Ellipse Fitting for Planar Hip Morphology and Kinematics
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Although the human hip is commonly thought to be a ball-and-socket joint, recent 3D studies suggest that the hip translates as well as rotates. This is of interest not only for biomechanics but also clinically, because hip translation may be important to physiotherapy and to computer-assisted surgery for total hip replacement. Previous 3D studies have evaluated hip kinematics as well as morphology by CT, MRI and surgical intervention. A minimally invasive, inexpensive, and accurate way of measuring hip shape and motion may be useful in basic science and clinical application. This work used prototype software to quantify planar hip morphology and kinematics from plain 2D radiographs. Ellipses were fit to the articular contours of the femoral head and acetabulum of plain 2D radiographs of arthritic and dysplastic patients. The prototype software was validated in a study performed by three board-certified orthopedic surgeons. It was found to be efficient and reliable, taking less than one minute to quantify planar hip morphology and having no statistical difference between the observers. The prototype software was used in a clinical study to quantify planar hip kinematics. Preoperative AP pelvic radiographs were taken of 11 THA patients in 4 different positions. The semi-automated fits detected a mean femoral head translation of 3.42mm, with no discernible directional pattern. Every one of these arthritic patients exhibited substantial planar hip translation. The findings are consistent with the hypothesis that hip morphology and kinematics can be quantified from plain 2D radiographs.