CHAPTER SYNOPSIS:

In response to the mechanical loosening, early wear, backside wear, and osteolysis, mobile-bearing technology was developed to lessen the constraints found in classic fixed-bearing knees and to improve the contact stresses that were found to lead to polyethylene failure. Kinematic studies demonstrated self-centering properties that could improve range of motion and patellar tracking. Approved by the U.S. Food and Drug Administration for use in the United States in 1984, mobile-bearing designs have an excellent long-term survivorship in both North America and Europe. The kinematic design of mobile-bearing knees requires the total joint surgeon to closely adhere to the principles of axial alignment as well as ligament and flexion-gap balancing. This chapter will review the evolution of mobile-bearing designs, principles of surgical implantation, and avoidance of specific pitfalls inherent to the design and provide a review of the short- and long-term outcomes.

IMPORTANT POINTS:

• 1
  

  It is important to understanding design rationales for fixed bearing, mobile bearing, meniscal bearing, and rotating platform.

  
  
• 2
  

  There are advantages and specific outcomes of mobile-bearing versus fixed-bearing total knee designs.

CLINICAL/SURGICAL PEARLS:

• 1
  

  Patient selection is one of the keys to the operation, which includes understanding the indications and contraindications to the use of mobile-bearing designed total knee arthroplasties.

  
  
• 2
  

  Understanding the measured resection versus gap-balancing technique is crucial to a stable mobile-bearing knee.

CLINICAL/SURGICAL PITFALLS:

• 1
  

  Poor balancing may lead to dislocation and bearing dissociation or bearing spinout, and these must be addressed prior to leaving the operating room.

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FIXED-BEARING DEVELOPMENT

In the 1970s came the beginning of the “modern total knee arthroplasty” with the U.S. Food and Drug Administration (FDA) approval of polymethylmethracrylate (PMMA) in the United States and the development of the total condylar knee, popularized by John Insall, which is recognized as the first widely successful cruciate sacrificing total knee arthroplasty (TKA). Early total knee systems demonstrated good results, with a 90% to 95% survival rate when using revision as an end point, with conventional fixed-bearing TKA. However, the patients in these early studies were older and more sedentary than are patients today. These early long-term survival studies used all polyethylene components or fixed monoblock systems with very limited sizing and no modularity. Early failures of the total knees were typically due to aseptic loosening caused by malalignment and subsequent ligamentous instability.

The 1980s can be referred to as the “age of the engineers,” with the development of improved instrumentation and surgical techniques to minimize malalignment, instability, and subsequent loosening. Implants including the Low Contact Stress (DePuy Orthopaedics, Inc., a Johnson & Johnson company, Warsaw, IN), Porous Coated Anatomic (PCA Howmedica, Rutherford, NJ), Press Fit Condylar (PFC Johnson & Johnson, New Brunswick, NJ), and Miller/Galante (Zimmer, Warsaw, IN) were developed with modularity and cementless fixation, giving surgeons the ability to better size and fit components even after cementing. Early designs had increased constraint to provide improved stability of the implants. Torsional, coronal, and sagittal stresses, normally shared by the surrounding soft tissues, were more fully transferred through the implant to the fixation interface. These additional forces resulted in premature total knee failure secondary to aseptic component loosening. To minimize the transfer of these stresses to the implant–bone interface, this decade also saw the evolution of less-constrained designs through the creation of less-conforming articular surface geometry, which typically incorporated the use of round-on-flat or flat-on-flat articular geometries. These designs allowed rotation and multiplane translations to occur at the articulating interface and relied more on the surrounding soft tissues than conformity of articular surfaces for the stability and dispersion of applied load. Despite the reduction in stresses being transmitted to the implant–bone interface, flat-on-flat designs resulted in small contact areas and subsequent increased contact stresses on the polyethylene. This resulted in the development of premature polyethylene wear. The use of finite element analysis ( Figs. 12–1 and 12–2 ) demonstrated that, although these less-conforming implants reduced the stress at the implant bone interface, they produced point-and-line contact or loading conditions at the articular surface with small contact areas, which greatly increased the stress on the polyethylene. Polyethylene inserts that were too thin or with reinforced polyethylene composites failed catastrophically.

Polyethylene surface area of contact for mobile-bearing (A) and three fixed-bearing implants demonstrating partial contact (B) , line contact (C) , and point contact (D) . Histogram demonstrating the contact forces associated with the contact areas (E) .

(Reprinted with permission.)

Histograms of a high kinematic knee simulator analysis demonstrating polyethylene wear per million cycles of a fixed-bearing versus rotating-platform total knee arthroplasty with identical femoral geometry.

(Reprinted with permission.)

