Three-dimensional kinematic change of hindfoot during full weightbearing in standing: an analysis using upright computed tomography and 3D-3D surface registration

The joint of the hindfoot, i.e., the ankle-joint complex (AJC), consists of the ankle joint and the subtalar joint. One of the major functions of the AJC is adjusting the lower limb alignment while weightbearing. It is difficult to assess the kinematic change of the AJC under the weightbearing condition using 2D radiography because of the complex bony shapes of the joint. Thus, the 2D-3D registration technique was developed to measure foot bone and AJC kinematics of patients [1, 2] or asymptomatic volunteers [3‐6]. Although the accuracy of the 2D-3D registration technique is below 1.0 mm and 1.0°, the number of steps is required to achieve accuracy. First, computed tomography (CT) scans are crucial to build the bone model. Second, 2D fluoroscopic images require calibration to adjust enlarged images projected from radiographs. Finally, a matching algorithm involves a number of mathematical calculations and optimization. Therefore, a method to analyze the kinematics of AJC without such time and cost is needed.

Many studies have analyzed the effect of weightbearing on the hindfoot using magnetic resonance imaging [1] or conventional CT with loading devices [7‐15] or upright cone beam CT [16‐23]. However, the effect of natural full weightbearing in a standing position has not been evaluated due to limitations of acquiring foot images under a physiological weightbearing condition using those imaging modalities. Major limitations of these modalities include low resolution and longer scanning time (MRI), prone position and non-physiological weightbearing (conventional CT), and motion artifact and partial weightbearing (cone beam CT). Approximately 1 to 2° of bony motion were observed in the AJC under weightbearing [10, 11, 15], while no study has reported the kinematic change in the AJC between non weightbearing and full weightbearing positions. Recently, we developed an upright CT with an area detector with Canon Medical Systems [24], in which a CT scan under full weightbearing in a natural standing condition can be acquired.

The present study aimed to analyze the effect of full weightbearing on hindfoot motion of asymptomatic feet using an upright CT with a 320-row multidetector and 3D-3D surface registration technique. We hypothesized that the upright CT and 3D-3D registration technique clearly reveals kinematic change in the AJC due to natural full weightbearing.

Image qualities of the 144 AJC scans of 24 subjects were good (diagnostic quality with minor artifacts) or excellent (diagnostic quality without any artifacts) [29] (Table 1). The intra- and interobserver correlation coefficients for the present study were 0.996 (95% confidence interval, 0.994–0.998) and 0.995 (95% confidence interval, 0.992–0.997). These data indicated that the present measurement was highly reliable.

Figure 3 shows the amount of change in each direction under each condition, and Fig. 4 summarizes the movement directions, with full weightbearing in one figure.

In the ankle joint, the talus plantarflexed (50%/100% weightbearing, 5.07 ± 4.52/6.77 ± 4.84 degrees), inverted (50%/100% weightbearing, 1.28 ± 1.37/2.01 ± 1.58 degrees), and internally rotated (50%/100% weightbearing, 2.40 ± 4.18/4.30 ± 4.64°) relative to the tibia as the weight load increased. Conversely, at the subtalar joint, the calcaneus dorsiflexed (50%/100% weightbearing, 2.76 ± 1.42/3.82 ± 1.68°), everted (50%/100% weightbearing, 5.29 ± 2.56/7.99 ± 3.55°), and externally rotated (50%/100% weightbearing, 2.96 ± 1.95/4.13 ± 2.43°) relative to the talus as the weight load increased (Figs. 3 and 4). Three-dimensional kinematics were opposite between the ankle joint and the subtalar joint on their respective axes, and each angle increased as the weight load increased. Regarding the absolute value, sagittal and axial plane movements were larger in the ankle joint, while the coronal plane movement was larger in the subtalar joint.

Our approach using the upright CT and 3D-3D registration technique clearly described the effect of full weightbearing in AJC kinematics, and the results support our hypothesis.

The bony motions in the AJC under weight load in the past studies [10, 11, 15] were lower than those in the present study, likely because of insufficient and unphysiological weightbearing (Table 2).

Our method to analyze AJC kinematics has several advantages over the methods using fluoroscopy or other imaging modalities (Table 3). First, 3D-3D registration on CT images requires fewer steps to match the bone and evaluate AJC kinematics and it is easier to match 3D to 3D than 2D to 3D models. Analysis of foot bone and AJC kinematics using fluoroscopy and the 2D-3D registration technique has been reported [1‐6]; however, its major limitation is the complex nature of the steps required to build and match the bones. The 2D images taken by fluoroscopic imaging are shadow pictures, and a 3D bone model based on CT images is required to accurately match the bones on the 2D images. Image calibration is also required to adjust enlarged images when using the X-ray system. Several matching algorithms have been developed, but the significant time and cost required to analyze the kinematics of the bones limit its use. The accuracy of the 3D-3D registration was below 0.2° in rotation [31]. Second, only minor motion artifacts were found in AJC images with upright CT in the present study (Table 1). Changes in hindfoot alignment have been assessed using upright cone beam CT [16‐23], but it takes as long as 20 to 48 s to acquire images, and it is necessary for participants to support the body to reduce artifacts. In fact, moderate to severe motion artifacts were observed in the cone beam CT images of the knee and ankle [27]. In addition, participants must put their foot in a small tube of the cone beam CT, and thus the participants must set their contralateral foot somewhere aside from the tube or stabilize their body using supportive tools such as a pole. This position is not a natural standing position, and only partial weight is loaded on the foot. Third, physiological weightbearing while standing can be acquired in the upright CT, while simulated weight with loading devices was applied in the studies using conventional CT [7‐15]. In those studies, the hip, shoulder, or knee must be fixed to reproduce the hypothetical loading conditions, and the lower limb muscles used to maintain the standing position was not active in the prone position. Those limit the representation of physiological loading and tarsal bone alignment while standing.

Several limitations of the present study should be noted. First, there were no patient data, and only asymptomatic subjects were included. However, our method using an upright CT and 3D-3D registration technique can be a powerful tool to investigate kinematic change in the AJC of the patients. The clinical relevance of the hindfoot motion during natural full weightbearing should be studied in the near future. Second, the imaging was divided into three categories, i.e., no weightbearing, 50% weightbearing, and full weightbearing, and static imaging was performed. Although continuous imaging in 4D was possible using an upright CT with 320-row multidetector, the image quality of 4D CT was insufficient to capture the tarsal bones; thus, we separately scanned the three loading conditions. To analyze the continuous dynamics of the hindfoot, we need to increase the observation points under different weightbearing conditions in a future study.

An upright CT and 3D-3D registration technique clearly described the kinematics of the AJC in a static full weightbearing condition. Our findings demonstrated that 3D motions were opposite between the ankle and subtalar joints on their respective axes.