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Evolution of the Low Contact Stress (LCS) Complete Knee System
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Abstract
The low contact stress knee design was one of the first designs to become widely available and popular for total knee arthroplasty. The low contact stress knee was initially designed to address wear due to high contact stresses seen in fixed-bearing designs. Today, nearly 30 years after its introduction, the technology behind the knee remains just as relevant. Many of the original innovations in early low contact stress knee systems are still in use today. New innovations, like a new instrument set and the advent of computer-assisted surgery, have added to the success of the technology. The history of this innovative design is discussed with special consideration given to the specific design principals that have improved the basic design over the past three decades.
The articular geometry of the present low contact stress (LCS) system has not changed in over 28 years and remains unchanged with the introduction of the LCS Complete Knee System (DePuy Orthopaedics Inc, Warsaw, Ind). What began in 1977 as a joint orthopedic-engineering venture by Fred Buechel, MD, and Michael Pappas, PhD, led to the introduction of the New Jersey LCS Complete Knee System. The LCS Complete Knee System has enjoyed a long and successful clinical experience and continues to be the model for other mobile-bearing knee systems. Instrumentation has improved dramatically with the introduction of the Milestone instrument set and the incorporation of computer-assisted surgeries (CAS). Enhancements to the LCS knee have been driven by clinical experience. Well into the third decade of use, the LCS Complete continues the legacy of the New Jersey LCS knee.
The Early LCS History
Introduced in 1977, the LCS Complete Knee System was one of 300 total knee designs clinically available. At that time, there were three LCS knee system options, a bicruciate-retaining option, a posterior cruciate-retaining option, and a cruciate-sacrificing option (the rotating-platform design). In 1980, the Food and Drug Administration granted DePuy an investigational device exemption to begin clinical trials of LCS with cement. Marketing of the cemented models began in 1985. The uncemented version of these three designs was granted an investigational device exemption in 1983. Successful marketing of the posterior cruciate-retaining option began in 1990 and for the cruciate sacrificing option in 1994. The earliest clinical results of LCS knee implantation were published by Buechel and Pappas1 in 1986.
In the early years, the majority of LCS knees implanted were of the posterior cruciate-retaining meniscal-bearing design. However, early clinical results initiated a change in use.2,3
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| Figure 1: Polyethylene contact forces for a given contact area. |
In a review that included a minimum 10-year follow-up on 373 LCS implants, Buechel et al4 reported that Kaplan-Maier Survivorship for revision for any reason was 83% (16-year follow-up) for the cementless meniscal-bearing group, 97.7% for the cemented rotating-platform group, and 98.3% (18-year follow-up) for the cementless rotating-platform group. With time, it became apparent that the rotating-platform prosthesis was easier to implant and yielded superior survivorship rates with fewer complications – an interesting finding given that the rotating-platform was initially designed for the more complicated and difficult procedures.
Unique Features of the LCS Complete Knee System
The LCS knee was initially designed to address problems of fixed-bearing designs, notably the problem of wear due to the high contact stress on the polyethylene common in the designs of the 1970s. To accomplish this, several innovative design features of the LCS knee were continued with the LCS Complete Knee System. Given the recent surge in the use of mobile-bearing knee arthroplasties, it is important to review these features:
- Maximal conformity of the femoral component and the tibial polyethylene in gait
- Maximal conformity of the femoral component and the patellar component through 110° of flexion
- Decoupling of the rotational forces of the femoral component on the tibial polyethylene by providing rotational freedom of the polyethylene insert on a polished chrome cobalt tibial tray
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| Figure 2: Surface area of contact for mobile-bearing (a = 4.9 MPa) and fixed-bearing knee arthroplasties (b = 25 MPa; c = 28 MPa; d = 32 MPa). | |
Polyethylene Conformity (Fatigue Failure)
Manufacturers of medical-grade polyethylene have recommended that the contact stress on the polyethylene in the total knee be less than 10 MPa (preferably less than 5 MPa). This is accomplished by maximizing the surface area of contact between the femoral component and the polyethylene insert (Figures 1 and 2). For any given femoral geometry, the only way to increase this area of contact is by maximizing the conformity of these surfaces. In the LCS knee, this has been accomplished by matching the curvature of radii between the femoral component and the tibial insert, as well as between the femoral component and the patellar polyethylene (Figures 3-5). This matching conformity occurs between the femur and tibia in gait (0°-30°) and between the patella and femur from 0-110° of flexion. Note that the anteroposterior radius of curvature of the femoral component in segment 2 (the weight-bearing segment) is the same A radius found in the medial lateral radius. Therefore, the area of contact, while in gait, is a large spherical area averaging 877 mm2 (Figure 2).
