Heterogeneous three-dimensional strain fields during unconfined cyclic compression in bovine articular cartilage explants

Abstract

Articular cartilage provides critical load-bearing and tribological properties to the normal function of diarthrodial joints. The unique properties of cartilage, as well as heterogeneous deformations during mechanical compression, are due to the nonuniform microstructural organization of tissue components such as collagens and proteoglycans. A new cartilage deformation by tag registration (CDTR) technique has been developed by the authors to determine heterogeneous deformations in articular cartilage explants. The technique uses a combination of specialized MRI methods, a custom cyclic loading apparatus, and image processing software. The objective of this study was to use the CDTR technique to document strain patterns throughout the volume of normal bovine articular cartilage explants during cyclic unconfined compression at two physiologically-relevant applied normal stress levels (1.29 and 2.57 MPa). Despite simple uniaxial cyclic compressive loading with a flat, nonporous indenter, strain patterns were heterogeneous. Strains in the thickness direction (Eyy) were compressive, varied nonlinearly with depth from the articular surface from a maximum magnitude of 11% at the articular surface, and were comparable despite a 2-fold increase in applied normal stress. Strains perpendicular to the thickness direction (Exx and Ezz) were tensile, decreased linearly with depth from the articular surface from a maximum of 7%, and increased in magnitude 2.5-fold with a 2-fold increase in applied normal stress. Shear strains in the transverse plane (Ex:) were approximately zero while shear strains in the other two planes were much larger and increased in magnitude with depth from the articular surface, reaching maximum magnitudes of 2% at the articular cartilage-subchondral bone interface. In general, strain patterns indicated that cartilage osteochondral explants exhibited depth-dependent nonisotropic behavior during uniaxial cyclic loading. These results are useful in verifying constitutive formulations of articular cartilage during cyclic unconfined compression and in characterizing the micromechanical environment likely experienced by individual chondrocytes throughout the tissue volume.

In situ deformation of cartilage in cyclically loaded tibiofemoral joints by displacement-encoded MRI

“…Explant studies have shown that transverse strains are tensile and maximal at the articular surface [18, 26, 27]. In contrast, the transverse strains in this study, which were normalized to the depth of the cartilage, showed maxima at the tibial subchondral interface (Figure 5) and within the thickness of the femoral cartilage, although transverse strains at all levels of depth were similar.…”

“…In newborn bovine cartilage explants compressed between porous platens, axial strains were highest at the superficial zone and decreased nonlinearly through a 1-mm layer of cartilage [9], although the full cartilage depth was not investigated. Other studies have demonstrated maximal strains in the loading direction at the articular surface of juvenile [26, 27] and adult bovine articular cartilage [26]. Another study showed axial compressive strains to be maximal in the middle of the full-thickness young bovine cartilage [18].…”

“…One study found that shear was maximal at the subchondral interface of young bovine cartilage [27], while another found that the maximum shear occurred at the articular surface of immature cartilage [26]. Yet another study showed that shear strains were minimal through the thickness of the cartilage when compared to the magnitude of strains in the loading and transverse directions [18].…”

Characterization of engineered tissue construct mechanical function by magnetic resonance imaging

“…carbon fiber piston on glass) pneumatic cylinder. This configuration allowed for load-controlled compression cycles to be applied to constructs with millisecond timing precision (Neu et al 2005). Biochemical studies (e.g.…”

“…Tissue deformation measures through the 3D volume of cartilage are based on tag registration (Neu et al 2005) and phase contrast (Neu and Walton 2008) methods. These techniques provide a local measure of deformation to a high (e.g.…”