Prolonged exposure to tool vibration causes vascular, neurological and musculoskeletal abnormalities in hand-arm system, which is collectively known as hand-arm vibration syndrome (HAVS). HAVS is a major musculoskeletal disorder (MSD) affecting large numbers of construction workers and miners. One of the major symptoms of HAVS is vibration white finger (VWF) caused by exaggerated vasoconstriction of the arteries and skin arterioles. A significant number of construction workers, miners and even dentists are affected by HAVS. The precise mechanism or pathogenesis of the syndrome still remains unclear, and there is a need for better understanding of various biodynamic responses induced by vibration. Accurate analysis of hand-arm vibration response is very difficult due to the complexity of the hand-arm structure such as redundancy of the musculo-tendon unit, active participation from central nervous system, inherent non-linearity and heavy damping effect. Three fundamental modeling methods are developed in this study to understand the cause of musculoskeletal responses and vascular system responses of the hand-arm system.
The first model is a musculoskeletal kinematic model developed by adopting a modified Hill's model to describe the behavior of the muscle and tendon. At first, the force in each muscle necessary to generate the grip force is determined by a quadratic optimization process. Using the force determined, the activation level of the muscle is found by assuming that the muscle-tendon undergoes an isometric contraction during the gripping. With the activation level known, the muscle-tendon system is simplified by a set of spring-mass-damper based on the assumption that the muscle response to the vibration is passive. From dynamic analysis it is shown that smaller intrinsic muscles are vulnerable to high frequency vibrations even if the overall response of the hand-arm is small.
The second model addresses vasospasm in finger artery caused by vibration, therefore a small artery is modeled as a fluid filled elastic tube whose diameter changes along the axial direction. Equations of motion are developed by considering interactions between the fluid, artery wall and soft-tissue bed. It is shown that the artery system shows a spatial resonance which responds with the highest amplitude at the location determined by the vibration frequency. This implies that a long-term use of one type of tool will induce high-level stresses at a few identical locations of the artery that correspond to the major frequency components of the tool. Hardening and deterioration of the artery at these locations may be a possible cause of VWF. The wave model is applied also to arteries of larger size such as coronary arteries, employing the equivalent circuit representation.
The third model is a flexible finite element based model of human index finger, which is created with most anatomical substructures, taking into account nonlinear tissue properties to study localized stress and strain pattern under dynamic loading. High strain and energy absorption can cause local tissue and nerve damage leading to numbness.
All these unique modeling techniques provide much needed methodology to understand various aspects of HAVS and its precursor.