There were two purposes for this study. One was to identify risk-factors on dynamics which affected humeral head abnormality on MRI using logistic regression analysis. The other was to simulate stress distribution in humeral head at maximum external rotation (MER) of throwing phase using Finite Element Method analysis (FEM). Then we evaluated the validity of the simulation using MRI findings of humeral head abnormality. The subjects were 18 asymptomatic collegiate baseball players who took part in both MRI and pitching motion analysis. The shoulder joint reaction force was calculated using multi-body dynamics analysis with musculoskeletal model analyzed from foot contact to ball release of pitching motion. We did logistic regression analysis to identify whether the force was a risk-factor or not. As a result, the risk-factor of humeral head abnormality on dynamics was the shoulder joint reaction force (shearing force) at the acceleration phase. We simulated the stress distribution in the humeral head at MER using FEM model and input the shoulder reaction force. Stress distribution in the simulation was almost the same as that of humeral head abnormality in MRI. This simulation is expected to be a useful predictor of the humeral head abnormality caused by internal impingement of throwing shoulder injury.
