Abstract
Purpose
In the literature, the hip is near-ubiquitously described as a mechanical ball-and-socket joint. This implies purely rotational motion as well as sphere-on-sphere contact geometry. However, previous works, by several authors, have quantitatively demonstrated asphericity of the articular hip surfaces in a variety of populations. This in turn implies the true kinematics of the hip joint may be more complex than purely rotational motion.
Previously, general ellipsoidal shapes have been used to model the articular surface of the acetabulii of dysplastic hips. This work aims to orient the major axis of these ellipsoids with respect to the anterior pelvic plane (APP).
Method
The source data for this study were CT segmentations done in routine preparation for computer-assisted periacetabular osteotomy (PAO) procedures. Seventeen patients, aged 3510 years, were included in this study. Segmentations were performed manually by skilled technicians using Mimics (Materialize, Belgium) and saved as triangulated surface meshes. These segmentations were manually processed using Magics (Materialize, Belgium) to isolate the acetabulum, removing any non-articular features such as the acetabular ridge and notch, as well as any segmentation artefacts. The vertices of this processed mesh were extracted, and fit to general ellipsoids using Markovskys Adjusted Least Squares (ALS) algorithm. The APP was defined by the left and right anterior superior iliac spines (ASIS) and the midpoint of the pubic tubercles, with the ASIS forming the mediolateral axis. Landmarks were manually chosen mesh vertices, chosen from the approximate centre of the anatomical landmark.
Orthogonal projections of the primary axis of the ellipsoid of best fit were examined in the APP and the two perpendicular planes (pseudo-axial and sagittal).
Results
Two distinct groups of fit orientation were observed. 12 hips had the major axis directed approximately towards the cup interior (group A); the other five had the major axis directed across the cup edge (group B). Group A axes were directed 68 degrees superiorly of the medial direction in the APP; 718 degrees posteriorly of the medial direction in the pseudo-axial plane, and 6027 degrees posteriorly of the superior direction in the sagittal plane. Group B axes were directed 4412 degrees inferiorly of the medial direction in the APP; 1421 degree anteriorly of the medial direction in the pseudo-axial plane, and 7120 degrees posteriorly of the superior direction in the sagittal plane.
Conclusion
In patients with hip dysplasia, ellipsoid fits of the acetabulum suggest two subgroups in orientation of the major axis of the ellipsoid: those axes that tend to be directed postero-medially (towards the center of the acetabulum) and those that tend to be directed infero-anteriorly (across the rim of the acetabulum). Information about the morphology of the acetabulum may have implications to diagnosis, treatment, and implant design for the hip.
