Imaging Modalities for Osteoarthritis
Radiographs are no longer de rigeur in making the diagnosis of knee osteoarthritis, according to guidelines released by the European League Against Rheumatism last month. Clinical signs, symptoms, and risk factors are sufficient to make the diagnosis.
Yet x-rays and other imaging modalities continue to have a role in osteoarthritis, according to Dr. Ali Guermazi and Dr. David J. Hunter. At present, imaging should primarily be used in research rather than in clinical practice (where its use should be limited to ruling out other likely diagnoses), they agreed.
Imaging conveys information about the pathophysiology of OA and has provided insight into symptoms and progression, said Dr. Hunter, chief of the research division of New England Baptist Hospital in Boston. Ultimately, “it will help to define the best treatments for osteoarthritis,” he added. Imaging has the potential to “have wider clinical application, when we have an opportunity to intervene in earlier arthritis through modifying joint structural changes.”
Yet, there continues to be “widespread use of different imaging modalities in the clinical setting, where it's clear there is osteoarthritis.” Dr. Hunter estimated that 60%–70% of patients who present to him in the clinic have an MRI on CD with them. “It doesn't change the diagnosis or what I'm going to do for them, so I'm not sure that there's much rationale for having that at present.”
Each imaging modality has a role to play when assessing the pathophysiology of the whole joint. “Some of that role is complementary, but much of it is unique to that particular modality. … Each has its strengths and weaknesses.”
Dr. Guermazi and Dr. Hunter shared their thoughts on how various imaging modalities can further understanding of the pathophysiology of OA.
Radiograph
An x-ray is typically used only to rule out other diagnoses, said Dr. Hunter. These could include rheumatoid arthritis, gout, intra-articular loose bodies, and trauma. “These would be reasons to think about doing additional imaging.”
X-ray is still the most widely used imaging modality because it's relatively inexpensive, it's available, and it's relatively easy to interpret. X-rays are useful primarily for outlining the two-dimensional geometry of the bones. “You infer from the x-ray what the joint space is, and from that, the health and integrity of the cartilage, but it's an indirect inference and you really can't make any direct representations of other tissues,” Dr. Hunter said.
In addition, x-ray evaluates features that don't contribute to pain, which is the primary symptom of OA, said Dr. Guermazi, director of the quantitative imaging center and section chief of musculoskeletal imaging at Boston University. “If we look at joint space narrowing, thinking that this is the cartilage measure, [x-ray is] useless for many reasons. Arthritis is defined clinically as pain. Cartilage can't be painful because there are no nerves in it.”
Synovitis, effusions, and bone marrow lesions, however, can all be painful. “The only features that are not painful are the ones we are looking at on a radiograph, which are cartilage [inferred from the joint space narrowing] and osteophytes.”
Dr. Hunter noted that the imaging community is divided about whether the best way to monitor the structural progression of underlying disease is to measure joint space on x-ray or to measure other features—such as the cartilage itself—on an MRI.
MRI
The main strength of MRI as a research tool is its ability to provide information on many tissues in the joint. “On MRI, you're able to … see changes in the curvature of the bone, lesions within the bone [which demonstrate where focal loading is occurring, and] alterations within the cartilage itself. … You can see inflammation within the joint [such as synovitis or an effusion], the ligaments, and the muscle,” said Dr. Hunter.
“The more we're learning about osteoarthritis, the more we're realizing that much of the reason why a person has pain [and] functional limitation—and much of the reason why the joint progresses—has a lot more to do with the tissues in the joint other than cartilage,” Dr. Hunter said. “Bone marrow lesions, synovitis, and effusions appear to account for the majority of the reason why a person has pain. MRIs provide a lot of insight there,” he said.
Beyond the local-tissue reasons for a person to have symptoms, there is also some local alteration in the neurophysiology. “The communication between different nerve pathways that leads to sensitization of nerves occurs both peripherally and centrally, “Dr. Hunter said.
“If we are focused on symptomatic response, it's helpful to think about the contribution of all of those factors.” Nerve endings are present in specific tissues, particularly the bone and synovium.
