Dynamic Bone V1.1.7 .rar
Penny Bozic
Despite it being known that subchondral bone affects the viscoelasticity of cartilage, there has been little research into the mechanical properties of osteochondral tissue as a whole system. This study aims to unearth new knowledge concerning the dynamic behaviour of human subchondral bone and how energy is transferred through the cartilage-bone interface.
The cartilage-bone interface in articulating joints is key to moderating the transmission of tensile, compressive, and shear forces from the articular cartilage to the subchondral bone [1]. The complex organisation of collagen fibres within cartilage, in part, enables it to store and dissipate energy [2], and articular cartilage is considered to be a frequency-dependent viscoelastic structure [3,4,5,6]. Studies that have analysed this interface have primarily focused on its structure and composition, characterising the calcified cartilage and underlying tidemark where collagen type I and II integrate [7,8,9,10]. More recently, biological signalling between articular cartilage and subchondral bone have been identified through vascular microchannels that traverse the subchondral bone and calcified cartilage, allowing diffusion of small molecules [11].
The aim of this study is to characterise the viscoelastic properties of human osteochondral tissues and assess the dissipation of energy by these tissues. More specifically, an approach that characterises viscoelastic behaviour of the osteochondral core and isolated tissues in a physiological frequency range has advantages in being able to assess the significance of the interactions between the two tissues. Therefore, energy dissipation has been analysed for osteochondral tissues. By using dynamic mechanical analysis (DMA), the viscoelastic properties of the human cartilage-bone unit were directly compared to the subchondral bone and articular cartilage. Furthermore, the bone mineral density (BMD) of the subchondral bone was determined, by micro-computed tomography (μ-CT), to identify any relationships with its mechanical properties, or the viscoelastic properties of cartilage.
Flow diagram illustrating femoral head specimen preparation and coring: a Preparation of specimen using a surgical saw, b Example of cartilage-bone block prior to μ-CT analysis demonstrating where core was taken, c Coring of specimen, and d Example of cartilage-bone core prior to dynamic mechanical analysis
Energy dissipation was calculated using Matlab R2018a (Matlab R2018a, MathWorks, Inc., Natick, Massachusetts, USA). Time, force, and displacement data was collected during DMA. Plotting displacement vs force at a given frequency for each specimen produced a hysteresis loop (Fig. 2a). For each complete loop, the area between the arcs was assumed to be the total energy dissipated for that DMA cycle. This was calculated by finding a polynomial approximation of each arc, which was then solved for the range zero to the maximal displacement of that cycle. The area below each polynomial arc was approximated by the trapezoid rule [31] (Fig. 2b); the difference between the two arcs equated to the energy dissipated for that cycle. Finally, the values across all cycles were averaged, resulting in the total energy dissipation per cycle for each specimen at a given frequency.
No significant relationships existed between BMD and the storage or loss moduli of the cartilage, or the storage and loss moduli of the bone. Furthermore, there were no significant relationships found between total energy dissipated and the thickness of the isolated cartilage, isolated bone, or the osteochondral specimens respectively.
As well as investigating isolated tissues, this research aimed to better understand the osteochondral core as a whole system. Storage stiffness for the cartilage-bone system was logarithmically frequency dependent and lower than cartilage and bone for all frequencies tested (Fig. 4). Loss stiffness for the cartilage-bone system was independent of frequency and lower than isolated specimens across the range of frequencies tested. These results are in line with previous work, which looked solely at bovine cartilage-bone cores and found loss stiffness to be frequency-independent [13, 18]. The difference in behaviour of cartilage isolated from and attached to subchondral bone is emphasised here and has demonstrated that cartilage should not be considered in isolation when determining properties representative of in vivo behaviour. The data obtained in our study supports the development and testing of whole tissue-replacement systems as opposed to cartilage replacement materials in isolation.
Prior studies of bovine cartilage both on- and off-bone found the loss modulus of on-bone cartilage to be frequency-independent, whereas cartilage off-bone has a frequency-dependent modulus [13, 18]. Lawless et al. [13] found that there was no dependency of the storage stiffness on the presence or absence of the underlying subchondral bone, and therefore proposed that on-bone cartilage may be more predisposed to failure than off-bone cartilage due to the storage/loss ratio being higher for cartilage on-bone. The findings of the present study report the same frequency-independence for cartilage on-bone loss modulus, with isolated cartilage displaying a frequency-dependent trend. Thus, findings reported in prior studies support the current results, although it should be noted that the aim of the present study was focused on the viscoelasticity of subchondral bone and the role of the cartilage-bone interface, rather than the cartilage itself. Hence, a more detailed discussion on the viscoelastic properties of cartilage both on- and off-bone is provided elsewhere [13].
A significant difference was identified between loss stiffness values for the cartilage-bone system and cartilage specimens. In contrast, when comparing the cartilage-bone system and bone specimens this was not observed, signifying that the loss stiffness of the osteochondral core is more closely aligned to the loss stiffness of bone than of isolated cartilage. This suggests a key property to consider is the energy transfer, including dissipation of energy, through the osteochondral junction. In order to characterise this in the present study, the total energy dissipated from the cartilage at each frequency during DMA was calculated. The benefit of this approach is the entire system is considered as a whole by determining absolute values rather than material properties. The results demonstrated that cartilage dissipated higher total energy than bone and the cartilage-bone system across all frequencies tested (Fig. 5). While expected, as one of the key roles of articular cartilage is to dissipate energy into the underlying bone, to the best of the authors knowledge this is the first report of such a relationship from human tissue analysis.
The complexity of the frequency-dependent viscoelastic properties of cartilage and bone described in this study demonstrate that there is a sophisticated interaction between these two tissues in regard to storage and dissipation of energy. High-frequency loading results in an increased storage of energy in the subchondral bone, a likely mechanism to prevent cartilage damage, which is a factor that should be further investigated in relation to the progression of OA. The results obtained in this study provide details of native tissue behaviour in vitro at physiologically relevant frequency ranges, the first reported values in human osteochondral tissue under dynamic loading.
The authors thank the members of the Healthcare Technologies Institute and Department of Mechanical Engineering at the University of Birmingham for their technical support. We also thank the Institute of Inflammation and Ageing at the University of Birmingham for providing the specimens.
EPSRC provided funding through a studentship at the Centre for Doctoral Training in Physical Sciences for Health (EP/ L016346/1). The materials testing equipment used in this study was funded by Arthritis Research UK, now a part of Versus Arthritis (Grant number: H0671). The funding bodies had no involvement in the design of the study; collection, analysis and interpretation of data; in the writing of the manuscript; and in the decision to submit the manuscript for publication.
Ethical approval was provided by the United Kingdom National Research Ethics Service (East of Scotland Research Ethics Service, 11/ES/1044) and informed written consent for the use of their tissue for research was given by all patients.
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