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Figure 3: The coronal radii of curvature between the femoral and tibial components are congruent, as are the patello-femoral radii. Figure 4: The sagital geometry of the low contact stress knee system. Note that the radius of Segment 2 (the weight-bearing segment) is the same A radius as the coronal femoral radius. |
As more mobile-bearing total knee options appear on the market, conformity of the design becomes an important consideration. Kuster and Stachowiak5 studied the effect of conformity and load using a finite element method. A conformity ratio equal to zero represents a flat tibial insert in contact with the femoral condyle, whereas the highest mild ratio of 0.99 represents a fully conforming contact between the femoral condyle and the tibial insert. Kuster and Stachowiak found that a high conformity ratio had a much greater effect on the polyethylene insert stress parameters than a large load reduction. For example, all stresses calculated were much higher for a 1000 N (standing) on a flat inlay than for 6000 N (running) on a conforming design. Even a small increase in the conformity ratio from 0.95 to 0.99 had a greater effect on the surface and shear stress parameters than a load increase of 3000 N. Kuster and Stachowiak concluded that a total knee arthroplasty with a high conformity ratio may reduce surface delamination and accelerated wear, whereas activity restrictions cannot prevent this type of wear in a nonconforming design. In gait, the LCS has a conformity ratio of 0.99.
Decoupling of Rotational Forces (Unidirectional Wear)
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The quality of commercially available medical-grade polyethylene has improved significantly in recent years. Avoidance of gamma sterilization in air (gamma irradiation in a vacuum pouch) and use of polyethylene resins free of calcium stearate have significantly reduced the potential for oxidation, which can dramatically reduce the mechanical properties of the polyethylene. Despite this improvement, recent studies continue to show a significant reduction in volumetric wear produced by the LCS knee under high kinematic conditions compared with fixed-bearing designs.6 This is believed to be due to the coupling of rotational forces experienced at the femoral component/tibial polyethylene insert surface as well as at the polyethylene insert/tibial tray surface. Unidirectional motion is known to produce low wear.7,8 Most of the rotation of the LCS knee occurs at the more distal tibial tray/polyethylene insert surface, which produces a predominant unidirectional wear pattern. Therefore, at the femoral component/polyethylene insert articulation, the motion is also preferably unidirectional (flexion-extension) and similarly has a low wear rate. Thus, the unique design of the LCS mobile-bearing knee translates complex input motions into more unidirectional motions, thus benefiting from a reduced wear rate (Figure 6).5
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Bridging Bearings
To accomplish maximal conformity, it is essential to match exactly the sizes of the femoral component and the tibial polyethylene. The classic LCS was available in six sizes (small, small plus, standard, standard plus, large, and large plus), but the cone of the tibial insert was only made in three sizes to accommodate the internal dimensions of the central stem of the small, standard, and large trays. A standard tibial polyethylene fit well with a standard femoral component and a standard or a standard plus tibial tray. However, if a standard plus femoral component was used with a large tibial tray, a “bridging bearing” had to be added to provide a standard plus femoral-polyethylene articulation with a large tibial tray cone diameter. While effective, the use of these bridging bearings was always a source of confusion.
The LCS Complete
Despite the outstanding clinical results of the LCS knee over the past 25 years, a number of enhancements have been made to the system. The LCS Complete Knee System is designed to improve ease of use, expand indications for use, and improve the accuracy and precision of the implantation. These refinements are the result of suggestions provided by many surgeons who have used the LCS Complete Knee System for many years (Table; Figures 7 and 8).
