Total Knee Contact Pressures: The Effect of Congruity and Alignment

William Petty, MD, Gary J. Miller, PhD, Donald L. Bartel, PhD, Timothy M. Wright, PhD, and Albert H. Burstein

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In This Article

Mechanics of Total Knee Replacement

Mechanics and kinematics of the knee joint following total knee replacement are related to the mechanics and kinematics of the normal knee. Some of the important new variables introduced include the material properties of the prosthetic components, the conformity of the articulating surfaces, the thickness of the polyethylene material, the elastic modulus of the polyethylene material, and the knee alignment following prosthetic arthroplasty.

The pits and delamination seen in tibial components are due to fatigue loading, which causes cracks to propagate from surface or subsurface defects (Figs. 2C and 2D).[8] The stresses associated with these failures are affected by the conformity of articulating surfaces, by the thickness of the polyethylene component, and by the elastic modulus of the polyethylene.[8] Minimization of surface damage can be achieved by reducing the contact stress between the components of the total knee prosthesis.

Failed polyethylene 8 years after total knee arthroplasty. (A) anterior posterior x-ray; (B) lateral x-ray; (C) delamination of the polyethylene prosthesis; (D) close-up view of the lateral tibial plateau prosthesis surface.

Failed polyethylene 8 years after total knee arthroplasty. (A) anterior posterior x-ray; (B) lateral x-ray; (C) delamination of the polyethylene prosthesis; (D) close-up view of the lateral tibial plateau prosthesis surface.

Failed polyethylene 8 years after total knee arthroplasty. (A) anterior posterior x-ray; (B) lateral x-ray; (C) delamination of the polyethylene prosthesis; (D) close-up view of the lateral tibial plateau prosthesis surface.

Failed polyethylene 8 years after total knee arthroplasty. (A) anterior posterior x-ray; (B) lateral x-ray; (C) delamination of the polyethylene prosthesis; (D) close-up view of the lateral tibial plateau prosthesis surface.

In principle, the contact stress could be reduced by increasing the conformity between the femoral and tibial components in both the lateromedial and the anteroposterior directions. However, the contact stress is relatively insensitive to changes in conformity in the anteroposterior direction, and changes in radii of curvature in the anteroposterior geometry can be rather limited if appropriate motion of the femur with respect to the tibia during flexion is to be maintained.

Contact pressure varies substantially with different loading configurations. Figures 2A-2D show the x-rays and failed polyethylene component after 8 years of service in a 69-year-old woman. Figures 3A-3C show the contact pressure in vitro of a prosthesis of the same design as that depicted in Figures 2A-2D. There are relatively low contact stresses when physiologic loads are applied in extension but when the prosthesis is loaded in 60 and 90 degrees of flexion, which occurs during stair-climbing or getting in or out of a chair, the contact stress exceeds, by a substantial margin, the yield strength of polyethylene. The prosthetic specimen in Figures 2C and 2D failed in its posterior portion due to the high contact stresses created as the knee was loaded in flexion. Therefore, in order to produce more durable prostheses, designs must include features to keep contact stresses low in various degrees of flexion as well as in full extension. The contact pressures from such a design are shown in Figures 4A-4C.[9]

Contact pressures of a prosthesis of similar design to the failure shown in Figure 2A-2D demonstrated in vitro with physiologic loading. (A) Contact pressure in extension is relatively low; (B) greater at 60 degrees and (C) greatest at 90 degrees of flexion.

Contact pressures of a prosthesis that was designed for lower contact pressure in all loading modes. (A) Contact pressure in extension; (B) and (C), contact pressures with 60 and 90 degrees of flexion, respectively.

The polyethylene-bearing surfaces of most contemporary knee replacements are fixed with respect to the tibia. Complete conformity would eliminate rotational laxity about the long axis of the tibia, which is necessary for soft tissue structures to carry part of the torsional loads applied to the knee. If the torsional load is taken entirely by the prosthesis, the fixation surfaces between the host bone and implant, may be subjected to high stresses, and loosening is more likely. Any design represents a trade-off between several objectives: minimizing contact stresses, maintaining proper knee kinematics, and minimizing stresses at the fixation surfaces. Flat surfaces are completely conforming in the medio-lateral direction and have little rotational constraint. However, such designs have a distinct disadvantage if a varus or valgus motion occurs between the femur and the tibia (Fig. 5). These motions occur regularly with normal activities. With designs of this type, high contact stresses occur because of the highly nonconforming contact between the edge of the femoral component and the tibial plateau. Figure 6A shows a failed polyethylene tibial component removed 6 years after implantation. This failure was due to high peak edge loading. In a similar prosthetic design, experimental studies of contact pressure show high peak edge loading with varus or valgus loads (Fig. 6B). This area of high contact pressure corresponds with the delaminated and fractured polyethylene in the failed clinical specimen. In order to produce more durable prostheses, designs must include features to keep contact stresses low in varus/valgus loading (Fig. 7).

