Stable Fixation in Cementless Total Knee Arthroplasty Even for Low Local Bone Mineral Density.

BACKGROUND

Cementless fixation is increasingly popular for total knee arthroplasty (TKA). Prior research suggested that volumetric bone mineral density (vBMD) measured in preoperative computerized tomography (CT) scans could be useful to identify suitable candidates for cementless TKA with sufficient bone strength to avoid aseptic loosening. However, the clinically relevant thresholds of vBMD for cementless knees have not been defined. As a step toward defining such thresholds, we sought to relate the preoperative vBMD to the migration of tibial baseplates after TKA as a marker of aseptic loosening.

METHODS

We prospectively enrolled 15 patients undergoing unilateral primary TKA with cementless tibial baseplates and cruciate-retaining inserts. Patients received a preoperative CT scan as standard of care, including a BMD reference phantom, and postoperative CT scans the day of surgery, at 6 weeks postoperatively, and at 6 months postoperatively. We calculated the implant motion relative to the day of surgery scan and related it to the vBMD in the four mm immediately under the baseplate.

RESULTS

The maximum total point motion (MTPM) ranged from 0.1 to 0.9 mm at 6 weeks and between 0.2 and 1.1 mm at 6 months. The largest motion occurred vertically and, on average, was consistent with the posterior tilt of the baseplate. The preoperative vBMD was highest under the posterior-medial quadrant (mean: 229.8 mg/cm3 range, 113 to 329.2), and the medial-to-lateral vBMD ratio was proportional to the preoperative arithmetic hip-knee-ankle angle. However, neither the vBMD under the implant nor the limb alignment was related to any metric of implant motion at any time point.

CONCLUSIONS

This is, to our knowledge, the first study to characterize the postoperative implant motion relative to a pre-weight-bearing CT scan and relate such motion to the vBMD directly under the tibial baseplate. Our results suggest that BMD alone cannot predict early migration of tibial baseplates.