Effect of humeral head component size on hemiarthroplasty translations and rotations

Glenohumeral translation and rotation were measured in 6 grossly normal, fresh frozen shoulder preparations while a manual load was applied to the humerus. The same tests (maximum elevation, total rotation, anterior/posterior (A/P) translation, and inferior translation) were repeated for each shoulder through 8 series: 1 with the shoulder intact, 1 with the shoulder vented, and 6 with progressively larger humeral head components after hemiarthroplasty.There was an inverse linear relation between humeral head component size and all 4 outcome variables. Replacing the native head with a component of equal diameter reduced elevation 20%, rotation 40%, A/P translation 50%, and inferior translation 60% in the vented shoulder. Replacing the native head with a component of equal effective volume decreased elevation 8%, rotation 20%, A/P translation 25%, and inferior translation 40% in the vented shoulder. Increasing humeral head component size decreased rotation, A/P translation, and inferior translation by similar percentages and elevation somewhat less. Humeral head component size is better described in terms of volume than in terms of diameter or offset. (J Shoulder Elbow Surg 1998;7:591-8.)