Diagnosis of osteoarthritis: MRI of articular cartilage

MRI picture of articular cartilage reflects the totality of its histological structure and biochemical composition. Articular cartilage is hyaline, which does not have its own blood supply, lymphatic drainage and innervation. It consists of water and ions, fibers of type II collagen, chondrocytes, aggregated proteoglycans and other glycoproteins. Collagen fibers are strengthened in the subchondral layer of the bone as an anchor and run perpendicular to the surface of the joint where they diverge horizontally. Between the fibers of collagen are large proteoglycan molecules, which have a significant negative charge, which intensively attracts water molecules. Chondrocytes of cartilage are arranged in even columns. They synthesize collagen and proteoglycans, as well as enzyme degraders in an inactive form and enzyme inhibitors.

Histologically, there were 3 layers of cartilage in large joints, such as the knee and femoral. The deepest layer is a junction of the cartilage and subchondral bone and serves as the attachment layer of a vast network of collagen fibers extending from it to the surface with dense bundles connected by numerous cross-linking fibrils. It is called the radial layer. Toward the articular surface, individual collagen fibers become thinner and bind together into more regular and compact parallel arrays with fewer cross-links. The middle layer - transitional, or intermediate, contains more disorderly organized collagen fibers, most of which are obliquely oriented in order to withstand vertical loads, pressure and tremors. The most superficial layer of articular cartilage, known as the tangential cartilage, is a thin layer of tightly arranged, tangentially oriented collagen fibers that opposes tensile forces acting under compression loads and forms a watertight barrier to the interstitial fluid, which prevents its loss during compression. The most superficial collagen fibers of this layer are arranged horizontally, form dense horizontal plates on the joint surface, although the fibrils of the surface tangential zone are not necessarily connected to those of the deeper layers.

As it was noted, the aggregated hydrophilic molecules of proteoglycans are located inside this complex mesh network of fibers. These large molecules have negatively charged fragments of SQ and COO at the ends of their numerous branches, which intensively attract oppositely charged ions (usually Na ⁺ ), which in turn facilitates the osmotic penetration of water into the cartilage.The pressure inside the collagen network is enormous, and the cartilage functions as an extremely effective hydrodynamic pillow.The compression of the articular surface causes a horizontal displacement of the water contained in the cartilage, as the network of collagen fibers is compressed. When the compression after the joint load decreases or disappears, the water moves back, attracted by the negative charge of proteoglycans.This is the mechanism that maintains a high water content and thus a high proton density of the cartilage.The highest water content is noted closer to the articular surface and decreases towards the subchondral bone .. The concentration of proteoglycans is increased in the deep layers of the cartilage.

In the present MRI - this is the main method of obtaining images of hyaline cartilage, implemented mainly using gradient - echo (GE) sequences. MRI reflects the water content of the cartilage. However, it is important how many protons of water the cartilage contains. The content and distribution of hydrophilic molecules of proteoglycans and the anisotropic organization of collagen fibrils affect not only the total amount of water, i.e. Proton density, in cartilage, but also on the state of relaxation properties, namely T2 of this water, giving cartilage characteristic "zonal" or delaminating images on MRI, which, according to some researchers, correspond to the histological layers of the cartilage.

On very short echo (TE) images (less than 5 ms), the higher resolution of cartilage images typically shows a two-layered image: the deep layer is located closer to the bone in the pre-calcification zone and has a low signal, since the presence of calcium greatly reduces TR and does not gives images; The surface layer gives a medium-intensive or high-intensity MP signal.

In intermediate TE images (5-40 ms) the cartilage has a three-layered appearance: a surface layer with a low signal; a transition layer with a signal of intermediate intensity; a deep layer having a low MP signal. At T2-weighing, the signal does not include the intermediate layer, and the cartilage image becomes homogeneously of low intensity. When a low spatial resolution is used, an additional layer sometimes appears on the short TE images, which is due to the oblique cut artifacts and high contrast on the cartilage / liquid surface, this can be avoided by increasing the size of the matrix.

