Short communicationAccuracy of mobile biplane X-ray imaging in measuring 6-degree-of-freedom patellofemoral kinematics during overground gait
Introduction
Accurate measurement of in vivo six-degree-of-freedom (6-DOF) patellofemoral kinematics during walking is important for identifying the biomechanical causes of anterior knee pain and patellofemoral osteoarthritis and for improving total knee replacement design. Intracortical bone pins (Koh et al., 1992), single plane fluoroscopy (Stein et al., 1993) and biplane fluoroscopy (Nha et al., 2008) have been used to measure patellofemoral kinematics during functional activity. The use of intracortical bone pins is invasive, has the potential to alter patellofemoral kinematics, and is not well suited to clinical studies involving large numbers of participants. Single plane fluoroscopy, although adequate for measuring in-plane motion, is unable to capture the complex three-dimensional motion of the patella relative to the femur (Bey et al., 2008). Biplane fluoroscopy is currently the most accurate, noninvasive method for measuring in vivo kinematics during dynamic load-bearing activities. However, the limited size of the imaging volume of such systems has constrained the types of activities investigated to forward lunges (Nha et al., 2008) and the stance phase of treadmill gait (Farrokhi et al., 2015). No study to our knowledge has measured in vivo patellofemoral kinematics for one complete cycle of overground walking.
We have developed a Mobile Biplane X-ray (MoBiX) imaging system capable of tracking and imaging the knee at high speed during multiple strides of overground gait (Guan et al., 2016). While system errors associated with the measurement of 6-DOF tibiofemoral joint kinematics during overground walking have been evaluated (Guan et al., 2016), corresponding errors for patellofemoral joint motion are unknown. Thus, the aim of the present study was to evaluate the accuracy with which the MoBiX imaging system measures 6-DOF patellofemoral kinematics of the intact knee during overground gait.
Section snippets
Methods
The MoBiX system tracked and recorded biplane fluoroscopic images of two human cadaver knees during simulated overground walking. The images were then used to calculate 6-DOF patellofemoral kinematics using a noninvasive bone volumetric model-based method. These results were compared against those obtained using a bead-based method that served as the gold-standard technique. Approval for the study was obtained from the Human Research Ethics Committee at the University of Melbourne.
Experiments
Results
For both specimens, the bone-based kinematic measurements fluctuated in the neighborhood of the bead-based measurements during the entire stance phase (Fig. 3). The mean errors for joint translations were no more than 0.10 mm. The standard deviation for patellar lateral shift (0.35 mm) and superior translation (0.33 mm) were larger than that for patellar anterior translation (0.17 mm). RMS errors followed a similar trend: patellar lateral shift, superior translation, and anterior translation were
Discussion
We measured 6-DOF patellofemoral kinematics in the intact human knee during simulated overground walking at a speed of 0.7 m/s. Maximum RMS errors were 0.37 mm for translations and 1.46° for rotations based on measurements obtained from two cadaver specimens.
The maximum RMS error obtained for patellofemoral translations was approximately one-half that reported previously for tibiofemoral translations (0.78 mm) when the same MoBiX imaging system was used to measure knee joint motion during
Conflict of interest statement
All authors declare no conflict of interest.
Acknowledgements
This work was supported by a Discovery Projects Grant from the Australian Research Council (DP120101973). We thank Max Biegler for his assistance with the experiments.
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