Abstract
Three basic design concepts of cementless femoral fixation have emerged. They include: anatomic designs, straight stem designs, and tapered designs. In addition, there have been modular designs. The most successful have been designs that have a metaphyseal sleeve with a tapered stem. A more recent newer concept has been the double taper neck designs which have not performed well in general.
Anatomic Stem Designs: The rationale for an anatomic stem design was to design a component that matched the sagittal plane bow of the femur. The APR (Centerpulse, Austin, Texas) and the PCA (Howmedica, Rutherford, New Jersey) were the initial designs. Although these designs provided excellent micromotion stability, they are not used in their present length today because they could not fit in to all femurs.
Straight Stem Designs: The concept of a straight stem design was to machine the femur to accept the prosthesis. This was done with diaphyseal reaming, proximal broaching, and preparing a proximal triangle to accommodate the proximal metaphyseal portion of the stem. These had previously, and still do, come in proximally coated and distally coated designs. They have proven to be durable long term. The AML fully coated stem (DePuy, Warsaw, Indiana) was and still is the prototype device.
Tapered Stem Designs: The most popular designs today are the tapered stems. They are inserted either by a broach only, or ream and broach technique. Some only taper in the ML plane and are flat in the AP plane. These are called ML taper or blade devices (Taperloc and Trilock). These are usually inserted broach only. Some have a double taper with proximal fill and include the Zweymuller stem (Zimmer, Warsaw, Indiana), the Omnifit stem (Stryker, Mahwah, New Jersey), the Summit stem, and the Corail stem (both DePuy, Warsaw, Indiana). Some are hydroxyapatite coated (Omnifit and Corail), some are porous coated (Summit), and some are only grit blasted (Zweymuller). Some are broach only including the Zweymuller and Corail, and some are broach and ream including the Omnifit and Summit. Some are tapered throughout, one of which is a Wagner type design, Trilogy (Zimmer, Warsaw, Indiana). These Wagner type devices are useful in abnormal anatomy (CDH and Perthes).
Modular Stem and Dual Modular Neck Designs: Metaphyseal sleeve modular stem designs are extremely versatile and can be inserted press fit into just about any femoral anatomy. They are most commonly utilised in cases of hip dysplasia with marked femoral anteversion. The S-ROM device is the prototype design. The long term concern with these as well as the double neck tapered devices is fretting and corrosion at the extra modular junctions.
Short Stem Designs: Short stem designs were developed to provide metaphyseal only fixation and to enable easy insertion through small incision techniques, especially those performed through anterior and antero-lateral approaches.
