Initial stability of an uncemented femoral stem with modular necks. An experimental study in human cadaver femurs.

Background: Uncemented implants are dependent upon initial postoperative stability to gain bone ingrowth and secondary stability. The possibility to vary femoral offset and neck angles using modular necks in total hip arthroplasty increases the flexibility in the reconstruction of the geometry of the hip joint. The purpose of this study was to investigate and evaluate initial stability of an uncemented stem coupled to four different modular necks.

Methods: A cementless femoral stem was implanted in twelve human cadaver femurs and tested in a hip simulator with patient specific load for each patient corresponding to single leg stance and stair climbing activity. The stems were tested with four different modular necks; long, short, retro and varus. The long neck was used as reference in statistical comparisons. A micromotion jig was used to measure bone-implant movements, at two predefined levels.

Findings: A femoral stem coupled to a varus neck had the highest value of micromotion measured for stair climbing at the distal measurement level (60μm). The micromotions measured with varus and retro necks were significantly larger than motions observed with the reference modular neck, P<0.001.

Interpretation: The femoral stem evaluated in this study showed acceptable micromotion values for the investigated loading conditions when coupled to modular necks with different lengths, versions and neck-shaft angles.