The P.F.C. Sigma RP-F Total Knee Arthroplasty: Designed for Improved Performance

Abstract

The new P.F.C. Sigma RP-F (rotating-platform high-flexion) knee has been designed to permit 155° of flexion after total knee arthroplasty (TKA) without compromising wear, polyethylene contact stresses, patellofemoral tracking, or stability. The purpose of this article is to describe the design features of the P.F.C. Sigma RP-F TKA. It is our belief that future primary posterior-stabilized TKA will incorporate many of the design features of the RP-F knee for improved performance.

The traditional goal of total knee arthroplasty (TKA) has been to provide painless knee performance over a functional range-of-motion (ROM). Achieving maximal ROM, ie, up to and beyond 155°, has become a more recent desire based on patient demands, cultural influences, and marketing.

Most contemporary TKA implants are not designed to permit flexion beyond 120°. The P.F.C. Sigma RP-F (rotating platform, high flexion) knee (DePuy Orthopaedics, Inc, Warsaw, Ind) is specifically designed to attain up to 155° of flexion after TKA without compromising wear, polyethylene contact stresses, patellofemoral tracking, or stability. The purpose of this article is to describe the design features of the P.F.C. Sigma RP-F total knee replacement.

Design Features

The femoral component of the P.F.C. Sigma RP-F is based on the highly successful P.F.C. Sigma posterior-stabilized design. During the first 100° of flexion, the design features are the same in terms of the tibiofemoral sagittal geometry (the so-called “J-curve”), the coronal geometry, the trochlear groove, the articulating geometry during the gait cycle, the contact area at heel strike (400 mm²), and the post-cam engagement point (70°). The major design change in the femoral component occurs beyond 100°, in the posterior condyles where the sagittal radius of the posterior condyle has been markedly reduced.

In deep flexion, this reduced sagittal radius translates into increased stresses in the posterior condyles of the femoral component. As a result, the posterior femoral condyles have been reinforced to prevent material failure. To accommodate the increased thickness of the implant, an additional 2-4 mm of bone must now be resected from the posterior condyles. This has the added benefit of improving visualization in the posterior aspect of the knee to allow for adequate decompression of posterior osteophytes and tight capsular structures. The increased bone loss from the posterior condyles have been offset by modifications to the box width and slope, which have been reduced to allow for approximately 20% less bone removal from the intercondylar area.

The high contact stresses seen in the posterior condyles in deep flexion are also applied to the polyethylene. These stresses have been reduced by increasing the contact area in deep flexion by the creation of a third articulating surface behind the post-cam mechanism. This unique feature of the P.F.C. Sigma RP-F knee not only increases the contact area in deep flexion and, thereby, reduces contact stresses, but also ensures adequate femoral roll-back for improved clearance.

The design of the highly polished, chromium cobalt tibial component and the polyethylene insert in the P.F.C. Sigma RP-F is identical to the P.F.C. Sigma rotating-platform system in terms of the articulating geometry in the sagittal and coronal planes, the slope of the insert, and its dimensions. However, in the P.F.C. Sigma RP-F design, a 5.6-mm diameter cobalt chromium reinforcement pin has been added to reinforce the post in deep flexion. The reduced distal diameter of the pin prevents it from protruding distally, while barbs prevent the pin from moving proximally. The P.F.C. Sigma RP-F tibial post is positioned 2 mm posterior compared with the P.F.C. Sigma rotating-platform. The optimized shape and position of the post provides increased posterior femoral roll-back in flexion, with a similar jump distance (16 mm). Additionally, the post is beveled anteriorly to avoid soft-tissue impingement in deep flexion, and the undersurface of the insert is beveled posteriorly to allow for greater rotation on the baseplate.

Discussion

Achieving full ROM after TKA is necessary to maximize knee performance and patient satisfaction. However, patients who have undergone TKA can rarely flex their knees beyond 120°.^1,2 This is because ROM after TKA is dependent on several variables, some over which the surgeon has control, including the surgical technique, the implant design, pain control, and postoperative rehabilitation.^3-6 Variables that cannot be controlled by the surgeon are patient-dependent factors, including a patient’s age, gender, body-mass index, history of previous surgery, preoperative diagnosis, preoperative knee score, level of cooperation, and, most importantly, preoperative ROM.^3-6

The P.F.C. Sigma RP-F knee is designed to provide a ROM of 155° without compromising wear, polyethylene contact stresses, patellofemoral tracking, or stability. This motion is achieved by decreasing the radius of curvature of the posterior femoral condyle and providing a third articulating surface at higher degrees of flexion. This allows for higher contact areas in deep flexion and reduced polyethylene contact stresses.

All high-performance designs carry the potential risks of knee instability, excessive polyethylene wear, patellofemoral joint problems, and massive bone loss at revision surgery.

The newly designed P.F.C. Sigma RP-F has addressed all of these concerns. It is our belief that future primary, posterior-stabilized TKAs will incorporate many of the design features of P.F.C. Sigma RP-F knee for improved performance.

References

Ranawat AS, Rossi R, Loreti I, et al. Comparison of the PFC Sigma fixed-bearing and rotating-platform total knee arthroplasty in the same patient: short-term results. J Arthroplasty. 2004; 19:35-39.
Ranawat CS, Flynn WF Jr, Deshmukh RG. Impact of modern technique on long-term results of total condylar knee arthroplasty. Clin Orthop Relat Res. 1994; 309:131-135.
Ranawat CS, Ranawat AS. Total knee replacement rehabilitation protocol: what makes the difference? J Arthroplasty. 2003; 18(suppl 1):27-30.
Ranawat CS. Design may be counterproductive for optimizing flexion after TKA. Clin Orthop Relat Res. 2003; 416:174-176.
Ritter MA, Berend ME, Harty LD, et al. Predicting range of motion after revision total knee arthroplasty: clustering and log-linear regression analyses. J Arthroplasty. 2004; 19:338-343.
Sultan PG, Most E, Schule S, et al. Optimizing flexion after total knee arthroplasty: advances in prosthetic design. Clin Orthop Relat Res. 2003; 416:167-173.

Authors

Drs A. Ranawat and C. Ranawat are from the Ranawat Orthopaedic Center, LLC, New York, and Dr Gupta is from the Department of Orthopedic Surgery, Lenox Hill Hospital, New York.