The decade of the 1990s saw the development of the “science of articulation” seen with the design of the articulating surfaces that were reactions to the design and engineering errors of earlier implants. Polyethylene inserts were made thicker and dished in an attempt to avoid edge loading. Titanium was eliminated as an articulating surface. This period also saw the maturation of polymer chemistry and tribiology. As the 21st century approached, many of the early failures and mistakes had been sorted out and addressed with better implant designs and improved sterilization and packaging of polyethylene that minimized wear and debris formation. Unfortunately, the advent of modular tibial designs using a metal tibial base plate with modular polyethylene tibial insert created a problem of increased wear and osteolysis due to particulate debris from the nonarticulating surfaces. Retrieval studies demonstrated that significant wear and the subsequent generation of particulate debris occurred between the captured polyethylene bearing and the base plate . Moderate to severe wear was found at the nonarticulating surface that was frequently observed in all designs of knee prosthesis, independent of capture mechanism. This wear and debris formation was substantial enough to possibly induce osteolysis. Furthermore, polishing the tibial base plate counterface and improved polyethylene sterilization diminished but did not eliminate osteolysis. Retrieval and finite element analysis studies ( Fig. 12–3 ) have also demonstrated high stress and small contact areas as the femoral component rotates around the central posterior cruciate ligament (PCL) post of the polyethylene insert. This cam and post impingement leads to increased polyethylene wear and debris formation and has been associated with catastrophic failure of cruciate sacrificing total knee implants. At present, fixed-bearing designs compromise to produce more conformity between components, while still allowing limited coronal rotation, axial rotation, and sagittal rotation and translation. This design concept remains the basis for many knee replacement systems.

Contact stresses generated in a fixed-bearing cruciate sacrificing design demonstrating line contact (left) and tibial post impingement (right) .

(Reprinted with permission.)

MOBILE-BEARING DEVELOPMENT

In , John Insall stated, “The kinematic conflict between low stress articulations and free rotation cannot be solved by any fixed bearing design. … Fixed-bearing knee designs have reached their ultimate expression; often, this stage of development indicates impending obsolescence. Mobile-bearing knees offer an attractive avenue for future development.” Mobile-bearing knee replacement systems were designed to prevent mechanical loosening and wear, the two primary shortcomings of early knee replacement systems.

Doug Noiles, an engineer with U.S. Surgical Corporation, was one of the pioneers in recognizing that a duel articulation rotating-platform prosthesis would resolve the kinematic conflict between low stress articulation and high bearing conformity. He postulated that the high stresses at the tibial bone interface in conforming fixed-bearing implants would be significantly reduced by allowing rotation through the polyethylene–tibial base plate interface. Forces generated during normal ambulation were not transmitted to the prosthesis bone interface. This also allowed greater conformity between the femoral component and polyethylene, increasing contact areas and minimizing contact stresses ( Fig. 12–4 ). In 1976, Noiles obtained a patent for the PS Rotating Platform Knee and Revision System. Richard “Dickey” Jones performed many of the early clinical trials on the system and helped develope the P-ROM, a press fit condylar (PFC) femoral component, and a primary S-ROM tibial component with a mobile bearing. With extensive European experience and clinical trials, the Noiles PS Rotating Platform Knee eventually evolved into the PFC Sigma rotating-platform prosthesis (DePuy Orthopaedics Inc.).

Contact area and stress analysis demonstrating higher polyethylene contact areas (mm ² ) and lower peak stresses (MPa) of three mobile-bearing total knee arthroplasty designs.

(Reprinted with permission.)

Following their design of the “floating-socket” total shoulder, Fred Buechel, an orthopedic surgeon, and engineer Michael Pappas were convinced that the mobile-bearing concept could resolve the dilemma between congruency and constraint in TKA designs. The New Jersey Integrated Knee Replacement System was developed with a large radius of curvature in extension that was symmetrical in the sagittal and coronal planes and a narrower radius in the posterior condyle. This design maximized contact areas in extension where loading is highest and allowed for improved flexion. In 1984, the FDA approved the sale of the New Jersey Knee System for cemented knee replacement based primarily on the experience of 23 orthopedic surgeons and the results from studies on 918 TKA procedures.

In the early 1980s, DePuy Orthopaedics developed the Low Contact Stress (LCS) knee from the New Jersey Knee System. The LCS knee again maximized conformity with a matching coronal and sagittal radius of the femoral component and the radii of the tibial polyethylene. There was also high conformity between the patella and the anterior flange of the component. This all contributes to very low contact stresses, potentially minimizing polyethylene wear. The evolution of the LCS implant eventually offered a variety of surgical options. Meniscal bearings allowed the surgeon to retain the cruciate ligaments, and the rotating-platform option allowed sacrificing the cruciate ligaments whenever appropriate.

Most surgeons in North America initially did not accept the mobile-bearing concept because its basis in biomechanics was not well understood. Additionally, established total knee centers in North America were reporting good outcomes with modular fixed-bearing designs. Publications on mobile-bearing knee systems focused on rare reports of surgical difficulty and complications. As the turn of the 21st century approached, some of these misconceptions were eventually dispelled by midterm studies in North America, Europe, and Asia.

Although early acceptance of mobile bearings was limited in the United States, there was significantly more enthusiasm for the LCS total knee system in Europe. In November 1984, the first LCS was implanted in Europe. Intermediate- and long-term survival studies in Europe demonstrated reliable function and low failures in both cruciate retaining meniscal-bearing components and cruciate sacrificing rotating-platform implants. Polyethylene wear in older implants associated with gamma radiation in air sterilization demonstrated poor rates of bearing fatigue in traditional fixed-bearing designs; however, rotating-platform bearings failed in only 0.5% of the cases, a significantly lower rate than their fixed-bearing counterparts.

