ABSTRACT
Lumbo-sacral orientation in the sagittal plane is of utmost importance, as it plays a critical role in the function of the spine and the hip joints. Equilibrium of the trunk influences the tridimensional orientation of the acetabulum and the functional range of motion of the hips. Each patient is characterized by a "morphological" parameter named incidence angle; its sagittal balance is the consequence of a postural adaptation for other functional parameters (pelvic tilt, sacral slope, lumbar lordosis, acetabular sagittal tilt, functional anteversion). Understanding variations of the sacral slope on lateral pelvic X-rays is essential for planning total hip arthroplasty and identifying patients at risk of impingement, as lumbosacral posture influences functional anteversion of the acetabulum. Posterior pelvic version as in sitting position (sacral slope decrease) is linked to the increase of the functional acetabular anteversion. Anterior pelvic version as in standing position (sacral slope increase) is linked to the decrease of the functional acetabular anteversion.