The linear penetration rate is not relevant for evaluating wear of dual mobility cups: an explant study

The linear penetration rate (LPR) has been used to estimate volumetric wear ever since metal-on-polyethylene bearings were first introduced. Annual volumetric wear and osteolysis are interrelated. The use of dual mobility cups has increased; however, the presence of two bearings needs paraclinical monitoring of wear to anticipate potential wear-related complications. Is LPR correlated to volumetric wear?

Ninety-eight dual mobility liner retrievals were mapped in three-dimensions (3D) with a fully automatic stripe light scanner. Martell's method was applied to 3D maps of the explants to determine how much the centres of the inner and outer bearings had shifted in the three planes, and in particular, penetration of the femoral head in the "z" axis. The total, inner and outer wear volumes were then compared to the shift in the centres of rotation (COR) and the pre-operative X-rays.

There was no relationship between wear and head penetration into the liner. There was a statistically significant relationship between the 3D centre of rotation shift and inner wear or total wear. Wear was multidirectional.

LPR is ineffective for estimating wear of dual mobility inserts, since its wear not two-dimensional. In the future, a 3D imaging method with very thin slices could be used to monitor the wear of dual mobility inserts by measuring the shift in the centres of rotation, once artifacts are overcome.