Proximal humeral anatomy in shoulder arthroplasty: Implications for prosthetic design and surgical technique.

Resurfacing arthroplasty of the glenohumeral joint has been established for several decades as a means to restore comfort and function of the shoulder for many afflictions that derange the normal anatomy. Rigorous study of shoulder anatomy in terms relevant to prosthetic geometry, however, did not begin until the 1990s. It has become apparent that normal anatomy is highly variable from individual to individual and that it is aligned somewhat differently than the early modular prosthetic devices. Multiple studies have shown that retroversion is markedly variable, not only between individuals but also between the left and right sides of the same individual, ranging from 0 degrees to 55 degrees . The inclination of the proximal humeral articular surface relative to the shaft (head shaft angle) is also variable, ranging from 30 degrees to 55 degrees. The center of rotation is variably offset in 3 dimensions. The radius of curvature ranges from 20 to 30 mm and is smaller in women than in men. The thickness of the articular surface, head height, is equally variable but shows a striking proportionality to the radius of curvature. The last decade has seen a proliferation of humeral implants aiming to serve a better understanding of shoulder anatomy and function. A modern understanding of shoulder anatomy and contemporary design features is paramount for surgeons performing shoulder arthroplasty. Head size selection is dependent on multiple factors, the most important of these, other than the patient's original head size, are the osteotomy performed by the surgeon and the inclination angle(s) of the prosthetic system. Systems with variable inclination angles instruct the surgeon to resect the humeral head along the anatomic neck as best possible and then provide either adjustable or variable prosthetic geometries to match the resultant inclination angle. Other prosthetic systems have a fixed inclination angle somewhere within the normal range and instruct the surgeon to make an osteotomy at this inclination, adjusting the fit with additional preparation of the canal and revising the osteotomy as necessary. Over the last decade, most systems have offered heads that are offset by 3 to 4 mm, allowing the surgeon to dial in the most suitable position for the head on the stem. Component design and surgical technique are inextricably intertwined. Most surgeons with experience using modern systems feel a greater sense of predictability in achieving their surgical goals when using these systems as compared with earlier ones. How these developments impact implant longevity will only be known with time and further follow-up.