OBJECTIVETo establish a three-dimensional finite element model of the lower limb bones, and investigate the changes of the contact characteristics of the subtalar joint after using laterally wedge insole intervention.

METHODSUsing the reverse modeling technology, the lower limb bones of normal adult volunteers was scanned by CT. Mimics 10.0 and Geomagic Studio 6.0 software were used to reconstruct the 3D morphology of bones and external soft tissue of the feet. The laterally wedge insole was designed in ProE 5.0. And then all the models were imported into Hyperwork 10.0 and meshed, and given the material properties. The finite element analysis was carried out in ABAQUS 6.9.

RESULTSA three-dimensional finite element model of the lower extremity was established, which was consisted of 95 365 nodes and 246 238 elements. The contact area of the standing state of the lower joint was larger than that of the anterior middle joint surface. The peak stress was concentrated in the anterior lateral part of the posterior articular surface, and the average stress value was(3.85±1.03) MPa. Compared with the model of 0°, the contact area of the subtalar joint was reduced accordingly. There was a significant correlation between anterior middle joint ||=0.964,=0.008, and posterior articular ||=0.978,=0.002. The equivalent stress of 0° model distributed from(3.07±1.14) MPa to(3.85± 1.03) MPa, which had no statistically difference. Compared with the 0° model, the equivalent stress of the anterior and middle joint surfaces of the 8° model was significantly reduced(<0.05), but the peak stress of the posterior articular surface was significantly increased(<0.05). In the 12° model, the peak stress was sharply increased to(10.51±3.53) MPa. Compared with 8° model, there was no statistically difference(<0.05). Although the peak stress was slightly increased in 16° model, but compared with 12° model, there was no statistically differences(>0.05).

CONCLUSIONSAlthough a certain valgus can be obtained in subtalar by wearing LWI, the result comes at the cost of the stress concentration on posterior surface. Through this study, we can find that LWI with 8° tilt angle could provide appropriate valgus moment without causing excessive concentration. Therefore, in order to avoid secondary ankle complications, we should not increase the tilt angle blindly.