Osteometry of the glenohumeral joint.

Measurements made from radiographs of the shoulder are often used for evaluation of the position of a shoulder prosthesis after surgery. The distances measured from a radiograph are subject to projection errors arising from positioning of the patient in front of the x-ray cassette and from the direction of the x-ray beam at the time of the exposure. The projection error was studied on radiographs of the glenohumeral joint in cadaver preparations. Measurements of the spatial relationship between the glenohumeral joint and the scapula changed by 10% to 50% when the specimen was rotated by 20 degrees around an axis through the glenohumeral joint. The values measured on radiographs of the proximal humerus changed by less than 10% when the humerus was rotated by 20 degrees (exorotation and endorotation). Proposals are made for specific measurements in evaluating the shoulder before and after a prosthesis is implanted.