UHMWPE for hip joint implants

Hip joint replacements are the most common type of joint replacements, with 300 000 operations in the USA in 2000. The aim of this case study is to highlight the wear resistance and low friction properties of UHMWPE, to discuss the effects of sterilisation on the properties of plastics, and the effects of wear debris in the body. The order of presentation is that followed by the polymer, through fabrication, sterilisation, implantation and wear, to the reactions caused by wear debris. 

The ram extrusion process for UHMWPE powder was described in Section 5.4.4, and the effects of diffusion on the strength of the particle boundaries in Section 6.4.3. Ram extruded rods are machined on a lathe into the required cup shape. It is not essential to remove all machining marks, since the smooth metal counter-face polishes the PE in contact with it. 

The external surface of the acetabular cup is grooved to allow bone cement (ceramic-filled acrylic resin) to key mechanically to the cup. This cement also binds to the excavated part of the pelvis. Further design variables are the cup wall thickness and whether it has a metal backing. For a 

FEA of the stresses in the UHMWPE cup is difficult, as the stresses exceed the elastic limit. Teoh et al. (2002) considered an 8 mm thick cup with a metal backing, a 32 mm diameter ball and a peak load of 2.2 kN (about 2.5 X body weight) for walking. Using the unrealistic condition that the compressive stress on the ball/UHMWPE interface could not exceed the uniaxial compressive yield stress (of 8 MPa), they predicted the compressive stress to be at this level over a surface region of diameter about 8 mm. However, a von Mises type yield criterion should be used. It requires a pressure of nearly three times the uniaxial yield stress to extrude the PE to the side of the joint (Section 8.2.4). 

Olley et al. (1999) examined the microstructure of fabricated acetabular cups inside the particles, there is no obvious spherulitic microstructure. Regions of about 6 xm diameter contain lamellae of width typically 0.5 xm (Fig. 15.15). These are surrounded by a looser boundary of 2 xm wide lamellae, consisting of lower molecular weight material. Such material diffuses to the boundaries to effect the bonding process. 

---