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Rationale for the Posterior-stabilized Rotating-platform Knee

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Abstract

The nonposterior-stabilized mobile-bearing knee and the posterior-stabilized fixed-bearing design have been successfully used for the past three decades. Lessons learned from their use have culminated in the design of a posterior-stabilized mobile-bearing knee that incorporates beneficial features of both. The posterior-stabilized mobile-bearing knee is aimed at addressing polyethylene wear issues related to the fixed-bearing design, and improve upon stability and range of motion typical of nonposterior-stabilized mobile-bearing knee designs.

Mobile-bearing knees for total knee arthroplasty (TKA) have been in use since 1978, when Buechel and Pappas¹ introduced knee implants with a meniscal-bearing design (cruciate retaining), and a rotating-platform design (cruciate sacrificing). The rationale was that mobile-bearing knee implants allowed increased tibiofemoral articular conformity without restricting axial rotation. This increased conformity enhanced surface contact area and reduced contact stresses, potentially reducing polyethylene wear.

The rotating-platform knee introduced by Buechel and Pappas for cruciate-sacrificing use was fundamentally different from the fixed-bearing cruciate-sacrificing device introduced by Insall et al 1978.² The difference was that the fixed bearing cruciate sacrificing device had a post-and-cam mechanism to provide anteroposterior stability otherwise afforded by cruciates and also helped guide posterior roll-back of the femur on the tibia. This is the posterior-stabilized fixed-bearing design. The mobile-bearing cruciate-sacrificing device had no such post-and-cam mechanism, instead providing stability by curved design of the tibial insert articulation surface, and a stringent surgical technique to achieve a balanced flexion gap. This is the low contact stress rotating-platform design.

When we trace the use of these devices over the past 20 years, both have produced good results as seen in a number of reports.^3-5 However, with such extensive use, over a long period of time, desirable and undesirable features of both designs became apparent.

Posterior-Stabilized Fixed-Bearing and the Low Contact Stress Rotating-platform Design: The Good and Bad

Let us first look at the posterior-stabilized fixed-bearing design. A desirable aspect of the posterior-stabilized fixed-bearing was that it afforded good stability compared with its forerunner, the total condylar knee, and also produced better range of motion (ROM), owing to the improved femoral roll-back. Undesirable aspects of the posterior-stabilized fixed-bearing knee relate to abnormal wear at the articular surface, and additional undersurface wear with the late version of modular components. Also, the issue of polyethylene wear of the post in the post-cam mechanism was problematic. This was attributed to the fact that rotational movement in a fixed-bearing knee occurred at the femorotibial insert articulation, which resulted in impingement in the box of the femoral component on the polyethylene post of the tibial insert (Figures 1 and 2). The post wear could be aggravated by a surgical error of rotational malalignment of the femoral and tibial components relative to each other.

With regard to the low cotact stress rotating-platform design, the most desirable feature was reduction in articular surface wear due to a larger contact area, and less undersurface wear compared with modular fixed-bearing devices. The downside of the LCS rotating-platform design was the occurrence, in some cases, of subluxation or dislocation of the bearing (ie, “spin-off”) due to uncontrolled mobility of the insert. The incidence of subluxation or dislocation was higher with the cruciate-retaining meniscal-bearing design than in the cruciate-sacrificing rotating-platform design.^5-7 In clinical practice around the world, the LCS rotating platform produced satisfactory results. More than 20 years of experience with the LCS rotating-platform design did not show any significant issues related to osteolysis due to polyethylene wear.

Experience with the LCS rotating platform started a trend towards increased use of mobile-bearing knees. Manufacturers of knee implants incorporated favorable design features of the posterior-stabilized fixed-bearing insert into the LCS rotating-platform insert. The post-and-cam mechanism of the posterior-stabilized fixed-bearing was incorporated into the LCS rotating-platform design to provide anteroposterior stability to circumvent the occurrence of spin-off.

