Anterior dislocation after total hip replacement - effects of pelvic rotation and femoral head size.

We investigated the effects of pelvic rotation and femoral head diameter on the anterior stability of the hip joint after total hip replacement. Computer navigation and cadaveric bone were used to simulate the range of motion after total hip replacement. The hip was put at 0 degrees of flexion and it was gradually externally rotated until the hip dislocated. The degree of external rotation when the hip was dislocated was recorded. The test was repeated with the hip at +10, -10, -20, -30, -40 and -50 degrees of flexion. The acetabular component was positioned with abduction of 45 degrees and anteversion at 20 degrees. There was a significant difference amongst each group of pelvic rotation from 0 to -50 degrees in the degree of external rotation when the hip was dislocated. The degree became insignificant when pelvic rotation was increased from 0 to 10 degrees. From -10 to -50 degree of pelvic rotation, 36mm head had significant better stability compared with 32mm and 28mm femoral heads. The presence of significant pelvic sagittal malrotation can increase the risk of anterior dislocation. A larger femoral head is more stable than smaller heads. When pelvic sagittal malrotation is not present, there is no difference in stability between large and small femoral heads.