Surface damage of bovine articular cartilage-off-bone: the effect of variations in underlying substrate and frequency

Abstract

Background: Changes in bone mineral density have been implicated with the onset of osteoarthritis, but its role in inducing failure of articular cartilage mechanically is unclear. This study aimed to determine the effect of substrate density, as the underlying bone, on the surface damage of cartilage-off-bone, at frequencies associated with gait, and above.

Methods: Bovine articular cartilage samples were tested off-bone to assess induced damage with an indenter under a compressive sinusoidal load range of 5-50 N at frequencies of 1, 10 and 50 Hz, corresponding to normal and above normal gait respectively, for up to 10,000 cycles. Cartilage samples were tested on four underlying substrates with densities of 0.1556, 0.3222, 0.5667 and 0.6000 g/cm³. India ink was applied to identify damage as cracks, measured across their length using ImageJ software. Linear regression was performed to identify if statistical significance existed between substrate density, and surface damage of articular cartilage-off-bone, at all three frequencies investigated (p < 0.05).

Results: Surface damage significantly increased (p < 0.05) with substrate density at 10 Hz of applied frequency. Crack length at this frequency reached the maximum of 10.95 ± 9.12 mm (mean ± standard deviation), across all four substrates tested. Frequencies applied at 1 and 50 Hz failed to show a significant increase (p > 0.05) in surface damage with an increase in substrate density, at which the maximum mean crack length were 3.01 ± 3.41 mm and 5.65 ± 6.54 mm, respectively. Crack formation at all frequencies tended to form at the periphery of the cartilage specimen, with multiple straight-line cracking observed at 10 Hz, in comparison to single straight-line configurations produced at 1 and 50 Hz.

Conclusions: The effect of substrate density on the surface damage of articular cartilage-off-bone is multi-factorial, with an above-normal gait frequency. At 1 Hz cartilage damage is not associated with substrate density, however at 10 Hz, it is. This study has implications on the effects of the factors that contribute to the onset of osteoarthritis.

Keywords: Articular cartilage; Bone mineral density; Damage; Frequency; Mechanical loading; Osteoarthritis.

Fig. 1

Representative image captured of bovine articular cartilage specimen, prior to testing on confirmation of the absence of damage with India ink. Scale bar is included (mm)

Fig. 2

Example set-up at actuator (a) for mechanical testing with stainless-steel indenter (b) at off-bone cartilage specimen (c) and lowest density underlying substrate (d), within customised test rig (e). Cartilage off-bone specimen is positioned above the substrate. Stainless-steel indenter is lowered onto the cartilage-off-bone specimen for testing, movement operated with testing machine. Load cell component (f) for experimental load control

Fig. 3

Representative images of bovine articular cartilage-off-bone samples after testing, at 1 Hz frequency of loading. Image a-d display a sample result at substrates 1–4, respectively. Damage as cracks and indentation were identified with application of India ink. Cracks formed are highlighted with the black ellipse for clear observation. Indentation can be observed across most of the cartilage-off-bone specimen surface, at this frequency of loading. Scale bar (mm) included for quantifying results

Fig. 4

Representative images of bovine articular cartilage-off-bone samples after testing, at 10 Hz frequency of loading. Image a-d display a sample result at substrates 1–4, respectively. Damage as cracks and indentation were identified with application of India ink. Cracks formed are highlighted with the black ellipse for clear observation, notably of multiple parallel straight-lines. Scale bar (mm) included for quantifying results

Fig. 5

Representative images of bovine articular cartilage-off-bone samples after testing, at 50 Hz frequency of loading. Image a-d display a sample result at substrates 1–4, respectively. Damage as cracks and indentation were identified with application of India ink. Cracks formed are highlighted with the black ellipse for clear observation, notably of single-line configurations of varying lengths. Scale bar (mm) included for quantifying results

Fig. 6

Mean crack length plotted with substrate density at 1, 10 and 50 Hz for off-bone articular cartilage, represented by the square, circle and diamond, respectively. Linear regression displayed by eq. (1) fit the data at R² values of 0.485, 0.909 and 0.524 at 1, 10 and 50 Hz, respectively. Error bars represent standard deviations

Fig. 7

Mean area of indentation plotted with substrate density at 1, 10 and 50 Hz for off-bone articular cartilage, represented by the square, circle and diamond, respectively. Linear regression displayed by eq. (1) fit the data at R² values of 0.487, 0.0386 and 0.851 at 1, 10 and 50 Hz, respectively. Error bars represent standard deviations