The foot and ankle have high tissue density and are highly affected by the motion artifacts of the soft tissues, measured using marker-based measurements. Thus, kinematic analysis of the foot and ankle is difficult because of the lack of accuracy of small kinematics. Consequently, ankle, subtalar, and talonavicular joint kinematics have been quantified as a whole, rather than individually [1]. Marker-based measurements of foot and ankle kinematics during gait are believed to include distance errors of 2.7–14.9 mm [2] and angle errors of >10° [3].

To overcome such limitations, a pipeline using two-dimensional (2D)‒three-dimensional (3D) registration between biplane X-ray video and 3D images [computed tomography (CT) and magnetic resonance imaging] has recently become a standard method for the analysis of foot and ankle kinematics [[4], [5], [6], [7], [8], [9], [10]]. However, they require laborious manual operations, which are complex and require expert knowledge, making its routine clinical application challenging. Furthermore, given the complexity of the process, simultaneous evaluation of multiple joints was painstaking. Moreover, the variations in evaluation protocols have made it difficult to compare the kinematics results across studies, which was also suggested previously [11]. Accordingly, we proposed a fully automated pipeline, “4D-Foot” [12] using biplane X-ray videos and CT images. From these, convoluted neural networks (CNNs) perform bone segmentation and landmark extraction and quantify the kinematics of each bone with six-degree-of-freedom parameters (three translation and three rotation parameters) using the intensity-based 2D‒3D registration. This enables us to quantify ankle, subtalar, and talonavicular joint kinematics with a registration error of 0.38 ± 1.95 mm in translation and 0.38 ± 1.20° in rotation [12], making it possible to evaluate and compare multiple joints automatically and simultaneously.

To date, several studies have analyzed foot kinematics with and without orthosis using biplanes [5,8,13]. Arch support is primarily used to compensate for the effects on the medial longitudinal arch and is believed to redistribute the foot pressure, causing beneficial changes in the ankle kinematics [14,15]. A previous work demonstrated that the shape of the medial longitudinal arch affects subtalar joint and ankle kinematics [16], suggesting that the medial longitudinal arch is a critical component of foot and ankle kinematics. Furthermore, as the medial longitudinal arch decreases, foot protonation occurs, and the height of the navicular tubercle decreases [13] (i.e., it affects the talonavicular joint). However, different types of foot orthoses were employed in these studies, and it remains unclear whether arch support affects foot and ankle kinematics. Additionally, to our knowledge, no studies have reported simultaneous evaluation and comparison of the dynamics of the ankle, subtalar, and talonavicular joints exist. Instead, these studies individually evaluated the ankle and subtalar joint kinematics [5] or navicular joint kinematics [13].

This study aimed to evaluate the effects of arch support on ankle, subtalar, and talonavicular joint kinematics using a fully automated analysis system, “4D-Foot.” We hypothesized that the arch support may inhibit the dynamics of each joint and that a strong connection exists between the dynamics of the talonavicular and subtalar joints.