Fluoroscopic studies had shown erratic movement (roll-forward instead of rollback) in a significant percentage of LCS rotating-platform implanted knees.⁷ Using a post-and-cam mechanism would lead to consistent posterior rollback, which, in turn, would lead to better flexion. This fundamental realization is the rationale behind development of a posterior stabilized rotating-platform knee. The posterior-stabilized rotating-platform design promised the benefits of both previous designs, reducing wear on the polyethelene surface and minimizing unwanted roll-forward. Compared with the posterior-stabilized fixed-bearing knee, it was designed to address polyethylene wear at the articular surface and undersurface and provide better rotational kinematics, addressing the issue of post wear. Also, stability and consistency of posterior femoral rollback were improved, and, as a result, ROM also improved.

The posterior-stabilized rotating-platform knee, specifically the P.F.C. Sigma rotating-platform knee system (DePuy Orthopaedics, Inc., Warsaw, Ind), was introduced in the United States in October 2000. This new design combined LCS rotating-platform features with the design of the P.F.C. Sigma fixed-bearing (DePuy Orthopaedics, Inc). The femoral component remained the same, and the tibial component resembled the LCS rotating-platform tibial component. The polyethylene insert resembled the P.F.C. Sigma insert on the articular surface but resembled the LCS rotating-platform insert on the undersurface (Figure 3). As flexion increases, the engagement of the post of the polyethylene insert to the cam of the femoral component guides posterior roll-back of the femur (Figure 4).

The Posterior-stabilized Rotating-platform Knee: Clinical Experience

From 1997 to 2001, 71 LCS rotating-platform knees were implanted in the author’s practice; and between 2001 and 2005, 260 P.F.C. Sigma rotating-platform knees were implanted. The average ROM at 1 year with LCS knees was 109° (range: 90°-135°) compared with 120° (range: 95°-155°) with the P.F.C. Sigma rotating-platform (Figure 5). Certainly, there is better ROM with the posterior-stabilized rotating-platform knee compared with a nonposterior-stabilized rotating-platform (specifically the LCS rotating-platform) knee. Spin-off occurred in 1 of 71 patients with an LCS rotating-platform knee compared with none in 260 implanted with the P.F.C. Sigma posterior-stabilized rotating-platform knee. As predicted, stability in the new design is improved.

There is published evidence to support these findings. Average ROM reported with LCS rotating platform is 108° in nonweight-bearing stance and 99° in weight-bearing stance. Average ROM in the posterior stabilized fixed bearing is 127° in nonweight-bearing implants and 113° in weight-bearing implants.⁷ In a study comparing posterior-stabilized fixed-bearing and posterior-stabilized rotating-platform designs, where each patient was implanted with one design in each knee, Ranawat et al⁸ showed that the posterior-stabilized rotating-platform device had the same ROM as the posterior-stabilized fixed-bearing design. Reported incidence of spin-off with the nonposterior-stabilized design of the LCS rotating-platform knee is 0.2% to 9.3%, whereas no spin-off has been reported with use of the P.F.C. Sigma rotating-platform knee.^4,7,9,10

The improved stability afforded by the posterior-stabilized rotating-platform knee has allowed the author to use them in a larger percentage of patients. In Asia, there is a large population of patients presenting relatively late for treatment. Often they have severe deformities needing significant ligament releases at the time of surgery, making it difficult to achieve balance in flexion, which is crucial for mobile bearings. Between 1997 and 2001, the LCS rotating-platform implant was used in 15% of TKA surgeries performed by the author; the rest were done with fixed-bearing implants. With the increased stability afforded by the post-cam of posterior-stabilized rotating-platform since 2001, the posterior-stabilized rotating-platform (P.F.C. Sigma rotating-platform) has been used in 40% of TKA. In the last year, the use of the posterior-stabilized rotating-platform has further increased to 50% because of the use of a tensioner and because of titrated ligament release that the author now does with computer navigation.

