An anatomically based protocol for the description of foot segment kinematics during gait
Introduction
The human shank and foot complex is a multi-joint mechanism which determines the critical interaction between the lower limb and the ground during locomotion [1]. Any injury, lesion or neuromuscular disorder of this key element of the lower limb drastically affects the normal interaction between muscles, bones and ligaments and will cause degradation, instability or disability of locomotion. The characterisation of the kinematics of specific segments of the shank and foot complex is therefore a major goal in clinical evaluation of many foot pathologies as well as in biomechanical investigations of foot–ground interaction during gait.
The motion of the ankle–subtalar complex during the first rocker, the elastic deformation of the mid-foot during the loading response, the three-dimensional (3D) rotation of the 1st metatarsophalangeal joint during the third rocker and the push-off phase are fundamental information for the dynamic functional evaluation of many foot diseases [1]. A recent paper [2] has shown that there are very few relationships between foot structure, usually derived from radiographs, and the foot function, examined by movement analysis. The authors therefore pointed out that only dynamic analyses of the patient during activity allows clinicians to distinguish between normal and pathological foot function, to discriminate between the various levels of impairment, and to quantitatively assess clinical outcomes. The current gait analysis practice uses a single rigid body representation of the foot [3], [4]. More detailed experimental protocols are therefore required for the in vivo tracking of the major foot segments.
Many authors have proposed different approaches for 3D in vivo kinematics of the shank and other selected foot segments, based on high-speed video systems and motion analysers. However, most of them have analysed a limited number of segments [5], [6], [7], [8], [9] and only preliminary studies have included mid-foot and forefoot segments [10], [11], [12], [13]. The small number of studies is probably related to the difficulty in tracking many foot segments by the traditional use of markers directly attached to the skin. They have also been limited by a lack of rigorous definition of the anatomical axes [5], [6], [8], [11], for which joint rotations can be better compared. Other techniques have involved X-ray exposure of patients and measurements on the relevant pictures in order to index external markers to the anatomical geometry of the underlying bones [11], [12]. Both these papers presented data from one test subject only and explicitly stated that some measurements were difficult to obtain from the radiographic views and others even impossible, such as forefoot position in the coronal plane. Because of the radiography and the complexity of data reduction, the technique suggested is inappropriate to routine clinical settings. Other studies, aimed to establish anatomically based co-ordinate systems, suggest collecting extra trials with the subject in an adjusted position according to a visual check [7], [8] or using a special jig [9], preventing the quantitative assessment of structural foot deformities and alterations in the posture. A recent paper [14] estimated the location of the ankle and subtalar axes with respect to external target markers on individual subjects using optimization criteria based on the critical assumption of a fixed axis of rotation for both the joints. Most of these studies have also failed to show consistent patterns of rotation due probably to the very limited number of subjects analysed [5], [6], [7], [8], [9], [10], [11], [12], [13].
Some recent investigations have questioned the reliability of foot bone tracking from external markers because of the interposition of soft tissues [15], [16], [17], [18]. However, Tranberg et al. [18] reported very small displacements of the skin markers and mostly in the vicinity of the ankle joint, whereas Reinschmidt et al. [17] found that skeletal rotation of the calcaneus with respect to the tibia is generally well reflected by external markers.
Bone embedded reference frames should be rigidly associated with the anatomy of the bones and therefore be identified by anatomical landmarks. Landmark positions are traditionally tracked by corresponding markers directly mounted on the skin. These landmarks cannot be appropriate for marker visibility and subject disturbance, and therefore may not represent ideal locations for marker placement. Cappozzo et al. [4], have proposed to distinguish between technical frames, determined from external markers, and anatomical frames defined from bone anatomy. The geometric relationship between technical frames and relevant anatomical landmarks can be obtained by an anatomical landmark calibration procedure [4], [19].
The present study deals with methodological problems related to the in vivo reconstruction of the position and orientation of human shank and foot segments in space during locomotion using stereophotogrammetry. Following preliminary studies [20], [21], an experimental protocol is presented for the tracking of five rigid segments in the shank and foot complex using the minimum number of cameras. The study is also intended to contribute to the definition of standardised anatomical frames for these segments.
Section snippets
Subjects
Nine asymptomatic normal subjects (five males and four females) aged between 25 and 45 yr, with shoe sizes between 38 (5 UK) and 44 (10 UK), volunteered for the study. They were pain free and no previous history of musculo-skeletal disease was reported. They were selected according to the ‘Biophysical Criteria for Normalcy’ proposed by Root et al. [22].
The five segment shank–foot model
The shank and foot complex was represented by five rigid segments: shank (including tibia and fibula bones), calcaneous bone, mid-foot
Results
The pattern of rotation was observed to be highly repeatable within each subject in all joints and planes. The DP, PS, and IE rotations for the four joints considered are shown in Fig. 3 as obtained from 10 trials of a representative subject. The standard deviation zone (grey band) around the mean pattern (solid line) indicates measurement and subject variability.
Fig. 4 shows the mean and standard deviation for each of the 12 curves as obtained from three trials in all nine subjects, in the
Discussion
The proposed technique for the description of the 3D motion allows for foot joint rotations to be investigated using anatomically based co-ordinate systems.
The method presents advantages over previously described studies. The acquisition protocol is non-invasive and can be carried out efficiently. The rigid clusters of markers using metallic clamps can embrace the underlying bones better than skin-mounted markers. Additionally, the cluster orientations permit a limited number of cameras while
Conclusion
Human foot segment kinematics have been investigated by means of stereophotogrammetry. A new protocol for the tracking of shank, calcaneus, mid-foot, 1st metatarsal, and proximal phalanx segments during gait has been suggested. The application of the CAST experimental protocol [4] enabled a valuable description of ankle and other foot joint rotations during the stance phase of walking on an anatomical base, with no further requirement of radiographs to refer external markers to internal bony
Acknowledgements
This work was supported by the Italian Ministry of Health Care and the British Council – Conferenza dei Rettori delle Università Italiane.
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