Osteoarthritis (OA) of the Knee

Osteoarthritis (OA) is one of the most common disorders of joints. The joints most frequently affected are the hip, knee, shoulder, the big toe and the base of the thumb. Degeneration of articular cartilage, leading to osteoarthritis, can occur naturally over time with a age or as a secondary condition to an associated injury, such as a trauma or repetitive-stress or occupational injury. Articular cartilage degeneration is a gradual process.

The sagittal proton density image with fat saturation, obtained near trochlear midline, illustrates the moderate (partial-thickness) articular cartilage loss, with subarticular bone marrow edema (arrow). At the most proximal patella, the articular cartilage remains normal (arrowhead). At the trochlear aspect, the prefemoral fat pad (arrowhead) contacts the patella. At the medial trochlea further distally, there was a small region of partial-thickness cartilage loss (not shown).

MR imaging represents a dramatic improvement in the evaluation of joint involvement by OA. Radiographic evaluation is limited to demonstrating changes in bony mineralization affecting architecture and density, a “mineral map”, while MRI can directly show detailed morphology of the articular cartilage and also of all mineralized and non-mineralized joint structures.

Knee Arthritis, MRI

Imaging for Osteoarthritis: An Overview

Knee Arthritis, MRI

In addition to being very common in humans, and having been identified in skeletal remains from thousands of years ago, OA is also frequent in other mammalian species. Still, OA has not been well understood in terms of its cause. The long held concept of OA as an inevitable feature of old age, or caused by long and hard work as a simple “wear-and-tear” phenomenon similar to the slow gradual wear down of treads of a car tire with a predetermined mileage limit, has been found to be entirely inaccurate. Instead of accepting OA as a disease limited to or even likely starting in articular cartilage (“primary” or idiopathic OA), several conceptual models have been proposed, with either a more mechanical or a more metabolic emphasis. One model describes OA as a process resulting from imbalance in the mechanical stresses affecting the entire joint, causing articular cartilage matrix degradation to dominate over matrix synthesis, thereby preventing cartilage self-repair and resulting in chondral loss 4 . Other proposed models see OA as a systemic metabolic disorder, in which circulating factors linked to altered lipid and glucose metabolism, or dysregulated tissue turnover in many tissues with common mesenchymal origin, may explain the diversity of pathophysiological changes found in generalized OA, including an association with obesity and with vascular disease 5 .

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Findings

The findings from radiographic imaging depend on the stage of a person’s OA. The doctor who orders the imaging will use the description and gradation of osteoarthritis (“localized or diffuse” and “mild, moderate or severe,” etc.) to determine treatment options. Treatments include conservative, nonsurgical therapy or surgery.

On coronal proton density image with fat saturation, mild (partial-thickness) articular cartilage loss is shown also in the medial compartment, femoral aspect (arrow). The vague region of signal loss at the lateral tibial plateau articular cartilage (arrowhead) is artifactual. The menisci are normal.

In order to detect early cartilage wear, HSS uses special X-ray views in place of or in addition to these standard views. These specialized views are designed to increase the sensitivity of the conventional radiographic study.

Osteoarthritis (OA) of the Knee

Rheumatoid arthritis MRI

Regardless of the joint that is affected, osteoarthritis is revealed on conventional radiographs (X-rays) by characteristics that are distinct from other joint disorders, such as rheumatoid arthritis. Specifically, an X-ray of a joint with osteoarthritis will show a narrowing of the space between the bones of the joint where the cartilage has worn away, as shown in the image below.

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What are the different types of imaging examinations for osteoarthritis?

Medial compartment OA, with full-thickness articular cartilage loss at both femoral and tibial aspects, exposing the pink vascularized underlying bone (arrow). Note the slightly yellowish white appearance of the normal adjacent cartilage (asterisk), and the normal cartilage at the lateral femoral condyle and trochlea.

Up to 90% of forces across the knee joint are routinely absorbed by active mechanisms when the knee flexes, mainly through opposing muscles distributing the impact over time and over surface 4 . In addition, much of articular impact is absorbed by the trabecular bone immediately deep to the cartilage 4 . In normal joints, the actual load per articular cartilage surface area during use has been found to be remarkably constant within most joints, around 23 kg/cm square, both in large weight-bearing joints and in small joints in the hand and fingers 6 . When this load is significantly exceeded, the chondrocytes in cartilage react, with various extent of degradation dominating synthetic activity. Chondrocytes, like osteocytes in bone, have been found to serve as both mechano-sensors and osmo-sensors, altering their metabolism in response to local physicochemical changes in the microenvironment. This recent discovery elegantly links extracellular environment events to intracellular signaling cascades.

