Current status and perspectives of shoulder replacement

Basis of the modern shoulder implants is the Neer II-system, a nan constrai ned total shoulder prosthesis with conforming radii of curvature and improv ed protection against dislocation. The second generation of shoulder prosth esis is based on the geometric shaft design of the Neer II prosthesis and o ffers not only a variety of modular head- and shaft-sizes but also through different radii a physiologic rotation-translation-mechanism. The third gen eration of humeral head prosthesis carries the concept of an anatomic recon struction one step further and enables the surgeon to adjust the inclinatio n and the eccentric offset of the humeral head to restore the centre of rot ation. The latest development in shoulder arthroplasty are humeral head pro sthesis with a fully variable 3-dimensional modularity to independently adj ust the prosthestic head position regardless of the individual shaft geomet ry. Th is achieves a 3-dimensional adaptability of the prosthetic head abou t the stem axis in the coronary and in the sagittal plane. Besides of the h umeral shaft prosthesis an alternative concept of shoulder joint replacemen t is established - the replacement of the humeral head articular surface. A hemispheric surface prosthesis - cup arthroplasty - is cemented onto the r esidual humeral head, which eliminates the obligatory humeral head resectio n and the reaming of the medullary canal. Bipolar shoulder prosthesis are h umeral shaft prosthesis with a bi-rotational head system. Their indication is limited to pre-existing lesions of the rotator cuff and/or the glenoid s urface. The inverse totals boulder prosthesis reverses the articular surfac e morphology of the humeral head and the glenoid. The hemispheric glenoid c omponent serves as the centre of rotation for the concave epiphyseal proxim al humerus component. This implant is especially used in cases of massive r otator cuff deficiences. The role of shoulder prosthesis in treating acute humeral head fractures needs special consideration. A fracture prosthesis h as to restore the exact length of the humerus, the centre of rotation, and the anatomical retroversion. Positioning of the tubercula and their adequat e osteosynthesis is most critical and fundamental to ensure a correct heali ng process. A failed consolidation of the tubercula does not lead to a sati sfying result. The shoulder joint replacement can be sufficiently fixated i n cemented, cementless or hybrid techniques. Today several component design variations of cemented glenoid implants exist. Their main distinction is t he fixation system which can be divided into two main groups - the keel - a nd the peg-shaped glenoid components. The peg-shaped anchorage system shall guarantee a greater stability against shear-forces. Cementless glenoid com ponents consist of a polyethylen inlay and a surface treated metal-back wit h an integrated fixation system. These fixation systems are object of inten sive biomechanical research and range from conventional screw fixation to s pecialised cone systems and self-cutting cage-screw-systems. The critical a rea of cementless glenoid components is the transition zone of the PE-inlay and the metal-back because of high force development. The question of impl anting a hemi- or total shoulder prosthesis is answered by the morphologic changes of the glenoid articular surface, which includes the size of the su bchondral defect and the underlying etiology of the shoulder joint disease, and the age of the patient. Preoperative planning must consist of an adequ ate radiologic work-up - X-ray, CT or MRI - to accurately assess the glenoi d morphology. G. Walch categorised the different glenoid lesions and develo ped a very important classification of possible glenoid deformations. To compare and evaluate the operative results one must consider the differe nt shoulder prosthesis and the discrepancies between a hemi- and a total sh oulder prosthetic replacement. Looking at the loosening and survival rate o f the implant the results are related to the type of prosthesis and the pre operative diagnosis. The Neer total shoulder prosthesis has a 15 year survi val rate of 87%, compared to 74% of the hemi-prosthesis. The objective far the future has to be to further advance the development of prosthetic compo nents, especially for primary joint replacement in acute humeral head fract ures. Another point of interest is how to reduce the still existing high lo osening rates of the glenoid components. A fairly new research field is the computer-assisted surgery, e.g. navigation systems and robotics. The compu ter-assisted navigation could be of great advantage to accurately find the individual resection plane (inclination and restroversion) of the humeral h ead. The use of a surgery-robot could be Very helpful to reproducibly achie ve the desired conformity of the articular surface when preparing the gleno id.