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Uncemented Tapered Femoral Components With Single/Double/Rounded Tapered Implants


Key Points

  • Single-taper stems are tapered in the medial/lateral (ML) plane but are flat in the anteroposterior (AP) plane, appearing as a rectangle on a lateral radiograph.

  • Double-taper stems maintain the ML taper design but taper in the AP plane as well.

  • Tapered round stems expand on prior stem designs by maintaining a geometric taper in both the ML and AP planes and adding an increased rounded taper in the axial plane.

  • Generally, proximally coated tapered stems achieve reliable fixation at a high rate.

  • Ongrowth stems also achieve reliable fixation at a high rate. However, stems that involve femoral neck modularity with mixed alloys (i.e., cobalt-chrome neck and titanium stem) have had excessively high failure rates.

Total hip arthroplasty is a markedly successful and reproducible surgical procedure. Reliable fixation has been demonstrated in multiple registries with both cemented and cementless femoral stem fixation. While the initial stem designs were cemented, the failures due to polyethylene-induced osteolysis led to the search for reliable uncemented fixation. Over the past 3 decades, particularly in North America, the clinical use of cementless femoral fixation has greatly increased. The technique has reduced the need for cement, which reduces the potential interfaces for failure (bone-cement and cement-prosthesis vs. bone-prosthesis). This biologic fixation is desirable, especially in younger patients, as it is an evolving and living fixation method that appears to be very robust even with long-term follow-up. The reproducible technique has resulted in a more streamlined workflow. Numerous current designs allow reliable biologic fixation through bone ingrowth into a porous metal coating or bony ongrowth onto a nonporous roughened coating.

Several design styles have emerged with different overall rationales of uncemented fixation. Straight-stem implants are designed for largely diaphyseal fit and are generally inserted with a combination of diaphyseal reaming and proximal broaching. While reproducible results can be obtained in a variety of femurs, intraoperative diaphyseal fractures are a risk and long-term proximal stress shielding has been noted. Anatomic stem implants were designed with a curve in the sagittal plane to match the bow of the femur. Preparation and insertion of these stem designs can be more demanding.

Proximally engaging tapered stems obtain fixation at the metaphyseal diaphyseal junction and load the proximal femur to reduce the risk of proximal stress shielding. This category of stem design comes with many variations: single versus double versus rounded taper, standard versus extended offset, standard length versus short length, hydroxyapatite (HA) coating versus porous coating versus grit blasting, and neck modularity. The combinations of these stem geometries, coatings, and base materials give implanting surgeons a wide variety of options to match patient anatomy and demands.

Tapered-Stem Design

Categorizing stem geometry has been nicely organized by Khanuja et al., whose classification will be followed here. They define metaphyseal engaging stems as single-taper, dual-taper, or rounded-taper stems.

Single-taper stems are tapered in the medial/lateral (ML) plane but are flat in the anteroposterior (AP) plane, appearing as a rectangle on a lateral radiograph. They are intended to obtain their fixation in the cortical bone of the proximal femur in the ML plane, generally with 3-point fixation or wedge fit. Single-taper stems are generally inserted with a broach-only technique without reaming the diaphysis. Examples of this type of stem include M/L Taper (Zimmer, Warsaw, IN), Tri-Lock BPS (DePuy, Warsaw, IN), Taperloc (Biomet, Warsaw, IN), Anthology (Smith & Nephew, Memphis, TN), and Accolade/Accolade II (Howmedica, Rutherford, NJ).

Double-taper stems maintain the ML taper design but taper in the AP plane as well, transitioning from thicker proximally to thinner distally on a lateral radiograph. This is intended to allow them to fill the proximal calcar of the femur with a snug fit in both the ML plane and the AP plane. Double-taper stems are intended to obtain fixation in the metaphysis but also may engage the diaphysis, which may allow for increased rotational stability. Due to this possible diaphyseal engagement, double-taper stems are often inserted with conical diaphyseal reaming followed by metaphyseal broaching. Examples of this type of stem include the Summit (DePuy, Warsaw, IN), Synergy (Smith & Nephew, Memphis, TN), Omnifit-HA (Stryker Howmedica Osteonics, Mahwah, NJ), and Secur-Fit (Stryker, Kalamazoo, MI).

Tapered round stems expand on the prior stem designs by maintaining a geometric taper in both the ML and AP planes and adding an increased rounded taper in the axial plane. This was developed to theoretically provide additional rotational stability and distribute forces over a larger surface area, ostensibly reducing subsidence. Examples of this type include the Mallory-Head (Biomet, Warsaw, IN).

Ongrowth stems are a unique category in which a blade-type stem is implanted to achieve press fit and a different surface texture pattern promotes bony ongrowth. The most commonly used example of this stem type is the Zweymüller-Alloclassic (Zimmer, Warsaw, IN).

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