Sunday February 25, 1996

Socket loosening boosts wear rate in ceramic components

Socket loosening stimulates wear rates up to 90 times higher than in cases with well-positioned, stable integrated ceramic components, according to a study of retrieved alumina ceramic heads and sockets from failed hip replacements.

A correct socket positioning and the use of a socket with a minimal loosening rate is of great importance and leads to a prolonged lifespan of the endoprosthesis, according to scientific paper 387 presented Saturday.

The resulting forces at the articulating surfaces become too high, thus initiating accelerated wear.

Orthopaedic researchers performed an analysis of 448 retrieved components after long-term implantation (average time: 9.3 years).

A total of 133 retrieved ceramic sockets and 315 retrieved ceramic heads were investigated. Joint diameter was 38 mm in 164 heads and 87 sockets; 32 mm in 151 heads and 46 sockets.

Fifty seven sockets and 99 heads had been in vivo more than five years.

To determine the three-dimensional wear, 36 measurements of the largest and lowest diameters were performed over the pole of every component (every 10 degrees of angle). The difference of these diameters determined the maximum linear wear.

The surface roughness was investigated at femoral heads of three different groups, classified under the aspect of the total amount of linear wear-less than 250m in the first group, between 250m and 800m in the second group, and more than 800m in the third group-using a surface testing device.

Stereomicroscopy, enlarged 20 times, was used to investigate the bore of the ceramic femoral head (female taper) with respect to changes of microstructure and signs of fretting corrosion.

In addition, rastered electron microscopy was performed on 10 heads with an implantation time over five years, and compared to new ceramic balls.

Wear was correlated to the preoperative diagnosis (socket- or stem-loosening or loosening of both); the initial position of the socket and its position at the time of revision surgery (steep/tilted), the change of the socket position, and the amount of axial socket migration.

Reasons for revision included 54 cases had loosening of the stem and the socket; 45 cases had socket loosening only; and 91 cases had stem loosening only. The results of the wear analysis of the remaining heads and sockets were withdrawn. Among these, 83 heads were paired with polyethylene sockets.

In 42 heads and 34 sockets, irregular conditions, such as septic loosening, (sub)luxations, and breakage of the ceramic components had to be considered.

In failed total hip replacement cases with loosening of stems and sockets, results of the study shows that average linear wear was 77m/year for the heads and 70m/year for the sockets.

In cases with socket loosening only, the average linear wear was 96m/year for the heads and 60m/year for the sockets. In cases with loosened stems only, the average linear wear of the heads was 22m/year.

There was no relationship between implantation time and the linear wear.

Heads with a linear wear less than 20m/year showed a significant relation between wear and the socket positioning. In these cases a socket inclination of more than 50 degrees, or a difference of more than 10 degrees of the inclination angle between the primary socket position and its position at revision surgery, showed significantly higher wear.

Researchers found a positive relation between the axial migration of the socket, and wear. However, the diameter of the head had no significant influence on the wear.

A linear relationship between the wear of the head and the corresponding socket could be detected.

Surface roughness was unchanged (Ra: 0.07m on average) except in areas of wear. In these areas, the average roughness coefficient (Ra) was 0.232m in the first group; 0.233m in the 2nd group; and 0.351m in the last group, showing macroscopic gross wear. The difference of the last group to the first two groups was statistically significant.

At the bore of the femoral head, a degradation of the microstructure did not occur and there were no signs of fretting corrosion.

These results cannot be interpreted as the normal tribological behavior of a properly implanted firm endoprosthesis with ceramic/ceramic articulation because the wear rates and changes in the surface roughness were all achieved with retrieved components of failed hip joint replacements. However, this study shows that loosening of the prosthesis leads to an increase of the wear.

Co-authors of the study are Romain Seil, MD; Ekkehard W. Fritsch, MD; and Heinz Mittelmeier, MD, all from Orthopaedic University Hospital, Hamburg, Germany; and Gerd Willmann, ScD, Plochingen, Germany.

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