The glenohumeral offset ratio: A radiographic study

Abstract

A systematic method of component selection for total shoulder arthroplasty is needed. The method must take into account the soft-tissue constraints of a degenerative joint and optimize joint biomechanics by placing the joint line in the best possible position. The purpose of our study was to determine radiographically the normal glenohumeral joint line position based on a ratio of distances between the joint line and fixed landmarks on the humerus and scapula. We studied modified anteroposterior radiographs of the glenohumeral joint in 86 volunteers (51 men and 35 women; ages ranging from 21 to 47 years). Two measurements were made on each radiograph: (1) the perpendicular distance from the most medial portion of the glenoid to the inferior base of the coracoid process at its attachment to the scapular blade, and (2) the perpendicular distance from the midline of the humeral shaft to the most medial point on the humeral head. The joint line position was described as the ratio of the glenoid measurement to the sum of the two measurements (i.e., the glenohumeral offset ratio). The validity and reliability of glenoid offset measurements were determined by comparing radiographic and anatomic measurements of glenoid offset in cadaveric human scapulae. Radiographs were made with rotational error to determine its effects on the measurement of humeral offset. Humeral offsets and glenoid thicknesses of five different total shoulder systems were then determined from template overlays. The mean glenohumeral offset ratio was 0.31 (range 0.18 to 0.39). We detected no significant difference in the ratio between men and women volunteers. There was close agreement between radiographie and direct (anatomic) measurements of glenoid offset in cadaveric scapulae. Values for humeral offset were not significantly affected by radiographic rotational error. The evidence indicates that a fairly constant glenohumeral offset ratio in normal shoulders can be reliably calculated from a single radiograph. In addition to the prosthesis specifications, we suggest that the glenohumeral offset ratio is a potentially useful preoperative planning tool for total shoulder arthroplasty.