In addition, some of these zones (layers) may not be visible under certain conditions. For example, when the angle between the cartilage axis and the main magnetic field changes, the shape of the cartilaginous layers may change, and the cartilage may have a homogeneous image. This phenomenon is explained by the anisotropic property of collagen fibers and their different orientation within each layer.

Other authors believe that obtaining a layered image of cartilage is not reliable and is an artifact. The opinions of the researchers diverge also regarding the intensity of the signals from the obtained three-layer cartilage images. These studies are very interesting and, of course, require further study.

[1], [2], [3], [4], [5], [6]

Structural changes of cartilage with osteoarthritis

In the early stages of osteoarthritis, the collagen network degrades in the surface layers of the cartilage, leading to the disintegration of the surface and increased permeability to water. As more proteoglycans break down, more negatively charged glycosaminoglycans appear that attract cations and water molecules, while the remaining proteoglycans lose the ability to attract and retain water. In addition, the loss of proteoglycans reduces their inhibitory effect on the interstitial water current. As a result, the cartilage swells, the mechanism of compression (retention) of the liquid does not work and the compression resistance of the cartilage decreases. There is an effect of transferring most of the load to the already damaged solid matrix, and this leads to the fact that the swollen cartilage becomes more susceptible to mechanical damage. As a result, the cartilage either recovers or continues to degenerate.

In addition to damage to proteoglycans, the collagen-new network is partially destroyed, which is no longer being restored, and vertical cracks and ulceration appear in the cartilage. These lesions can spread down the cartilage to the subchondral bone. Decay products and articular fluid spread to the basal layer, which leads to the appearance of small areas of osteonecrosis and subchondral cysts.

In parallel with these processes, cartilage undergoes a number of reparative changes with an attempt to restore the damaged joint surface, which include the formation of chondrophytes. The latter eventually endochondral ossification and become osteophytes.

Acute mechanical trauma and compression load can lead to the development of horizontal cracks in the deep calcified layer of cartilage and the detachment of cartilage from the subchondral bone. Basal cleavage or delamination of the cartilage in a similar way can serve as a mechanism for degeneration of not only normal cartilage under conditions of mechanical overload, but also for osteoarthritis when there is instability of the joint. If the hyaline cartilage is completely destroyed and the articular surface is exposed, then two processes are possible: the first is the formation of dense sclerosis on the surface of the bone, which is called eburnesis; the second is the damage and compression of the trabeculae, which on X-rays looks like subchondral sclerosis. Accordingly, the first process can be considered as compensatory, the second is clearly a phase of joint destruction.

Increasing the water content in the cartilage increases the proton density of the cartilage and eliminates the T2-shortening effects of the proteoglycan-collagen matrix, which has a high signal intensity in the areas of matrix damage on conventional MRI sequences. This early chondromalacia, which is the earliest sign of cartilage damage, can be noticeable before even a slight thinning occurs. At this stage, there may also be a slight thickening or "swelling" of the cartilage. Structural and biomechanical changes of the articular cartilage are constantly increasing, the loss of the basic substance occurs. These processes can be local or diffuse, limited surface thinning and defibration, or complete disappearance of the cartilage. In some cases, local thickening or "swelling" of the cartilage can be observed without rupturing the joint surface. In osteoarthritis, it is often possible to observe a local increase in the signal intensity of cartilage on T2-weighted images, which is confirmed arthroscopically by the presence of surface, transmural and deep linear changes. The latter can reflect profound degenerative changes, beginning mainly as a detachment of cartilage from the kalıdifikirovannogo layer or the tide line. Early changes can be limited only to deep layers of cartilage, in this case they are not found when arthroscopic examination of the joint surface, although local dilatation of deep layers of cartilage can lead to damage to adjacent layers, often with the growth of the subchondral bone in the form of a central osteophyte.

