Tassos Natsakis

An in vitro approach to the evaluation of foot-ankle kinematics: Performance evaluation of a custom-built gait simulator

Despite their well-known limitations, in vitro experiments have several benefits over in vivo techniques when exploring foot biomechanics under conditions characteristic of gait. In this study, we present a new setup for dynamic in vitro gait simulation that integrates a numerical model for generating the tibial kinematics control input, and we present an innovative methodology to measure full three-dimensional joint kinematics during gait simulations. The gait simulator applies forces to the tendons. Tibial kinematics in the sagittal plane is controlled using a numerical model that takes into account foot morphology. The methodology is validated by comparing joint rotations measured during gait simulation with those measured in vivo. In addition, reliability and accuracy of the control system as well as simulation input and output repeatability are quantified. The results reflect good control performance and repeatability of the control inputs, vertical ground reaction force, center of pressure displacement, and joint rotations and translations. In addition, there is a good correspondence to in vivo kinematics for most patterns of motion at the ankle, subtalar, and Chopart’s joints. Therefore, these results show the relevance and validity of including specimen-specific information for defining the control inputs.

In vitro analysis of muscle activity illustrates mediolateral decoupling of hind and mid foot bone motion

Activity of the extrinsic ankle-foot muscles is typically described for the whole foot. This study determines if this muscle activity is also confirmed for individual foot segments defined in multi-segment foot models used for clinical gait analysis. Analysis of the individual bone motion can identify functional complexes within the foot and evaluates the influence of an altered foot position on muscle activity. A custom designed and built gait simulator incorporating pneumatic actuators is used to control the muscle force of six muscle groups in cadaveric feet. Measurements were performed in three static postures in which individual muscle force was incrementally changed. The motion of four bone embedded LED-clusters was measured using a Krypton motion capture system and resulting motion of calcaneus, talus, navicular and cuboid was calculated. Results indicate that primary muscle activity at bone level corresponds with that described for the whole foot. Secondary activity is not always coherent for bones within one segment: decoupling of the movement of medial and lateral foot bones is documented. Furthermore, secondary muscle activity can alter according to foot position. The observed medio-lateral decoupling of the foot bones dictates the need to extend some of the multi-segment foot models currently used in clinical gait analysis.

Specimen-specific tibial kinematics model for in vitro gait simulations

Until now, the methods used to set up in vitro gait simulations were not specimen specific, inflicting several problems when dealing with specimens of considerably different dimensions and requiring arbitrary parameter tuning of the control variables. We constructed a model that accounts for the geometric dimensions of the specimen and is able to predict the tibial kinematics during the stance phase. The model predicts tibial kinematics of in vivo subjects with very good accuracy. Furthermore, if used in in vitro gait simulation studies, it is able to recreate physiological vertical ground reaction forces. By using this methodology, in vitro studies can be performed by taking the specimen variability into account, avoiding pitfalls with specimens of different dimensions.

Inertial control as novel technique for in vitro gait simulations

In vitro gait simulations are a preferential platform to study new intervention techniques or surgical procedures as they allow studying the isolated effect of surgical interventions. Commonly, simulations are performed by applying pre-defined setpoints for the kinetics and kinematics on all degrees of freedom (DOFs) of the cadaveric specimen. This however limits the applicability of the experiment to simulations for which pre-defined kinematics and kinetics can be measured in vivo. In this study we introduce inertial control as a new methodology for gait simulations that omits the need for pre-defined setpoints for the externally applied vertical ground reaction force (vGRF) and therefore allows the effect of interventions to be reflected upon it. Gait simulations of stance (1 s) were performed in 10 cadaveric specimens under three clinically relevant conditions: native ankle, total ankle prosthesis (TAP) and total ankle prosthesis plus triple arthrodesis (TAP+TA). In the native ankle, simulated vGRF was compared against the vGRF measured in vivo in 15 healthy volunteers and high correlations were found (R(2)=0.956, slope of regression line S=1.004). In TAP and TAP+TA, vGRF changed, therefore confirming the sensitivity of the method to kinematic constrains imposed with surgery. Inertial control can replicate in vivo kinetic conditions and allows investigating the isolated effect of surgical interventions on kinematic as well as kinetics.

