The influence of conformity and constraint on translational forces and frictional torque in total shoulder arthroplasty.

Glenoid loosening in total shoulder arthroplasty (TSA) may result from translational forces and frictional torque generated at the articular surfaces and transmitted to the fixation interface. The effect of glenohumeral articular design on the magnitude of these translational forces and frictional torques has not been evaluated previously. Seven different glenohumeral designs were evaluated on a specially designed fixture integrated with a materials test system. The degree of conformity and constraint in the glenohumeral designs directly affected the forces generated during translation and rotation of the humeral head. Maximum translational forces occurred close to the midline in the more conforming design. The less conforming designs generated lower translational forces and frictional torques. Because frictional torque and translational forces at the glenohumeral articulation may contribute to glenohumeral loosening, the use of less conforming and less constrained designs is advocated where possible to reduce the potential for mechanical loosening of the glenoid component.