Implant Materials

Implant Shape

The concept of “cement disease” became popular as a reason for loss of fixation of cemented acetabular implants. By eliminating cement, the hope was that acetabular fixation would be more durable. The first generation of cementless acetabular components were designed with several different geometries and were classified by Morscher and associates²² into five different types: (1) cylindrical, (2) square, (3) cone type, (4) ellipsoid or oblong, and (5) hemispherical. Of these design types, the most popular designs used are presented in the following paragraphs.

Among the more common early “cementless” designs were threaded acetabular components, designed in the early 1970s. Most of these designs did not possess a surface coating and therefore relied upon a mechanical interlock between the device and the host bone, rather than biological fixation, to achieve stable implant fixation. Some specific designs included the Lord cup, along with Mittelmeier, Mecring, T-Tap, and S-ROM Anderson implants. Several studies reported initial satisfactory results, but longer-term follow-up revealed loosening rates of nearly 60% and early revision rates for aseptic loosening as high as 31% at 10 years.^23–28

Several theories regarding the reasons for unacceptable rates of migratory failure with first-generation threaded implants became prevalent. It was thought that screwing in a cementless implant led to increased stresses at the implant-bone (thread) interface, resulting in subsequent pressure necrosis, bone resorption, and fibrous tissue interposition.^23,29 Retrieval studies supported the postulation that fibrous tissue filled in between the threaded surfaces, yielding inadequate host-bone contact to support long-term fixation.^30,31

As compared with hemispherical cup designs, threaded components were more difficult to reproducibly insert in the proper position. Acetabular bed preparation for threaded components was challenging and often led to poorer mechanical stability compared with hemispherical cups because of a lesser degree of surface area contact with host bone. In addition, many systems used an awkward insertional handle, which predisposed to vertical cup positioning, especially in obese patients.²¹

The failure of first-generation cementless devices brought about so-called second-generation threaded cementless implants that incorporated a porous or grit-blasted surface to enhance biological fixation via bone ingrowth or ongrowth, respectively. Despite the shortcomings of threaded designs, these devices relied upon the threads for initial mechanical stability and the surface coating for long-term biological fixation. Cups with this design included the Zweymüller (grit-blasted titanium; Zimmer), the S-ROM Super Cup (titanium-sintered beads; DePuy), and the Arc-2f (hydroxyapatite-coated; Osteonics, Stryker Orthopaedics, Mahway, NJ) (Fig. 66-2). Medium-term results demonstrated extremely low rates of radiographic failure; however, these devices had numerous problems that resulted in a move toward alternative designs, including difficulty with insertion and high rates of polyethylene wear.^28,32,33 Incorporation of a surface coating with the early threaded design feature supported the importance of initial mechanical stability in conjunction with biological fixation for long-term implant stability.

Hemispherical cups are classified as having single or dual geometry design. Dual geometry components exhibit an enlarged radius at the rim of the component and were designed to maximize the surface area of contact for biological ingrowth. The dual geometry design gained enthusiasm in attempts to avoid adjunctive fixation to enhance the initial mechanical stability of the implanted acetabular component. Concern has arisen that design features may decrease total contact between the component and the surrounding host bone, specifically at the dome portion of the implant, leading to decreased bone ingrowth. Bauer and coworkers³⁴ evaluated two dual geometry cup designs at the time of postmortem retrieval and found evidence of increased bone ingrowth at the peripheral rim as compared with the dome. The authors compared this design to threaded cups and determined that a greater degree of bone apposition was seen with the dual geometry design, but bone ingrowth was less than was suggested by radiographs.³⁴ Most cementless acetabular devices in use today are single geometry in design, mostly because of the ease of acetabular bed preparation and reproducible insertion.²¹

The final shape to be considered is the hemi-ellipsoid shape of the Zimmer trabecular metal component. This device exhibits a gradual transition in interference from the peripheral rim (2 mm) to the dome (0 mm). The rationale behind this design is that it maximizes peripheral interference stability, minimizes the potential for early bottoming out, and eliminates the discontinuity associated with dual geometry designs. This interference fit plays an important role in monoblock-type implants, into which screws cannot be inserted for supplemental fixation (Fig. 66-3).

Surface Coatings

With the understanding that surface coatings were of critical importance in achieving clinical success with an uncemented acetabular component, several different surface preparations were developed. Hydroxyapatite (HA) coatings gained popularity as osteoconductive surface coatings for acetabular components, secondary to good early and intermediate results when applied to cementless femoral components.^35,36 Unfortunately, when applied to the back of a cementless acetabular component without an underlying porous surface, when the HA resorbed over time, a high rate of failure ensued, because no ingrowth or ongrowth surface could provide long-term fixation.³⁷ Hydroxyapatite is still popular, however, and improved results have been associated with the use of HA in conjunction with porous-coated or grit-blasted surfaces.^38–40

Supplemental Fixation

Although supplemental fixation with screws has been associated with high rates of osseointegration,^31,49 concerns have been raised regarding screws and associated screw holes as pathways for wear particles that lead to osteolysis.⁵⁰ Further, fretting corrosion between the screws and the metal shell is an additional concern with the use of screws for adjunctive fixation.

