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. 2011 Aug 17;101(4):916-24.
doi: 10.1016/j.bpj.2011.07.006.

Contribution of proteoglycan osmotic swelling pressure to the compressive properties of articular cartilage

EunHee Han  1 , Silvia S Chen, Stephen M Klisch, Robert L Sah
Affiliations

Contribution of proteoglycan osmotic swelling pressure to the compressive properties of articular cartilage

EunHee Han et al. Biophys J. .

Abstract

The negatively charged proteoglycans (PG) provide compressive resistance to articular cartilage by means of their fixed charge density (FCD) and high osmotic pressure (π(PG)), and the collagen network (CN) provides the restraining forces to counterbalance π(PG). Our objectives in this work were to: 1), account for collagen intrafibrillar water when transforming biochemical measurements into a FCD-π(PG) relationship; 2), compute π(PG) and CN contributions to the compressive behavior of full-thickness cartilage during bovine growth (fetal, calf, and adult) and human adult aging (young and old); and 3), predict the effect of depth from the articular surface on π(PG) in human aging. Extrafibrillar FCD (FCD(EF)) and π(PG) increased with bovine growth due to an increase in CN concentration, whereas PG concentration was steady. This maturation-related increase was amplified by compression. With normal human aging, FCD(EF) and π(PG) decreased. The π(PG)-values were close to equilibrium stress (σ(EQ)) in all bovine and young human cartilage, but were only approximately half of σ(EQ) in old human cartilage. Depth-related variations in the strain, FCD(EF), π(PG), and CN stress profiles in human cartilage suggested a functional deterioration of the superficial layer with aging. These results suggest the utility of the FCD-π(PG) relationship for elucidating the contribution of matrix macromolecules to the biomechanical properties of cartilage.

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Figures

Figure 1
Figure 1
Multiscale schematic of the effects of FCD and πPG on the CN. (A and B) At the macroscopic tissue scale, compression (A->B) leads to increased FCD and πPG. (C–F) At the microscale, both compression (C->E and D->F) and increasing depth (superficial C, E to deep D, and F) lead to higher FCD and πPG, and a fluid shift from IF space into EF space.
Figure 2
Figure 2
(A and B) Four-segment, piecewise curve-fitting to data from Williams and Comper (30) and Basser et al. (11). An inset of A at lower FCD values is shown in B. (C) The nomograms of CS and KS contents (downward tic marks) for the corresponding FCD (upward tic marks) provide the conversion between CS or KS contents to FCD. The FCD contributions from CS and KS can also be summed and used to estimate the πPG from the curves in A and B.
Figure 3
Figure 3
FCD (A–C) and πPG (D–F) for bovine fetal (A and D), calf (B and E), and adult (C and F) femoral condyle cartilage calculated using the total water or EF water content.
Figure 4
Figure 4
FCDEF for bovine (A) and human (B) femoral condyle cartilage (∗p < 0.05 versus fetal; p < 0.01 versus young).
Figure 5
Figure 5
Values of πPG, σEQ, and σCN for bovine fetal (A), calf (B), and adult (C) femoral condyle cartilage (#p < 0.05 versus fetal).
Figure 6
Figure 6
Values of πPG, σEQ, and σCN for human young (A) and old (B) femoral condyle cartilage (# for πPG and ∧ for σCN, p < 0.01 versus young).
Figure 7
Figure 7
CN prestress (A and B) and compression level at σCN = 0 (C and D) for bovine (A and C) and human (B and D) cartilage (∗p < 0.01 versus young).
Figure 8
Figure 8
Strain (A–E), πPG (F–K), and σCN (L–Q) for human young (A, F, and L) and old (B, G, and M) cartilage with initial normalized depth of the tissue at each compression level (0%, 10%, 20%, and 30%; ∗p < 0.05 versus young).
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