IN-VIVO FOOT KINEMATICS: DEFINITION OF A FLUOROSCOPIC GOLD STANDARD FOR THE EVALUATION OF MARKER-BASED PROTOCOLS

INTRODUCTION The quantification of foot kinematics is an issue in a large number of pathologies. The number of protocols recently proposed demonstrates the great clinical interest. These protocols are prone to accuracy limitations like any marker-based protocol, but even more due to the deformability of the foot throughout the gait cycle. Therefore, a validation or at least a quantification of the accuracy of foot protocols is required to assess their adequacy to fulfil the requirements for clinical applications. Therefore, a method for the accurate quantification of foot kinematics in physiological conditions, without limitations to range of motion and skin sliding, is necessary. 3D fluoroscopy (1-2) can provide the required accuracy. This technique was previously applied to different joints. It can accurately (in the order of 1° and 1mm) reconstruct the relative kinematics of bony segments using an anatomical 3D model of the relevant bony segments, but requires function-related models for the quantification of accurate relative kinematics of compound bony segments (2-3). In foot kinematics, the rear-, mid- and forefoot are compound segments, intrinsically deformable. The aim of the present study is the definition of a fluoroscopic gold standard based on a functional-anatomical model for the assessment of marker-based foot protocols. CLINICAL SIGNIFICANCE The large number of foot protocols recently proposed demonstrates the clinical interest in the quantification of foot segmental kinematics. A method for the validation of these protocols is necessary to support the clinical reliability of their results. METHODS One subject (female, 26 years, 174 cm, 61 kg) was analysed. The kinematics of foot and ankle was synchronously acquired using stereophotogrammetry (SMART-D, BTS, Italy) and fluoroscopy (Sirecon 40hd, Siemens). Reflective markers were positioned according to Sawacha et al. (4). Flex-extension and Inv-eversion cycles were acquired, together with neutral and maximal flexion, extension, inversion and eversion static postures. Simplified movements were used to analyze the specificity of the modelling approach. Bone models of the foot were reconstructed from MRI (Simpleware, ScanIP Version 4.2) and the anatomical-model based alignment used to estimate bony segments kinematics. A function-based model was then adopted for the reconstruction of the kinematics of the 3 foot segments. Functional axes of each segment were associated to specific anatomical features of the relevant bony segments, e.g. for the forefoot, the antero-posterior axis was associated to 129 the long-axis of the first ray and the vertical one to the plane containing the first and the fifth ray. RESULTS Figure 2: Rear-foot flexion/extension: stereophotogrammetry, fluoroscopy calcaneus, fluoroscopy calcaneus-talus. Figure 3: Fore-foot flexion/extension vs tibia: stereophotogrammetry, fluoroscopy solid forefoot model, fluoroscopy deformable forefoot model. For the rear-foot, angles quantified using stereophotogrammetry overestimated the motion quantified using 3D fluoroscopy. On the other hand, during unloaded motion no significant difference could be observed between the motion of the calcaneus and the motion of the whole rear-foot (talus-calcaneus), reconstructed using 3D fluoroscopy particularly for large motion ranges. For the forefoot, instead, significant difference can be observed between the kinematics reconstructed considering the whole forefoot geometric model using 3D fluoroscopy and the one reconstructed from the 3D kinematics of the I, II and V metatarsal separately. DISCUSSION The preliminary results show that the function-based model constrained to bony segment kinematics provides a good specificity in describing the relative kinematics of foot sub- segments, while the kinematics of the different bony segments within the sub-segments can hardly be generalised, without the proposed functional approach. The present methodology is...