Displaced fractures of the proximal humerus present the surgeon with one of the more difficult challenges in fracture surgery. Prosthetic replacement in three and four-part fractures has frequently led to poor functional outcomes despite adequate fixation and healing of the prosthetic-tuberosity construct. Functional failures in these cases may be related to failure to reproduce the normal anatomical relationships of the proximal humerus during reconstruction. Common errors include failure to restore proper height and retroversion. The geometry of first and second generation prostheses made it difficult to reproduce the anatomic gleno-humeral relationship and subsequently accurate restoration of the relationship of the tuberosities to the articular surface. The Polarus Modular Shoulder System has been designed to allow the surgeon to consistently and accurately reproduce the normal proximal humeral anatomy.