patient satisfaction after TKA.
Noise was expected to be correlated with joint awareness. Nevertheless, it had only a weak correlation with FJS-12 and KSS-2011. Although the patients with TKA are aware of a noise, some studies have revealed that it has minimal influence on patient satisfaction if the functional aspects, such as ROM, are good . The previous study was done with a TKA model using a unique tricondylar mechanism; therefore, the noise perception influence on PROMs for patients with conventional TKA models needed to be elucidated. In this study of TKA mechanisms, a remarkable difference was found between KSS-2011 and postoperative ROM, and the BCS results were good. A remarkable difference was confirmed regarding noise, and the BCS score was the lowest. Nam et al.  reported that noise was recognized in 27% of the patients and the likelihood of noise generation was different among the TKA mechanisms; PS design was the greatest, followed by rotating-platform, sex-specific, and CR factors. Furthermore, patient-recognized noise was reportedly associated with residual symptoms, such as difficulty in getting in and out of cars, limp, stiffness, or swelling. The inconsistency of the findings between the previous studies and our study can be caused by the difference in assessing noise frequency. The previous studies categorized subjects into two groups based on with or without noise, whereas our study used an ordinal scale. From our findings, the noise score had remarkable correlations both with FJS-12 and KSS-2011, but the correlation coefficients were small. It was suggested that even if noise was recognized, patient satisfaction postoperatively would be high if functions, such as the ROM, were good. Therefore, noise itself had a limited effect on joint function, overall satisfaction, and joint awareness after TKA. Nevertheless, it should be encouraged to find better implant design and surgical techniques to reduce the noise with high function after TKA.
This study had several limitations. First, there was a question with low response rate in FJS-12. As reported in previous studies [19, 20], the response rate for item 12 was particularly low. It may be reasonable to consider that some patients who underwent TKA were relatively inactive. Nevertheless, FJS-12 has been confirmed to be valid with a few missing items . In addition, the collection rate of scores was low (65.7%). The reasons for this are as follows: in the collection process of the questionnaires by mail, we found some patients with newly confirmed death, unresponsiveness owing to dementia, and serious disabilities other than knee disabilities. In addition, some patients relocated and returned invalid responses. The process of eliminating the invalid responses would improve the response reliability even though it would reduce the response rate. Furthermore, the effect of bias owing to the low responsive rate on the results is estimated to be small because it is a cross-sectional study that examines the correlation within one sample and the final number of respondents was over 200.Second, this was a retrospective cross-sectional study, and there were time variations in data collection in the answers of FJS-12. In fact, some studies have found that the FJS-12 scores change over time . The differences in questionnaire sampling time might have influenced the results. Nevertheless, we recruited patients for whom at least 1 year had passed postoperatively. Third, various TKA models had been used. Although the mechanisms can be categorized into four systems, several models remained within the same category. Furthermore, preoperative patient conditions were not matched among the groups. Therefore, this study did not conclude that BCS was superior in clinical outcomes than in any other TKA mechanisms. We did not attempt to investigate which mechanism had the best performance. The point of this analysis was to show that some TKA models exhibit greater noise generation while showing higher KSS-2011 score. Thus, this finding suggested that noise perception has a limited effect on clinical outcomes. Lastly, the noise score used in this study has not been validated previously. Because there is no score as a reference to evaluate the noise perception, the same questionnaire expression method as the FJS-12 was used to evaluate noise.