Rear-foot, mid-foot and fore-foot motion during the stance phase of gait

Abstract

This paper proposes a new protocol designed to track a large number of foot segments during the stance phase of gait with the smallest possible number of markers, with particular clinical focus on coronal plane alignment of the rear-foot, transverse and sagittal plane alignment of the metatarsal bones, and changes at the medial longitudinal arch. The shank, calcaneus, mid-foot and metatarsus were assumed to be 3D rigid bodies. The longitudinal axis of the first, second and fifth metatarsal bones and the proximal phalanx of the hallux were also tracked independently. Skin markers were mounted on bony prominences or joint lines, avoiding the course of main tendons. Trajectories of the 14 markers were collected by an eight-camera motion capture system at 100 Hz on a population of 10 young volunteers. Three-dimensional joint rotations and planar angles were calculated according to anatomically based reference frames. The marker set was well visible throughout the stance phase of gait, even in a camera configuration typical of gait analysis of the full body. The time-histories of the joint rotations and planar angles were well repeatable among subjects and consistent with clinical and biomechanical knowledge. Several dynamic measurements were originally taken, such as elevation/drop of the medial longitudinal arch and of three metatarsal bones, rear-foot to fore-foot rotation and transverse plane deformation of the metatarsus. The information obtained from this protocol, consistent with previous clinical knowledge, enhanced our understanding of the dynamics of the human foot during stance.

Introduction

The human shank and foot complex is an intricate, multi-joint mechanism, which is fundamental for the interaction between the lower limb and ground during locomotion [1]. The critical effect of abnormal foot motion on lower limb function has been demonstrated [2], [3], [4]. The quantitative assessment of abnormal function and of the effects of treatment requires a more detailed analysis than that offered by standard gait analysis, which considers the foot as a single rigid segment or a vector. Dynamic modelling of the foot also requires multi-segment motion analysis, which takes into account deformity [5], [6]. Special techniques based on X-rays [7] and on more modern MRI [8], [9] or videofluoroscopy [10] are not applied routinely because of the invasive data acquisition, the restricted field of measurement, and the intense data reduction. Skeletal tracking in vivo [11] is inappropriate in routine clinical assessments. In vitro measurements on cadavers by simulators of locomotion [12] have been criticised for the lack of realistic conditions.

An increased interest in shank and foot multi-segment kinematics analysis in vivo by stereophotogrammetry [13] is documented in the literature. Initially [14], [15], [16], [17], [18], a limited number of segments were analysed, and subsequently mid-foot and fore-foot segments were included in the models [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], probably because of the availability of more reliable instrumentation. Even a 19-marker, 9-segment, 8-joint model was proposed [26], though marker-to-bone association and validation was limited. All techniques but one [25] utilised stereophotogrammetry. Only a few addressed explicitly adolescents’ and children's feet [26], [28]. Another recent technique [30] was not based on standard three-dimensional (3D) kinematics, bur rather on isolated planar angles. Despite X-ray-based association between external markers and underlying bones [19], [31] or the inclusion of special additional trials for defining the anatomical reference frames [16], [17], [18], consistent patterns of joint rotations were rarely observed. These were observed in a previous work by the present authors [21], but the technique involved uncomfortable marker clusters and time-consuming anatomical landmark calibration. The analysis was limited also by the probable rigid motion of the clusters with respect to the underlying bones, and restricted to the first ray of the foot. Many clinical and biomechanical foot concepts, such as foot segment alignments and deviations, medial longitudinal arch angle, navicular drop, first ray mobility, etc., are implied in clinical or radiographic examinations [32], [33], but rarely addressed in overall multi-segment foot function analyses in vivo [34], [35], [36], [37], [38].

In the present proposal, the selection of the foot segments, the design of the marker set and anatomical reference, and the calculation of the kinematic variables were based on the following clinical interests and general technical indications. In most foot-related functional abnormalities, frontal plane alignment of the rear-foot is essential, both in relation to the shank and the fore-foot [5]. Transverse and sagittal plane deformations of the metatarsus under load have been subject to limited analysis. Deformation under load of the medial longitudinal arch is mostly assessed statically on radiograms or footprints [39], [40], [41]. As for the technical design, single markers directly mounted on the skin surface over relevant anatomical landmarks were pursued. More dorsal locations for the fore-foot markers were sought, because of clearance in severely deformed gait, along with locations which can avoid the course of the main foot tendons. In addition, joint lines were used as marker locations, to represent characteristic landmarks of the two adjacent bones, and allow easy and repeatable identification. Other relevant landmarks were calibrated either using an instrumented pointer [42] or an additional marker to be removed before gait trials. The smallest possible marker size was tested, compatible with the additional objective of complete foot marker tracking with the usual number and configuration of TV cameras for full body gait analysis. Finally, clinically oriented definitions of the foot joint rotations were required.

The objective of this work was to design a technique for the in vivo description of ankle and foot joint motion using optoelectronic stereophotogrammetry to be applied in patients with foot pathologies for clinically oriented functional evaluation according to the criteria and goals stated above.