Effects of the soft tissue artefact on the hip joint kinematics during unrestricted activities of daily living

Abstract

Soft tissue artefact (STA) affects the kinematics retrieved with skin marker-based motion capture, and thus influences the outcomes of biomechanical models that rely on such kinematics. In order to be compensated for, the effects of STA must be characterized across a broad sample population and for different motion activities. In this study, the error introduced by STA on the kinematics of the hip joint and of its individual components, and on the location of the hip joint center (HJC) was quantified for fifteen THA subjects during overground gait, stair descent, chair rise and putting on socks. The error due to STA was computed as the difference between the kinematics measured with motion capture and those measured simultaneously with moving fluoroscopy, a STA-free X-ray technique. The main significant effects of STA were: underestimation of the hip range of motion for all four activities, underestimation of the flexion especially during phases of the motion with higher flexion, overestimation of the internal rotation, and lateral misplacement of the HJC mostly due to the functional calibration. The thigh contributed more to the STA error than the pelvis. The STA error of the thigh appeared to be correlated with the hip flexion angles, with a varying degree of linearity depending on the activity and on the phase of the motion cycle. Future kinematic-driven STA compensation models should take into account the non-linearity of the STA error and its dependency of the phase of the motion cycle.

Introduction

Skin marker-based motion capture (MC) is one of the most commonly used methods for human motion analysis. Yet, its accuracy is limited by the soft tissue artefact (STA), which is caused by the relative movement between skin markers and bone due to inertial effects, skin sliding and muscle contraction (Camomilla et al., 2017a, Leardini et al., 2005). STA was identified to be the largest source of error of skin markers-based motion capture (Benoit et al., 2006, Peterset al., 2010) and it is hard to resolve in a non-invasive manner.

Quantification of the effects of STA on the kinematics of the hip joint is important for reliable interpretation of results from clinical gait analysis (Benedetti et al., 2017, Ku et al., 2015, Wrenet al., 2011), for detection of femoro-acetabular impingement (Rylander et al., 2013), for wear analysis of the prosthetic hip (Hua et al., 2016, Mellonet al., 2013), and for validation of the estimates from musculoskeletal models. In studies on simulated gait propagation of STA had the highest impact among all sources of uncertainty on the hip joint moment outputs, and produced variations of 37.2 ± 20 N on the predicted muscle forces (Myers et al., 2015). Lamberto et al. (2017) reported that STA causes variations of up to 1.8 times the body weight for hip contact forces and of 30% to 50% for some muscle forces. Stagni et al. (2000) estimated that misplacements of the hip joint center (HJC) by 30 mm produce errors as high as 22% and 15% in hip flexion-extension and adduction-abduction moments, respectively. These studies concluded that propagation of STA to the estimates of current biomechanical models should be compensated for, especially when accurate subject-specific kinetics and bone strains are required.

For compensation of STA, a thorough in-vivo characterization of STA and of its effects during motion activities are required. Recent methods measured STA in-vivo by comparing the skin marker positions with the 3D poses of the underlying joint segment retrieved from X-ray video fluoroscopic images. While several studies have used this technique to quantify the STA for the knee joint during different activities (Akbarshahi et al., 2010, Barré et al., 2013, Garling et al., 2007, Kuo et al., 2011, Da Li et al., 2017, Stagniet al., 2005, Tsaiet al., 2011), fewer have been performed for the hip joint. Recently, Fiorentino et al. (2017) have used dual-plane fluoroscopy and CT to quantify STA and its effects on the angles and range of motion of the native hip joint during level and inclined treadmill gait, and during hip adduction and hip rotation motions. They reported large STA, especially for the markers attached to the femur, and underlined its dependency on anatomical direction, subject and activity. STA caused an underestimation of the overall range of motion, with values up to 21.8 deg for the internal-external rotation during hip rotation. The effects of STA on the HJC location were also analyzed in-vivo using video-fluoroscopy. Functional methods for determination of the HJC location were reported to carry errors of 11 ± 3 mm, with STA the primary contributor (Fiorentino, Kutschke, et al., 2016). During dynamic activities, STA caused about 2.2 mm of spurious HJC motion, contributing to overall HJC errors of 16.6 ± 8.4 mm (Fiorentino, Atkins, et al., 2016).

To date, characterization of STA is still a complex challenge due to its dependency on subject (Benoit et al., 2006), on motion activity (Holden et al., 1997, Leardini et al., 2005) and on skin-marker configuration (Schwartz et al., 2004), its non-linearity with respect to the pose parameters of the underlying bone (Tsaiet al., 2009, Barré et al., 2013), and the scarcity of reference in-vivo estimations (Cereatti et al., 2017, Peterset al., 2010). Consequently, propagation of STA on the kinematic estimates has not been thoroughly characterized. The objective of this study was to extend the existing knowledge of the effects of STA on the kinematics of the hip joint, by quantifying them for a sample total hip arthroplasty (THA) population, for a broader range of activities of daily living (ADLs). A moving fluoroscopic system was used to measure the hip joint during unconstrained motions such as overground gait, stair descent and chair rise, which were not previously assessed due to the use of stationary X-ray imaging equipment. Errors due to STA were quantified across the whole motion, including the relative contribution of each joint segment. The overall error on the HJC location was also analyzed with respect to both its components: a static component, arising from STA during the functional calibration task and dependent on the functional algorithm, and a dynamic component, cause by inertial STA occurring during motion.