The kinematic relationship between sitting and standing posture and pelvic inclination and its significance to cup positioning in total hip arthroplasty.

PURPOSE: The aim of this study is to describe the influence of sitting and standing posture on sagittal pelvic inclination in total hip replacement patients to assist with correct acetabular component positioning.

METHODS: Lateral radiographs of the pelvis and lumbar spine in sitting and standing positions were extracted. Pelvic tilt was measured using the vertical inclination of a line from the anterior superior iliac spine (ASIS) to pubic tubercle. Sacral inclination, Cobb angle of the lumbar spine and hip flexion were recorded.

RESULTS: Sixty patients were identified with a mean age of 63. Men were more likely to flex the lumbar spine in sitting (p = 0.004); 80° of hip flexion is required for seated posture. Stiff hips required compensatory pelvic flexion and lumbar flexion in sitting. There is a linear relationship between hip flexion and pelvic tilt, hip flexion and lumbar lordosis.

CONCLUSIONS: Pelvic orientation is determined by lumbar and hip stiffness. This impacts on acetabular version.