From inverse dynamics hand modelling to distal radius fracture healing simulation

Abstract

Musculoskeletal research questions regarding the prevention of chronic overloading or rehabilitation of the hand can be addressed using inverse dynamics simulations when experiments are not possible or ethically questionable. To date, no complete human hand model implemented in a holistic human body model has been established. The aim of the first part of this work was to develop, implement, and validate a detailed hand model using the AnyBody Modelling System (AMS) (AnyBody, Aalborg, Denmark). To achieve this, a consistent multiple cadaver dataset, including all extrinsic and intrinsic muscles, served as a basis. Various obstacle methods were implemented to obtain the correct alignment of the muscle paths. For model validation, experimental datasets from the literature were used, which included the comparison of numerically calculated moment arms of the wrist, thumb, and index finger muscles. In general, the results displayed good comparability of the model and experimental data. In contrast to this validation, the aim of the next study was to further validate the hand model by analyzing numerically calculated muscle activities in comparison to experimentally measured electromyographical signals of the muscles. Therefore, the electromyographical signals of 10 hand muscles of five test subjects performing seven different hand movements were measured. The kinematics of these tasks were used as an input for the hand model, and the numerical muscle activities were computed. To analyze the relationship between simulated and measured activities, the time difference of the muscle on- and off-set points were calculated. The results showed that the hand model fits the experiment quite accurately despite some limitations. Therefore, this study is a further step towards patient-specific modelling of the upper extremity. This developed hand model was then applied to a specific hand gripping task, which patients were urged to perform after a conservatively treated distal radius fracture. Especially the transferred wrist load was of great interest, as it is the main mechanical stimulus to the fractured radius healing performance. The musculoskeletal hand model was adapted and used to determine muscle and joint forces in the wrist that occur as a consequence of this hand grip contraction. The movement has been captured precisely by motion capture analysis in combination with electromyographic (EMG) data of all essential muscles in the forearm and hand. On top, the acting contact forces have been determined with the aid of pressure mapping sensors attached between the fingers and hand grip tool. By determining a characteristic force distribution among the fingers, it was possible to apply the actual forces which act on the individual fingers during the simulation. A comparison with in vivo studies showed a good accordance of the predicted muscle force distribution and could determine a prediction of the applied mechanical stimulus onto the fractured radius. Simulating diaphyseal fracture healing via numerical models has been investigated for a long time. It is apparent from in vivo studies that distal radius fracture healing should follow similar biomechanical rules, although the speed and healing pattern might differ. To investigate this hypothesis, a pre-existing, well-established diaphyseal fracture healing model was extended to study metaphyseal bone healing. Validation through clinical data of distal radius fractures compared to corresponding geometrically patient-specific fracture healing simulations was successful. Therefore, the model appeared appropriate to study metaphyseal bone healing under differing mechanical conditions and metaphyseal fractures in varying bones and fracture types. Nevertheless, the model was conducted in a simplified rotational symmetric case. Further studies identified crucial numerical problems in the algorithm to be able to establish three dimensional simulations. We solved this by applying an iterative convolution approach and decreased the computational demands with factors up to 70. Preliminary results indicated, that the modelling approach, as depicted for rotational symmetric geometries also showed plausible results in three dimensions. This model and results will help optimizing clinical treatments on radial fractures, medical implant design and foster biomechanical research in fracture healing.