Size and direction of hip joint forces associated with various positions of the acetabulum

When total hip replacement is performed, the position.,tl of the acetabular component may affect wear and component survival time. We considered the q uestions: In what way does displacement of Ihc hip joins center alter (1) t he magnitude and (2) the direction of the resultant force? Biomechanical te sts were carried out on a human multibody model. After displacement of the hip joint center the resultant forces were calculated for the single leg st ance. With the flexed single leg stance, maximum hip joint forces were obse rved with lateral, cranial, posterior displacement. The peak Forces were af fected by the modeling of a gluteus maximus wrapping point ar the ischial t uberosity and were overestimated when this was removed. With the straight s ingle leg stance. posterior displacement decreases the total load on the hi p joint because of the increased leverage of the rectus femoris. With regar d to the direction of the resultant force, medial displacement increases th e angles in both planes, cranial displacement increases it in the sagittal plane (cranial, posterior-caudal, anterior), and anterior displacement decr eases the angle in the sagittal plane and increases it in the frontal plane (medial, cranial-lateral. caudal). The direction of the force is relativel y insensitive to displacement of the hip joint center. Thr results presente d here indicate a marked increase in the force after lateral, cranial poste rior displacement of the center in the flexed single let: stance. To avoid extreme joint loading and to reduce the wear after total hip arthoplasty, t he cranial and posterior regions of the acetabulum should be fully reconstr ucted. A high hip joint center has an adverse effect on the magnitude of th e force. although the directions are hardly affected by it.