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| Figure 7: The LCS rotating-platform knee prosthesis. | Figure 8: The LCS Rotating-platform Knee System. The system is available with a keeled tibial or traditional nonkeeled tibial component. |
Perhaps of more importance is what has not been changed. The sagittal and coronal geometry of the femoral component has remained unchanged. The 0.99 conformity ratio between the femoral component and the tibial polyethylene insert during gait remains unchanged. The patellar femoral articulation, with conformity through 110° of flexion, remains unchanged. The ability to provide cemented or uncemented fixation also remains unchanged.
Femoral Component Enhancements
Several subtle design changes have been made to the femoral component of the LCS knee to allow it to be more forgiving in size and fit and to provide greater ease in use. A medium size has been added (Figure 9) based on worldwide anthropometric data. Two minor changes have been made to the anterior flange of the femoral component: (1) the mediolateral width of the flange was reduced by several millimeters to avoid any potential for overhang; and (2) the height of the anterior flange was extended by several millimeters to provide for better patellar engagement and to provide additional stability to flexion forces (Figure 10). Impaction slots were added to the distal femoral condyle to facilitate mechanical removal in case of infection. The internal shape of the trochlea groove has been straightened, the shape of the fixation pegs has been smoothed and made more uniform, and the depth of the cement pockets was lessened slightly.
As noted above, the articular geometry of the femoral component has not been altered at all to ensure that the surface contact area of the polyethylene inserts (tibial and patella) remains unchanged.
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| Figure 9: The medium size has been added to the LCS Complete system. |
Tibial Component Enhancements
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Several changes have been made to the tibial component of the LCS Complete Knee System based on input from many experienced LCS knee surgeons. In general, these changes improve the design by adding to the system’s flexibility and ease of use.
The shape of the tibial component has been changed from the dumbbell shape of the classic LCS tibia to the more traditional kidney shape of the LCS Complete Knee System (Figure 11). This allows for greater coverage of the proximal tibia and is especially helpful in uncemented fixation. Two additional sizes of tibial components have been added, further enhancing the ability to fully cover the proximal tibia. The tibial tray is now offered in the traditional LCS nonkeeled geometry as well as in a keeled design (Figure 8).
The polyethylene insert of the tibial component has also undergone change. The height of the anterior and posterior lips of the insert have been modified, with the anterior flange increasing in height by 2.4 mm (to 10 mm) and the posterior height decreasing by 0.4 mm (to 2.7 mm). This will effectively increase the resistance to tibiofemoral subluxation (spinout) (Figure 12) while avoiding posterior impingement of the polyethylene and the posterior femur in deep flexion.
The shape of the cone of the polyethylene insert was changed from the traditional tapered sides. The new shape adds a cylindrical section to the distal aspect of the cone to enhance the cone’s resistance to subluxation from the tray. This is particularly important in revision applications (Figure 13). Perhaps the most significant change in the tibial component is that the internal dimensions of the central stem are now universal between the tibial component and the polyethylene inserts. This allows use of any size polyethylene insert with any size tibial base plate, makes the components universally interchangeable, and eliminates the need for the cumbersome bridge bearings while still allowing matching of the polyethylene insert and the femoral component.
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| Figure 11: The shape of the original LCS tibial component (A) and the shape of the new LCS Complete tibial component (B). | |
Patellar Enhancements
The patellar component of the LCS Complete Knee System has undergone two modifications: (1) the classic metal-backed mobile-bearing patellar component has undergone a slight decrease in size to prevent the possibility of the component overhanging the patellar bed; and (2) an all-polyethylene patellar button has been added. While an all-polyethylene component may seem to contradict the mobile-bearing philosophy of the LCS Complete Knee System, the unique nature of this design still provides for maximal conformity between the patella polyethylene and the femoral component. With the all-polyethylene design, the lateral facet of the patella maintains its highly conforming biconcave design (similar to the metal-backed mobile-bearing patella design) while the medial facet mimics a domed component. While providing only point contact medially, the lateral facet of the all-polyethylene patella will still provide maximal conformity and reduce polyethylene stresses through 110° of flexion.
