More than 30 years ago, Neer opined that for shoulder arthroplasty to be successful, it must be considered a soft tissue procedure. Now, present day understanding of the sophisticated biomechanics of the shoulder only further reinforces this concept. The principles of joint stability, biomechanics, and kinematics must be considered, understood and respected in all human joint replacements. This is particularly critical in consideration of shoulder joint arthroplasty because the shoulder has the least intrinsic stability of any human joint and depends primarily on accurate and physiologically correct soft tissue tensions for stability, motion, and function. Appropriate myofascial sleeve tension, balanced musculoligamentous structures and accurate reproduction of component version are critical to re-establish during shoulder arthroplasty if the surgeon is to reproduce those parameters which insure established biomechanical principals. Because the shoulder is not weight-bearing, biomechanical principals are often thought to be irrelevant and unimportant. However, the shoulder is very much load-bearing and adherence to the biomechanical principles of muscle length-tension relationships and load transmissions across prosthetic interfaces ensures joint survival, maximal function, and patient satisfaction. This lecture will review and discuss the most critical biomechanical principles as they relate to replacement arthroplasty of the shoulder. Specifically, the principles will be related to the most recent generations of shoulder implant designs, bone preparation, and cement techniques.
Reconstructing a shoulder fracture with an arthroplasty is analogous to solving a jigsaw puzzle. Before beginning the task of assembling the pieces of a puzzle, the final “picture” must be seen, as this will guide the proper location for the individual puzzle pieces. And, only when all the pieces are properly placed will the final picture be defined and recognised as a piece of art! In the same way, following a three or four part displaced fracture of the shoulder, reassembly of the various pieces requires time and skilful appreciation of the three dimensional anatomy of bone, ligament and muscle. These tissues are like puzzle pieces which, when assembled according to the rules of anatomy and physiology, will lead to successful outcome measured by minimal pain and functional recovery of shoulder motion. This presentation will examine each of these puzzle pieces – the bone fragments, the muscle, and the ligaments – individually and functionally integrated. We will review their importance in the overall scheme of shoulder reconstruction following a complex fracture. The critical concerns of humeral length, humeral version and myofascial sleeve tension will be discussed in detail. It is the recognition of these anatomic and physiologic necessities, which allow the final shoulder radiograph and the patient, to indeed resemble a piece of art.