In the foreign literature there are data on the possibility of obtaining quantitative information on the composition of the articular cartilage, for example, the content of the water fraction and the diffusion coefficient of water in the cartilage. This is achieved with the use of special programs MP-tomograph or in MR-spectroscopy. Both these parameters increase when the proteoglycan-collagen matrix is damaged in cartilage damage. The concentration of mobile protons (water content) in the cartilage decreases in the direction from the articular surface to the subchondral bone.

A quantitative evaluation of the changes is possible on T2-weighted images. Summarizing the data of the images of the same cartilage obtained with different TE, the authors evaluated the T2-weighted images of the cartilage using an appropriate exponential curve from the obtained signal intensity values for each pixel. T2 is evaluated in a particular area of the cartilage or displayed on the map of the entire cartilage, in which the signal strength of each pixel corresponds to T2 at this location. However, despite the rather large possibilities and relative ease of the above-described method, the role of T2 is underestimated, in part because of the increase in diffusion-related effects with an increase in TE. Basically, T2 is underestimated in cartilage with chondromalacia, when water diffusion is increased. If special technologies are not used, the potential increase in T2, measured with these technologies in the cartilage with chondromalacia, will slightly suppress the diffusion-related effects.

Thus, MRI is a very promising method for identifying and monitoring the early structural changes that are characteristic of articular cartilage degeneration.

Morphological changes of cartilage in osteoarthritis

Evaluation of morphological changes in cartilage depends on high spatial resolution and high contrast from the surface of the joint to the subchondral bone. This is best achieved by using a fat-suppressed T1-weighted 3D GE-sequence that accurately reflects the identified local defects and verified both in arthroscopy and autopsy material. The cartilage image can also be obtained by transferring the magnetization by subtracting the images, then the articular cartilage has the form of a separate band with a high signal intensity distinctly contrasting with a number of underlying low-intensity articular fluid, intra-articular fat tissue and subchondral bone marrow. However, when using this method, the image is obtained 2 times slower than the fat-suppressed T1-VI, therefore it is less widely used. In addition, images of local defects, surface roughness and generalized thinning of the articular cartilage can be obtained using conventional MP sequences. According to some authors, the morphological parameters - thickness, volume, geometry and topography of the cartilage surface - can be quantified using 3D MRI images. By summing the voxels constituting the 3D reconstructed cartilage image, the exact value of these complex-linked structures can be determined. Moreover, measuring the total volume of cartilage obtained from individual slices is a simpler method due to smaller changes in the plane of one cut and more reliable in spatial resolution. In the study of whole amputated knee joints and patella specimens obtained with arthroplasty of these joints, the total volume of the articular cartilage of the femoral, lumbar and bones was determined and a correlation was found between the volumes obtained with MRI and the corresponding volumes obtained by separating cartilage from bone and measuring it histologically . Consequently, this technology can be useful for dynamically assessing the changes in cartilage volume in patients with osteoarthritis. Obtaining the necessary and accurate cuts of articular cartilage, especially in patients with osteoarthritis, requires sufficient skills and experience of the doctor conducting the study, as well as the availability of the corresponding software of the MR-tomograph.

Total volume measurements contain little information on common changes and are sensitive, respectively, for local loss of cartilage. Theoretically, the loss of cartilage or its thinning at one site can balance the equivalent increase in cartilage volume elsewhere in the joint, and measuring the total cartilage volume would not show any abnormality, so such changes would not be detectable by this method. The division of articular cartilage with the help of 3D reconstruction into separate small regions made it possible to estimate the volume of cartilage in certain areas, in particular, on surfaces experiencing a force load. However, the accuracy of the measurements decreases, since very little separation is performed. In the end, an extremely high spatial resolution is necessary to confirm the accuracy of the measurements. If sufficient spatial resolution can be achieved, the prospect of mapping the cartilage thickness in vivo becomes possible. Cartilage thickness maps can reproduce local lesions in the progression of osteoarthritis.