Two designs of total knee replacement are shown. (A) In this design, each plateau consists of a single medial-lateral radius of curvature. (B) In this design, each plateau consists of three radii of curvature in the medial-lateral direction (a large radius forming a nearly flat surface, with smaller radii of curvature at both edges). When the joint contact load is evenly distributed between both plateaus, design B shows a larger contact area (less severe load distribution) than design A. When sufficient varus or valgus moment is applied so that all the load shifts to one plateau (right), the load is much more concentrated for design B. (Burstein, A.H. and Wright, T.M.: Fundamentals of Orthopaedic Biomechanics. Williams and Wilkins, 1994. By permission.)

Effect of valgus or varus load on a relatively flat tibial polyethylene component. (A) A clinical specimen removed after 6 years, showing the effects of high peak edge loading. (B) The contact pressures in a polyethylene tibial component measured in vitro. There are high peak stresses along the edge of the component that correspond with the failed portion of the clinical specimen.

Effect of valgus or varus load on a relatively flat tibial polyethylene component. (A) A clinical specimen removed after 6 years, showing the effects of high peak edge loading. (B) The contact pressures in a polyethylene tibial component measured in vitro. There are high peak stresses along the edge of the component that correspond with the failed portion of the clinical specimen.

Contact pressures of a prosthesis that was designed for low contact pressure in all loading modes. (A) Contact pressure in 5 degree varus or valgus load. Even though the contact stress is all on one plateau, the pressure remains low and evenly distributed.

An alternative design option to achieve greater conformity while maintaining rotational laxity is for the polyethylene to conform to the femoral component, but move with respect to a metal tibial tray, which is fixed to the bone. Some designs (LCSreg., Oxfordreg.) include small polyethylene components that slide in tracks machined into the metal tibial tray. These polyethylene components are intended to function as artificial menisci. For such moving-bearing designs to be completely conforming, the femoral component must have a single anteroposterior radius. Wear has been observed in retrieved acetabular components even in completely conforming designs. Use of a single radius to approximate the anteroposterior geometry of the femoral condyles changes the kinematic motion of the femur with respect to the tibia during flexion and extension. As a result, some compromise in the function of the collateral ligaments may occur. This disadvantage, as well as the increased complexity in surgical technique and prosthesis design, and the possibility of polyethylene-bearing dislodgment, must be weighed against the potential benefits of increased conformity with moving-bearing designs.

As the thickness of the polyethylene component increases, the contact stress decreases and becomes less sensitive to further increases in thickness.[8] At a thickness less than 8mm, and especially less than 6mm, the contact stress rises dramatically. Very thin polyethylene components are associated with high contact stresses in nonconforming designs. Consequently, a minimum polyethylene thickness of 6mm should be maintained for tibial components using classic total condylar style designs. The thickness of the polyethylene may be limited with metal-backed components. Addition of the metal backing necessitates a decrease in the thickness of the polyethylene if the overall thickness of the component is to be maintained. The measurement that is important is not the nominal thickness of the prosthetic component, but the thickness of the polyethylene from the deepest location of the tibial articular surface to the bottom of the polyethylene, excluding the metal base plate (Fig. 8). When measured in this manner, some designs have a polyethylene thickness as low as 4mm (Fig. 9).[10]

The important measurement for polyethylene thickness is at the deepest location of the tibial articular surface to the bottom of the polyethylene part of the prosthesis (C) rather than the nominal thickness of the prosthesis (D).

Knee Tibial Components Minimum Poly Thickness
AMK 4
Natural 4.1
AGC 4.2
MG II 4.9
Genesis 5.1
Advantim 5.1
PFC 5.3
OpteTrak 6.5
* Adapted from Chillag, KJ and Barth E. An analysis of polyethylene thickness in modular total knee components. Clin Orthop 273: 261-262, 1991
Figure 9. True polyethylene thickness in the thinnest component of various designs. (Adapted from Chillag, K.J. and Barth. E.: An analysis of polyethylene thickness in modular total knee components. Clin. Orthop. 273: 261-262, 1991.)

Attempts to reduce surface damage by increasing the strength of the polyethylene have not been successful because they have resulted in stiffer materials (higher elastic modulus). As the stiffness increases, the contact area decreases, and the stresses associated with surface damage also increase.[8] Unless the increase in strength is greater than the increase in contact stresses, such materials have no advantage. The elastic modulus of the polyethylene also increases with length of implantation. The resulting change in modulus varies according to the depth beneath the articulating surface. This increase in modulus results in higher stresses associated with surface damage. Because the greatest change in material properties occurs in the region with the greatest maximum shear stress, the increase in damage probably is due to both mechanical loading and chemical degradation.[8]

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