Abstract
Total hip arthroplasty for developmental dysplasia of the hip (DDH) remains a difficult and challenging problem. How to reconstruct acetabular deficiencies has become increasingly important. One of the major causes inducing loosening of acetabular reinforcement ring with hook (Ganz ring) is insufficient initial stability. In this study, three-dimensional finite element models of the pelvis with different degrees of bone defect and acetabular components were developed to investigate the effects of the number of screws, screw insert position (Fig. 1), and bone graf quality on the initial stability under the peak load during normal walking. The size of pelvic bone defect, the number of screws and the position of screws were varied, according to clinical experience, to assess the change of initial stability of the Ganz ring. The Ganz ring was placed in the true acetabulum and the acetabular cup was cemented into the Ganz ring with 45 degrees abduction and 15 degrees of screws. The Insert position, nodes on the sacroiliac joint and the pubic symphysis were fixed in all degrees of freedom as the boundary condition. The peak load during normal walking condition was applied to the center of the femoral head (Fig. 2). According to the Crowe classification, as the degree of acetabular dysplasia was increased, the relative micromotion between the Ganz ring and pelvis was also increased. The peak micromotion increased as the stiffness of bone graft decreased. Increasing the numbers of screws, the relative micromotion tended to be reduced and varied the screw insertion position that affects the relative micromotion in the Ganz ring-pelvic interface (Fig. 3). This study showed that increasing the number of inserted screws can reduce the relative micromotion. Both the insert position and graft bone property affect the stability of the Ganz ring while the insert position has a greater impact. The current study is designed to lay the foundation for a biomechanical rationale that will support the choice of treatment.
