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.

Abstract

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. 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. 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). 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.