Mechanistic Characterization of Shoulder Function and Tissue Loading

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Shoulder function is crucial for human activities. In the modern world, shoulder tissues experience loads and scenarios that often cause damage, decreases in quality of life, and reduced productivity. Over half of Canadians will have shoulder damage in their lifetimes. The guiding objective of the proposed research program is to systematically improve quantification of shoulder tissue mechanical exposures, failure tolerances, and their interactions to better understand healthy shoulder function and its implications for human life. Shoulders and humans are complex, symbiotic mechanical systems. The incredible flexibility of the shoulder allows diverse capabilities, but comes at the cost of low mechanical joint stability. Biological variability across the human population further complicates shoulder evaluation. My proposed research will resolve crucial fundamental uncertainties in shoulder mechanics through three linked objectives. Objective 1: Characterize the influence of multi-scale population variability on tissue exposures in silico: Five major metrics will be treated as variable parameters within my existing shoulder model to define which metric(s) influence tissue response the most. These are: 1) tissue composition, 2) glenohumeral stability, 3) shoulder rhythm, 4) segmental kinematics, and 5) task demands. Objective 2: Quantify the effects of shoulder muscle fatigue on kinematic responses in vivo: The shoulder must function in the presence of many stressors. Muscular fatigue is one of the primary factors that alters normal shoulder mechanics. We will use multi-dimensional symbolic motion segment characterization to identify experimental fatigue progression. The data will be used to improve the dynamic fatigue response of the probabilistic model (Obj. 1) and to guide tissue tolerance testing protocols (Obj. 3). Objective 3: Define tissue tolerances to realistic mechanical loads in vitro: Physiological supraspinatus tendon loading is inseparable from the shoulder's incredible postural flexibility, which causes untested complex tendon deformation. Specimens will be tested with a novel focal tissue system capable of applying complex load profiles, yielding advanced data on the influences of combined torsional, focal, and longitudinal exposures to help clarify supraspinatus tendon failure mechanisms (Obj. 1). In summary, shoulder mechanics from bone movement to specific tissue loads and failure modes - will be assessed with integrative, cutting edge approaches from across the field of biomechanics. The data-driven probabilistic model will exceed current attempts to describe human shoulder mechanics, providing descriptions of shoulder behavior that reflect human variability. Ultimately, these fundamental insights into shoulder function will provide guidance on methods to maintain and exploit the incomparable potential of our upper limbs.