Cartilage-on-cartilage cyclic loading induces mechanical and structural damage

Abstract

Cartilage breaks down during mechanically-mediated osteoarthritis (OA). While previous research has begun to elucidate mechanical, structural and cellular damage in response to cyclic loading, gaps remain in our understanding of the link between cyclic cartilage loading and OA-like mechanical damage. Thus, the aim of this study was to quantify irreversible cartilage damage in response to cyclic loading. A novel in vitro model of damage through cartilage-on-cartilage cyclic loading was established. Cartilage was loaded at 1 Hz to two different doses (10,000 or 50,000 cycles) between -6.0 ± 0.2 MPa and -10.3 ± 0.2 MPa 1st Piola-Kirchhoff stress. After loading, mechanical damage (altered mechanical properties: elastic moduli and dissipated energy) and structural damage (surface damage and specimen thickness) were quantified. Linear and tangential moduli were determined by fitting the loading portion of the stress-strain curves. Dissipated energy was calculated from the area between loading and unloading stress-strain curves. Specimen thickness was measured both before and after loading. Surface damage was assessed by staining samples with India ink, then imaging the articular surface. Cyclic loading resulted in dose-dependent decreases in linear and tangential moduli, energy dissipation, thickness, and intact area. Collectively, these results show that cartilage damage can be initiated by mechanical loading alone in vitro, suggesting that cyclic loading can cause in vivo damage. This study demonstrated that with increased number of cycles, cartilage undergoes both tissue softening and structural damage. These findings are a first step towards characterizing the cartilage response to cyclic loading, which can ultimately provide important insight for delaying the initiation and slowing the progression of OA.

Keywords: Articular cartilage; Cartilage-on-Cartilage; Cyclic loading; Damage; Fatigue.

Figure 1.

Experimental setup for cartilage-on-cartilage repetitive cyclic loading and testing of material properties. (A) schematic showing backplate with bottom and control samples attached, and loading platen with top sample attached, in a DPBS+PI bath. (B) experimental image showing bone and cartilage layers of the top and bottom samples in mechanical loading setup.

Figure 2.

Material properties were determined by fitting linear (Eq. 1) and exponential (Eq. 2) expressions to the cartilage stress-strain loading curve and calculating the area between the loading and unloading curves.

Figure 3.

Representative images showing pre and post test surface damage. Raw images were converted to binary using threshold values based on the pre test images. Intact surface area was calculated from the binary images.

Figure 4.

Mechanical properties pre and post-loading. (A) average and standard deviations of loading stress-strain curves qualitatively show reduced mechanical stiffness with increasing cycles; (B) linear modulus decreased with cyclic loading; (C) 10% strain modulus decreased after 50,000 cycles; (D) dissipated energy decreased after 50,000 cycles. * indicates significance, ► indicates trend.

Figure 5.

Structural integrity pre and post test. (A) cartilage thickness decreased after 50,000 cycles; (B) intact area trended towards decreasing after 50,000 cycles. * indicates significance, ► indicates trend.