SHANK LATERAL BAR IN MOTION ANALYSIS PROTOCOLS: ARTIFACTS AFFECTING ANKLE KINEMATICS

INTRODUCTION Several motion analysis protocols exploit the lateral shank bar for the identification of the frontal plane of the shank. The use of lateral bar can result critical both for static alignment with respect to the relevant anatomy and for motion of the bar with respect to the relevant segment during walking. The present work aims at quantifying the propagation of bar motion to relevant joint rotation angles. CLINICAL SIGNIFICANCE The quantification of 3D joint kinematics performed using markers-based motion analysis is exploited to support clinical decision process. Therefore the reliability of the quantified joint kinematics is fundamental and the possible quantification errors associated to the use of lateral bar in specific protocols needs to be quantified. METHODS Five subjects, with no reported musculoskeletal alteration, were analysed. The kinematics of the shank and foot was acquired during walking at self-selected speed by means of stereophotogrammetry (Elite and SMART-D, BTS, Milan). Five trials were acquired for each subject. Markers were attached to the shank and foot according to an integrated Davis protocol [1] allowing the quantification of ankle 3D kinematics: 4 markers were attached to the shank (Head of the Fibula, Lateral Bar, Tibial Tuberosity, Lateral Malleolus) and 3 to the foot (Heel, I Metatarsal Head, V Metatarsal Head). During the acquisition of the standing static posture 2 additional hemispherical markers were used: Medial Malleolus on the shank and II Metatarsal Head on the foot. The anatomical reference frame of the shank was defined according to ISB recommendations [2]. Two anatomical reference frames were defined for the foot: both references had the vertical axis defined as orthogonal to the plane defined by Heel, I Metatarsal Head, V Metatarsal Head, while one had the antero-posterior axis defined by Heel and midpoint between the I and V Metatarsal and the other by heel and II Metatarsal Head. The anatomical reference frames were defined in the standing static position and then localized with respect to the relevant cluster of markers, in order to analyse specifically the effect of lateral bar motion. For the shank 2 different clusters were analysed: i) cluster Head of the Fibula, Lateral Bar, Lateral Malleolus; ii) cluster Head of the Fibula, Tibial Tuberosity, Lateral Malleolus. Based on previous literature [3] cluster (ii) is assumed as reference, being more rigidly connected to the shank. For the foot only one cluster was analysed: Heel, I Metatarsal Head, V Metatarsal Head. Ankle rotation angles were calculated based on the Grood and Suntay [4] convention according to ISB recommendations [2]. The motion of the lateral bar marker was quantified with respect to cluster (ii) during motion. Moreover, Root Mean Square Distance (RMSD) and Maximal Distance (MD) were quantified for ankle rotation angles calculated using cluster (i) and (ii). RESULTS The lateral bar marker resulted to move with respect to the relevant shank reference in the order of the centimetre during walking, the motion was time-varying and particularly associate to the inertial phenomena at toe-off and heel contact. The RMSD(MD) between ankle rotation angles calculated with (cluster (i)) and without (cluster (ii)) the lateral bar was in the order of 1°(3°) for Dorsi/Plantar-flexion and Prono/Supination and 9°(20°) for Internal/External rotation (Figure 1). The use of different anatomical references (using either the II Metatarsal Head or the midpoint between the I and V Metatarsal for the definition of the AP foot axis) of the foot affected only ankle Internal/External rotation, with a maximal difference of 3°. DISCUSSION The motion of the shank lateral bar resulted to significantly affect the ankle rotation angles quantified during motion. The effect of lateral bar motion particularly affects prono/supination, with errors up to the 20% of the range, and internal/external rotation, with errors up to 120% ...