Contact finite element stress analysis of porous ingrowth acetabular cup implantation, ingrowth, and loosening.

Two-dimensional linear and contact finite element analyses were conducted of total hip arthroplasty using metal-backed, porous ingrowth acetabular components. The stress transmission characteristics from the component to the surrounding bone were given special attention. Resultant loads of 20 and 40 degrees medial of vertical were studied, and the influence of adding a metal flange to the rim of the cup was evaluated. The results indicated that when a conventional metal-backed component (without a flange) is initially implanted and subjected to normal loading, these components may experience distraction between the component and the surrounding bone at inferior sites. Compressive stresses in the superior dome cancellous bone, however, will be substantial. If complete porous ingrowth is achieved, the superior dome compressive stresses will be reduced and substantial shear stresses created. In addition, high local bone stresses were found at the component rim. If bone ingrowth is achieved only in specific locations, stress transmission will be dictated by those locations and may differ markedly from the case of complete bone ingrowth. In the event that no porous ingrowth is achieved and a fibrous layer forms around the component, the interface stresses will be similar to those calculated for the natural hip. The addition of a flange to the rim of the cup will reduce the magnitude of the radial stresses transmitted to the cancellous bone superiorly and medially by directly transferring some of the load to the lateral wall of the pelvis. The flange will also help to relieve the high local stresses that are found at the component rim.