Musculoskeletal Actuation System of Thumb Metacarpal of the Human Hand

Abstract

The thumb is a remarkable digit that is essential for our capability to carry out a variety of complex tasks, from power grip operations to fine precision movements. The thumb metacarpal has a very special role in the functioning of the human hand due to the presence of the Trapeziometacarpal joint at its base. The intricate anatomy, kinematics and muscular actuation system that controls the movement and stability of the thumb metacarpal bone is responsible for its exceptional adaptability. It is essential to understand the musculoskeletal actuation system and the control of the thumb metacarpal through the Central Nervous System for better prosthetics, hand surgery, tendon-based rehabilitation mechanisms, and robotic hands. Understanding and evaluating the extensions and force patterns generated in the Muscle-Tendon Units responsible for the motion of the thumb metacarpal is crucial. This thesis investigates the dynamic behaviour of musculoskeletal actuation mechanism of the thumb metacarpal by examining the interaction between muscles tendons, and bones. This research seeks to enhance our understanding of the muscle tendon actuation mechanism of the thumb metacarpal, to elucidate the extensions and force patterns generated, and the role of the Central Nervous System, for commonly required motions of flexion-extension, adduction-abduction, and circumduction. Initially, the model has been developed for the muscle-tendon actuation system for one of the phalanges of the hand for desired general hand motions, viz. flexion extension, adduction-abduction, and circumduction considering a generalized approach. The phalange is represented by a cylindrical rigid body, actuated by four muscle-tendon units attached to it symmetrically. The muscle-tendon unit is based on the modified Hill’s muscle model. Word Bond Graph Objects (WBGOs) have been used to model each of the component subsystems, including the bone, muscle-tendons, translational couplings, rotational couplings, etc. making the system flexible and modular. Every WBGO has clearly defined input and output to communicate with other WBGOs and the overall system. The role of the Central Nervous System which commands desired motions is emulated through a virtual domain in the model. The muscle-tendon controller decides the activation pattern for the muscle-tendon units. Accordingly, the muscle-tendon units apply forces on the phalange/bone to achieve the desired motion. The model facilitates a better understanding of the mechanical behaviour of the different components of the musculoskeletal system (joints, muscles tendons, etc.) and their interaction. The model has been extended to include research on the dynamics of the musculoskeletal actuation system of the actual thumb metacarpal in the human hand. The concept developed is then applied to the flexion extension motion of the thumb metacarpal considering the actual location and orientation of the joint axis and the associated asymmetric muscle tendons. The motion of the thumb metacarpal to scribe a round, circular segment in the air is known as circumduction. There is a distinctive motion characteristic of the thumb metacarpal in which the head and the base of the thumb metacarpal rotate in opposite directions during circumduction. The motion of the thumb metacarpal has been modelled using a bond graph model for verifying the shape of the articular profile generated at its base during circumduction motion and for determining the actuation patterns, and corresponding forces generated among the associated muscle-tendon units. Signals are provided for the targeted task via a virtual domain emulating the central nervous system. The muscle-tendon controllers activate the muscle-tendon units to determine the extensions and force patterns needed to be applied on the real thumb metacarpal, to follow the desired articular motion. The model is simulated to understand the musculoskeletal actuation system of the thumb metacarpal during circumduction. The extensions and force patterns of the muscle-tendon units among other important details were not taken into account in earlier static models. The work presented in this thesis makes a substantial contribution by offering a methodical and concrete foundation for taking these important effects into account.