CONTACT AREA AND PRESSURE LOAD AT THE IMP LANT BONE INTERFACE OF PRESS-FIT CUPS COMPARED WITH NATURAL HIP JOINTS/

The implantation of an endoprosthetic socket into the acetabulum alter s the mechanical stresses in the periacetabular region in a significan t manner compared with the natural hip joint. In this way, a remodelli ng process is initiated. Primary stability to achieve osseointegration and a loading of the acetabulum owing to the biomechanical interactio n between cup and bone that is similar to the natural joint, are impor tant prerequisites for a long-term bony integration of the implant. Th erefore, the intraarticular pressure distribution in eight hip joints of fresh-frozen human pelvic cadavers and in the bone/implant interfac e of two press-fit cups was investigated using pressure-sensitive Pres cale films. A modular cup with a pure hemispherical shape (PCA cup) an d a monoblock cup with a biradial surface and flattening of the pole ( press-fit cup) were tested. Loads of up to twice body weight were intr oduced into the sacrum, simulating a single-leg stance with the hip in neutral flexion. The results were extracted from the pressure prints applying digital image processing methods. It was demonstrated that in tra-articular contact occurs over the whole articular surface of the j oint, with contact areas between 39.0% and 56.9% of the hemisphere, sh owing zones of higher pressures where the acetabulum is supported by t he iliac, ischial and pubic bone. The biradial press-fit cup showed me an contact areas of 44.7% of its surface when inserted under press-fit only. This area was enlarged to 53.0% and to 64.2% with increasing lo ad. The corresponding figures for the PCA cup are 45.1%, 48.9% and 57. 1%. The low-profile PCA cup produces a small band of high pressure nea r the periphery, and with increasing load its pole area comes into con tact with the acetabular fossa. Occasionally, irregular gaps do occur. The press-fit cup enlarges the contact area from the periphery to the pole within its optimised biradial contact zone until the lunar surfa ce is loaded. The acetabular fossa remains unloaded owing to the flatt ening of the pole. This load distribution is close to that of the natu ral hip joint. The pressure distribution of both cups is also characte rised by three zones at the iliac, ischial and pubic bones showing a h igher load transmission.