The anteroposterior axis of the tibia is adjusted to approximately a right angle to the anterior pelvic plane in the standing position in patients with hip dysplasia similar to normal subjects: a cross-sectional study

For this study, 77 Japanese women were enrolled because patients with DDH are reported to have a 9:1 female dominance [19]. Twenty-nine patients (29 legs) with bilateral DDH (mean age, 35.8 ± 8.8 years) from our institution who had undergone curved periacetabular osteotomy [20] for the early treatment of osteoarthritis of the hip joint due to acetabular dysplasia and whose center-edge angle of the hip joint was < 25°, as evaluated in the anteroposterior view on a plain radiograph of the hip, were enrolled in the DDH group. Patients with DDH who had previously undergone hip surgery or those with Crowe stages 2–4 of subluxation or Tönnis grades 2 and 3 of arthritic change according to plain radiographs of the hip bilaterally were excluded from this study. We also included 48 women (48 legs; mean age of women, 54.0 ± 10.8 years) without lumbago and knee pain and without any abnormal findings of the knee and spine on radiographic examination recruited from the family of outpatients and medical staff in the normal group. Computed tomography scans from all participants were examined to reconstruct a 3D bone model. With regard to the DDH group, computed tomography scans were examined before their operation to plan for osteotomy. This study was performed with the approval of the institutional research board of Niigata University Medical and Dental Hospital and written informed consent was obtained from the participants of the normal group. With regard to DDH group, the need for informed consent was waived because of the cross-sectional nature of this study that did not provide an intervention.

Radiographic examinations, including biplanar computed radiography images, were performed in the standing position, where each subject adopted a relaxed position with their knees fully extended and the toes aligned to the shoulders. Computed tomography from the pelvis to proximal tibia was also performed in the supine position with the knees fully extended.

We used ZedHip® software (Lexi, Tokyo, Japan) to create three-dimensional (3D) digital bone models of the pelvis and femur and accurately reconstruct the spatial relationship between them [15, 21, 22]. We adjusted the 3D pelvis model to the APP [23], which contains both the anterior superior iliac spines and the pubic symphysis, which are the origin of this pelvis coordinate system. In the ZedHip® system, when the pelvis was adjusted to the APP, other bones such as the femur and tibia synchronously moved according to the pelvis position. With regard to the femoral coordinate system, the 3D model of the femur was positioned with the retrocondylar plane, which contains the bilateral posterior condyles and the most posterior point of the greater trochanter [24]. The femoral neck axis was defined as that in the method described by Sugano et al. [25] and measured in the plane just below the femoral head. Femoral neck anteversion (FNA) was measured as the angle between the femoral neck axis as above and the posterior condylar axis (PCA) (Fig. 1). Further, the CEA was defined as the line connecting the most prominent point of the medial epicondyle and the lateral epicondylar prominence. The condylar twist angle (CTA) was measured as the angle connecting the CEA and PCA (positive values indicate that the CEA is externally rotated relative to the PCA) (Fig. 2) [18]. The determination of the CEA and measurement of the CTA were also performed in the retrocondylar plane. The line through the midpoint of the lateral epicondylar prominence and the most prominent point of the medial epicondyle, and the line perpendicular to the CEA was defined as the femoral AP axis. With regard to the AP axis of the tibia, Akagi’s line [14] was selected. The knee rotation angle (KRA) was measured as the angle connecting the femoral AP axis and the AP axis of the tibia, projected onto the horizontal plane of the femoral coordinate system (Fig. 2). In the present study, negative values were defined as the internal rotation of the tibia relative to the femur and positive values as the external rotation.

FNA, CEA, and PCA relative to the APP were also measured in the standing position (APP-FNA, APP-CEA, and APP-PCA, respectively) using HipCAS® software (Lexi, Tokyo, Japan). The 3D digital bone models were projected onto the biplanar computed radiography images to match the contours of the 3D digital models with the computed radiography images for rotations and translations [15, 21, 22]. Kobayashi et al. [22] previously described the accuracy of HipCAS® in creating a 3D digital bone model that accurately reproduced the spatial relationship between the pelvis and the femur, and calculated the various alignment parameters within 1° and 1 mm of accuracy. Therefore, projection error and misalignment were estimated to be small in the current study. APP-FNA, APP-CEA, and APP-PCA were the angles that connected the FNA, CEA, and PCA projected onto the transverse plane of the pelvis and the APP was the line connecting both anterior superior iliac spines (Fig. 3).

Lastly, we calculated the estimated angle between the APP and AP axis of the tibia from the KRA in femoral coordination and APP-CEA with the formula: (AP axis of the tibia relative to the APP transverse axis) = (APP-CEA) − (KRA, the angle between the AP axis of the tibia and the line perpendicular to the CEA), as in our previous study [18] (Fig. 4).