## 4 Discussion
Most research on isometric points has been conducted to determine whether some of the points preset with a limited number (e.g., using grids) were isometric during cadaveric articular movements [17; 18; 19; 20]. Instead of pre-setting certain points before the articular movements, our method discovered the least-moving point from CTs already taken in different positions in a retrospective manner. With our method, an IP could be found more accurately than by conventional methods, as we could set a myriad of points. In addition, our study made in vivo measurements that had more clinical relevance than cadaveric studies. We expect that our method can be effectively applied to investigate IPs in other joints such as knees or ankles. Figure 6: Typical examples of smaller TUH and forearm rotation (**A**) and greater TUH and forearm rotation (**B**). The red area shows the points with \(\mathrm{D_{FR}}\) values in the lowest 10%. (**A**) When the TUH or forearm rotation was smaller, the RC was located closer to the center of the ulnar head. (**B**) When the TUH or forearm rotation was greater, the RC was located closer to the center of the ulnar styloid process. Note that the fovea ulnaris is not isometric in this case. Our study had limitations. First, our data were from patients with ulnar-sided wrist pain and not from normal, painless subjects. Although no patient in our study had instability in the DRUJ relative to the painless other side, a study enrolling normal, painless wrists is necessary to establish that our conclusions apply to normal biomechanics. Second, there was no control group and the number of cases was small. However, 21 cases were enough to identify significant correlations between the parameters in our study. Third, the thickness of the cartilage was not taken into consideration and the soft tissue was not evaluated, as the current study was CT-based. Fourth, the amount of forearm rotation and the position of the wrist and elbow were not uniformly controlled and the effect of active muscle contraction and the shape of the sigmoid notch was not analyzed. Future research including dynamic evaluations with controlled subjects may be helpful to strengthen our conclusions. In conclusion, the isometricity of the foveal insertion of the TFCC during forearm rotation may not be consistent in vivo. The foveal insertion of the TFCC may have physiologic laxity in certain forearm rotations. The center of rotation on the ulnar head during forearm rotation appears to shift ulnary with increasing amounts of TUH or forearm rotation. A tight foveal repair of the TFCC might not ensure DRUJ stability in all forearm rotations, and the ulnar styloid insertion of the TFCC might be more important for DRUJ stability at higher DRUJ translations or higher forearm rotations in vivo. Conceptualization, S.-H.S. and D.-S.K. ; Methodology, D.-S.K. ; Validation, S.-H.S. ; Formal analysis, J.P.; Investigation, J.P.; Writing--original draft preparation, S.-H.S. and D.-S.K. ; Writing--review and editing, J.P. and Y.-G.C. All authors have read and agreed to the published version of the manuscript. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2019 RI1 GA1100754) and the Industry-Academy Cooperation Program through the Industry Academic Cooperation Foundation of the Catholic University of Korea (5-2021-D0354-00001). The study was conducted in accordance with the Declaration of Helsinki and approved by the Institutional Review Board of The Catholic University of Korea, Seoul St. Mary's Hospital (KC19 RCSl0384, 12 September 2020). The data presented in this study are available on request from the corresponding author. The authors declare no conflict of interest.