[Preparation of the acetabulum to correct severe acetabular deficiency for total hip replacement--with special reference to stress distribution of the periacetabular region after operation].

The author analysed the radiological results of eighty-three total hip replacements in the patients with severely defective acetabulum secondary to congenital hip dysplasias or failed total hip replacements, after an average follow-up period of three years, paying particular attention to socket loosening. The patients with congenital hip dysplasias were divided into the following four groups: Group 1: The sockets were placed above the level of true acetabulum. Group 2: The sockets were placed in the true acetabulum with the cement used as a filler superolaterally. Group 3: The sockets were placed in the true acetabulum with bone grafts. Group 4: Eccentric sockets were placed in the true acetabulum. The incidence of radiological loosening of the socket was 50% in the group 1, 13% in the group 2, 6% in the group 3 and 75% in the group 4. The patients who underwent revision for socket loosening were also divided into two groups, the one whose acetabula were reconstructed with bone grafts and the other without bone graft. The incidence of radiological loosening of the socket was 28% in the group with bone grafts and 72% in the group without bone graft. The major factor that may have been responsible for socket loosening in these cases was considered to be poor mechanical condition around the socket due to bone deficiency. Two-dimensional finite element analysis was performed to establish the stresses in the periacetabular region after total hip replacement for acetabular deficiency. Model variations include, the conventional plastic socket of 44 mm outside, 28 mm inside diameter placed in false acetabulum or in high level, (2) in true acetabulum using bone cement as a filler, (3) in true acetabulum with bone graft and (4) eccentric socket placed in true acetabulum without bone graft. The ratio of von Mises stress to yield stress of each element were calculated. The results demonstrated that in the periacetabular region, the area superomedial to the socket was at high risk in each model. The area in periacetabular bone where the ratio of von Mises stress to yield stress was more than 20% was almost the same in model (1), (2), (4) and about a half of them in model (3). In conclusion, the most effective method to prevent the socket loosening after total hip replacement in the patient with deficient acetabulum was considered to reconstruct the new spherical acetabulum at anatomical position with bone graft.