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Osteoarthritis of the knee, MRI scan


Anteroposterior (front to back) X-ray image of the knee showing osteoarthritis. Note the narrower spacing on the right side of the image, where cartilage has degenerated.

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Conventional radiographs – Routine X-ray examinations

Interestingly, high but not excessive level of activity does not predispose a normal joint to develop OA, but may be considered beneficial, considering that the lack of any vascularity within cartilage makes intermittent loading an important factor promoting cartilage metabolism. Studies have shown that long distance middle aged and older runners develop OA with similar but not higher incidence as in the general population 6 . Furthermore, a very sedentary life style is a risk factor for development of OA, with a proposed mechanism attributing this to the lack of muscle strength and coordination 4 .

This 3D graphic representation demonstrates the densely packed large proteoglycan aggregates “trapped” between the bundles of collagen, both produced by the chondrocytes. In the background note the organization of collagen fibers, with an overall “upside-down U” configuration (Benninghoff’s arcades) leading to parallel fibers along the main articular force vector at the deep regions of cartilage, while at the joint surface the fibers are parallel to the surface. This organization is thought to be reflected in the MR signal from different regions of articular cartilage.

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When cartilage is lost, bone rubs against bone. This can cause to cysts or fluid-filled cavities can form in the bone, which will also be visible in an X-ray. The body also responds with sclerosis (increased bone density), in which more bone grows in where the cartilage used to be. The joint surfaces become misaligned and osteophytes (bone spurs) may form. There are basic routine X-ray views for imaging each joint:

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What kinds of specialized imaging can help diagnose osteoarthritis?

The older OA classification also includes “secondary” OA, resulting from another distinct disorder, such as pseudogout/CPPD, ochronosis and others causing selective tissue deposits that lead to quality changes within cartilage, and the accepted risk factors for OA including post-traumatic joint instability, joint malalignment, obesity, genetic predisposition, and also senescent changes increasingly frequent with advancing age.

When evaluating the MR imaging features of OA, it is beneficial to have a conceptual understanding of the basic components, architecture, and physiology of articular (hyaline) cartilage and adjacent tissues (6a). Cartilage is a highly specialized tissue, one of the very few avascular and aneural tissues in the body. Hyaline cartilage always exists in a thin layer, from a fraction of a mm, up to 5-6mm in maximal thickness at the mid-patella which is usually the thickest hyaline cartilage in the body. Importantly, cartilage contains no distinct laminar structures, but instead there is a particular functional arrangement of collagen fibril orientation, chondrocyte prevalence, proteoglycans, and water content, with predictable gradual variations from the surface to the deep aspect of cartilage. The orientation of the collagen fibers (Benninghoff,s arcades) include parallel thick collagen bundles at the surface, oblique orientation at the mid-section, and radial collagen bundles at the deep layers (6a). The presence of collagen fiber cross-linking is strongly associated with function and imparts the characteristic strength and resilience. The dense collagen meshwork functions to “trap” the extremely large proteoglycan molecules that represent some of the largest molecules in the body, combining into aggregates with molecular weight up to 100 million (compare to water MW of 18). Inside cartilage, these proteoglycan molecules are highly compressed to1/1000 of the size they would have outside of cartilage, and indeed if the collagen interfibrillar ties rupture, this allows proteoglycan expansion to force the matrix apart, producing swelling and fissuring, characteristic early manifestations of OA. These large proteoglycan molecules are highly negatively charged, and during weight bearing when cartilage is compressed, they are made to move even slightly closer together, further repelling each other, and thereby maintain volume and then re-expand, contributing to the tremendous cartilage resilience manifested by tolerance of high load and high repetition mechanical stresses. In addition, when cartilage is compressed such as in the knee during motion, a minute amount of fluid is squeezed from the surface layers of cartilage onto the contact surfaces, with the lubrication further lowering the friction. Hyaline cartilage has one of the lowest friction coefficients known, approximately 1/3 of the friction between two melting ice cubes, lower than what has been recreated in man-made mechanical devices 7 .

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