Tuesday, August 25, 2009

The History of Mobile-bearing Total Knee Replacement Systems


By Karel J. Hamelynck, MD, PhD

Abstract

The use of a mobile-bearing knee system is routine in modern total knee arthroplasty (TKA). There are indications for use of a mobile-bearing TKA for a growing number of patients. The design, however, was not always well appreciated. Fears of perioperative difficulties and lack of understanding of the design principles limited the acceptance of mobile-bearing technology. Recent evidence has shown that use of mobile-bearing prostheses in TKA has increased, and today, nearly three decades after its introduction, the mobile-bearing design remains relevant and important. The theories behind the design of mobile-bearing prostheses have shown in clinical practice what many already believed to be true: mobile-bearing TKA, when performed correctly, is reliable, and capable of providing substantial benefit for patients.

Mobile-bearing knee replacement systems were designed to prevent mechanical loosening and wear, the two primary shortcomings of knee replacement systems. Studies from the 1960s and 1970s demonstrated good results with conventional fixed-bearing total knee arthroplasty (TKA) systems at 10- to 15-years’ follow-up. However, patients at that time were older and less active than patients are today, and thus there was less demand on implants. Indications for TKA are changing rapidly; today’s patients are more active, and therefore require more durable knee replacement systems, and patients are seeking TKA for knees damaged by excessive weight, accidents, and sports injuries.


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It became clear to early designers of knee prostheses that TKA should entail more than simply replacing cartilage with metal and polyethylene and providing stability through the intrinsic constraint of the components. Clinical follow-up showed that achieving stability through intrinsic constraint was detrimental to fixation and that unnecessary prosthetic constraints should be avoided to minimize the transmission of forces to the bone-prosthesis interface. Free anatomic motion between components was recommended not only to improve function, but to prevent mechanical loosening of components.

These changes had consequences, however; in knee prostheses with fixed bearings, minimal constraint against displacement resulted in small contact areas and high contact stresses. Though minimal torque was transmitted to the fixation interfaces and mechanical loosening was reduced, there was an increased risk of polyethylene articulation damage.

In the 1970s and 1980s, polyethylene wear was not widely recognized as a major cause of aseptic loosening of total knee components. The concept was not well appreciated by orthopedic surgeons, despite the fact that it was demonstrated in laboratory settings and prostheses-retrieval studies. Many designers compromised to produce more conformity between components, while still allowing varus-valgus rotation and some axial rotation. This design concept remains the basis for many knee replacement systems.

Pioneers in Mobile-bearing TKA

Doug Noiles, an engineer with US Surgical Corporation, was probably the first in the United States to recognize that a dual-articulation rotating-platform prosthesis would resolve the kinematic conflict between a low-stress articulation and high bearing conformity. Noiles obtained a patent in 1976 for the Noiles PS Rotating Platform Knee and Revision System, which used metaphyseal sleeves and stems on the tibia and femoral sides. Richard “Dickey” E. Jones began working with Noiles in the mid-1980s, performing many clinical trials on the Noiles PS Rotating Platform knee. This knee system eventually evolved into the P.F.C. Sigma rotating-platform prosthesis (DePuy Orthopaedics, Inc, Warsaw, Ind).

In 1974 Fred Buechel, an orthopedic surgeon, and Michael Pappas, a mechanical engineer, developed the first mobile-bearing joint replacement at Martland Hospital, Newark, NJ, called the “floating-socket” total shoulder, patented in 1975.1,2

Shortly after, they became influenced by the work of John O’Connor, who presented the work of the Oxford group on mobile meniscal bearings used with intact cruciate ligaments to allow normal knee kinematics after TKA at the annual meeting of the American Association of Orthopaedic Surgeons in Las Vegas, Nev, in February 1977. The Oxford group, headed by John Goodfellow and O’Connor, first described the principle of creating congruent contact at the femorotibial interface while allowing the polyethylene tibial bearing to move relative to the tibial tray.3 The Oxford unicompartmental, meniscal-bearing knee, introduced in 1976, is still used today.