Posterior-stabilized Rotating-platform: Addressing Post Wear

With fixed-bearing knees, flexion and extension as well as rotation occur at the femorotibial articular surface. With mobile-bearing knees, rotational movement is decoupled from the flexion-extension movement. Rotation occurs at the undersurface. This occurrence has been confirmed by in vitro studies by D’lima et al,¹¹ who found that all mobile-bearing implants demonstrated rotational movement from 0° to 90° flexion. In another study analyzing cruciate-substituting design with and without rotational constraint, the results showed that mobile-bearing inserts rotate with the femur.¹² Increased conformity or rotational constraint in mobile-bearing knee prostheses do not affect knee kinematics adversely. There was some concern as to whether this rotational mobility occurs in vivo, and, if it does, whether it is maintained over of time. In an in vivo study, Komistek et al¹³ confirmed that polyethylene bearing mobility is maintained. Further, they determined that the polyethylene bearing is primarily rotating relative to the tibia, rather than relative to the femoral component. As the femoral component rotates axially, the polyethylene bearing is rotating at a similar magnitude in the same direction. This axial rotation averaged 8.5° at 3 months and 9.8° at 15 months. The study found that there was only minimal movement of the polyethylene bearing relative to the femoral component, averaging 1.9° at 3 months and 1.0° at 15 months. Komistek et al concluded that bearing mobility is maintained over time, dispelling the concern of soft tissue encapsulation of the bearing. This rotational decoupling occurring in posterior-stabilized rotating-platform knees eliminates post wear of rotational origin compared with the posterior-stabilized fixed-bearing knee.

Posterior-stabilized Rotating-platform knee: Addressing Articular Surface and Undersurface Wear

Ultra-high molecular weight polyethylene has shown improved wear characteristics with unidirectional motion compared with multidirectional motion.^14,15 A greater magnitude of multidirectional motion is seen at the femorotibial articulation in fixed-bearing knees. The rotating-platform design is unique in that it is designed to reduce multidirectional motion of the polyethylene on the proximal tibial-bearing surface (cross-shear). The rotating-platform design permits motion between the femoral-polyethylene insert (articular surface) and polyethylene insert-tibial articulating surfaces (undersurface). This reduces cross-shear motion at the articular surface and converts it to reciprocating unidirectional motion at the undersurface.¹⁶

McNulty et al¹⁹ kinematically tested the wear in fixed-bearing and rotating-platform knee designs on a knee simulator. The average rotating-platform wear rate was 94% lower than the fixed-bearing design.¹⁷ They concluded that the reduction in wear rate of the rotating-platform knee was a result of the reduction in cross-shear motion at the tibiofemoral articulation. With more rigorous knee simulator inputs, Bell et al¹⁶ noted a 77% wear reduction of the rotating-platform knee over a fixed-bearing design using the same implant components.

The increased undersurface wear with modular fixed-bearing devices has been shown in numerous reports.^5,18,19,20 This was attributed to limitations of the locking mechanism, leading to undersurface micromotion, compounded by the rough finish on titanium base plates in these devices. In contrast, the mobile-bearing devices used a highly polished cobalt chrome base tray with full undersurface conformity, which has nearly eliminated concerns over undersurface wear and related osteolysis.

Conclusion

The rationale of posterior-stabilized rotating-platform knee design is to combine the beneficial features of the mobile-bearing rotating-platform knee along with the beneficial features of posterior-stabilized fixed-bearing design. This includes improvements in articular surface wear and undersurface wear characteristic of the mobile-bearing rotating-platform design and simultaneous improvement in knee flexion owing to consistent posterior roll-back and good stability, which reduces the potential for spin-off. The posterior-stabilized rotating-platform knee thus incorporates benefits of the post-and-cam mechanism from the posterior-stabilized fixed-bearing design.

References

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2. Insall JN, Lachiewicz PF, Burstein AH. The posterior stabilized condylar prosthesis: a modification of the total condylar design. Two to four year clinical experience. J Bone Joint Surg Am. 1982; 64:1317-1323.
3. Buechel FF, Pappas MJ. Long-term survivorship analysis of cruciate-sparing versus cruciate-sacrificing knee prostheses using mensical bearings. Clin Orthop Relat Res. 1990; 260:162-169.
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15. Jones VC, Barton DC, Fitzpatrick DP, et al. An experimental model of tibial counterface polyethylene wear in mobile bearing knees: the influence of design and kinematics. Biomed Mater Eng. 1999; 9:189-196.
16. Bell CJ, McEwen HM. Comparision of wear in fixed and mobile-bearing knee designs. Transactions of: The 49th Annual Meeting of the Orthopaedic Research Society. 2003 New Orleans, La.
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Author

Dr Maniar is from the Lilavati Hospital and Research Centre, Mumbai, Maharashtra, India.