## 4 Discussion  
Our results can be summarized into two major findings. First, a truly isometric point was found on the ulnar head in limited cases and the location of the RC varied during forearm rotation in vivo. Second, the RC shifted ulnary (toward the ulnar styloid process) as the amount of TUH or the amount of forearm rotation increased. There has been a biomechanical report that the foveal insertion of the TFCC is nearly isometric during forearm rotation [4]. It highlighted the importance of the foveal insertion of the TFCC for DRUJ stability. The average location of the RC on the ulnar head in our study (2.4 mm ulnary and 0.7 mm volarly from the center of the ulnar head) was consistent with the location of the center of the fovea previously reported (2.4 mm ulnary from the center of the ulnar head) [13], and this supported the importance of the foveal insertion of the TFCC. However, the fact that a true IP was found in only three out of 21 cases and the location of the RC varied (from \(-\)0.4 to 5.2 mm from the center of the ulnar head in the radioulnar direction) rather than being constant suggests that the isometricity of the foveal insertion  
Figure 4: The location of the RC in the radioulnar direction and the amount of translation of the ulnar head (TUH). (**A**) The location of the RC in the radioulnar direction was significantly correlated with the amount of TUH ([PERSON]’s correlation coefficient = 0.775, \(p\) < 0.001). (**B**) The color-gradient diagram shows that the RC was located more ulnary when the TUH was greater. Figure 5: The location of the RC in the radioulnar direction and the amount of forearm rotation. (**A**) The position of the RC in the radioulnar direction was significantly correlated with the amount of forearm rotation ([PERSON]’s correlation coefficient = 0.454, \(p\) = 0.039). (**B**) The color-gradient diagram shows that the RC was located more ulnary when the amount of forearm rotation was greater. of the TFCC may not be consistent in vivo. Moreover, the point with the lowest 1% \(\mathrm{D_{FR}}\) showed an average movement distance of as great as 2.5 mm during forearm rotation. The lowest 5% \(\mathrm{D_{FR}}\) point moved an average of 5.0 mm, and the lowest 10% \(\mathrm{D_{FR}}\) point moved an average of 6.8 mm. Considering that the footprint of the foveal insertion of the TFCC occupies more than 5-10% of the distal ulnar head [13; 14], the foveal insertion of the TFCC may have physiologic laxity in certain forearm rotations. Figure 6 shows that sometimes the foveal insertion of the TFCC cannot be isometric in vivo. Such a lack of isometricity may result in either insufficient stabilization or overconstraint after TFCC repairs, and this may explain why sometimes there are disappointing outcomes including recurrent instability after TFCC repairs [15]. The other major finding of our study was the shift in the RC toward the ulnar styloid process with increasing amounts of TUH or forearm rotation. This suggests that the forearm rotation in the wrist is not a pure circular motion of the radius around a certain point on the ulnar head, and also suggests that the ulnar styloid insertion of the TFCC might be more important for DRUJ stability at higher DRUJ translations or higher forearm rotations in vivo. Figure 6 shows a typical example of an ulnarly shifted RC with greater amounts of TUH or forearm rotation. Overall, our results suggest that a foveal repair of the TFCC might not ensure DRUJ stability in all forearm rotations, even if the repair is tight enough to be stable on the \"hook\" test, a lifting test using a probe [16]. Factors other than the foveal insertion of the TFCC that may affect DRUJ stability should also be meticulously evaluated. In addition to the above major findings, our novel method of overlaying 3D reconstructions with surface mapping has the advantage that we could precisely measure the movement of the ulna with respect to the radius in vivo.