Pappas and Buechel4,5 were convinced that the mobile-bearing concept could resolve the dilemma facing designers of total knee prostheses at that time: congruency versus constraint. O’Connor viewed the posterior femoral condyle as a circle with the same femorotibial contact in flexion and extension and used a constant radius (the radius of the posterior condyle) for the posterior and distal part of the femoral component. Pappas, using mobile bearings, studied the material properties of polyethylene and calculated the contact area needed to bring the contact stresses well under the maximum allowable level (10 MPa) of ultra high molecular weight polyethylene. Pappas created a femoral component with a large curve in extension, which was the same in both the coronal and sagittal planes. The radius was 50 mm, much larger than the radius of the posterior condyle. The radius created large contact areas between the femoral component and the tibial mobile bearing and also between the femoral component and the patellar component. Pappas, understanding that polyethylene wear would occur during cycles of peak loading of the knee, during heel strike and before toe-off while the knee is extended 5° to 20°, attempted to achieve congruency in extension. To facilitate flexion, the large femoral radius was reduced posteriorly to maintain line contact.4,5

DePuy Orthopaedics, Inc, entered the “floating socket” industry in 1977 when it received a licensing agreement to manufacture and sell a shoulder prosthesis that implemented the technology. The company later entered the knee prosthesis market in 1979, after Buechel and Pappas presented evidence that the New Jersey Integrated Knee Replacement System improved wear performance of meniscal bearings, compared with conventional fixed bearings, after just 2 years of clinical study.6 In 1979, Barry Sorrells became one of the leading advocates of the design and continues, after 30 years, to use the DePuy rotating-platform knee replacement system successfully. Sorrells is one of the original participants in FDA-mandated trials for the New Jersey Knee System and has become an influential advocate of the rotating-platform knee system, also known as the Low Contact Stress (LCS) knee.

Initially, the FDA declined to allow the sale of the New Jersey Knee as a 510K device and required premarket approval and investigational device exemption clinical trials to prove its safety and efficacy. Several prominent surgeons, including Blackwell Sawyer, Emmet Lunceford and Peter Keblish, participated in these trials along with Louis Jordan and P. Fenning. Keblish, interested in the meniscal-bearing and rotating-platform concepts, became known as a speaker and training surgeon both in the United States and abroad. 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 results from studies on 918 TKA procedures followed for a minimum of 2 years. The evidence submitted to the FDA included data from Seth Greenwald showing minimal wear and an improvement over fixed bearings in a 10 million-cycle simulation on each of three sets of meniscal bearings. Clinical trials for cementless knee replacement were completed successfully between 1984 and 1991.6

Most orthopedic surgeons in the United States at the time did not accept the mobile-bearing concept, because its basis in biomechanics and materials was not well understood. In addition, established knee replacement centers in Boston, New York, and Baltimore, which used fixed-bearing designs, contributed to lack of response. Further, articles about the New Jersey knee were seldom published unless they addressed complications of mobile bearings.7,8 Such reports fostered misconceptions about complications and surgical difficulty. Gradually, however, some orthopedic surgeons began to use the new design and in 1998, John Insall reported the merits of mobile bearings over fixed bearings to improve long-term wear performance in TKA.9

There was more enthusiasm for the LCS total knee system in Europe, as the possibility of good anatomical motion and relief of stresses to the interface appealed to many surgeons. Additionally, the New Jersey knee offered a variety of surgical options. Meniscal bearings allowed the surgeon to retain cruciate ligaments, the natural stabilizers of the knee, whenever they were sufficient and well functioning, and the rotating-platform option allowed sacrificing cruciate ligaments whenever this option seemed more appropriate. In November 1984, the first New Jersey, or LCS, knee, was implanted by a European surgeon in Amsterdam, and in March 1988, the first European congress on mobile-bearing TKA was held, also in Amsterdam. As in the United States, in Europe opinion on the mobile-bearing knee system was divided. Some surgeons, influenced by Sorrells, advocated sacrificing the cruciate ligaments at all times, whereas others, influenced by Keblish, did not consider this sound biomechanical practice. Knee surgeons in Europe were also strongly influenced by Professor Mueller of Bruderholz, Switzerland, whose knowledge of the anatomy and physiology of the knee, and especially the role of the cruciate ligaments, was renowned in TKA.10 Mueller is credited with the wide use of cruciate-retaining meniscal-bearing prostheses in early LCS TKA.

