Pelvic and Femoral Coordinates and Implant Alignment Representations in THA

Abstract

For three-dimensional study of total hip arthroplasty, including preoperative planning and biomechanical studies, it is important to indicate the systems used to define the cup and stem alignment and the pelvic and femoral coordinate systems that are used. Cup alignment may be represented radiographically, operatively, or anatomically. Stem alignment is based on the representation of stem anteversion. The pelvis may be defined using the anatomical coordinate system, the functional coordinate system, or the coordinate system recommended by the International Society of Biomechanics (ISB). The anatomical coordinate system uses the anterior pelvic plane as the reference plane, which consists of the bilateral anterosuperior iliac spines and the midpoint of the bilateral pubic tubercles. The functional coordinate system incorporates the pelvic sagittal inclination, or pelvic tilt, in the supine position. The ISB pelvic coordinate system uses the plane consisting of the bilateral anterosuperior iliac spines and the midpoint of the bilateral posterosuperior iliac spines. The two major femoral coordinate systems are the femoral retrocondylar coordinate system and the ISB femoral coordinate system. The femoral retrocondylar coordinate system uses the retrocondylar plane as a reference plane, which consists of the posterior edge of the great trochanter and the bilateral posterior condyles. The ISB femoral coordinate system uses the plane consisting of the femoral head center and the bilateral femoral epicondyles as a reference plane. We must recognize that these differences in the representation of cup and stem alignment and the reference bone coordinate system significantly influence the clinical outcome of total hip arthroplasty whether using three-dimensional planning, navigation, or robotics. Such misunderstanding could result in an incorrect clinical interpretation of biomechanical analysis results.