Additional LCS Complete Knee System Developments
The LCS Complete Knee System in Revision
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The LCS Complete Knee System in revision has evolved from the rudimentary classic stemmed LCS knee revision components to the LCS knee modular revision components (which allowed augmentation of the femoral and/or tibial components) to the LCS Complete Knee System in revision. This complete revision knee system will allow complex revision using the LCS knee philosophy. It will include stems, augments and metaphyseal filling sleeves and will provide a range of stability from the LCS dished insert to a posterior stabilized insert to a constrained (TC3) insert. The LCS Complete Knee System in revision will merge with the hinged components of the DePuy Limb Preservation System and will add much needed flexibility to the performance of these difficult revision cases.9
Instrumentation
The surgical technique of the LCS system implantation has always been integral to the success of the LCS knee system. The Milestone instrumentation set has served its purpose for many years and will continue to remain available. Computer-assisted surgery has now been integrated into the LCS implantation technique. At present, the surgeon can navigate the classic Milestone instruments or perform the surgery using the minimally invasive workflow. This allows the surgeon to cut the tibia first (as with the non-navigated technique), establish adequate alignment, record the flexion and extension gaps, and then perform the distal femoral resection as the second bony cut. This workflow may allow for a smaller incision and more limited exposure. The LCS instrumentation set is currently being developed for more minimally invasive non-navigated purposes and is undergoing clinical trials at this time.
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| Figure 13: The shape of the original LCS tibial component (A) and the shape of the new LCS Complete tibial component (B). | |
Summary
The LCS Complete Knee System has maintained the heritage of the LCS knee while expanding its use. Subtle design changes in the femoral component have improved the fit and have made insertion easier. The tibial component changes allow for better proximal tibial coverage, universal flexibility in tibial femoral interchangeability, and greater subluxation resistance. The patella modifications provide more accurate sizing while allowing for an all-polyethylene option.
The LCS Complete Revision Knee System has greatly enhanced the ability to include the philosophy of low contact stress arthroplasty to the more clinically challenging revision situation. Instrumentation is now designed to provide greater accuracy and precision and will incorporate computer navigation.
The philosophy underlying the LCS knee has been successfully applied for over 25 years. The LCS Complete Knee System has retained the principles of low contact stress arthroplasty while moving toward its third decade of use. Increasingly difficult clinical situations will continue to be addressed using these principles.
References
- Buechel FF, Pappas MJ. The New Jersey Low Contact Stress Knee Replacement System: biomechanical rationale and review of the first 123 cemented cases. Arch Orthop Trauma Surg. 1986; 105:197-204.
- Buechel FF, Pappas MJ. New Jersey Low Contact Stress Knee Replacement System. Ten-year evaluation of meniscal bearings. Orthop Clin North Am. 1989; 20:147-177.
- Sorrells RB, Fenning JB, Davenport JM. Comparison of the clinical results and survivorship of noncemented cruciate sacrificing vs. cruciate sparing total knee replacements. Presented at: 60th Annual Meeting of the American Academy of Orthopaidic Surgeons. Feb 18-23, 1995, San Francisco, Calif.
- Buechel FF Sr, Buechel FF Jr, Pappas MJ, D’Alessio J. Twenty-year evaluation of meniscal bearing and rotating platform knee replacements. Clin Orthop Relat Res. 2001; 388:41-50.
- Kuster MS, Stachowiak GW. Factors affecting polyethylene wear in total knee arthroplasty. Orthopedics. 2002; 25(suppl 2):235-241.
- McEwen HMJ, McNulty DE, Auger DD, et al. LCS Mobile Bearing Knee Arthroplasty: 25 Years of Worldwide Experience. Berlin: Springer-Verlag; 2002:67-80.
- Marrs H, Barton DC, Jones RA, et al. Comparative wear under four different tribiological conditions of acetylene enhanced cross-linked ultra high molecular weight polyethylene. J Mater Sci Mater Med. 1999; 10:333-342.
- Wang A, Stark C, Dumbleton JH. Mechanistic and morphological origins of ultra-high molecular weight polyethylene wear debris in total joint replacement prostheses. Proc Instn Mech Engrs [H].1996; 210:141-155.
- Lippe CN, Crossett LS. Low contact stress (LCS) complete knee system in revision surgery. Orthopedics. 2006; 29(suppl 1):S88-S94.
Author
Dr Crosset is from the University of Pittsburgh School of Medicine, Pittsburgh, Pa.