MRI of joint components in osteoarthritis

The accessory joint apparatus, i.e. ligaments, menisci, tendons, labrum are important in maintaining static and dynamic stability, mechanical load distribution and functional integrity of the joints. Loss of these functions increases biomechanical wear and is a cause of joint damage, apparently due to the large decrease in the risk of osteoarthritis after meniscectomy, cruciate ligament and rotator cuff tears. These structures are composed predominantly of collagen, which provides tensile force and also holds water protons. The T2 of collagen is usually so fast (< 1 ms) that in most cases it appears as a low-intensity signal in all pulse sequences, delineated by high-intensity structures such as adipose tissue or synovial fluid.

Intact ligaments appear as dark bands. Their interruption is a direct sign of ligament rupture. However, it should be taken into account that imitation of ligament rupture can occur when obtaining an oblique plane of section through an intact ligament. A special choice of plane may be required to depict some ligaments. The anterior cruciate ligament of the knee joint is best seen on oblique sagittal images of the knee in a neutral position or on direct sagittal images with a slight abduction of the tibia, while the inferior lig. glenohumerale of the shoulder joint is, in principle, statically stable in shoulder abduction and difficult to visualize if not for the position of the shoulder in abduction and external rotation. Multiplanar 3D reconstruction analyzes the integrity of the ligaments quite completely, but is not the original image obtained.

The menisci are composed of fibrocartilage and contain a large number of collagen fibers spatially arranged to resist tensile forces under weight-bearing loads. The fibers are predominantly circularly oriented, especially in the peripheral portion of the meniscus, which explains the tendency for tears to occur longitudinally, so that linear cracks between collagen fibers are more common than across the fibers. When focal collagen loss occurs, such as in myxoid or eosinophilic degeneration, which is also usually accompanied by focal water gain, the effect of T2 shortening is reduced and the water signal is not masked and appears as a rounded or linear area of intermediate signal intensity within the meniscus on short-TE images (T1-weighted proton density SE or GE), which tends to fade with long-TE. These abnormal signals are not tears, as would be the case with meniscal integrity. A meniscus tear may be due to gross deformation of its surface. Sometimes a large amount of synovial fluid outlines the meniscus tear and it is visualized on T2-weighted images, but in most cases undetected meniscal tears are not visible on long TE images. Short TE images are thus highly sensitive (>90%) but somewhat nonspecific for meniscus tears, whereas long TE images are insensitive, although highly specific when visible.

MRI is sensitive to the full spectrum of tendon pathology and detects tendinitis and ruptures with greater accuracy than clinical examination in most cases. Normal tendons have smooth margins and homogeneous low signal intensity on long T2-weighted images (T2WI). Tendon rupture may be partial or complete and is represented by varying degrees of tendon interruption with high signal intensity within the tendon on T2WI. In tenosynovitis, fluid may be visible under the tendon sheath, but the tendon itself appears normal. Tendinitis is usually the result of widening and irregularity of the tendon, but a more reliable finding is increased signal intensity within the tendon on T2WI. Tendon rupture may result from mechanical wear resulting from friction over jagged osteophytes and sharp edges of erosions, or from primary inflammation within the tendon itself. The tendon may be torn from its attachment site acutely. The most common tendons to rupture are the extensor tendons of the wrist or hand, the rotator cuff of the shoulder, and the tendon of the posterior tibial muscle in the ankle. Tendinitis and rupture of the rotator cuff of the shoulder and the tendon of the long head of the biceps in most cases manifest as pain and instability of the shoulder joint. A complete rupture of the rotator cuff of the shoulder is the result of anterior subluxation of the head of the humerus and is often the leading one in osteoarthritis.

Muscles contain less collagen and therefore have a medium signal intensity on T1 and T2-weighted images. Muscle inflammation sometimes accompanies arthritis and has a high signal intensity on T2-weighted images because in both cases, with the development of interstitial edema, the water content increases and the prolongation of T2 is associated with the loss of collagen. Conversely, postinflammatory fibrosis tends to have a low signal intensity on T2-weighted images, while marbled fatty atrophy of muscles has a high signal intensity of fat on T1-weighted images. For muscles, the localization of the process is typical.

It can be concluded that MRI is a highly effective diagnostic, non-invasive method that provides information on all components of the joint simultaneously and facilitates the study of structural and functional parameters in joint diseases. MRI can detect very early changes associated with cartilage degeneration, when clinical symptoms are minimal or absent. Early detection of patients at risk of disease progression detected by MRI allows for appropriate treatment much earlier than with clinical, laboratory and radiological methods. The use of MR contrast agents significantly increases the informativeness of the method in rheumatic joint diseases. Moreover, MRI provides objective and quantitative measurements of subtle, barely perceptible morphological and structural changes in various joint tissues over time and is therefore a more reliable and easily reproducible method that helps monitor the course of osteoarthritis. MRI also facilitates the assessment of the effectiveness of new drugs for the treatment of patients with osteoarthritis and allows for rapid research. Further optimization of these measurements is needed as they may be used as powerful objective methods to study the pathophysiology of osteoarthritis.

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