Patellar component implants

Total knee replacements can be implanted with or without cement, the latter relying on porous coating for fixation. The femoral components are typically made of CoCr alloy and the monolithic tibial components are made of UHMWPE. In modular components, the tibial polyethylene component assembles onto a titanium alloy tibial tray. The patellar component is made of UHMWPE, and a titanium ahoy back is added to components designed for uncemented use. The relatively small size of the pateUar component compared to the forces that travel through the extensor mechanism, and the small area of bone available for anchorage of the prosthesis, make the pateUa vulnerable. 

Infection, loosening, and patellar complications have been identified as prevalent reasons of TKA revision (Figure 8.10). In a study of 440 revision surgeries performed between 1982 and 1999, Fehring and colleagues reported that infection was the single largest cause of knee arthroplasty revision within the first 5 years of implantation (2001). In a more recent study of 212 knee revisions performed between 1997 and 2000 by Sharkey and associates, infection was responsible for 25% of revisions within the first 2 years of implantation, but only 7.8% of components implanted over 2 years were infected (2002). 

FIGURE 17.4 CFR UHMWPE (Poly II) tibial component (A) and patellar component (B) retrieved after 32 years of implantation. Implanted in 1975, this Total Condylar knee replacement was revised in 2006 for instability and patellar dislocation. Image provided courtesy of Francisco Medel, PhD, Drexel University. 

Gunston and MacKenzie s response to severe patello-femoral pain was to develop a separate patello-femoral arthroplasty, which consisted of a metallic patellar button articulating against an UHMWPE track implanted in the femur (1976). Because this solution required the implantation of two additional components, they cautioned against using this solution for patello-femoral replacement "indiscriminately" (1976). 

Although the cemented all-polyethylene Total Condylar Prosthesis was intended to provide improved fixation over previous designs, troubling radiolucencies were nonetheless observed at the cement interface within the first 10 years of implantation [41]. To address the problem of implant fixation, metal tibial trays were initially used to improve the integrity of the cement-prosthesis interface. Finite element analyses later demonstrated that metal backing had the further theoretical benefit of lowering the stresses in the cement and in the subchondral bone [42, 43]. The use of metal backing was later adapted to patellar and unicondylar components for similar reasons. 

Sharkey et al. [38] also found that 25% of knee revisions were associated with wear or surface damage of the tibial or patellar insert. Interestingly, the timing of knee revisions associated with UHMWPE wear has two major peaks (Figure 8.11), the first occurring before 5y and the second peak in revisions occurring between 6-lOy. For components revised for wear within the first 5y of implantation, patellar... 

Patient age, weight, and implantation time were statistically comparable for both groups. However, Poly II had a wider implantation range (3.7-32.8y) than UHMWPE (3.5-17.0y) with the same average (ll.ly). Patellar complications (n = 22) prevailed in patients with carbon fiber-reinforced components, but the incidence of wear and metalosis was statistically indistinguishable. Poly II inserts had less surface damage within all regions when compared to conventional UHMWPE components. Also, Poly II was... 

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