In the 1990s, there was more general acceptance of the LCS concept, and by 1994, 10 years after the LCS knee prosthesis was introduced in Europe and Asia, 75% of all LCS prostheses were sold abroad, compared with only 25% in the United States.11 Mobile-bearing TKA was still new enough, however, that speakers presenting 10- and 15-year follow-up data on the LCS knee system at major orthopedic meetings were relegated to sessions on “new” products or on “the future of TKA.”

Cruciate-retaining versus Cruciate-sacrifcing TKA

Some surgeons still considered TKA using mobile bearings difficult to perform, and there was some belief that the two articulating surfaces would lead to twice as much wear. Proponents of fixed-bearing knees published studies with long-term follow up showing more durability than mobile-bearing knees, data that had not yet been shown except by its designer.12-15

There was considerable debate among advocates of LCS knee replacement on whether to retain or sacrifice the cruciate ligaments in TKA. With rare exception,16 both surgical approaches produced good clinical results.6,17-22 In nearly all cases, less favorable results could be attributed to insufficient surgery and improper indication, such as cruciate ligament retention in the elderly, rather than to defective design of the prosthesis. Very few surgeons attempted to retain both cruciate ligaments; it was considered too difficult by most surgeons, despite reports of good clinical results.24 Retaining the posterior cruciate ligament (PCL) was preferred in young and active patients, because of its important mechanical and proprioceptive qualities. According to Mueller,10 it was unclear whether the PCL (the lateral collateral ligament of the medial compartment of the knee and an important stabilizing force against varus-valgus rotations)25 could also help provide stability in the anteroposterior direction and in femoral roll-back during flexion of the knee in the absence of the anterior cruciate ligament. Some PCL-retaining knees proved to be unstable in the anteroposterior direction, sometimes even demonstrating a negative roll-back movement, with negative consequences for the meniscal bearings, including breakage, dislocation, and wear.26

Some surgeons used the rotating-platform LCS knee in all patients, whereas others limited its use to elderly patients with insufficient cruciate ligaments. Results were generally very good, with minimal loosening of components and virtually no wear-induced osteolysis.21,27 In the United States in the mid-1990s, 75% of LCS knees implanted were of the cruciate-retaining, meniscal-bearing type, and 25% were of the rotating-platform type, compared with 63% and 37%, respectively, outside the United States.11

In 2002, Hamelynck et al28 published a multicenter outcome study comparing results in cruciate-retaining TKA versus cruciate-sacrificing TKA. Twenty-seven surgeons from the United States, Europe, Asia, and South Africa submitted results from procedures performed on 4743 knees: 324 with a bicruciate-retaining, meniscal-bearing tibial component, 2165 with a PCL-retaining, meniscal-bearing tibial component, and 2254 with a rotating-platform tibial component. All patients were followed a minimum of 5 years. The study confirmed results of the LCS total knee system as reported in the literature, that fixation of tibial components with a central tapered cone was reliable. Failure of fixation occurred in 1.1% of the cementless PCL-retaining meniscal-bearing rotating-platform TKAs; 0.7% of the cemented PCL-retaining meniscal-bearing TKAs; and 0.5% of the cemented rotating-platform TKAs. Loosening of femoral and patellar components did not occur. No metal components were revised because of wear-induced osteolysis. Polyethylene wear was reported in some of the older rotating platforms and meniscal bearings, primarily associated with gamma-irradiation in air sterilization. Bearing problems occurred in 1.8% of all patients: 2.2% in bicruciate-retaining meniscal-bearing tibial component knees and 3.0% in PCL-retaining meniscal-bearing tibial component knees; the rotating-platform bearing caused problems in only 0.5% of the cases.

