Load transfer and periprosthetic fractures after total hip arthoplasty: Comparison of periprosthetic fractures of femora implanted with cementless distal-load or proximal-load femoral components and measurement of the femoral strain at the time of implantation.

BACKGROUND

Little is known about the causes and mechanisms underlying periprosthetic fractures around femoral components particularly in relation to the stem design. In an in vitro study 20 pairs of fresh cadaveric femora were loaded to fracture axially and transversally.

FINDINGS

When proximal femoral strain was measured at the time of impaction of cementless stems the load transfer was determined by the underlying anatomy rather than by the shape of the stem, so that the so-called "load transfer" properties - proximal or distal - ascribed to stem designs are a myth. The axial-load and the transverse-load model were then exposed to loads to failure (fracture) and showed a biphasic pattern throughout independent of the impact direction. In the second phase, the fracture phase proper, the bone behaved like a brittle solid. Failure occurred very rapidly within less than 5 milliseconds. The forces to failure were between 2 and 11 kN. Most of the fractures (82.5%) occurred above the stem tip.

INTERPRETATION

Note that the study was confined to early preosteointegration fractures. Neither the stem design nor the impact direction, i.e. on the knee or on the side of the hip, was related to the fracture morphology.