## 4 Discussion
In this study, the participants reported high scores for the user experience-related items of device helpfulness (mean 8.5 over10) and satisfaction with app design (mean 7.7 over 10), indicating high user satisfaction for the assistance of MSD in rehabilitation and the design of user interface. We believed that the Doctor and Patient app used in the MSD offered an informative and interactive interface, which is the key factor contributing to the excellent outcomes of the MSD-assisted rehabilitation and high user satisfaction in this feasibility study. However, the score for ease of operability reported by the participants was relatively low (mean 5.0 over 10), which may indicate the potential difficulty of using the MSD and, therefore, reflect on the ordinary score for the user's interest in future use (mean 5.8 over 10). In addition, the user experience feedback indicated that the complexity of the MSD design with a relatively steep learning curve could be further improved. For elderly users, simplifying the operability for the MSD will be a development target in the future. Overall, the positive results obtained justify further work on the application of the MSD in telerehabilitation following joint replacement. The telemedicine industry in the orthopedic field is poised for growth in response to the 2019 coronavirus (COVID-19) pandemic [35]. An example of a successful telemedicine model is the application of virtual exercise rehabilitation in-home therapy, which can provide similar effects as traditional home or clinic physiotherapy and can significantly decrease 3-month healthcare costs after TKA [36]. In contrast to virtual physiotherapy, the MSD-assisted rehabilitation model presented in this study supports image-based and sensor-based telerehabilitation using an activity recognition model, an interactive 3D avatar in mobile phone applications, and a wireless telecommunicated network which enables more comprehensive physiotherapy for patients following TKA. The main limitation of this study is the small sample size, which prevents this study from robust conclusions on the effectiveness of MSD-assisted home-based rehabilitation for patients following total knee replacement. However, the value of this study was to offer novel but preliminary results on the feasibility of the use of MSD in implications for post-TKA rehabilitation. We, therefore, recorded and presented the user experience from the participants in the MAR group, anticipating that this experience may facilitate the future development of MSD in telerehabilitation, especially in the post-TKA telerehabilitation scenario. Second, owing to the large national differences in post-TKA rehabilitation protocol, the protocol used in this trial, including the exercise contents, the different study time points, and the short follow-up period, may create potential bias resulting in difficulty in making conclusive statements. Third, we only recorded the postoperative pain score during follow-ups without documenting the analgesic use, which may mask the influence of pain on patient adherence to the training program. Fourth, the allocation of patients was not randomized because of their varying levels of familiarity with mobile apps and MSDs. Older adult patients may not favor new rehabilitation technology [37]. The gap in the acceptance of new technology among elderly patients may have caused a potential selection bias. The patients who accepted MSD-assisted rehabilitation and who were allocated to the MAR group were relatively highly educated, although no significant difference was noted. Furthermore, these patients may have had stronger motivation for rehabilitation training. Future studies should consider a training program for MSD usage before enrollment to prevent any potential selection bias. Finally, the four item questionnaire used in measurement of the user experience is not a standardized instrument, so that the questionnaire may carry the limitation of unidentified psychometric properties.