Meniscal-bearing complications such as dislocation and subluxation were found to result from surgical technique issues, including insufficient ligament balancing in flexion, improper positioning of the tibial or femoral component, and retention of an insufficient PCL.

Use of the rotating-platform knee design increased in the United States; the rotating-platform knee was used in approximately 25% of all TKAs in 1994 and in 88% in 1998. Data from Europe and Asia show that a rotating-platform knee was used in 37% of TKAs in 1994 and 72% in 1998.11 The use of meniscal bearings in LCS TKAs in Europe and Asia declined after DePuy launched the LCS AP-glide, a posterior cruciate-retaining tibial component with a gliding polyethylene platform. Movement in the new design was controlled by a guide arm rather than a central cone. Today meniscal-bearing knees are use in less than 4% of LCS knee systems.11

Retrieval studies also demonstrated the superiority of the mobile-bearing design compared with the fixed-bearing designs. Collier et al29 observed important differences in wear in mobile-bearing and fixed-bearing designs in a study comparing 206 mobile-bearing knee devices (144 meniscal-bearing devices and 62 rotating-platform devices) with 619 fixed-bearing knee devices. In the fixed-bearing knees, fatigue mechanisms of cracking and delamination had been dominant. Mobile-bearing knees were found to have a significantly lower incidence of delamination than fixed-bearing knees, despite these bearings all having been sterilized with gamma irradiation in an air environment and then stored in air. It had become clear from earlier research that gamma radiation and air storage resulted in degradative oxidation of the polyethylene.30-33 The superior results among mobile-bearing knees in the study, however, suggested that a design factor, namely congruity, which affords a large contact area to reduce contact stress, accounted for this apparent discrepancy.

Collier et al29 also reported a lower incidence of significant abrasion in mobile-bearing knees, an important factor given the advent of sterilization methods that limit bearing oxidation and its accompanying fatigue mechanisms. Advances in sterilization methods, specifically the use of ethylene oxide instead of gamma irradiation, indicated that abrasion would likely become the wear mode of greatest concern in knee prostheses. According to Collier, the concern that mobile-bearing knees have a dual articulation, and therefore the potential for increased debris generation, appears to be mitigated by the observation that even fixed bearings fret against their metal counterfaces and produce backside wear debris. The wear data indicated very low wear rates for the dual-articulation mobile-bearing system. The constant observation that machining lines remained visible on the mobile bearing back surface was interesting. Backside surface wear did not result in appreciable removal of material, even in bearings of longest duration.

The importance of reducing wear by increasing the contact areas has been confirmed.23,34-36 Recently, McEwen et al37 demonstrated that LCS mobile-bearing rotating-platform knee designs result in a significantly lower mean volumetric wear rate of polyethylene than fixed-bearing knee designs, especially when subjected to high internal-external rotational kinematic inputs. McEwen postulated that the reason for this reduced wear rate is that rotating-platform, mobile-bearing designs decouple the motion between the femoral insert and tibial tray insert. As a result, most of the rotation occurs at the distal tibial articulation surface, which generates unidirectional rotation motion. Unidirectional motion is known to produce low wear.38 In other words, backside wear is minimal, and the proximal femoral articulating interface also generates very low axial rotation. Therefore, at the femoral insert articulation, motion is mostly unidirectional and lower wear on the polyethylene is produced.

The mobility characteristics and gait patterns of patients with mobile bearings have also been studied extensively,26,28,39-47 confirming the superiority of mobile-bearing TKA over fixed-bearing TKA in long-term follow-up.27

Summary

Mobile-bearing systems were designed to prevent mechanical loosening and wear, the two primary complications of knee replacement. Today, after more than 25 years in clinical use, and with overwhelming evidence from clinical experience, laboratory experiments and retrieval analysis, the mobile-bearing concept underlying rotating-platform knee systems has proven to be reliable. The LCS rotating-platform knee, unchanged in almost 30 years, remains a relevant and important design concept.

