Optimizing glenosphere position and fixation in reverse shoulder arthroplasty, Part One: The twelve-mm rule

The Aequalis Reversed Shoulder Prosthesis (Tornier, Inc., Edina, MN) is a successful treatment option for older, low-demand patients with rotator cuff arthropathy. Scapular notching is the most common radiographic complication and is associated with poorer intermediate-term clinical outcomes. Aligning the lower border of the glenosphere baseplate with the inferior glenoid rim has been recommended to reduce this complication, but guidelines for achieving this positioning are currently lacking. The purpose of this study is to develop a rule of thumb that will facilitate proper glenosphere component positioning intraoperatively. Utilizing a computed tomography-templating protocol, we found that the ideal location of the drill hole for the baseplate post was 11.5 +/- 1.0 mm above the inferior glenoid rim. On the basis of measurements from 10 implanted cadaveric specimens, we concluded that drilling the baseplate posthole 12 mm above the inferior glenoid rim-the 12-mm rule-will result in excellent glenosphere position in most cases.