Further studies on the anisotropic distribution of collagen in articular cartilage by μMRI

Abstract

To further study the anisotropic distribution of the collagen matrix in articular cartilage, microscopic magnetic resonance imaging experiments were carried out on articular cartilages from the central load-bearing area of three canine humeral heads at 13 mm resolution across the depth of tissue. Quantitative T 2 images were acquired when the tissue blocks were rotated, relative to B 0 , along two orthogonal directions, both perpendicular to the normal axis of the articular surface. The T 2 relaxation rate (R 2 ) was modeled, by three fibril structural configurations (solid cone, funnel, and fan), to represent the anisotropy of the collagen fibrils in cartilage from the articular surface to the cartilage/bone interface. A set of complex and depth-dependent characteristics of collagen distribution was found in articular cartilage. In particular, there were two anisotropic components in the superficial zone and an asymmetrical component in the radial zone of cartilage. A complex model of the three-dimensional fibril architecture in articular cartilage is proposed, which has a leaf-like or layer-like structure in the radial zone, arises in a radial manner from the subchondral bone, spreads and arches passing the isotropic transitional zone, and exhibits two distinct anisotropic components (vertical and transverse) Structural architecture of articular cartilage plays a critical role in the biomechanical functions and morphological properties of the tissue as a load-bearing material in joints (1-6), whose degradation is the hallmark of clinical joint diseases such as osteoarthritis. As the collagen fibril is the principal macromolecule that provides a depth-dependent structural integrity to articular cartilage (7), continuing efforts have been focused on the specific features of the three-dimensional (3D) collagen structure in cartilage. Histologically, the collagen matrix in noncalcified cartilage is commonly considered to contain three structural zones from the articular surface to the cartilage bone interface, namely, the superficial zone (SZ) with the collagen fibrils parallel with the tissue surface, the transitional zone (TZ) with mostly random fibrils, and the radial zone (RZ) with the perpendicular fibrils anchored to the underlining bone. These depth-dependent features of the collagen matrix become the fundamental components in several fibril models in literature, including the arcade model (8), where the collagen fibrils arise in a radial manner from the subchondral bone, pass toward the surface through the TZ obliquely, and return to the bone; the columnar arrangement (9,10), where the collagens are arranged in a columnar manner, that could be traced from the calcified cartilage to their oblique orientation in the tangential tissue matching the concept of the arcade model; and the leaf model (3,5,10,11), where the collagens are arranged in a series of closely packed layers or leaves in the RZ and arches in the TZ to form the horizontally orientated leaves in the SZ.The structural orientation of collagen ...

Discrimination of healthy and osteoarthritic articular cartilage by Fourier transform infrared imaging and Fisher’s discriminant analysis

“…The primary molecular components of the extracellular matrix in AC are type II collagen and proteoglycan (PG) [3]. Collagen forms the essential structural framework of fibro network that enmeshes PG molecules [4]. PG keeps the resiliency and compressive strength of AC [5].…”

Translational and Rotational Dynamics of Water Molecules in Aligned Collagenous Tissues: Implications for Assessing Collagen Organisation with Magnetic Resonance Imaging

“…This is commonly known as transverse spin anisotropy, or the Magic Angle (MA) Effect, and has been observed in ligament and tendon [33,43,177], articular cartilage [32,176] and the anulus fibrosus of the intervertebral disc [34,37]. The orientation dependence means that T 2 has the potential for interrogating collagen fibre alignment in these tissues [34,[43][44][45]. …”

“…The anisotropy of T 2 , or the Magic Angle Effect, is generally thought to be the result of intramolecular residual dipolar couplings, a result of anisotropic water rotation at water bridge sites. This orientation dependence has been interpreted in terms of fibre alignment in the anulus fibrosus and ligament [34,43] and in terms of a fibre orientation distribution in articular cartilage [44,45].However, due to the lack of understanding of the molecular mechanism behind the effect, the full potential of T 2 to interrogate collagen fibre orientation may not be fully realised. While some studies confirm the ice-like water bridge model 2005.…”

“…The orientation dependence of T 2 is commonly known as the Magic Angle Effect and has been observed in ligament and tendon [33,41], articular cartilage [32,42] and the anulus fibrosus of the intervertebral disc [34,37]. The phenomenon has been exploited to assess collagen fibre alignment in ligament [43] and fibre orientation distributions in articular cartilage [44,45] using an empirically derived formula. This process is not straight forward; it usually involves measuring the T 2 of the sample at various orientations [43][44][45] relative to the static magnetic field and comparing the measured T 2 values to those expected from theoretical fibre distributions [34,44,45].…”

“…The phenomenon has been exploited to assess collagen fibre alignment in ligament [43] and fibre orientation distributions in articular cartilage [44,45] using an empirically derived formula. This process is not straight forward; it usually involves measuring the T 2 of the sample at various orientations [43][44][45] relative to the static magnetic field and comparing the measured T 2 values to those expected from theoretical fibre distributions [34,44,45]. Conversely, measurement of the water diffusion tensor in collagenous tissues in vivo is at present limited, but its interpretation in terms of fibre organisation is relatively straight anisotropically, and the direction of greatest water diffusion corresponds to the predominant direction of collagen fibre alignment [36,38].…”