Design of Humeral Stems

Abstract

The first attempt at prosthetic replacement of the humeral head was performed in 1893 by Pean to treat a shoulder infected with tuberculosis. Modern total shoulder arthroplasty, however, really began in the 1950s with the cemented Neer prosthesis [1] (Fig. 14.1) which gave reasonable short and long term results in terms of pain relief, function and movement and a low incidence of humeral loosening [2]. Since then there has been significant evolution both in terms of materials but also in design of the humeral prostheses. Initially humeral components were monoblocks available in limited sizes. These evolved into second generation implants which allowed a degree of modularity in terms of variable head sizes to better match the resected head (Fig. 14.2). This purported to allow better soft tissue tensioning and hence a better outcome although clinical studies have not yet bourne this out [3, 4]. They also facilitated revision surgery by allowing separate head removal which exposes the bone-cement interface proximally as well as the glenoid. The third-generation implants added further modularity in terms of eccentricity of the head compared to the stem, head thickness and diameter as well as head-neck angulation (in some systems) (Fig. 14.3). These are commonly referred to as an anatomic replacement [5]. More recently the introduction of platform systems has added further complexity to the design of humeral stems as the ability to change from an anatomic stem to a reverse polarity stem requires further modularity of the body of the stem. Finally there has also been the development of both resurfacing systems (Fig. 14.4) and short stem, (or metaphyseal fit/stemless) prostheses (Fig. 14.5). At this time again there is little clinical evidence to support one over the other. However, there is no doubt that the use of resurfacing and stemless prosthesis again makes revision simpler.