Early postoperative fixation of tibial components: an in vivo roentgen stereophotogrammetric analysis.

The fixation of cemented and noncemented tibial components in 19 total knee arthroplasties was examined 3 to 10 weeks postoperatively with roentgen stereophotogrammetric analysis. Physiologic external forces were applied either in outward-inward rotation or as eccentric posterior loading generated by squatting. In one case there was no motion, while in the remaining 18 cases relative interface motion of up to 1.3 mm was found. In some cases, the largest motion was found during inward-outward rotatory stress, while in others, the largest motion was found during squatting. Posterior tilt was weakly correlated with posterior displacement of the femoral component on the tibia during squatting (r2 = 0.323, p < 0.05). For the noncemented cases, the initial fixation to bone was probably insufficient, and ingrowth of bone would not have been achieved. For the cemented cases, motion of the implant was reduced. Given the short postoperative time and the probable absence of any substantial fibrous tissue membrane, we suggest that the observed motion represented elastic deformation of the bone.