The effects of acetabular component position and hip ball diameter on posterior stability after total hip replacement

Objective To study the effects of acetabular component position and hip ball diameter on the posterior stability of the hip joint after total hip replacement.Methods Computer navigation system and synthetic bone model were used to simulate the motion of hip after total hip replacement. The hip was flexed to 90 degree and was adducted 0 and 30 degree as indicated by the computer navigation, after then it was progressively internally rotated until the hip was dislocated. The degree of internal rotation when the hip was dislocation was recorded.Results When the acetabular component abduction was 45 degree and anteversion were 0, 5, 10, 15 degrees, and the hip was flexed to 90 degree and adducted to 0 and 30 degrees, there was significant difference between 28 mm and 36 mm hip balls in the degrees of internal rotation when the hip was dislocated (P0.05). 36 mm diameter hip ball was significantly better in stability. The degree of internal rotation when the hip dislocated was less when the hip was adducted 30 degree compared to 0 degree. The stability was diminished with less acetabular component anteversion. With 0 degree anteversion and the hip adducted to 30 degree, a 28 mm diameter hip ball would lead to dislocation even if the hip was in external rotation. Conclusions The increasing anteversion of acetabular component can increase the stability of total hip replacement. Large hip ball is more stable than small hip ball. However, when anteversion of acetabular component is sufficient, there is no difference in stability between large and small hip balls.