Estimation of joint contact pressure in the index finger using a hybrid finite element musculoskeletal approach

The human hand is a sophisticated biological tool involved in many activities, as gripping tasks that are essential for most movements of everyday life. The pinch grip, which consists in holding objects between the thumb and the index fingertips, is one of the most used gripping techniques. Because of its specific biomechanical configuration, the pinch grip technique can lead to high joint loadings that expose the fingers to injuries such as osteoarthritis. Therefore, knowledge of the local stress distribution in hand joints is crucial to understand injuries and osteoarthritis occurrence. Today, determining cartilage contact stresses remains a challenge. Direct measurements of joint pressures on patients or cadaver specimens are highly invasive and ethically questionable. For this reason, the development of numerical models is required to represent the hand biomechanics thus providing a tool to better understand joint disease occurrence and development. Among the various advantages offered by Simulia Abaqus for finite element modelling, it was chosen mostly because of the possibility to develop homemade material properties (UMAT), the power of the solver to manage complex contacts, and the interaction with python scripting to easily generate a large cohort of models. These capacities are necessary to model the complex environment of the index finger. The scope of this project was to develop a hybrid biomechanical model of the index finger to estimate in-vivo joint contact pressure during a static maximal strength pinch grip task. A finite element model including bones, cartilage, tendons, and ligaments was developed, with tendon force transmission based on a tendon-pulley system. This model was driven by realistic tendon forces estimated from a multi-rigid body model and motion capture data of six subjects. Here following the main steps of the finite element modelling:

• Bone geometries were obtained using medical imaging data from computer tomography (CT-scan) and the material properties of the bones were calculated element by element by the apparent density of the CT-scan images.
• Cartilage was modelled at each index finger joint and modelled using a hyper-elastic material specifically developed for the hand;
• Ligaments of the index finger were modelled as nonlinear spring elements with a tension-only behaviour;
• All flexor and extensor tendons composing the index finger were modelled with linear elastic properties and represented the complex assembly of fibres in CATIA V5;
• Multi-articular tendons were held tight to the bone by annular pulleys providing via-points for tendons and modelled in CATIA V5;
• The contact at each cartilage joint surface was modelled as a softened pressure-overclosure relationship with a friction coefficient. The Abaqus Explicit solver was used for making the contact model robust;
• The proximal end of the metacarpal bone was fully constrained and index fingertip nodes were restricted to one degree of freedom to model the pinch grip task.

The model was used to compare mechanical loadings in the three finger joints of six healthy participants. The resulting mean contact pressures for the maximal voluntary contraction task performed by the six subjects showed significant inter-subject differences. Two groups of subjects could be identified: for one group, joint contact pressure was higher in the distal joint while for the other subjects it was higher in the proximal joint. These differences indicated that the same movement could derive from different biomechanical neuromuscular strategies leading to different postures. Some of them may be more disadvantageous and compromising than others, thus representing a higher risk factor of joint disease. Other information relative to the osteoarthritis occurrence could be obtained from the contact pressure values. Mean contact pressures obtained in the finite element simulation did not suggest excessive or traumatic use of the cartilage. Indeed, this value could be considered as normal loadings needed for cartilage development and renewal. However, peak joint contact pressures corresponded to an excessive mechanical load which could cause chondrocyte death and extracellular matrix damage. Nevertheless, it should be kept in mind that only two repetitions of the pinch grip task at extreme force levels were asked to the subjects in the experimental protocol. Thus, the studied task could be considered as a worst-case scenario. The project presented here combines two numerical approaches (finite element model and multi-rigid body) commonly used in computation biomechanics to estimate the local stress distribution in the three index finger joints using non-invasive in-vivo data. This model has been recognized as a breakthrough for hand biomechanics, overcoming years of lack of data. It represents a remarkable tool to deeper understand the mechanical determinants of hand joint diseases as osteoarthritis. Other studies based on this finite element model are in progress, using this numerical modelling for the improvement of surgical procedures and the development of innovative medical devices, such as arthrodesis implants.