In situ comparison of A-mode ultrasound tracking system and skin-mounted markers for measuring kinematics of the lower extremity
Introduction
Accurate measurements of joint kinematics are essential to comprehensively understand the mechanism of joint motion. Most of kinematic data on the lower extremity are obtained from motion capture systems, in which the trajectories of skin-mounted markers are recorded to represent the motion of underlying skeletal structure (Lafortune et al., 1992). However, the estimated three-dimensional (3D) position and orientation of bone segments and the related kinematics are subject to soft tissue artifacts (STA) (Cereatti et al., 2017). Since the movement of the skin, muscles, and fat relative to the underlying bone is an inevitable phenomenon under dynamic motion tasks (Dumas and Jacquelin, 2017), the inherent mismatch between skin and bone movement is difficult to remove under all circumstances (Cappozzo et al., 1995, Cappozzo et al., 1996). It has been reported that STA can cause measurement errors of up to 30 mm in the thigh (Akbarshahi et al., 2010). The propagation of STA to knee joint kinematics has been reported to lead to average rotational errors of up to 4.4° and 13.1° and average translational errors of up to 13.0 mm and 16.1 mm for walking and cutting motions, respectively (Benoit et al., 2006).
Extensive researches have been conducted on quantification, assessment, and compensation of STA for different motor tasks (Andersen et al., 2009, Bonnechère et al., 2015, Cappozzo et al., 1997, Charlton et al., 2004, Duprey et al., 2010, Lu and O’Connor, 1999). Multi-body kinematics optimization (MKO) has been used with the intent to compensate the STA and to limit the propagation of STA to joint kinematics estimation (Andersen et al., 2009, Lu and O’Connor, 1999). The typical mechanical linkages representing the knee joint and embedded in MKO are the hinge joint and the spherical joint, which involve major simplifications with respect to the actual joint and reduce the degrees of freedom (DOF) of the joint (Ojeda et al., 2014, Reinbolt et al., 2005). The hinge joint only allows the flexion-extension rotation (Andersen et al., 2009). The spherical joint allows all three rotations but no translation (Clement et al., 2017). Researchers have realized that motion analysis research community should make more efforts in search of more advanced subject-specific joint models or error models, or a new measurement modality in order to improve the accuracy of estimated joint kinematics (Andersen et al., 2010, Cereatti et al., 2006, Richard et al., 2017).
Instead of compensating STA from various perspectives of mathematical models and optimizations, we developed a new method to directly measure spatial information of underlying bones in order to produce an effective and valid representation of skeletal motion and the corresponding joint kinematics. A novel ultrasound (US) tracking system was developed to measure tibiofemoral kinematics dynamically, non-invasively, and without radiation. As A-mode ultrasound transducers are capable of detecting underlying bone surfaces non-invasively through multiple layers of soft tissues under dynamic movement, a combination of multiple A-mode ultrasound transducers and conventional skin-mounted markers provides a new approach of measuring the trajectories of multiple A-mode ultrasound transducers attached on the thigh and shank as well as detecting respective underlying bone surfaces from received ultrasound signals. Subsequently, the trajectories of bone surfaces could be obtained and the relevant kinematics can be quantified. To evaluate our system relative to conventional motion capture systems with skin-mounted markers, cadaveric experiments were conducted.
The aim of the presented work is to compare tibiofemoral kinematics derived from our ultrasound tracking system with those kinematic outcomes derived from skin-mounted markers using two typical joint models (hinge and spherical) in cadaveric experiments. Another goal is to demonstrate the potential for being less affected by STA and achieving high accuracy in estimated tibiofemoral kinematics with our ultrasound tracking system.
Section snippets
Experimental setup
A full body cadaveric specimen (male, 79 kg, 179 cm) was obtained from and approved by the Anatomy Department of the Radboud University Medical Center (RUMC), Nijmegen, the Netherlands. There was no history of illness, injury, or treatment affecting the knee or hip functions. Tracked intra-cortical bone pins equipped with optical markers were inserted into the right femur (with two pins) and right tibia (with two pins), which provided ground truth kinematics as a reference. A full leg (from the
Results
The mean rotational errors ranged from 3.23° to 6.25° and from 2.17° to 6.12° for the spherical model and hinge model, respectively. The mean translational errors ranged from 4.65 to 5.82 mm and from 4.56 to 6.39 mm for the spherical model and hinge model, respectively. For the ultrasound tracking system, the mean rotational errors (0.85°–2.65°) and mean translational errors (2.00–4.35 mm) were lower than those errors of the two types of kinematic measurements (obtained from the spherical and
Discussion
In this study, we performed cadaveric experiments in order to compare the measurement of our novel ultrasound tracking system with skin-mounted marker measurement for assessing the accuracy in estimated tibiofemoral kinematics in a highly controlled experimental scenario. As the results have shown, the ultrasound tracking system could achieve relatively high accuracies in flexion/extension (1.54° RMS error) and abduction/adduction (1.00° RMS error), which is close to the accuracy of the mobile
Conclusion
We proposed a quantitative comparison of tibiofemoral kinematics estimated using an ultrasound based tracking system versus traditional skin-mounted markers system with hinge and spherical knee joint models. The ultrasound tracking method resulted in lower kinematic errors, in the experimental conditions investigated, and could represent a viable alternative to traditional system, which could improve the measurement accuracy of bone and joint kinematics. This new ultrasound based kinematic
Acknowledgements
The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement n. 323091 awarded to N. Verdonschot. The authors thank for the generous helps of Léon Driessen and Richard van Swam in the cadaveric preparation from Orthopedic Research Lab, Radboud University Medical Center, Netherlands. The authors also thank for the technical help of Mats Boeve in the design of ultrasound
Conflict of interest statement
The authors have no conflict of interests to report related to this work.
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