Diagnosing knee osteoarthritis: from clinical examination to imaging

How a diagnosis of knee osteoarthritis is established

The diagnosis is primarily based on a medical history : location and type of pain (mechanical or inflammatory), context of onset, history of knee trauma or surgery, and impact on daily activities. Theclinical examination completes this assessment: the doctor evaluates your gait, the alignment of your lower limbs, checks for joint effusion, and assesses knee mobility in flexion and extension.

The X-ray then allows for the visualization of characteristic joint changes:

• The joint space narrowing : a reduction in the space between bone surfaces, which reflects the thinning of the cartilage
• The osteophytes : bony growths that form at the edges of the joint
• The geodes : small cavities in the bone located just beneath the cartilage (subchondral bone)
• The subchondral densification : thickening of the bone in response to cartilage loss

The stage of knee osteoarthritis is assessed based on these signs, according to the Kellgren-Lawrence classification, which includes 5 stages (grades 0 to 4), ranging from grade 0 (no visible signs) to grade 4 (severe osteoarthritis with near-total disappearance of the joint space).

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X-ray, MRI, arthro-CT scan: which test for which situation?

These three tests are not interchangeable. Each one answers a specific question at a specific point in the patient's journey.

X-rays are the first-line examination for any persistent knee pain in adults. In the majority of cases, they are sufficient to confirm the diagnosis and assess its stage. They must be performed while standing to be interpretable.

MRI is prescribed in targeted situations:

• Young patient (under 50 years old) with an atypical clinical presentation
• Suspected associated meniscal or ligamentous injury
• Discrepancy between reported symptoms and X-ray findings
• Preoperative assessment before certain procedures

It is not routinely recommended for diagnosing knee osteoarthritis in adults over 50 who present with consistent clinical and radiological findings.

CT arthrography (a CT scan with contrast dye injected into the joint) is used when an MRI is contraindicated (due to metal implants or claustrophobia) or to precisely evaluate cartilage lesions that are not visible on an MRI.

An expert review of imaging results by a specialized orthopedic surgeon can change the interpretation of the osteoarthritis stage and lead to a different treatment plan. A second opinion is based on the complete file, not just the written imaging report.

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Conditions that can mimic knee osteoarthritis

Several conditions can present with symptoms similar to knee osteoarthritis. Identifying them is helpful in understanding why additional testing is sometimes necessary before starting treatment.

• Chondrocalcinosis (calcium pyrophosphate deposition disease): crystal deposits in the cartilage or joint fluid can cause pain and radiological images similar to osteoarthritis. A joint aspiration or blood tests can help guide the diagnosis.
• Inflammatory rheumatism (including rheumatoid arthritis): prolonged morning stiffness, symmetrical involvement of multiple joints, and biological markers of inflammation point toward this diagnosis. A consultation with a rheumatologist is recommended in these cases.
• The Gout : Gout attacks can affect the knee and mimic an acute inflammatory episode (rapid swelling, intense pain). A uric acid blood test and joint aspiration confirm the diagnosis.
• Septic arthritis (joint infection): A hot, swollen, and very painful knee accompanied by a fever is a medical emergency that should not be dismissed as an osteoarthritis flare-up without prior evaluation.

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When the diagnosis is unclear or multiple hypotheses exist, a consultation with a rheumatologist or an orthopedic surgeon helps provide clarity before beginning treatment.