Impingement between femoral neck and endoprosthetic cup is one of the causes for dislocation in total hip arthroplasty (THA). Choosing a correct combined orientation of both components, the acetabular cup and femoral stem, in manual or computer-assisted implantation will yield a maximized, stable range of motion (ROM) and will reduce the risk for dislocation. A mathematical model of a THA was developed to determine the optimal combination of cup inclination, cup anteversion, and stem antetorsion for maximizing ROM and minimizing the risk for cup-neck impingement. Single and combined hip joint motions were tested. A radiographic definition was used for component orientation. Additional parameters, such as stem-neck (CCD) angle, head-neck ratio, and the design of the acetabular opening, were also considered. The model showed that a maximized and safe ROM requires compliant, well-defined combinations of cup inclination, cup anteversion, and stem antetorsion depending on the intended ROM. Radiographic cup anteversion and stem antetorsion were linearly correlated. Additional internal rotation reduced flexion, and additional external rotation reduced extension, abduction and adduction. The articulating hemispheric surface of acetabular cups should be oriented between 40" and 45" of radiographic inclination, between 20" and 28" of radiographic cup anteversion, and should be combined with stem antetorsion so that the sum of cup anteversion plus 0.7 times the stem antetorsion equals 37". Final component orientation must also consider cup containment, implant impingement with bone and soft tissue, and preoperative skeletal contractures or deformities to achieve the optimal compromise for each patient.

No abstract

The implantation of an endoprosthetic socket into the acetabulum alters the mechanical stresses in the periacetabular region in a significant manner compared with the natural hip joint. In this way, a remodelling process is initiated. Primary stability to achieve osseointegration and a loading of the acetabulum owing to the biomechanical interaction between cup and bone that is similar to the natural joint, are important prerequisites for a long-term bony integration of the implant. Therefore, the intra-articular pressure distribution in eight hip joints of fresh-frozen human pelvic cadavers and in the bone/implant interface of two press-fit cups was investigated using pressure-sensitive Prescale films. A modular cup with a pure hemispherical shape (PCA cup) and 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 introduced 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 intra-articular contact occurs over the whole articular surface of the joint, with contact areas between 39.0% and 56.9% of the hemisphere, showing zones of higher pressures where the acetabulum is supported by the iliac, ischial and pubic bone. The biradial press-fit cup showed mean 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 load. 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 near the periphery, and with increasing load its pole area comes into contact 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 surface is loaded. The acetabular fossa remains unloaded owing to the flattening of the pole. This load distribution is close to that of the natural hip joint. The pressure distribution of both cups is also characterised by three zones at the iliac, ischial and pubic bones showing a higher load transmission.

No abstract