Biomechanics of the Trapeziometacarpal Joint during Smartphone Use

Increased smartphone use has altered the functional role of the thumb; from providing stabilisation during daily activities such as gripping objects, to acting as the dominant digit during touchscreen interaction. This shift has raised clinical concerns regarding excessive loading of the trapeziometacarpal (TMC) joint and its potential contribution to early-onset TMC osteoarthritis. However, TMC joint loading during smartphone use remains poorly understood, due, in part, to limitations in experimental measurement of the loading during smartphone use and in relevant computational models of the joint. This thesis investigated the biomechanical response of the TMC joint during smartphone interaction by integrating experimental measurements of thumb kinematics and kinetics and developing a finite element (FE) model able to predict stresses in the joint. A novel smartphone-sized force plate was designed and validated to capture the low-magnitude thumb forces during natural touchscreen interaction. Combined with optical motion capture, this system enabled synchronised acquisition of thumb-joint kinematics and applied forces during representative smartphone tasks of healthy volunteers. These provided input data for FE simulations of the TMC joint during smartphone activities. An FE modelling framework of the TMC joint was developed to enable prediction of articular cartilage stress under smartphone-related loading. Systematic sensitivity analyses quantified the influence of key modelling uncertainties, including variability in ligament delineation method and bone geometry, both of which were found to substantially affect predicted cartilage stress distributions. FE simulations revealed that specific thumb postures during smartphone use produced elevated cartilage stresses, indicating increased mechanical demand at the TMC joint, and therefore risk in developing osteoarthritis. Collectively, this work establishes a robust experimental-computational framework for evaluating TMC joint loading during smartphone use and provides biomechanical evidence to inform ergonomic smartphone interface and accessory design aimed at reducing excessive TMC-joint loading and mitigating osteoarthritis risk.