Abstract
A socket of the total hip prosthesis is 1) fixed with bone cement or 2) placed without bone cement. The stability of both methods was investigated under severe loading conditions. A two-dimensional finite element model was used and its cross section was cited from CARTER, VASUS and HARRIS. Sockets were set by four methods. In Cases 1 and 2, bone cement was used for the installation and fixation of sockets, although the two differed from each other in the mode of anchoring. The anchoring of Case 2 is based on the theory that the inner wall of the cortical bone is expected to be a supporting area, in Cases 3 and 4, bone cement was not used. Acetabular dysplasia was absent in Case 3 and present in Case 4. Boundary non-linear elements were considered at the interface between the bone and bone cement in Cases 1 and 2, and at the interface between the bone and a screw-socket in Cases 3 and 4. It was assumed that HDP and bone cement as well as HDP and a screw-socket were united. The program used in the analysis was MSC/NASTRAN. The load employed was 10kg in all cases, and it was considered that as a three-dimensional effect, a socket would be given rotatinal force to some extent. Therefore, Moment M=2.55kg.mm was made to distribute on the inside surface of HDP. These loading patterns are severe conditions from the clinical point of view. A model for analysis was fixed with its top and bottom ends. Cases 1 and 2 present considerable deviated distribution of contact area pressure. On the back of the cortical bone, no contact pressure was detected. In Case 3 and 4, the interface between the bone and implant was again separated. This displacement is likely to occur in clinical cases. The amount of displacement in Cases 1 and 2 was considerably larger than that in Cases 3 and 4. In Cases 3 and 4, support by the cortical bones on the back resulted in a smaller amount of displacement. In the case of restraining both sides of the cortical bone, there is considerable estimation of rigidity on within-area bending deformation of the cortical bone. Both sides of the cortical bone were thus restrained. In all the cases including Case 1D through Case 4D, distribution of the contact area presure was uniform. Subsequently, the displacement was small.
