Torsional fixation of the femoral component in total hip arthroplasty. The effect of surgical press-fit technique.

Rotational loading of the femoral component has recently emerged as a prime factor for causing loosening and failure of total hip replacements. In this study the effect of surgical press-fit technique on torsional fixation of the femoral component was evaluated. The commonly used line-to-line reaming technique was compared to an underreaming technique using both four-fifths and one-third porous-coated anatomic medullary locking (AML) implants. Rotational micromotion, permanent rotational displacement, and slop displacement between bone and implant were measured with linearly variable differential transducers under torsional loading. The line-to-line reamed group showed significantly greater motion than both underreamed groups for all micromotion parameters. This was especially true for the slop displacement test, which revealed large amounts of displacement after a single moderate torsional load, whereas in the underreamed groups significantly less loosening was found. The line-to-line reaming technique did not achieve good rotational fixation of the femoral component. Superior rotational stability was accomplished only when the diaphysis was underreamed and tight diaphyseal fixation was achieved. The extent of the porous coating on the stem did not have a significant effect.