The radiographic study in the relationship of the glenohumeral joint.

Accurate reproduction of anatomic relationship is important in non-constrained prosthetic arthroplasty. The accurate lateral glenohumeral offset, which indicates a parameter of the lever arm of the deltoid and supraspinatus muscles, is one of the most important elements in achieving the efficient shoulder functions after prosthetic reconstruction. However, to our knowledge, there has been no detailed study on the influence of minute changes in the neck shaft angle, within the normal range, on lateral glenohumeral offset. In this study, we evaluated the relationship between the neck shaft angle and various geometric measurement values in the glenohumeral joint. Radiographs of 471 shoulders without osseous lesions in the glenohumeral joint and greater tuberosity were reviewed. There were 265 males and 206 females, and a mean age was 53.5 years old. The final diagnosis was impingement syndrome in 269 patients, rotator cuff tear in 147, and control group in 55 patients. Eight parameters were measured on true anteroposterior radiographs. The lateral glenohumeral offset correlated with the radius of the curvature of the humeral head and the humeral head diameter. The neck shaft angle was not correlated with the offset value. However, when the offset value was divided by the humeral head diameter, to eliminate the affection of the size of the humeral head toward the offset, a correlation was observed with the neck shaft angle. Based on these correlations, we could obtain the equation of the lateral glenohumeral offset according to the neck shaft angle. Using the equation, when the neck shaft angle is decided, the lateral glenohumeral offset can be estimated based on the correlation between the parameters, which achieve the efficient shoulder functions after prosthetic components. Therefore, inaccurate determination of the neck shaft angle in the humeral head component design and selection may induce dysfunction of the abductor muscles.