Foot-ankle simulators: A tool to advance biomechanical understanding of a complex anatomical structure.

In vitro gait simulations have been available to researchers for more than two decades and have become an invaluable tool for understanding fundamental foot–ankle biomechanics. This has been realised through several incremental technological and methodological developments, such as the actuation of muscle tendons, the increase in controlled degrees of freedom and the use of advanced control schemes. Furthermore, in vitro experimentation enabled performing highly repeatable and controllable simulations of gait during simultaneous measurement of several biomechanical signals (e.g. bone kinematics, intra-articular pressure distribution, bone strain). Such signals cannot always be captured in detail using in vivo techniques, and the importance of in vitro experimentation is therefore highlighted. The information provided by in vitro gait simulations enabled researchers to answer numerous clinical questions related to pathology, injury and surgery. In this article, first an overview of the developments in design and methodology of the various foot–ankle simulators is presented. Furthermore, an overview of the conducted studies is outlined and an example of a study aiming at understanding the differences in kinematics of the hindfoot, ankle and subtalar joints after total ankle arthroplasty is presented. Finally, the limitations and future perspectives of in vitro experimentation and in particular of foot–ankle gait simulators are discussed. It is expected that the biofidelic nature of the controllers will be improved in order to make them more subject-specific and to link foot motion to the simulated behaviour of the entire missing body, providing additional information for understanding the complex anatomical structure of the foot.

Insertion of a pressure sensing arrayminimally affects hindfoot bone kinematics

BackgroundUnderstanding the development of ankle osteoarthritis (OA) is of high importance and interest; however its causality is poorly understood and several links to joint loading conditions have been made. One way of quantifying joint loading conditions is by measuring the intra-articular pressure distribution during gait simulations performed by in-vitro experimental set-ups. However the effect of inserting a pressure sensing array in the ankle joint could potentially disturb the proper kinematics and therefore the loading conditions.MethodsIn this study, we performed in-vitro gait simulations in 7 cadaveric feet, before and after inserting a pressure sensing array and quantified the effect on the joints range of motion (ROM). The gait was simulated with a stance phase duration of one second using a custom build cadaveric gait simulator (CGS).ResultsThe results show a limited effect in the ROM for all the joints of the hind foot, not exceeding the variability observed in specimens without a sensor. However, no consistent direction (increase/decrease) can be observed.ConclusionThe results suggest that even though the effect of inserting a pressure sensing array is minimal, it needs to be evaluated against the demands/requirements of the application.

Inertial control: a novel technique for in-vitro analysis of foot function

Background In-vitro gait simulations have great potential, allowing a systematic analysis of the foot function. However, it is important that the loading conditions are realistic i.e. physiologic ground reaction forces (GRF). In most experiments, in-vivo measured GRF can be imposed [1,2]. However in experimental designs that evaluate the effect of altered muscle forces on foot motion this is more complex; the effect of the altered muscle activity on the loading and kinematics cannot be taken into consideration. Therefore, we investigated the use of a new technique to simulate such cases with realistic loading conditions.

Effects of extrinsic foot musculature on hindfoot kinematics during stance phase: Implications for flatfoot pathology

Background Flatfoot deformity is a common condition, characterized by a collapse of the medial foot arch. Specific muscle dysfunctions relate to kinematic changes of the hind foot (plantarflexion, abduction and valgus) inducing the onset of flatfoot deformity. However, to determine a causal relation between individual muscle action, foot bone motion and flatfoot, in vitro experiments are needed. Our hypothesis states that inducing altered muscle forces in cadaveric feet causes alterations in kinematics, representative for flatfoot deformity [1,2]. Materials and methods A gait simulator was used to test seven cadaveric feet. Pneumatic actuators applied forces to the foot tendons, simulating flatfoot related pathologies: contracture of M. Triceps Surae (C-TS) and Mm.Peronei (C-PE); weakness of M.Tibialis Posterior (W-TP) and the pretibial muscles (W-PT); combined contracture of TS and PE (P1) and combined TS contracture with TP weakness (P2). Trajectories of bone-embedded LED clusters were measured during a one second roll-off and resulting ankle, subtalar and talonavicular joint motion was calculated.