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the results also showed that the energy dissipated by the cartilage-bone system was frequency-dependent and a significant difference was identified between each frequency. the frequency-dependence of energy dissipation is indicative of a viscoelastic property. as expected, the energy dissipated by cartilage-bone was higher than the energy dissipated by bone alone. this suggests that the osteochondral junction dissipates energy from the system. the implications of this are that the osteochondral junction should not be considered as a static interface. previous work, using sheep femoral heads, showed that the energy dissipated from cartilage-bone was frequency-dependent and that the dissipated energy exceeded the elastic energy stored in the cartilage and bone [ 22 ]. to the best of the authors knowledge, this is the first study to report the frequency-dependence of energy dissipation from human osteochondral samples and it supports the argument that the osteochondral junction should not be considered as a static interface.
in order to investigate if the observed dissipated energy was dissipated as heat within the specimens, a time-resolved temperature measurement was performed. this showed that cartilage-bone absorbed less energy than bone alone, suggesting that the energy transferred was dissipated as heat in the system (fig. 6 ). the dissipated energy for the cartilage-bone system increased from 8hz to 90hz, indicating that this mechanism is frequency-dependent. this supports the findings of ohyama and colleagues [ 14, 17 ] who also reported a frequency-dependence of dissipated energy in cartilage-bone. the results also demonstrated that the increase in dissipated energy across the range of frequencies was not uniform and while the total dissipated energy increased, the dissipated energy per cycle decreased. this implies that the dissipated energy is not dissipated at all frequencies but is accumulated over the course of the frequency-sweep. this is also supported by ohyama and colleagues [ 14, 17 ] who found that the dissipated energy increased over the course of the frequency sweep but decreased at a faster rate than the increase. the dissipated energy for bone alone is lower than cartilage-bone at all frequencies and this is attributed to the difference in stiffness. the dissipated energy for cartilage-bone is higher than bone at higher frequencies, which is attributed to the difference in dissipated energy per cycle. in conclusion, this study shows the potential of osteochondral cores as a whole to emulate the energy dissipation properties of articular cartilage. future work will consider the loss and storage moduli as a function of frequency, temperature and humidity to determine if there is a link between these and the dissipation mechanism in cartilage-bone. furthermore, further work will consider the viscoelastic properties of the osteochondral core at different locations on the articular surface of the femoral head to determine if the dissipation is dependent on the location of the load application.
the mechanical properties of the three tissues were determined (fig. 3). the storage modulus was determined for each of the three tissues (fig. 3 a). it is observed that the storage modulus of cartilage was significantly higher than that of bone and core. similarly, the loss modulus was determined for each of the three tissues (fig. 3 b). it is observed that the loss modulus of cartilage was significantly higher than that of bone and core. the trends observed in these data are comparable to those found in the literature [ 3, 19 ], however, the large variation in these properties and the complex hierarchical organisation of the tissues is evident. the trends identified here are important, as they demonstrate that cartilage and bone have a different viscoelastic response. given the presence of bone on the surface of the cartilage, the trend in the data suggests that bone is a better dissipator of energy than cartilage [ 3, 4, 19 ], which is supported by the biological function of cartilage as a lubricant to dampen the movement of the bones [ 22 ].
this study has demonstrated that the young’s modulus of bone decreases with increasing frequency, and that cartilage is viscoelastic at all frequencies, whereas bone is a viscoelastic at low frequencies and a purely elastic at higher frequencies. young’s modulus is a constant that represents the degree of stiffness of a material, and can be calculated as the ratio of applied stress over strain. previous work on bone has shown that young’s modulus decreases with increasing frequency [ 9 ], which is a result of the influence of fluidity in bone in the higher frequencies. this fluidity is attributed to the fact that bone consists of a heterogeneous material, including a honeycomb-like structure with pores containing fluid, called the haversian system [ 15 ]. bone consists of a combination of organic and inorganic constituents, with the inorganic phase consisting of hydroxyapatite crystals [ 16 ]. from histological studies, a decrease in pore size in bone in proportion to an increase in the concentration of inorganic content has been observed [ 17 ]. the results of the present study show that cartilage has a higher young’s modulus than bone at low frequencies, which is consistent with previous findings that show that the cartilage-bone interface in the human joint has a greater stiffness than isolated bone [ 14 ].