Rheometer enabled study of cartilage frequency-dependent properties

Abstract

Despite the well-established dependence of cartilage mechanical properties on the frequency of the applied load, most research in the field is carried out in either load-free or constant load conditions because of the complexity of the equipment required for the determination of time-dependent properties. These simpler analyses provide a limited representation of cartilage properties thus greatly reducing the impact of the information gathered hindering the understanding of the mechanisms involved in this tissue replacement, development and pathology. More complex techniques could represent better investigative methods, but their uptake in cartilage research is limited by the highly specialised training required and cost of the equipment. There is, therefore, a clear need for alternative experimental approaches to cartilage testing to be deployed in research and clinical settings using more user-friendly and financial accessible devices. Frequency dependent material properties can be determined through rheometry that is an easy to use requiring a relatively inexpensive device; we present how a commercial rheometer can be adapted to determine the viscoelastic properties of articular cartilage. Frequency-sweep tests were run at various applied normal loads on immature, mature and trypsinased (as model of osteoarthritis) cartilage samples to determine the dynamic shear moduli (G*, G' G″) of the tissues. Moduli increased with increasing frequency and applied load; mature cartilage had generally the highest moduli and GAG depleted samples the lowest. Hydraulic permeability (K_H) was estimated from the rheological data and decreased with applied load; GAG depleted cartilage exhibited higher hydraulic permeability than either immature or mature tissues. The rheometer-based methodology developed was validated by the close comparison of the rheometer-obtained cartilage characteristics (G*, G', G″, K_H) with results obtained with more complex testing techniques available in literature. Rheometry is relatively simpler and does not require highly capital intensive machinery and staff training is more accessible; thus the use of a rheometer would represent a cost-effective approach for the determination of frequency-dependent properties of cartilage for more comprehensive and impactful results for both healthcare professional and R&D.

Figure 1

Biopsy process, cleaning legs carefully in order to remove all dirt (a); skin removal with a blade gently to not damage the cartilage underneath (b); joint opening with a scalpel (c, d); full depth explants excised using biopsy punches (e, f).

Figure 2

Scheme of (a) Peltier plates geometry used for the tests, (b) stress response to oscillatory strain deformation for a viscoelastic material and (c) relation between complex, elastic and loss moduli.

Figure 3

Storage modulus (G') and loss modulus (G″) of cartilage as function of applied strain at 1 Hz.

Figure 4

Dynamic shear modulus (right) and phase angle (left) of normal and GAG depleted cartilage samples at different frequencies under applied of load of 5 N (a, b), 25 N (c, d) and 50 N (e, f).

Figure 5

Hydraulic permeability (K_H) of normal and GAG depleted cartilage at different applied loads.

Figure 6

(a) Schematic representation of the structure/composition of different tissues used. (b) correlation among GAG, collagen and mechanical properties (G^*_1 MHz, G^*_10 Hz and hydraulic permeability (K_H)) at 5 N applied load. Size of the symbols is proportional to the value of the parameter represented.