Evaluation of the contact forces developed in the lower limb/orthosis interface for comfort design

Abstract

Ankle-Foot Orthoses (AFOs) are technical aids that promote locomotion and rehabilitation of individuals with gait pathologies. Every pathology has its own requirements and this has led to classical solutions that are far from optimal in terms of comfort and overall performance for restoring the human gait. Formal scientific approaches are therefore necessary to take full advantage of the available technological potential and produce more efficient modern devices that can significantly improve the quality of life of people with locomotion impairment. In this work, a computational multibody dynamics approach is adopted. A multibody model of an active AFO is developed and integrated in a whole-body multibody human model developed in the MATLAB environment. In the adopted procedure, normal gait motion data is used as input to the biomechanical model. The experimental data was obtained in the gait lab by acquiring the kinematic and kinetic data of a stride of a healthy female subject. The aim of this integrated model is to estimate the loads transmitted at the foot-leg-orthosis interface. To achieve this, it is essential to decide where the contact between the lower limb and the orthosis should occur, and what are the forces generated in such interface. A contact model between a sphere and a plane is applied together with a friction model to determine the contact forces, and the properties used for the surfaces in contact were based on the available data in the literature and on experimental tests. The results revealed that the contact pressures obtained are below the PPTs (Pain Pressure Thresholds), which represent the patient’s comfort. However, the pressure is dependent on the contact properties between surfaces, meaning that these must be experimentally determined for each material. It was also uncovered that the weight of the orthosis is an important issue in the design of an active AFO as it greatly influences the moments at the ankle, knee, and hip and, therefore, the patient effort when wearing the orthosis.