Tracking Translations in the Hip and Shoulder Joints
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The hip joint is often described as a ball and socket joint, which has two key implications: congruent interacting bony joint surfaces and purely rotational relative motion. This model can be challenged in two ways: that the geometry of the hip does not have the congruency required for sphere-on-sphere contact, and that the joint translation occurs when the joint is moving within its accepted range of motion. The closely related shoulder joint is known to have translatory motion; however, the extent of these translations attributable to different soft-tissue constraints is not well understood. The subject of this dissertation is the measurement of translatory motions and understanding the role of surrounding soft tissues in the overall kinematics of human hip and shoulder joints. Here, human cadavers were used as a model for studying detailed passive motion. A surgical navigation system, with sub-millimeter accuracy, was used to measure the translatory motion of these cadavers in different soft tissue states. Every cadaver hip and shoulder exhibited subtle but definite aspherical motions. Soft tissues partially constrained the hip but did not correct for underlying morphological asphericity. There was no consistent pattern of translation. Changes in the shoulder soft tissues led to distinct changes in the translatory motion that could be used to describe the etiology of shoulder instabilities.