THE 3-DIMENSIONAL GEOMETRY OF THE PROXIMAL HUMERUS - IMPLICATIONS FORSURGICAL TECHNIQUE AND PROSTHETIC DESIGN

We have studied the three-dimensional geometry of the proximal humerus on human cadaver specimens using a digitised measuring device linked to a computer. Our findings demonstrated the variable shape of the pro ximal humerus as well as its variable dimensions. The articular surfac e, which is part of a sphere varies individually in its orientation as regards inclination and retroversion, and it has variable medial and posterior offsets. These variations cannot be accommodated by the desi gns of most contemporary humeral components. Although good clinical re sults can be achieved with current modular and non-modular components their relatively fixed geometry prevents truly anatomical restoration in many cases. To try to restore the original three-dimensional geomet ry of the proximal humerus, we have developed a new type of humeral co mponent which is modular and adaptable to the individual anatomy. Such adaptability allows correct positioning of the prosthetic head in rel ation to an individual anatomical neck, after removal of the marginal osteophytes. The design of this third-generation prosthesis respects t he four geometrical variations which have been demonstrated in the pre sent study. These are inclination, retroversion, medial and posterior offset.