A composite beam stem is demonstrated on the left. The stem is bonded to cement (B), and shear forces are transmitted directly to the cement-bone interface (arrows). A taper-slip stem is demonstrated on the right. Shear forces are transmitted to the stem-cement interface, where subsidence occurs (arrows) and the cement-bone interface is protected (B)
Shen,an engineer, suggested that cemented femoral components can be divided into two basic types . Taper-slip or force-closed stems are exemplified by the original polished flatback Charnley (Zimmer, Warsaw, Indiana) and the polished Exeter (Stryker Inc., Newbury, United Kingdom) stems that routinely subside within the cement mantle without fracturing it. With these stems, stability is maintained by a balance of forces across the stem-cement interface with no form of bond between the two. Composite-beam or shape-closed stems cannot, by definition, subside within the cement, and stability is maintained by the bond that arises when the surface contours of the stem are matched and cement is applied to it. Kärrholm showed in a radiostereometric assay (RSA) study that subsidence of the stem at the stem-cement interface regularly occurred not only with taper-slip stems, but also with composite-beam stems, thus calling into question the whole concept of the composite-beam stem.
Good results can be obtained using composite-beam stems, but they are less forgiving in terms of surgical technique.The basic surgical techniques described here for contemporary femoral cementing are valid for both types of stem. If a composite-beam stem is to be used, it is essential that a complete cement mantle of adequate thickness is established, as well as solid fixation at both the stem-cement interface and the cement-bone interface. Scheerlinck points out that although in vivo both concepts of stem fixation have proved effective, they cannot work together, and it is important to understand on which principle a particular stem relies.
In contrast to the variable results of composite-beam components, taper-slip stems have been demonstrated to yield excellent results in all existing national joint registries, as well as in individual publications from multiple centers. In engineering practice, the taper is one of the strongest and most reliable methods of transmitting not only axial but also torsional forces between one component and another. In the context of the cemented stem, subsidence of the tapered stem is analogous to engagement of the taper; if the stem is to function effectively as a taper, it must not in any way be fixed to the cement or end-bearing. Shen and Howie point out that the taper-slip system is more forgiving in terms of surgical technique, and the success of the concept is demonstrated by the fact that all major manufacturers of hip implants now have such stems in their portfolio. In contemporary practice, they are by far the most widely used cemented stems throughout the world.