Contact stress assessment of conventional and highly crosslinked ultra high molecular weight polyethylene acetabular liners with finite element analysis and pressure sensitive film.

Stress magnitude and distribution of both conventional polyethylene versus a crosslinked polyethylene in the liner of a total hip replacement (THR) were examined using finite element analysis and pressure sensitive film. Both types of polyethylene were assessed against head sizes of 22 and 28 mm with 5-mm thick polyethylene liners and head sizes of 28, 38, and 46 mm with 3-mm thick polyethylene liners. Liners with 5-mm conventional polyethylene represented successful combinations with long track records. Our hypothesis was that although the combination of the large head and the lower modulus of the highly crosslinked polyethylene would lead to lower stresses, the stresses would be excessive if the liner was extremely thin at 3 mm. Von Mises stresses at the articulating surface of the highly crosslinked liners were lower, when compared to conventional polyethylene, in every THR size examined. Specifically, however, the 38- and 46-mm inner diameter (ID) highly crosslinked polyethylene even at the extreme of only 3-mm thick had lower stresses than the 22-mm ID conventional liner of 5-mm thickness. These data indicate that the use of a large head against highly crosslinked material even at 3-mm thickness results in lower stresses than in an existing conventional 22-mm head and 5-mm thick combination. Obviously, other considerations will influence the minimum thickness to be recommended.

The measurement of creep in ultrahigh molecular weight polyethylene: a comparison of conventional versus highly cross-linked polyethylene.

Quantification of creep of highly cross-linked polyethylene would enable separation of creep from wear when evaluating femoral head penetration into polyethylene. We compared creep magnitude of a highly cross-linked versus conventional polyethylene in the laboratory. Twelve acetabular liners of each material were tested, 6 of which had a 32-mm inner diameter (ID) and 6 had 28-mm ID. Creep was measured using coordinate measuring machines during loading at 2 Hz without motion to 4 million cycles. Penetration into 32-mm ID conventional liners reached 97 microm versus 107 microm for highly cross-linked material, not significant. Penetration into 28-mm conventional liners was 132 microm versus 155 microm for highly cross-linked material (P = .017). Ninety percent of the creep had occurred by 2.5 million cycles.