Uncemented Tapered Femoral Components With Blade-Shaped Parallel-Sided Implants

Functional Design of Uncemented Tapered Femoral Stems

The main goal of successful total hip arthroplasty (THA) is to recreate a painless biomechanically stable hip joint without short- or long-term complications. Femoral component stability is one of the main factors to obtain successful clinical outcomes. There are many variables that ultimately affect femoral stem stability, including but not limited to native femoral anatomy, surgical technique, patient profile, and implant design.

Because of complications reported with cylindrical and anatomic stem designs, attention has shifted toward the manufacturing and use of cementless tapered femoral stems, especially in the United States. In general, cementless tapered stems are proximally porous coated. Mechanically, they are designed to allow three-point fixation, contacting endosteal bone posterior proximally, anterior in the midportion, and posterior distally. As a result of three-point fixation, the implant should maintain axial and rotational stability, allowing a gradual transfer of load to the proximal femur and decreasing stress-shielding and potentially thigh pain. Khanuja et al. have further categorized cementless implants into six categories based on osseous contact and location of stem fixation. Based on this classification system, nonmodular tapered stems are categorized as type 1, type 2, and type 3A, 3B, and 3C ( Table 67.1 ). Type 1 stems are wedge shaped with only a single taper in the medial to lateral plane. Type 2 stems have a doubled wedge or metaphyseal filling design to accomplish fixation both in the medial to lateral and anterior to posterior planes. Type 3 stems are subcategorized into three subtypes according to the geometry and location of fixation. Overall, these stems taper in the medial-lateral and anterior-posterior planes with fixation at the metadiaphyseal junction as opposed to the metaphyseal junction in types 1 and 2.

Initial fixation of tapered implants primarily relies on stable impaction or press-fitting the stem into the metaphysis to minimize micromotion between the implant and bone. Minimizing bone-implant micromotion ultimately results in greater osseous ingrowth. Pilliar et al. and Engh et al. were able to demonstrate fibrous ingrowth with micromotion exceeding 150 µm in canine models. Jasty et al. revealed almost complete osseous ingrowth with micromotion of less than 20 µm. Fibrous ingrowth could lead to implant loosening or subsidence. In addition to an adequate press-fit, the type of coating also aids in bony ingrowth and implant stabilization. As mentioned, cementless tapered stems are proximally coated to allow for metaphyseal fixation. There are two general mechanisms for implant-bone integration. Bony ongrowth occurs when bone grows onto a roughened surface; ingrowth refers to bone interdigitation within a porous surface. Most surfaces are metal based because ceramics and polymeric materials were limited in mechanical strength. In general, tapered stems are porous coated allowing for bony ingrowth. Examples of porous surfaces include sintered beads, fiber meshes, and porous metals. Sintered beads consist of microspheres of metal alloys that are attached to a base implant using extreme temperatures to create a porous surface; fiber meshes involve an interconnection of woven metal fibers that attach by diffuse bonding. Porous metals such as Trabecular Metal (Zimmer-Biomet, Warsaw, IN) provide porosity of up to 85% with greater resistance to shear compared with sintered beads and meshes that reach porosities of up to 50%. Grit-blasting and plasma spraying are two ways to create an ongrowth surface. Grit-blasting involves creating a textured surface by spraying pressurized metal particles. Plasma spraying textures implants by using metal powder mixed with an inert, ionized argon gas under pressure. One commonly used plasma sprayed material is hydroxyapatite. Despite the type of coating used, the overall goal for a cementless tapered stem is to achieve circumferential metaphyseal osseointegration to prevent stress-shielding and loosening.

In addition to implant geometry and proximal coating properties, the composition of the metal may affect clinical outcomes. Cobalt-chromium (Co-Cr) and titanium alloys are the most commonly used metals because of their inherent biocompatibility, mechanical properties, and success over many decades. The main difference between the two alloys when referring to stem manufacturing is the difference in modulus of elasticity. Titanium has a lower modulus of elasticity, which is closer to that of bone. The increased stiffness of the Co-Cr alloy has been proposed to be a component of thigh pain after THA. In a study by Burkart et al., titanium Mallory-Head stems (Zimmer-Biomet) were associated with less thigh pain (3% vs 23% at 2 years) than Co-Cr Porous Coated Anatomic (Howmedica, Rutherford, NJ). However, controversy exists around the idea of modulus of elasticity mismatch and thigh pain. Lavernia et al. prospectively evaluated 241 primary total hips using the Trilock (DePuy) single tapered-wedge stem with different metal compositions. Before 2000, this stem was manufactured with Co-CR-molybdenum (Co-Cr-Mo). The exact stem designed with titanium was introduced in 2000. They concluded no difference in thigh pain between both groups when controlling for all variables including age, bone quality, bone morphology, and stem size. They did however associate thigh pain with larger stem sizes. In a prospective randomized study of 423 Trilock stems (DePuy), reoperations and Harris Hip Scores were significantly better in patients who received the titanium stem. The Co-Cr stem was associated with twice the incidence of thigh pain. Because of continued controversy regarding modular mismatch, most currently designed cementless tapered stems are composed of a titanium alloy.

In general, variations in the proximal femoral anatomy can be categorized according to the Dorr classification. Dorr type A femurs can be described as a champagne flute, with thick proximal femoral cortices and a metaphyseal canal that tapers toward the diaphysis. Tapered femoral stems are designed to securely fit the metaphysis of this femur type that is typically seen in the young, active patient population with great success. However, there have been reports of failed metaphyseal fixation due to distal diaphyseal engagement and subsidence in patients with Dorr type A femurs. Dorr type B femurs are referred to as funnel shaped, where the medullary canal of metaphysis tapers distally. Like Dorr type A femurs, theoretically a tapered stem design would allow for greatest metaphyseal fixation. McLaughlin et al. revealed only one revision out of 282 hips due to aseptic loosening of the femoral component using the Taperloc (Zimmer Biomet) stem in Dorr type A and B femurs ( Fig. 67.1 ). Of the stems that were not revised, 98% achieved complete osseous ingrowth, 1% achieved stable fibrous ingrowth, and 1% were considered radiographically loose. Similarly, Issa et al. reported survivorship of 98% and excellent clinical outcomes in patients with Dorr type A and B femurs using the Omnifit HA (Stryker, Mahwah, NJ) cementless tapered stem at a mean of 8 years of follow-up. Dorr type C femurs are stovepipe in contour, with thin femoral cortices without tapering in the metadiaphyseal junction that are commonly noted in osteoporotic patients of older age. Conceptually, the tapered stem design does not fit the contour of this type of femoral morphology and has historically been discouraged. Cooper et al. noted that the most important factor predisposing to failed osteointegration was greater canal fill at the mid to distal aspects of the tapered femoral stem in patients with wide canals. Additionally, tapered stems have been shown to have an increased risk of revision and is an independent risk factor for periprosthetic fracture. However, despite stem-bone contour mismatch, there is growing evidence supporting the use of cementless tapered stems in patients with Dorr C femurs showing similar aseptic loosening, revision, and fracture rates compared with Dorr type A and B femurs.