No published outcome study has demonstrated superior results of mobile-bearing TKA over fixed-bearing TKA. Callaghan has stated, however, that the results of mobile-bearing TKA should be at least equivalent to the results of fixed-bearing TKA. Callaghan et al. reported 97% survival of the low contact stress mobile-bearing knee at 15 years. Sorrells and Stiehl reported 88% survival at 13 years in younger patients, under the age of 65, using the same system. Long-term studies of the low contact stress mobile-bearing knee system by Beuchel et al. reported 98% survival at 20 years. None of these studies found more than 1% periarticular osteolysis at medium- and long-term follow-up, validating the concept of minimal surface wear with mobile-bearing TKA. The scientific basis of mobile-bearing TKA is now firmly established. Wear testing data and dynamic kinematic motion studies highlight potential advantages of mobile-bearing TKA. These include decreased contact stresses and wear on the polyethylene inserts and reduced stresses transmitted to the implant–bone interface. Further clinical studies are ongoing.

PATIENT SELECTION FOR MOBILE-BEARING, HIGH-FLEXION TOTAL KNEE ARTHROPLASTY

Our practice has identified patients with disabling knee pain unresponsive to conservative measures as good candidates to receive a mobile-bearing TKA. Patient selection is guided by patient expectations. Patients implanted with a mobile-bearing TKA may benefit from potential increase in range of motion (ROM) over fixed-bearing conventional knee systems, especially with higher flexion requirements. Patients with significant coronal deformity, 20 degrees of valgus, and 25 to 30 degrees of varus may not have the adequate remaining soft tissues needed to create well-balanced flexion and extension gaps necessary to prevent instability and possible spinout. A more constrained polyethylene should be available for these cases as well as a fixed-bearing component.

Preoperative fluid exercises, such as biking or swimming, prepare a patient for surgery and enhance return to function. Multimodal pain management programs encourage early flexion range, as do educational programs to decrease patient anxiety and improve patient knowledge and expectations.

Individuals with significantly limited preoperative ROM may have a better result with the use of a high-flexion rotating-platform TKA. Based on early clinical result studies from R. E. Jones and A. S. Ranawat, patients who have a preoperative ROM of 100 degrees or less can expect a significant increase in ROM after implantation with the high-flexion designed mobile-bearing TKA.

Traditionally, postoperative ROM after TKA is most closely correlated to preoperative ROM for fixed-bearing designs. Dennis and Komistek showed in dynamic fluoroscopic kinematic studies that abnormal kinematics are often observed only in gait with active weight-bearing testing versus passive non–weight-bearing conditions. Their study cited a mean ROM difference between active and passive of 4 degrees for normal knees, 20 degrees for fixed cruciate retaining TKAs, and 15 degrees for fixed cruciate substituting TKAs, but only a difference of 4 degrees in rotating-platform cruciate substituting TKA. Jones and de Jong demonstrated that there was not a positive correlation between postoperative ROM and preoperative ROM using a mobile-bearing cruciate substituting system. In a series of 153 consecutive TKAs, a mean ROM of 123 degrees (range, 95 to 145 degrees) was demonstrated at 1 year postoperatively, whereas in a cohort of 32 knees with preoperative ROM of less than 95 degrees, mean ROM was 120 degrees (range, 100 to 140 degrees) at 1 year.

Improvements in ROM resulted from kinematics of femoral rollback in the cruciate substituting design and the self-aligning character of the rotating platform Ranawat et al. showed increased axial rotation and less condylar lift-off when comparing a matched series of cruciate substituting fixed-bearing TKAs versus cruciate substituting rotating-platform TKAs.

Improved ROM in TKA is an important issue for many patients with certain types of employment, cultural lifestyles, and activities that require in high flexion. Knee flexion up to 150 degrees is found routinely in cultures in the Middle and Far East. These issues have led to a new generation of TKA designs that allow flexion beyond 125 degrees. The designs share features of increased posterior condylar offset to maintain bearing conformity and decrease line contact loading, extended trochlear grooves for congruent patellar contact, and an anteriorly recessed ultrahigh-molecular-weight polyethylene bearing to accommodate the increased excursion of the extensor mechanism. High-flexion activities, such as kneeling or praying, are done with internal rotation, external rotation, or neutral rotation. Maximum flexion requires a mobile-bearing tibial insert to accommodate the increased rotational forces.

Jones in a pilot study with bilateral TKAs, compared the PFC Sigma RP (DePuy Orthopaedics, Inc.) cruciate substituting in one knee and the high-flexion PFC Sigma RPF in the other knee with the TKAs being performed sequentially by the author. Twenty-four patients with PFC Sigma RP demonstrated a postoperative ROM of 118 degrees. The 24 TKAs in the contralateral group with PFC Sigma RPF had a mean postoperative ROM of 128 degrees. This study, as well as results from Gupta et al., point to a design-specific increase in postoperative ROM with a mobile-bearing high-flexion design.