## 4 Discussion  
The osteolysis patterns in this study were similar to those reported previously for third-generation TAA systems [15; 16; 33; 34]. Osteolysis was primarily confined to peri-prosthetic regions, such as the talus body, rather than the talus neck. However, previous studies lacked detailed quantitative data. Osteolysis caused by stress-shielding may develop within periprosthetic bone regions with a small load due to the difference in material properties between bone and the metallic implant [38]. The fixation configurations, and the stress flow and distribution, of TAA systems are modifiable, and may not currently be optimal in certain regions of the distal tibia and talus [19; 26]. [PERSON] et al. found that osteolysis can develop if the stress flow and distribution in bone are suboptimal [18]. Low strain after TAA within periprosthetic bone, caused by changes in stress flow and distribution, may promote osteolysis [18]. The contribution of changes in stress flow and distribution to osteolysis (caused by stress-shielding) must be analyzed, because it may be possible to reduce the risk of osteolysis by inducing the appropriate stress within the distal tibia and talus. This study is the first to perform a 3D analysis of osteolysis characteristics in an attempt to improve TAA design. Extensive osteolysis was generally confined to regions near the talar pegs. These regions (Talar VOIs 4-6) exhibited 24.2 \(\pm\) 4.5% more osteolysis than other talar regions (Talar VOIs 1-3 and 7-9). [PERSON] et al. radiographically evaluated several TAAs, including the SALTO (Tornier SA, Saint Ismier, France), HINTEGRA, AES (Biomet, Warsaw, IN, USA), COPELIA (unknown manufacturer, France), STAR (Waldemar Link, Hamburg, Germany), RAMSES (Laboratoire Fournitures Hospitalieres Industrie, Heimsbrunn, France), and AKILE (Lavender Medical Limited, Stevenage, UK) [36]. Bone cysts were more frequent in patients treated with the SALTO and AES, which have larger tibial stems. Most bone cysts developed at the bone-prosthesis interfaces and periphery of fixation components. [PERSON] et al. reported that osteolysis was particularly common near the tibial heel of the SALTO prosthesis [16]. TAA fixation components, including the talar pegs and tibial stems, change stress flow and distribution, which can lead to osteolysis [39; 40]. [PERSON] et al. reported high stress between the periphery of fixation components and bone, and low stress (\(-\)0-1.1 MPa) at bone-prosthesis interfaces [40] caused by changes in stress flow mediated by stress-shielding, in turn induced by differences between the bone and prosthesis. The stress induced by these differences is lower than the critical stress (0-1.46 MPa) in the periphery of fixation components. According to bone remodeling theory, there is a high risk of extensive osteolysis if the stress is below a critical stress [32; 41]. However, stress flow and distribution can be altered by modifying the TAA fixation components to reduce osteolysis around the talar pegs. Thus, the osteolysis characteristics of the ankle joint after TAA may reflect the location and shape of TAA fixation components, which thus require modification. We found that the extent of osteolysis within the peri-prosthetic region (C_VOIs) was greater than within other regions. Particularly, in the talus, osteolysis within the posterior region (VOIs 7-9) was 9.9 \(\pm\) 4.4% more extensive than within the anterior region (VOIs 1-3), and osteolysis within the lateral region (VOIs 3, 6, and 9) was 9.7 \(\pm\) 4.6% more extensive than within the medial region (VOIs 1, 4, and 7). Our results differed from those of previous studies reporting that osteolysis was more common in the anterior of the talus [29; 35]. This may reflect the fact that screws were often placed in earlier studies, but infrequently in the current study. Insufficient stress was transferred to peripheral regions because the screws contributed to stress-shielding.