It is important to note the important design features of mobile-bearing knees. Polyethylene components need to be mobile in a manner that movement is restricted by soft tissues rather than by intrinsic constraint, and bearings must provide sufficient stability to compensate for the absence of the cruciate ligaments. Also, femorotibial contact areas should be large to prevent wear, especially considering condylar lift-off during gait. Finally, movement proximally and distally of the polyethylene bearing should be unidirectional rather than controlled by metal stops, also to prevent wear.

There are many mobile-bearing knee prostheses today. The original patent for mobile-bearing rotating-platform knee systems owned by DePuy expired in 1997, allowing manufacturers to develop their own mobile-bearing prosthesis. Although most rotating-platform knees look alike, there are major differences in design, and classification is needed as proposed by Briard46 and Heim and Greenwald.17 In some rotating-platform designs, the femorotibial contact area is very large, and in other designs, the contact area is rather small. Some designs incorporate unidirectional movement at the distal surface between the polyethylene bearing and tibial tray, whereas others utilize multidirectional gliding at the distal surface.

There also are significant differences in how bearing movement is controlled. Some mobile-bearing knee systems incorporate a certain amount of constraint. In others, bearing movement is controlled by metal cylinders, slots, or stops that may be effective in controlling bearing movement, but may produce suboptimal kinematics and stress on the polyethylene leading to damage and wear. Because of these design differences, long-term follow-up studies should be undertaken to determine variations in results.

References

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  41. Morra EA, Postak PD, Greenwald AS. The influence of mobile bearing knee geometry on the wear of UHMWPE tibial inserts: a finite element study. Paper presented at: 65th Annual Meeting of the American Academy of Orthopedic Surgeons; March 19-23, 1998; New Orleans, La.
  42. Haas BD, Komistek RD, Stiehl JB et al. Kinematic comparison of posterior cruciate sacrifice versus substitution in a mobile bearing total knee arthroplasty. J Arthroplasty. 2002;17:685-692.
  43. Schlepkow P. Three-dimensional kinematics of total knee replacement systems. Arch Orthop Trauma Surg. 1992; 111:204-209.
  44. Stiehl JB, Dennis DA, Komistek RD, Keblish PA. Kinematic analysis of a mobile bearing total knee arthroplasty. Clin Orthop Relat Res. 1997; 345:60-65.
  45. Stiehl JB, Komistek RD, Haas BR, Dennis DA. Frontal plane kinematics after mobile bearing total knee arthroplasty. Clin Orthop Relat Res. 2001; 392:56-61.
  46. Stiehl JB, Komistek RD, Dennis DA. In vivo kinematic comparison of posterior cruciate retention or sacrifice with a mobile bearing total knee arthroplasty. Am J Knee Surg. 2002; 13:13-18.
  47. Briard JL. Mobile bearing knee prosthesis: description and classification. In: Hamelynck KJ, Stiehl JB, eds. LCS Mobile Bearing Knee Arthroplasty: a 25 Years Worldwide Review. Heidelberg, Germany: Springer; 2002; 301-310.
  48. Heim CS, Postak PD, Plaxton NA, Greenwald AS. Mobility characteristics of mobile bearing total knee designs. Series II. Proceedings of the American Academy of Orthopedic Surgeons Annual Meeting. 2000;1:618.

Author

Dr Hamelynck is a former clinical professor of orthopaedic surgery at Slotervaart Hospital, Amsterdam, Netherlands.

4 comments:

  1. Yes I agree with you that the theories behind the design of mobile-bearing prostheses have shown in clinical practice what many already believed to be true: mobile-bearing TKA, when performed correctly, is reliable, and capable of providing substantial benefit for patients.

    Orthopedics Las Vegas

    ReplyDelete
  2. hose looks like quality made bearings. Great to look at. hydrodynamic bearings

    ReplyDelete
  3. Regardless of the materials, all bearing needs care to insure the proper working of the machine.
    white metal bearings

    ReplyDelete
  4. Hi,

    A great article indeed and a very detailed, realistic and superb analysis, of this issue, very nice write up, Thanks.


    Robert Tomlinson MD

    ReplyDelete