SYNOPSIS
This chapter reviews the history of acetabular component development, from cemented all-polyethylene components to uncemented porous-coated cups. It describes first- and second-generation component designs and discusses their early pitfalls. Current acetabular cup designs provide reliable fixation and improved modularity, which allows a variety of improved bearing options.
EVOLUTION OF THE ACETABULAR COMPONENT IN MODERN TOTAL HIP ARTHROPLASTY
Before 1950 arthroplasty designs demonstrated limited success in alleviating pain and improving function of patients with osteoarthritis of the hip. Early designs primarily involved resurfacing or replacing the diseased femoral head without addressing the acetabulum. One design, described by Smith-Peterson as “mold arthroplasty,” was composed of a polished vitallium head placed over the native femoral head, which articulated with the native acetabular cartilage. Another design, the Judet acrylic prosthesis, achieved fixation to the proximal femur with a short stem that traversed the canal of the femoral neck and exited the femur laterally below the vastus ridge. Although these designs demonstrated some clinical success, both had high failure rates from a loss of fixation.
In an effort to improve fixation of the femoral component, Moore, and later Thompson, developed endoprosthetic femoral components. These components provided better support of the head of the femoral component by achieving fixation through a metal stem introduced into the intramedullary canal of the proximal femur. Although these designs achieved reasonable initial fixation with an interference fit, they did not provide a means of definitive fixation and succumbed to subsequent loosening with continued patient activity. In addition to inadequate fixation, early prosthetic designs often clinically failed because they did not address pathology on the acetabular side of the joint or produced erosion of the acetabular cartilage, resulting in pain.
Thus, as of the mid 1950s, hip arthroplasty had two fundamental weaknesses: poor fixation of the femoral component and the lack of sound technique to resurface the acetabulum. As the necessity of resurfacing the acetabular side of the joint was recognized, the difficulty in creating a low-friction, wear-resistant bearing surface was realized. Several designs were produced that used different materials for the bearing surface. McKee and Watson-Farrar reported on the results of a cobalt-chromium, metal-on-metal design in 1966. Success was moderate.
In the late 1950s Sir John Charnley pioneered the development of modern total hip arthroplasty with his then-revolutionary ideas of bone cement (polymethylmethacrylate) for fixation of femoral and acetabular components to the host bone and a low-friction polymer (polyethylene) acetabular cup for articulation with the metal femoral head. Polymethylmethacrylate proved successful at providing secure and durable component fixation, and the low-friction articulation provided both wear resistance and a low-torque bearing surface, relieving stresses at the bone-prosthesis interface. The success of low-friction, cemented hip arthroplasty soon paved the way for the application of the technique to other disciplines of arthroplasty (e.g., the cemented total knee arthroplasty).
In 1965 Charnley and Ketterwell published a cadaveric study demonstrating superior fixation of the femoral prosthesis to the bone compared with the interference fit used by the Thompson and Moore prostheses. In 1970 Charnley reported 5-year follow-up data on 138 total hip arthroplasties that used a cemented high-molecular-weight polyethylene acetabular component and a cemented femoral stem. Regarding acetabular fixation, only two patients demonstrated radiographic evidence of loosening, but neither had undergone revision.
With the passage of time, the results of Charnley’s technique remained successful and the technique was soon applied to a more broad patient population. Although mid-term results of cemented components revealed durable results regarding aseptic loosening, surgeons began to recognize several unappealing aspects of cemented acetabular components. These included the inability to trial before final implantation, the time required to prepare the cement, the inability to correct positional errors during or after cement polymerization, and difficulties with maintaining a bloodless bony cavity to allow maximal cement interdigitation. In addition, at longer term follow-up a biologic reaction that caused bone loss surrounding hip replacement components was beginning to be seen with increasing frequency ( Fig. 12-1 ). Originally thought to be the body’s response to polymethylmethacrylate, the process was termed “cement disease.” The technical difficulties associated with implantation of a one-piece cemented cup coupled with the emergence of so-called cement disease prompted the development of newer components and techniques with a focus on modularity and different methods of fixation.
FIRST-GENERATION CEMENTLESS CUPS
Innovations in components and technique initially were explored with cemented, metal-backed components. These components demonstrated worse survival rates regarding fixation than their all-polyethylene counterparts and were soon abandoned. Smooth, threaded components also were introduced. These cups failed to achieve definitive fixation and demonstrated high loosening rates. The early to mid-1980s ushered in the era of the uncemented, porous-coated, modular acetabular component. New concepts such as bone ingrowth and osseointegration of the acetabular component were explored. Both hemispherical and threaded cup designs demonstrated excellent short- and mid-term success. Unfortunately, success with modularity would prove more difficult to achieve.
