Influence of a high hip center on abductor muscle function.

In 20 anatomic specimens with an acetabular defect (type Paprosky 3b), an acetabular component was implanted in the position of a high hip center. The vertical migration of the hip center ranged between 13 and 35 mm. It was accompanied by a lateralization and ventral migration of between 5 and 25 mm. The influence on the different abductor muscles was calculated through computer model comparing muscle force and muscle length before and after implantation of a high hip center. The increase in length of the gluteus maximus muscle and the posterior part of the gluteus minimus muscle ranged between 1% and 6%, while all other evaluated abductor muscles were shortened from 3% to 16%. The effect of the simultaneous changes of the lever arms was an increase in necessary muscle strength for pelvic stabilization from 140% to 250% compared with the original estimated strength prior to implantation. This may lead to insufficiency of the abductor muscles after placement of a high hip center. On the basis of these findings, we do not recommend the implantation of an acetabular component in the position of a high hip center.