In this study, the heel alignment state determined by subtalar compensation or ankle OA stage did not affect spatiotemporal gait parameters. The sagittal range of hindfoot motion decreased at the advanced ankle OA stage. Coronal hindfoot motion was disrupted through all gait phases regardless of the presence or absence of subtalar compensation. The compensated coronal position of the hindfoot seemed to be maintained throughout the gait cycle.
The spatiotemporal parameters were not affected by the alignment state of the heel resulting from subtalar compensation. Meanwhile, a significant difference in the step width by ankle OA stage was noted in this study. However, this could have been caused by the significant discrepancy in the sex ratio, which could affect the foot and ankle function or structure [18, 19]. We hypothesized that ankle symptoms such as pain degree might not affect their spatiotemporal parameters because all patients included in this study had suffered sufficient long-standing and substantial ankle pain to undergo surgical treatment regardless of ankle OA stage determined on simple radiographs.
Disrupted coronal movement of the subtalar joint had no correlation with ankle OA stage or hindfoot compensation state. Before this study, we expected that the reduced coronal movement of the subtalar joint is naturally led by the reduced sagittal movement of the pathologic ankle joint. However, we still observed limited supination at the subtalar joint in patients with a markedly conserved sagittal motion of the ankle joint. We expected that the ankle with neutral heel alignment maintained by subtalar compensation had more residual supination reservoirs in several phases such as the preswing phase compared with the ankle with a varus heel led by subtalar decompensation. However, we obtained the opposite result. Once the subtalar motion was disrupted at advanced ankle OA, its biomechanics did not seem to change according to the presence of subtalar compensation or ankle OA stage. Further studies are needed to determine whether surgical treatment, such as total ankle arthroplasty or ankle arthrodesis, could restore the disrupted coronal motion of the subtalar joint in patients with ankle OA.
Decreased sagittal range of motion in stages 3b and 4 compared with stage 3a implied that advanced ankle OA affects the sagittal range of hindfoot motion, although the range of sagittal motion in all three stages seemed to be mostly conserved compared with the ranges of coronal or transverse motion. Takakura stage 3a refers to the obliteration of the medial gutter but the cartilage remaining on the whole talar dome. Stages 3b and 4 commonly have bone contact on the talar dome to greater or lesser degrees. This might have led to the difference in the outcome, although studies with more populations are needed to draw definite conclusions.
We mainly focused on hindfoot rather than midfoot or forefoot motion in this study regarding the analysis using an MFM for two reasons. First, we excluded patients who had a simultaneous pathology of foot joints other than the ankle joint, which was our concern. Second, we believed that the ankle pathology would primarily affect hindfoot motion, while midfoot and forefoot motions would merely show adjustable motion according to abnormal hindfoot movement. To our knowledge, this is the first study to report on hindfoot motion using MFM analysis according to the presence of subtalar compensation and ankle OA stage.
Our study had several limitations that we could not address. First, because this was a retrospective analysis, we could not evenly assign the number of patients to each group or control sex or laterality in the comparison of the Takakura groups. However, there were no statistically significant differences in the groups sizes. Moreover, the effect of sex or laterality on gait pattern remains debatable. Therefore, we believe that the effects of sex and laterality on this outcome may be limited. Second, we could not separate subtalar motion from whole hindfoot movement in the analysis because we had no available marker set enabling such separation and complete validation. However, we believe that this may not have significantly affected the study outcomes because the coronal movement of the ankle joint is limited compared with that of the subtalar joint. Moreover, ankles with advanced OA are often stiff due to soft-tissue contracture or impingement from the osteophytes of the ankle mortise. Finally, we did not include all patients with ankle OA because there were a few cases of early ankle OA in our database, and we excluded several ankles that could not be classified using the Takakura system. Therefore, we might not draw definite conclusions from the study outcomes.