CHAPTER SYNOPSIS

Mobile-bearing technology has been available since the late 1970s for primary knee arthroplasty. The introduction of rotating platform technology in revision total knee arthroplasty affords the revision surgeon tangible benefits as the surgeon approaches challenging knee revisions. These benefits address concerns of loosening and post wear in highly constrained devices. Both laboratory and clinical data exist to support the use of these devices.

IMPORTANT POINTS:

• 1
  

  Understand the goals revision surgery, and understand what you as the surgeon expect to achieve for the patient prior to the operation.

  
  
• 2
  

  Using a mobile bearing in a revision can alleviate some of the difficulties, such as the concern of precisely aligning a constrained device.

  
  
• 3
  

  Metaphyseal sleeves can be used in conjunction with the mobile bearing to achieve excellent long-term fixation.

CLINICAL/SURGICAL PEARLS:

• 1
  

  Ream and broach the femur posteriorly to more effectively manage the flexion gap. This tends to move the femoral component posteriorly and close this gap.

  
  
• 2
  

  Preparing the distal femoral canal reduces the influence of the canal axis thus obviating the need for an offset stem.

  
  
• 3
  

  Reliance on metaphyseal fixation reduces the need for long intramedullary stems.

  
  
• 4
  

  Trial with less constrained poly to get a better feel for the balance of the knee without being fooled by the secure feel of the constraining post.

CLINICAL/SURGICAL PITFALLS:

Imbalanced flexion and extension gaps can lead to instability, and further revisions. Use spacer blocks or some sort of balancer or tensor to ensure that the knee is balanced with equal, rectangular gaps to minimize this issue.

VIDEO AVAILABLE:

None.

HISTORY/INTRODUCTION/SCOPE OF THE PROBLEM

Mobile-bearing technology has been available since the mid-1970s in various forms. The original mobile bearing total knee was designed for a hinged knee arthroplasty (Noiles knee prosthesis; US Surgical, Stamford, CT). In 1977, the Low Contract Stress (LCS) Knee was developed, and later received U.S. Food and Drug Administration (FDA) approval for cemented and cementless application after a lengthy clinical investigation. Since then, the mobile-bearing concept has progressed in terms of its broad applications to various forms of knee arthroplasty. Many designs are available around the world for primary total knee arthroplasty (TKA) application from the major implant manufacturers in a variety of different configurations, including cruciate retaining, posterior cruciate ligament (PCL) sacrificing, and, in differing technologies, a rotating platform (RP) type, pin in slot, or some sort of anterior-posterior translation in addition to the axial rotation element. Currently only one manufacturer (DePuy Orthopaedics, Inc., a Johnson & Johnson company) provides a mobile-bearing TKA prosthesis in the United States. Recently, another manufacturer (Zimmer: ZMH Form 10-k Annual Report, February 28, 2007) announced its intent to market a mobile-bearing knee in the United States. It is expected that other mobile-bearing revision knee systems will follow in the short-to mid-term in the United States with further expansion internationally ( Fig. 23-1 ).

Unidirectional versus multidirectional wear of ultra high-molecular-weight polyethylene.

The benefit of mobility in a primary application is two-fold. The main benefit is based on wear reduction by leveraging the scientific knowledge of polyethylene that unidirectional motion reduces wear, while multidirectional motion increases wear. Figure 23-1 illustrates this point. It is important to understand the design of these devices, as some mobile-bearing knees provide unidirectional topside wear, and unidirectional backside wear and show significant wear reduction in the laboratory, while others, particularly those that “slide and glide,” allow for multidirectional motion, and therefore higher wear rates.

The second benefit is one of loosening. Several long-term LCS studies have shown a very low instance of loosening in both a cemented and cementless mobile-bearing device.

THE CASE FOR ROTATION IN A REVISION

To understand the impact of rotation in revision knee arthroplasty, it is important to recognize what the goals are for this surgery. These objectives are to:

• •
  

  Replace lost bone

  
  
• •
  

  Achieve sufficient soft tissue/ligamentous support

  
  
• •
  

  Balance flexion and extension gaps

  
  
• •
  

  Restore joint line

  
  
• •
  

  Set appropriate femoral rotation

  
  
• •
  

  Establish correct mechanical alignment

Successfully obtaining the above should give the patient the best chance to avoid additional revision surgeries. While the goals listed are not inconsequential to achieve in a primary, several complicating factors exist in revisions, making these even more difficult. The first of these factors is the need to properly align the tibial and femoral component with a constrained implant. Studies have shown that the knee can rotate in excess of 16 degrees during flexion and extension, while revision components such as a DePuy Sigma TC3 Fixed Bearing or a Zimmer CCK can limit rotation to as little as 2 degrees due to the fixed-bearing construct. Clearly, this creates a kinematic conflict that places strain on the polyethylene post, which transmits stress to the tibial tray and may ultimately lead to tibial loosening. This task of alignment becomes even more difficult when the patient anatomy is compromised to the degree of the attached ( Fig. 23-2 ).

Intraoperative photograph of femur showing central metaphyseal defect.

In this case, if a fixed-bearing device is used, the surgeon must attempt to precisely align the tibial and femoral components to minimize post wear and tibial strain, while at the same time trying to best cover the remaining bone, recognizing that regardless of how well aligned these components are, the knee will still try to rotate. Using a mobile-bearing prosthesis in this case allows the surgeon to place the mobile-bearing tray in a position that best covers the tibial plateau and then allows the bearing mobility to address the rotary kinematics of the knee with less concern for the strain placed on the tibia and post of the polyethylene ( Fig. 23-3 ).

Mobile bearing TC3 RP polyethylene.

(Adapted from image courtesy of DePuy Orthopaedics, Inc.)

This argument has been substantiated in the laboratory, and the strain reduction on the proximal tibia by using a mobile-bearing prosthesis has been shown in these studies to be as great at 73%.

The first generation of hinged designs gave us further clinical support to the message regarding constraint and rotation. Hinged prostheses, created by linking the femur to the polyethylene and tibial tray, contain a great deal of constraint. This constraint is necessary to replace missing or deficient collateral ligaments. Early (fixed-bearing) hinge designs, however, only delivered satisfactory results in 65% to 75% of the cases, with a high rate of loosening and other complications. These early designs, like the Guepar and Walldius, had significant loosening issues because of the highly constrained implants that transmitted significant torsional forces to the bone–cement interface due to the lack of rotation. Recognizing this as an issue, all major manufacturers (DePuy, Zimmer, Stryker, Biomet, etc.) now market mobile-bearing hinged designs. These rotating designs still deliver the required constraint, but because they now allow rotation, the concern of loosening due to rotational forces is significantly reduced. This engineering concept drives all forms of hinged knee technology designed to reduce stress on the components as well as the bone–implant interface. It is fair to say that mobile-bearing technology coupled with the individual company-patented technologies are the key to the success of these implants long term.

The other concern with a fixed-bearing constrained implant is the wear on the post. Retrieval analysis confirms the intuitive, which is that post wear has been shown in nearly all posted retrievals, and that the more constrained the polyethylene, the higher is the wear. This topic has been explored in a recent laboratory testing environment, and the results are discussed in the next section.