Imaging of Osteoarthritis

Osteoarthritis (OA) is the most prevalent joint disorder in the elderly, and there is no effective treatment. Imaging is essential for evaluating the synovial joint structures (including cartilage, meniscus, subchondral bone marrow and synovium) for diagnosis, prognosis, and follow-up. This article describes the roles and limitations of both conventional radiography and magnetic resonance (MR) imaging, and considers the use of other modalities (eg, ultrasonography, nuclear medicine, computed tomography [CT], and CT/MR arthrography) in clinical practice and OA research. The emphasis throughout is on OA of the knee. This article emphasizes research developments and literature evidence published since 2008.

Key points

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Although conventional radiography is still the most commonly used imaging modality for clinical management of patients with osteoarthritis, and loss of joint space width represents the only end point approved by the US Food and Drug Administration for structural disease progression in clinical trials, magnetic resonance (MR) imaging–based studies have revealed some of the limitations of radiography.
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The ability of MR to image the knee as a whole organ and to directly and three-dimensionally assess cartilage morphology and composition plays a crucial role in understanding the natural history of the disease and in the search for new therapies.
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MR imaging of osteoarthritis is classified into the following approaches: semiquantitative, quantitative, and compositional.
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Ultrasonography can also be useful to evaluate synovial disorders in osteoarthritis, particularly in the hand.

Conventional radiography

Overview

Radiography is the simplest and least expensive imaging technique. It can detect bony features associated with osteoarthritis (OA), including marginal osteophytes, subchondral sclerosis, and subchondral cysts. Radiography can also determine joint space width (JSW), an indirect surrogate of cartilage thickness and meniscal integrity, but precise measurement of each of these articular structures is not possible with radiography. Despite this drawback, slowing of radiographically detected joint space narrowing (JSN) is the only structural end point currently accepted by regulatory bodies in the United States (US Food and Drug Administration) to prove efficacy of disease-modifying OA drugs in phase-III clinical trials. OA is radiographically defined by the presence of osteophytes. Progression of JSN is the most commonly used criterion for the assessment of OA progression and the complete loss of JSW characterized by bone-on-bone contact is one of the indicators for joint replacement.

However, previously held beliefs that JSN and its changes are the only visible evidence of cartilage damage have been shown to be incorrect. Recent studies have shown that alterations in the meniscus, such as meniscal extrusion or subluxation, also contribute to JSN. The lack of sensitivity and specificity of radiography for the detection of articular tissue damage associated with OA, and its poor sensitivity to change at follow-up imaging, are inherent limitations of radiography.

Another limitation is the presence of variations in semiflexed knee positioning, which occur during image acquisition in trials and clinical practice despite standardization. Kinds and colleagues showed that such variations have significant influence on the quantitative measurement of various radiographic parameters of OA including JSW. Thus, better standardization needs to be achieved during radiographic acquisition. Despite these limitations, radiography remains the gold standard for structural modification in clinical trials of knee OA.

Semiquantitative Assessments of Knee OA Features

The severity of OA can be estimated using semiquantitative scoring systems. Published atlases provide images that represent specific grades. The Kellgren and Lawrence (KL) grade is a widely accepted scheme used for defining the presence or absence of OA, usually using grade 2 disease as the threshold. However, KL grading has limitations; in particular, KL grade 3 includes all degrees of JSN, regardless of the extent. Felson and colleagues suggested a modification of KL grading to improve the sensitivity to change in longitudinal knee OA studies. They recommend that OA be defined by a combination of joint space loss and definite osteophytes on radiography in a knee that did not have this combination on the previous radiographic assessment. For OA progression, they recommend a focus on JSN alone using either a semiquantitative or a quantitative approach.

The Osteoarthritis Research Society International (OARSI) atlas takes a different approach and grades tibiofemoral JSW and osteophytes separately for each compartment of the knee. This compartmental scoring seems to be more sensitive to longitudinal radiographic changes than KL grading. A recent study using data from the OA Initiative highlighted the importance of centralized radiographic assessment in regard to observer reliability, because even expert readers apply different thresholds when scoring JSN.

Quantitative Assessments of JSW

Quantitative measures of JSW use a ruler, either a physical device or a software application, to measure the JSW as the distance between the projected femoral and tibial margins on the image ( Fig. 1 ). The femoral margin is defined as the projected edge of the bone, whereas the software usually determines the tibial margin as a bright band corresponding with the projection of the X-ray beam through the radiodense cortical shell at the base of the tibial plateau. Quantification of JSW using image processing software requires a digital version of the image, which can be provided for plain films by a radiographic film digitizer, or files can be analyzed directly for fully digital modalities such as computed radiography and digital radiography. Minimum JSW is the standard metric, but some groups have investigated location-specific JSW as well.