DISTAL STEM DESIGN AND THE TORSIONAL STABILITY OF CEMENTLESS FEMORAL STEMS

In cementless hip arthroplasty, the fit between the implant and the endosteal cavity is a critical determinant of implant stability. Although cementless implants may be stabilized through proximal fit within the metaphysis, many surgeons rely on diaphyseal fixation to provide the necessary resistance to rotational forces, especially in revision hip arthroplasty. The cross-sectional design of the femoral stem at the level of the femoral isthmus was investigated with respect to its effect on the rotational stability of the bone-stem interface. Four cross-sectional designs-a fluted stem, a finned stem, a porous-coated stem, and a slotted fluted stem-were implanted in 12 cadaveric femurs and loaded in torsion. A knurled stem, cemented into each specimen at the conclusion of testing, acted as a control stem. The torque required to cause 100 pm of displacement at the bone stem-interface ranged from 13.7 +/- 0.8 N-m with the porous-coated design to 30.1 +/- 3.7 N-m with the fluted design (P < .0001). Intermediate values of 19.5 +/- 1.4 and 19.9 +/- 2.3 N-m were observed with the finned and slotted fluted designs, respectively. In all of the cemented control stems, failure occurred at the bone-cement interface at an average torque of 34.0 +/- 3.0 N-m. Statistical analysis demonstrated that the porous-coated, finned, and slotted fluted designs were all significantly weaker in torsion than the cemented control stem; however, there was no significant difference between the torsional resistance of the solid fluted (unslotted) and cemented stems. With the exception of the fluted stem design, it is postulated that the cementless stem configurations evaluated would provide insufficient resistance to torsional forces to stabilize a femoral prosthesis solely through distal fixation within the medullary canal. Consequently, rotational stabilization of the cementless prosthesis necessitates proximal and;distal contact between the implant and the femur.