Abstract
Background
Hemiarthroplasty of the hip involves the replacement of the femoral side of the joint with a metal prosthesis, resulting in metal-on-cartilage articulation. The two most common types of hemiarthroplasty used are the Austin Moore and the Thomson, both of which are available in either Titanium (Ti) or cobalt chromium (CoCr). Hemiarthroplasty may be more cost effective in elderly patients who have lower life expectancy and are less active.
Materials and Methods
Three Ti and two CoCr hemiarthroplasty components were obtained following revision surgery. Four had an articulating diameter of 44mm and the other was 46mm diameter. These five hemiarthroplasties were analysed using a Mitutoyo LEGEX322 co-ordinate measuring machine (CMM) (manufacturer's claimed scanning accuracy of 0.8μm). In each case a wear map was generated and the wear volume from the articulating surface was calculated using a bespoke MATLAB program.
Results
The two CoCr prostheses had wear volumes of 1.3mm3 and 7.8mm3, while the three Ti prostheses had wear volumes of 85.4 mm3, 16.3 mm3 and 17.4 mm3.
Figure 1: CoCr Prosthesis with a small wear scar at the rim
Figure 1 shows the location of the wear mark on the bottom edge of the CoCr prosthesis, with a volumetric wear of 1.3 mm3, and also some scratching at the pole. The maximum wear depth was 0.0082mm.
The second CoCr prosthesis also showed small localised wear marks over the surface and at the pole giving a total wear volume of 7.8mm3 and a maximum linear wear depth of 0.016mm.
Figure 2: Ti Prosthesis with large wear
Figure 2 shows the Ti prosthesis with the highest wear volume of 85.4 mm3. The worn area (shown in blue) extended over much of the articulating surface area and the maximum linear depth was 0.080mm.
The second Ti prosthesis (the only sample of 46mm diameter) had a wear volume of 16.3 mm3 and a maximum linear wear depth of 0.045mm.
Figure 3: Ti Prosthesis with wear mark at the pole
The third Ti prosthesis analysed (shown in Figure 3) had a wear volume of 17.36 mm3. Unlike the others the wear was concentrated at the pole where the linear wear depth was 0.060mm.
Discussion
To the authors' best knowledge they are unaware of any other volumetric wear studies on explanted hemiarthroplasties. The CoCr hemiarthroplasties showed lower wear volumes and smaller linear wear depths than the Ti prostheses. The location of the wear marks on the articulating surface varied between the retrieved prostheses.
These results suggest that Ti hemiarthroplasties are wearing more than CoCr prostheses. It is unknown what effect the two different types of prostheses have upon the cartilage of the acetabulum, or whether there is any difference in the damage they cause.
Given the results obtained, and the known negative effects of particulate wear debris in the body questions should be asked regarding the suitability of Ti for use in hemiarthroplasty.
