Should total hip arthroplasty femoral components be designed to subside? A radiostereometric analysis study of the Charnley Elite and Exeter stems.

The Charnley Elite and the Exeter stems have different design concepts: The former is designed not to subside, whereas the latter is expected to subside. This radiostereometric analysis study compares the early migration of the 2 stems. For both implants, the 1st year migration was about 4 times faster than the 2nd year. The Exeter migration was predominantly distal (1 mm/y in the 1st year). It also showed slight collapse into valgus, and the head migrated slowly posteriorly (0.3 mm/y in the 1st year). In contrast, the Elite had slow distal migration (0.2 mm/y in the 1st year) and rapid posterior head migration (0.8 mm/y in the 1st year). Four Elites and no Exeters had rapid posterior head migration rates (mean 2.8 mm/y in the 1st year and 0.8 mm/y in the 2nd year). The Elite and the Exeter stems have fundamentally different early patterns of migration, which affect their long-term function; 20% of the Elites and none of the Exeters had rapid posterior head migration in the 1st year and the 2nd year and are likely to fail early. Polished, collarless, tapered designs, such as the Exeter, may be more forgiving than conventional stems designed not to subside.