Micromotion and subsidence of a cementless conical fluted stem depending on femoral defect size - A human cadaveric study.

Background: Cementless modular endoprostheses with tapered fluted stems cover a wide spectrum of femoral defects in reconstructive surgery of the hip. Nevertheless, for these hip stems the recommendations concerning the minimum diaphyseal anchorage distance differ widely. The present experimental study investigated the primary stability of a conical fluted revision stem depending on different types of femoral bone defects.

Methods: Using six fresh frozen human femora, the relative movement of a bi-modular revision stem within the implant-bone interface was examined under cyclic loading conditions. Implant subsidence as well as micromotions at the bone-implant interface were captured with linear variable differential transformers for the intact femora and three different defects ranging from Paprosky type II to type IIIB.

Findings: Compared to the intact femur, the infliction of a Paprosky type IIIB defect (3 cm of intact diaphysis) notably increased mean stem subsidence (13-389 μm per 500 load cycles; P = 0.116) but the mean interface micromotion vector sum remained unchanged (50 μm vs. 53 μm). In Paprosky IIIB defects the subsidence component resulting from rotation (horizontal plane) was significantly higher than with the intact femur and a Paprosky II defect (P ≤ 0.041).

Interpretation: With optimal bone quality and ideal femur preparation a 3 cm conical fixation was sufficient to meet the set criteria of bony ingrowth in vitro. A conical fixation of 7 cm should be recommended to limit rotational subsidence, especially in case of impaired diaphyseal bone quality or expected difficulties with partial weight-bearing.