Optimal spatial orientation which minimizes bias error in model-based radiostereometric analysis for a cementless tibial baseplate is not aligned with the anatomic orientation defined by the global coordinate system.

BACKGROUND: Bias error in determining tibial baseplate migration based on maximum total point motion (MTPM) in model-based radiostereometric analysis (MBRSA) should be minimized because this error can be large ranging from 0.21 mm to 0.47 mm based on previous reports for cemented baseplates. For a cementless tibial baseplate, the primary objective was to determine the optimal spatial orientation which minimizes the bias error in MTPM.

METHODS: A baseplate-sawbone construct was oriented in 24 different combinations of flexion and valgus angles with three pairs of radiographs acquired for each combination. Flexion was varied from 0° to 25° and valgus was varied from 0° to 15° each in 5° increments. Radiographs were processed in MBRSA software and the mean MTPM for each flexion/valgus angle combination was determined. Mean MTPM indicated bias error because no migration of the tibial baseplate occurred.

RESULTS: At the optimum practical angle combination of 15° flexion/5° valgus for the biplanar stereo calibration cage used herein, mean MTPM of 0.07 mm was minimum and increased more than 3-fold for the anatomic orientation defined by the global coordinate system (i.e. 0° flexion/0° valgus) (p = 0.015).

CONCLUSION: Since mean MTPM at the optimal angle combination and surrounding region was significantly below that of the anatomic orientation, migration results for cementless baseplates should be computed using a local implant coordinate system with the baseplate positioned in the optimal region. For the cementless baseplate herein, ideally an angle combination of 15° flexion/5° valgus should be insured to avoid inflating the bias error in clinical use.