Cook and coworkers conducted a histologic study comparing the degree of bone ingrowth attained with the use of components with spikes, pegs, or screws (Fig. 66-5). The authors were able to demonstrate that the distribution of bone ingrowth surrounded the pegs or spikes, but was greater surrounding cups secured with adjunctive screws, because this design allowed visualization of bone-implant contact upon insertion—a factor that has been shown to influence bone ingrowth. They concluded that the use of peripheral spikes or pegs may prevent adequate seating of the cementless device, thereby reducing the surface area of contact between the porous surface and the host bone. Concerns surfaced regarding difficulty in repositioning a malpositioned cup with peripheral fins or spikes. In addition, several retrieval studies have shown increased bone ingrowth adjacent to screws compared with other regions of the cup.^31,51

Biomechanical analyses of these modes of adjunctive fixation further demonstrated that the use of screws may convert torsional forces to compressive forces, thereby preloading the bone-prosthesis interface and increasing the contact area, further promoting bone ingrowth.⁵² Using in vitro models, Lachiewicz and associates⁵³ showed that significantly greater torque was required for screw failure as compared with spikes and pegs, and Stiehl and colleagues⁵⁴ revealed less micromotion with the use of screw fixation compared with pegs.

Although ample literature supports the use of screws, other studies have demonstrated excellent clinical success of components when dome spikes or peripheral pegs and fins were used. Engh and colleagues⁵⁵ reported 15-year survivorship of 95% for tri-spiked cementless acetabular components with revision for aseptic loosening as an endpoint.

With improved, low wear–bearing surfaces, concerns regarding screw holes as access channels for wear particles may no longer be relevant, because the particle load with alternative bearings and highly cross-linked polyethylene may be below the level required to induce osteolysis. Long-term studies are required to make a definitive conclusion, because this concept pertains to implant longevity.

An additional important consideration to be addressed regarding supplemental screw fixation is the risk of neurovascular injury associated with errant screw placement. Through the combination of defined acetabular quadrants, improved understanding of pelvic anatomy, and increased surgeon experience in placing acetabular screws, the risk for neurovascular injury has been dramatically reduced by placement of screws in the posterosuperior quadrant with careful measurement of screw length.⁵⁶

Press-Fit Technique

The concept of a pure press-fit technique without the use of screws for component insertion was developed to avoid the use of screws for the reasons previously described. The acetabulum is under-reamed by 1 mm to 2 mm, and then the component is impacted into place. This method of cup insertion is dependent upon the viscoelastic or time-dependent properties of bone to allow deformation and recoil to obtain initial implant stability.⁵²

On initial evaluation of the press-fit technique, it was evident that radiolucencies at the periphery of the acetabular shell were less prevalent; however, gaps at the dome were more common. By 2 years, dome gaps were no longer visible, suggesting bone formation at the pole of the component.⁵⁷ In vitro analysis, however, showed that gaps greater than 2 mm are not compatible with bone formation and subsequently increase the effective joint space for synovial fluid and wear with debris access to trabecular bone behind the component.⁵⁸

Both Won and associates and Steihl and colleagues demonstrated in a cadaveric model that press-fit insertion with under-reaming by 1 mm resulted in better component mechanical stability than line-to-line insertion with the use of supplemental screw fixation.^54,59 Based on this and other studies, it seems that the depth and accuracy of acetabular reaming may be more important than the presence of supplemental fixation modalities (e.g., spikes, pegs, screws) for attaining mechanical stability.

Finite element models have been used to evaluate loading conditions at the periphery of cementless devices. Acetabular strains registered at the periphery of hemispherical cups inserted using a press-fit technique were the highest, resulting in greater compressive bone forces, thus enhancing cup stability.^60,61 Component oversizing of 2 mm rather than 1 mm was needed to augment stability; however, greater amounts of under-reaming increase the risk for acetabular fracture. This may be dependent on acetabular component size; in general, larger amounts of under-reaming are acceptable for larger components.

Comparisons were made between hemispherical and dual geometry cup designs that possess a wider radius at the mouth of the implant compared with the dome; this is intended to increase press-fit at the rim of the acetabulum. Dual geometry designs when tested biomechanically produced higher strain rates at the periphery than were produced by hemispherical cups but resulted in less deformation of the acetabular bone stock. Theoretically, such dual geometry designs would require less force for proper implant seating and would potentially decrease the risk of acetabular fracture upon insertion. However, although higher strains were recorded at the periphery with dual geometry designs, hemispherical cups exhibited better overall implant stability; thus the dual geometry design concept, for the most part, has fallen out of favor.^60,61

The risk of creating a periprosthetic acetabular fracture is a concern that surfaced when the press-fit technique of insertion was introduced. This technique of insertion relies upon hoop stresses generated within the acetabulum to help maintain component stability. The tenet that is inherent to this technique is that the host bone is able to tolerate the hoop stresses that are generated. Insertion of significantly oversized components may result in inadequate acetabular component seating and possible fracture of acetabular walls or columns. Kim and associates⁶² showed that 18 of 30 (60%) acetabular fractures occurred during insertion of cementless devices using a press-fit technique. Acetabular fractures were more likely with components that were oversized by 4 mm than 2 mm. Sharkey and colleagues reported 13 periprosthetic acetabular fractures, of which 9 were recognized intraoperatively, during implantation of a hemispherical cup using a press-fit technique and under reaming by 1 mm to 3 mm. Of the 13 patients, 6 had osteoarthritis, 3 had osteonecrosis of the femoral head, 2 had rheumatoid arthritis, and the remaining patients were treated for developmental dysplasia or a hip fracture nonunion. Fractures identified intraoperatively were addressed by a variety of methods including screw augmentation in and around the cup, use of autologous bone graft at the fracture site, and/or protected weight bearing and immobilization postoperatively. Two cases identified postoperatively required acetabular component revision because of radiographic evidence of component migration. As a result, the authors identified two additional predisposing clinical risk factors for periprosthetic acetabular fracture—osteoporosis and rheumatoid arthritis.⁶³