The effect of tibial stem design on component micromotion in knee arthroplasty.

Rigid body mechanics with computer data acquisition and analysis techniques were used to determine the three-dimensional motions of any point on the tibial component of a total knee arthroplasty. Three stem configurations were compared: (1) no stem; (2) short stem (40 mm); and (3) long stem (75 mm). In addition, three loading conditions were analyzed for each stem configuration: (1) central loading; (2) posterior loading; and (3) medial loading. The longer stem implants were associated with increased micromotion, especially under eccentric loading. Cemented implants seemed to have more stable fixation, compared with noncemented implants. It was thought that the increased motion was secondary to a toggling of the implant under load, secondary to uneven medullary cortical contact. Overall, the results indicated that short and long stems do not enhance initial fixation with cemented or cementless implantation in routine knee arthroplasty.