Tribological and material properties for cartilage of and throughout the bovine stifle: support for the altered joint kinematics hypothesis of osteoarthritis

Abstract

Objective: Prior studies suggest that ligament and meniscus tears cause osteoarthritis (OA) when changes in joint kinematics bring underused and underprepared regions of cartilage into contact. This study aims to test the hypothesis that material and tribological properties vary throughout the joint according to the local mechanical environment.

Method: The local tribological and material properties of bovine stifle cartilage (N = 10 joints with 20 samples per joint) were characterized under physiologically consistent contact stress and fluid pressure conditions.

Results: Overall, cartilage from the bovine stifle had an equilibrium contact modulus of Ec0 = 0.62 ± 0.10 MPa, a tensile modulus of Et = 4.3 ± 0.7 MPa, and a permeability of k = 2.8 ± 0.9 × 10(-3) mm(4)/Ns. During sliding, the cartilage had an effective friction coefficient of μeff = 0.024 ± 0.004, an effective contact modulus of Ec = 3.9 ± 0.7 MPa and a fluid load fraction of F' = 0.81 ± 0.03. Tibial cartilage exhibited significantly poorer material and tribological properties than femoral cartilage. Statistically significant differences were also detected across the femoral condyle and tibial plateau. The central femoral condyle exhibited the most favorable properties while the uncovered tibial plateau exhibited the least favorable properties.

Conclusions: Our findings support a previous hypothesis that altered loading patterns can cause OA by overloading underprepared regions. They also help explain why damage to the tibial plateau often precedes damage to the mating femoral condyle following joint injury in animal models. Because the variations are driven by fundamental biological processes, we anticipate similar variations in the human knee, which could explain the OA risk associated with ligament and meniscus tears.

Keywords: Articular cartilage; Bovine; Friction; Knee; Osteoarthritis.

Figure 1

Definitions of sample sites within the bovine stifle and the regions classified for purposes of comparison. Left: the frontal plane view of a bovine stifle joint. Center: the sagittal plane view for three flexion angles in which different samples experience cartilage-cartilage contact. Right: sampling locations on the femoral condyles and tibial plateau. The comparisons of interest are the femoral condyles vs tibial plateau, medial vs lateral, and outer femoral condyles (O) vs central femoral condyles (C) vs inner femoral condyles (I) vs shielded tibial plateau (S) vs uncovered tibial plateau (U).

Figure 2

Illustration of the custom microtribometer used to measure the material and functional properties of bovine articular cartilage.

Figure 3

Representative data to illustrate the characterization of material properties. Left: force verses displacement curves for nominal speeds of 50, 0.5, 5, 20, and 10μm/s, in that randomized order for a representative high functioning sample. Following the last indent the stage is held fixed until equilibrium is reached. The equilibrium contact modulus is obtained directly from that point and the dotted line represents the predicted Hertzian relationship between force and deformation. Right: the fluid load fraction is calculated for representative high and low functioning samples as a function of the prescribed indentation rate. The dark labels correspond to the force-displacement data on the left. The fits to the biphasic model from Moore and Burris are shown in red and were used to determine tissue permeability and tensile modulus.

Figure 4

Correlations between the functional performance and material properties for bovine articular cartilage. The functional properties of interest are the effective friction coefficient (Left), effective contact modulus (Center) and fluid load fraction (Right). The Top row contains the best overall correlation for each functional metric. The Bottom row contains the best overall correlation for each functional metric against E_c0, k, and E_c0·k.

Figure 5

Comparisons of the tribological and material properties for the femoral condyles and tibial plateaus of the bovine stifle joint. Error bars represent 95% confidence intervals. Significant differences are indicated by dissimilar letters, P<0.05.

Figure 6

Comparisons of the tribological and material properties for the femoral central (C), outer (O), inner (I), and tibial shielded (S) and uncovered (U) regions of the bovine stifle joint. Error bars represent 95% confidence intervals. Significant differences are indicated by dissimilar letters, P<0.05.