Contact area and pressure load at the implant/bone interface of press-fit cups compared with natural hip joints

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.

The effects of implant surface roughness and surgical technique on implant fixation in an in vitro model

“…Finally, it is well known that the implantation of an implant in an undersized implant socket alters the mechanical stresses in the peri‐implant area in a significant manner compared with a press‐fit inserted implant (Widmer et al 1997; Taylor et al 1998; Skalak & Zhao 2000). In the present study, the mismatch between implant and drill hole diameter was 0.6 mm.…”

Acetabular revision in total hip replacement with a press-fit cup

“…The flattened dome area and the biradial eccentricity avoid gaps between the cup and the underlying bone bed (tripod stability). 37 Migration in cemented and uncemented cups which do not use the press-fit concept appears to be an indicator of subsequent aseptic loosening. 12 Similar conclusions from radiological assessments of migration with Radio-Stereometric analysis or EBRA (Einzell-Bild-Röntgen-Analyse) 63 do not apply to press-fit implants which are fixed without screws.…”

“…The transmission of force is as in the natural hip joint, to the periphery, and is achieved by flattening the cup's dome area. 37 Close implant-bone contact in the periphery, the main region for force transmission, is generated by a hemispherical design of biradial bicentricity. Therefore, the radius of the outer surface corresponds to that of the acetabular reaming.…”

3D video-based deformation measurement of the pelvis bone under dynamic cyclic loading

“…The femora were then fixed with polymethyl methacrylate (PMMA, Beracryl, Swiss-composite, Jegenstorf, CH) in the holding fixture, which was mounted on the table of the Servo-hydraulic testing machine to simulate a two-leg standing position. The load was applied vertically onto the sacrum by the axial actuator of the testing machine with an adjustable fixture at the sacrum, allowing unconstrained rotational and transverse motion [ 17 ]. After adjusting the pelvis on the testing machine, 80 reflecting markers (diameter: 6.5 mm, Prophysics AG) were fixed at anatomically defined positions on the bone with cyanoacrylate (Loctide 401; Henkel & Cie AG, Pratteln, CH).…”