Offset

Determining the appropriate offset in total hip arthroplasty (THA) is important from the perspective of the effect on the joint forces and range of motion applied to the hip by the acquisition of dislocation resistance and definition of the lever arm of the abductor muscle. In addition to femoral and acetabular offsets, the global offset, which is the distance from the pubic symphysis to the femoral axis, is evaluated for the hip offset (Fig. 44.1). In the past, preoperative and postoperative evaluations were often performed using two-dimensional (2D) frontal X-ray images of the hip joint. However, in recent years, three-dimensional (3D) evaluation using computed tomography (CT) (Fig. 44.2) has improved our understanding of the anatomy of the hip joint and enabled detailed preoperative and postoperative planning and evaluation. Factors that determine the offset include the thickness of the acetabular base, presence or absence of a double floor, joint congruity, deformation of the femoral head, and anteversion and cervical angles of the femur. The position of the hip center on the affected and healthy sides should be accurately evaluated in two and three dimensions using simple radiographs and CT, and the offset should be determined from the viewpoint of biomechanics (kinematics and kinetics) during movement.