Many first-generation porous-coated components, implanted by reaming to the same diameter as the porous-coated shell with screws used for initial fixation, demonstrated the clinical success of biologic fixation with durable long-term stability but illustrated new modes of failure, some of which were unique to certain designs. First-generation modular acetabular components such as the Arthopor (Joint Medical Products, Stamford, Conn.), ACS Triloc+ (DePuy, Warsaw, Ind.), and Harris-Galante (Zimmer, Warsaw, Ind.) cups are good examples.
The Arthropor was a hemispherical, porous-coated component with an internal rim used to lock the liner into the metal shell. The polyethylene liners of the Arthropor cups were sterilized by ethylene oxide and had an inadequate uniform thickness of 4.9 mm; increases in cup diameter were accompanied by increases in thickness of the metal shell, not the liner. The extremely thin nature of the polyethylene, coupled with a lack of polymer cross-linking from the sterilization technique, resulted in higher stresses in the liner and more rapid polyethylene wear in vivo. Long-term use of the component often resulted in the prosthetic femoral head completely wearing through the thin polyethylene liner, resulting in unintended metal-on-metal articulation within the joint.
The Triloc+ cup also was a porous-coated component with a hemispherical outer geometry; however, it had a cylindrical inner geometry. Thus the polyethylene liner for the Triloc+ cup was not of uniform thickness; it was thicker at the dome than at the cup face and did not conform well to the shell. Because of its geometry, the Triloc+ liner was exposed to increased stress and deformation of the liner lip, which often led to catastrophic failure at the superior rim of the liner ( Fig. 12-2 ). This cup did, however, demonstrate durable long-term fixation by osseointegration.
The Harris-Galante cup and liner also were hemispherical. The locking mechanism for this cup was exterior to the shell and did not interfere with the seating of the cup liner. Unlike the other first-generation cups mentioned, polyethylene thickness with the Harris-Galante cup increased with cup diameter. However, the cup was not porous coated; it was covered with a titanium fiber mesh that was used for osseointegration ( Fig. 12-3 ). Delamination of this mesh leading to late loosening among the Harris-Galante cups has been observed.
These examples illustrate specific complications of first-generation cup designs. In addition, polyethylene wear-related complications dominated failure modes of first-generation cups. The biologic reaction that caused bone loss surrounding hip replacement components, originally thought to be cement disease, was now being seen with uncemented components. This led to the realization that this bone loss, now termed osteolysis, was the body’s response to particulate debris, not a specific reaction to cement. The particulate debris was mainly that of the polyethylene liner produced both from the front side of the liner, which directly articulated with the metal femoral head, as well as the backside of the modular liner, which indirectly articulated against the inside of the metal shell. Increased stress on the polyethylene liner as a result of first-generation thin liner designs coupled with early sterilization techniques and poor locking mechanisms, which allowed motion of the liner within the shell, all resulted in increased polyethylene wear and osteolysis with first-generation components. Outcome studies of the first-generation AML (DePuy), PCA (Howmedica, Rutherford, NJ), and Harris-Galante cups (see Fig. 12-3 ) have documented pelvic osteolysis rates up to 56% at 10-years or more of follow-up.
SECOND-GENERATION CUPS
By the mid- to late 1990s, 10-year results of uncemented hemispherical cups became available. These components demonstrated equal or better success than cemented cups regarding maintenance of fixation. Second-generation cup designs focused on improvements with ingrowth surfaces, locking mechanisms (modularity), and bearing surfaces. Little attention was directed toward further improvements in cemented techniques. By the late 1990s the technique of cementing polyethylene acetabular components was no longer taught at most orthopedic residency programs in the United States. These trends are reflected in Table 12-1 .
| United States | Europe, Middle East, Asia, Australia | |
|---|---|---|
| Shell Type | ||
| Porous acetabular shell | 93% | 75% |
| Cemented shell | 7% | 25% |
| Bearing Type | ||
| Metal on polyethylene | 71% | 51% |
| Metal on metal | 20% | 12% |
| Ceramic on ceramic | 5% | 11% |
| Ceramic on polyethylene | 4% | 26% |
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