Abstract
Movement of skin markers with respect to their underlying bone (i.e. soft tissue artifacts (STAs)) might corrupt the accuracy of marker-based movement analyses. This study aims to quantify STAs in 3D for foot markers and their effect on multi-segment foot kinematics as calculated by the Oxford and Rizzoli Foot Models (OFM, RFM). Fifteen subjects with asymptomatic feet were seated on a custom-made loading device on a computed tomography (CT) table, with a combined OFM and RFM marker set on their right foot. One unloaded reference CT-scan with neutral foot position was performed, followed by 9 loaded CT-scans at different foot positions. The 3D-displacement (i.e. STA) of each marker in the underlying bone coordinate system between the reference scan and other scans was calculated. Subsequently, segment orientations and joint angles were calculated from the marker positions according to OFM and RFM definitions with and without STAs. The differences in degrees were defined as the errors caused by the marker displacements. Markers on the lateral malleolus and proximally on the posterior aspect of the calcaneus showed the largest STAs. The hindfoot-shank joint angle was most affected by STAs in the most extreme foot position (40° plantar flexion) in the sagittal plane for RFM (mean: 6.7°, max: 11.8°) and the transverse plane for OFM (mean: 3.9°, max: 6.8°). This study showed that STAs introduce clinically relevant errors in multi-segment foot kinematics. Moreover, it identified marker locations that are most affected by STAs, suggesting that their use within multi-segment foot models should be reconsidered.
Highlights
Skin-mounted marker-based multi-segment foot models are frequently used to measure foot kinematics during gait, for instance to assess foot and ankle problems in patient populations
Largest marker displacements were shown when the foot was in 40° plantar flexion
It was shown that Soft Tissue Artifacts (STAs) affect multisegment foot kinematics with joint angle errors up to 6.7° on average over the subjects and maximal individual values up to 11.8° when the footplate was positioned in 40° plantar flexion
Summary
Skin-mounted marker-based multi-segment foot models are frequently used to measure foot kinematics during gait, for instance to assess foot and ankle problems in patient populations. OFM and RFM provide a different kinematic output when measuring the same gait trial (Schallig et al, 2020). It is unclear how accurate each of these models are, which is especially of importance when the data is used for clinical decision-making. Soft Tissue Artifacts (STAs) are a well-known source of error in marker-based human motion analyses, affecting the accuracy of kinematic measurements (Leardini et al, 2005). When motions like gait are performed, soft tissue displacement causes relative motion between a marker and its corresponding bone, thereby affecting the derived kinematics. The intermarker distances are small, a displacement causes relatively large angular errors compared to markers that are further apart
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