Internal rotation of the lower limb following inappropriate antero-posterior placement of the acetabular component in total hip arthroplasty

Introduction The authors report the case of an internal rotation of the lower limb, late ly ascribed to a posterior placement of the acetabular component during tot al hip arthroplasty. Material and method A 58 years old female had an unreducible internal rotation of the right hip 3 years after total hip arthroplasty for arthritis. When the hip was exten ded, the lower limb showed an irreducible internal rotation of 45 degrees. In flexion of the hip this rotation disappeared. AP radiograms showed femur and femoral stem in internal rotation, a healed fracture of the acetabulum , and the acetabular component seemed to be in correct position. On CT scan the acetabular component was 4 cm posterior to the anatomic location, alth ough there was no abnormal anteversion of the stem and acetabular component . Revision, with relocation of the acetabular component, corrected lower li mb rotation. Discussion Posterior position of the acetabular component has not been described as a cause of lower limb malrotation. Normaly the strength of the external hip r otators muscles is three times as important as of the internal rotators, Th e tranverse acetabular fracture led to backwards placement of the acetabula r component and yelded in an automatic internal rotation of the femur, beca use the trochanter kept an anatomical position in the horizontal plane. The posteriorised rotation center of the hip had changed the balance of the di fferent rotator muscles, some of them, originaly external rotators, becomin g internal rotators.