On the hypotheses that determine the definitions of glenohumeral joint motion: with resolution of Codman's pivotal paradox.

HYPOTHESIS: Current terminology of shoulder motion does not fully explain several well-known paradoxes. We developed a model of glenohumeral joint motion that describes shoulder motion more accurately and provides the ability for computer modeling of shoulder motion.
METHODS: All glenohumeral joint motions can be defined by the model, as well as resolving the problem of well-known paradoxes of shoulder motion. The group model was also extended to infinitesimal rotations of the glenohumeral joint. This model then becomes a description known in physics as Eulerian angles, the description of rotations of a rigid body. By use of this model, computer modeling of shoulder motion is possible, whereas it is shown that it is not in the current terminology. CLINICAL APPLICATIONS: Several clinical situations are discussed and analyzed by the new model of shoulder motion. Among these are Codman's pivotal paradox and a paradox of shoulder motion described by Rowe. Both of these can be resolved mathematically using the group model of shoulder motion. Discussion centers on how the new model may affect therapy models for these motions.
CONCLUSION: We conclude that a symmetric group model of motion centered at the glenohumeral joint may allow computer modeling of shoulder motion, allow resolution of several important paradoxes concerning shoulder motion, and have importance in therapeutic rehabilitation of shoulder problems.