2.1

This retrospective cohort study was approved by our Institutional Review Board. The requirement for informed consent was waived owing to the retrospective nature of this study, as all analyses were conducted using anonymized, previously obtained radiographic data. This study was conducted in accordance with the principles of the Declaration of Helsinki.

We reviewed the records of 100 consecutive patients with end-stage ankle arthritis who underwent ankle arthrodesis at our institute between July 2020 and December 2023. These patients were classified as Takakura stages 3b and 4 and showed no significant improvement despite conservative treatment for at least 6 months.

Starting July 2020, our institution began routinely performing conventional whole-leg standing radiographs and H-C views for patients undergoing ankle arthrodesis, recognizing the clinical importance of comprehensive lower limb and hindfoot alignment assessments. However, these images were not originally taken for research purposes or specifically intended for comparative analysis. During retrospective data collection, we found that among the 100 patients initially reviewed, only 67 had taken both imaging modalities. From 2023 onwards, our institution gradually shifted clinical practice toward predominantly using the H-C view due to its sufficient clinical utility, resulting in fewer conventional whole-leg standing radiographs subsequently performed. This shift in clinical practice explains why not all patients received both views.

After excluding six patients with a history of hindfoot surgery (five who underwent revision ankle arthrodesis due to nonunion and one who underwent medial displacement calcaneal osteotomy), 61 patients were included in the analysis ( Fig. 1 ). Patients with nonunion were excluded because the joint line was indistinct on radiographic evaluation, complicating accurate parameter measurements. Additionally, the patient who underwent medial displacement calcaneal osteotomy was excluded as the endpoint of the calcaneus was altered, making it inconsistent with the reference used for alignment assessment.

Flow diagram of the study design.

2.2

The H-C view was obtained following the method previously described by Haraguchi et al. . Specifically, patients stood on a radiolucent board in a comfortable bipedal stance, with both patellae facing forward and ankles in a neutral position ( Fig. 2 ). Radiographs were taken in the posteroanterior direction, with the cassette positioned and extended below the level of the foot. In contrast, conventional whole-leg standing radiography was performed without the use of a radiolucent board. Patients maintained a similar bipedal stance, with both patellae directed forward and ankles positioned neutrally, and radiographs were taken in anteroposterior direction. Both imaging methods utilized a consistent distance of 2 m between the tube and the cassette, with imaging parameters set at a tube voltage of 85 kV and a current of 200 mA.

Technique and patient positioning for taking the hip-to-calcaneus view.

Radiographic measurements were obtained by using the Picture Archiving and Communication System. To assess intraobserver reliability, a single author (a board-certified foot and ankle surgeon with 5 years of clinical experience) repeated radiographic measurements at 1-month intervals. Interobserver reliability was determined by independent measurements performed by two authors (both board-certified foot and ankle surgeons, with clinical experience of 5 and 4 years, respectively).

In the conventional whole-leg standing radiograph and in the H-C view, the hip-knee-ankle (HKA) angle, percentage of the weight-bearing line, talar tilt, and medial distal tibial angle were measured, as previously described ^,, . The percentage of the weight-bearing line was defined as the ratio of the distance from the medial tibial plateau to the point of intersection in the knee joint, where the line connecting the centers of the hip and ankle was drawn, divided by the width of the tibial plateau. The correlation between the values obtained using the two imaging methods was calculated.

In the H-C view, the H-C axis was drawn to connect the center of the hip to the end point of the calcaneus ( Fig. 3 A). The percentage of the H-C axis crossing the talus was then calculated and was defined as the ratio of the distance from the medial talar dome to the point of intersection in the talar dome divided by the width of the talar dome ( Fig. 3 B). The asymmetry (i.e., hindfoot malalignment) was subsequently assessed. We defined percentages between 25 % and 75 % as “symmetric” and percentages below 25 % or above 75 % as “asymmetric”. A similar method of categorizing alignment into three groups has previously been reported by Spierenburg et al. , who divided the groups equally into thirds (each representing 33 %). However, to minimize ambiguity owing to measurement errors and eliminate the gray zone between symmetric and asymmetric classifications, we defined our alignment categories more distinctly using cutoff values of 25 % and 75 %. In cases of asymmetry, we evaluated the correlation between the TCA, talar tilt, and HKA angle. The TCA was defined as the angle between the tibial and calcaneal axes, as previously described ( Fig. 3 C). The tibial axis was determined by a line connecting two midpoints of the tibial shaft located 8 cm and 13 cm proximal to the tip of the medial malleolus ^, . The calcaneal axis was defined as a line connecting a point located 40 % from the lateral side on a horizontal line 7 mm above the most distal point of the calcaneus, and the midpoint on a horizontal line 30 mm above the most distal aspect of the calcaneus ^, .

Hip-to-calcaneus (H-C) view: (A) the H-C axis is drawn from the hip center to the end point of the calcaneus, (B) the percentage of the H-C axis crossing the talus is defined as the ratio of (a) the distance from the medial talar dome to the point where the H-C axis intersects the talus, divided by (b) the width of the talar dome, (C) measurement method for tibiocalcaneal angle.

2.3

SPSS Statistics 28 (IBM, Armonk, New York, USA) was used for the statistical analyses. The Pearson correlation coefficient ( r ) was calculated to assess the correlation of radiographic parameters between the two views, as well as the correlation with the percentage of the H-C axis crossing the talus and various angles in the H-C view. Student’s t -test was used to compare the means of the independent variables. Intraobserver and interobserver reliabilities were evaluated by using intraclass correlation coefficients. Statistical significance was defined as a P -value less than 0.05.

2.4

A post hoc power analysis using Power Analysis & Sample Size 16.0.12 (NCSS, Kaysville, Utah, USA) was conducted to assess the statistical power of the paired t -test comparing talar tilt between the two views. A sample size of 61 achieves 93 % power to detect a mean of paired differences of −2.3 with an estimated standard deviation of differences of 5.1 and with a significance level (alpha) of 0.05 using a two-sided paired t -test.