Changes in in vitro compressive contact stress in the rat tibiofemoral joint with varus loading

Abstract

Increased compression of the tibiofemoral joint, due to increased body mass or malalignment, is a risk factor for the onset and progression of osteoarthritis. This work investigates compressive stresses and contact areas in the articular cartilage of the rat tibiofemoral joint during standing with different applied varus loads. Cadaver rat knees underwent loading of the extensors combined with varus loading (0%, 50% or 100% of bodyweight) of the tibiofemoral joint. Articular cartilage contact stress was evaluated using stereophotogrammetric measurements of biplanar radiographs, high-resolution micro-computed tomography and discrete element analysis. Random coefficients regression models were used to analyze the relationship between peak and spatially averaged contact stresses and contact areas as a function of increasing varus loadings. The contact stresses increased linearly in the medial compartment. Peak stress significantly increased 0.042 MPa (p=0.006) and spatially averaged stress significantly increased 0.029 MPa (p=0.045) for each 10% increase in varus loading. There was a trend for a small decrease in contact areas in the lateral compartment with varus loading. This is the first report of the contact stresses in a rat tibiofemoral joints under simulated weight bearing conditions. The 0.42 MPa increase in peak contact stress at the cartilage-cartilage interface of the medial compartment with 100% bodyweight varus load is similar to the reported change in peak contact stress associated with development of symptomatic knee osteoarthritis in humans. Determination of contact stresses in rat tibiofemoral joints allows comparison to contact stresses in humans with the development of osteoarthritis.

Figure 1

(A) Rat tibiofemoral joint with the varus loading device (VLD) attached to the lateral side of the left femur and tibia with transcutaneous bone plates. The torque from the VLD torsional spring applies a lateral force (F) to the distal tibia multiplied by the tibia moment arm (L) creates a varus moment (M) about the tibiofemoral joint. Tantalum beads (●) implanted in cortical bone for measuring the joint position are shown on the medial side. At least four additional beads were placed in similar locations on the lateral side. (B) Anterior-posterior view of the left rat tibiofemoral joint. The varus moment (M) changes the normal compressive joint loadings (gray arrows) by increasing the compression (+ΔP) in the medial compartment and decreasing the compression (−ΔP) in the lateral compartment leading to altered compressive loads (black arrows). The target change in compression (ΔP) is equal to the varus moment (M) divided by the intercompartmental moment arm (D) (Roemhildt et al., 2010a, Roemhildt et al., 2012a).

Figure 2

Articular cartilage contact stresses of the cadaver rat tibiofemoral joint were estimated using five steps: A) implantation of tantalum beads into the femur and tibia, collection of biplanar stereoradiographs and Roentgen stereophotogrammetric analysis (RSA) of the femur and tibia positions with and without applied varus loadings, B) collection of high-resolution micro-computed tomography (μCT) images of the tibiofemoral subchondral bone and bead surface geometries, C) registration of the μCT surface geometries based on the common measured bead positions in the RSA and μCT images, D) calculation of the surface normal vectors and separation distances between the tibia and femur, and E) calculation of the compressive contact stresses based on cartilage properties.

Figure 3

Typical distributions of tibial compressive contact stresses and the associated peak contact stresses in the medial and lateral compartments of the tibiofemoral joint without and with varus loadings. The varus loadings increase the compression in the medial compartment by 0%, 50%, or 100% body weight (BW) and reduce load in the lateral compartment by an equivalent amount.

Figure 4

Significant linear increases in peak (stress_peak, p=0.006) and spatially averaged (stress_averaged, p=0.045) contact stresses in the medial compartment (left) of the rat tibiofemoral joint as a function of varus loading in percent bodyweight (%BW). Different symbols represent different animals. The thick straight lines are the estimated linear relationship as expressed by the parameters in Table 1. The dash lines are 95% confidence bands on the mean contact stresses. There were trends of decreasing peak and spatially averaged contact stresses with varus loading in the lateral compartment (right).