The Effect of a Lateral Flare Feature on Uncemented Hip Stems

Ideal goals for a primary uncemented femoral stem prosthesis are to transmit the loads to the femur proximally, and to minimise the interface migration. It has been proposed that the addition of a lateral flare which loads the lower region of the greater trochanter will contribute positively to these goals. Analytical and radiographic studies were used to study the load transfer between the stem and the bone, and the migration. A comparison was made between a straight stem, and a straight stem with the addition of a lateral flare. The finite element study showed that the straight stem migrated down the canal approximately four millimeters before stabilisation was reached. The forces were transmitted on to the proximal-medial femur and around the lower half of the stem. When the lateral flare was added, there was only one millimeter of migration to reach stability. The loads were transferred by a wedging effect between the proximal-medial femur and the around the lateral flare, with little force transfer from the stem. In a radiographic follow-up of an HA-coated lateral flare stem, trabeculae could be seen attaching to the lateral flare. The axial migration was significantly less for this stem design compared with that from a series of previously reported cemented stems. This study suggested that the lateral flare contributed positively to the goals of uncemented stem design, and that the stems could be made shorter than designs not incorporating the lateral flare feature.