Graphique de Localisation de la Douleur au Genou

Where a knee hurts narrows the list of likely causes considerably, because the structures that generate pain sit in distinct compartments. Front, inner, outer and back each point to a different group of diagnoses. Before location is useful, though, an acute injury needs a different question answered first: whether an X-ray is needed at all, which is something a validated decision rule answers better than a chart.

After an acute injury: the Ottawa Knee Rule

The Ottawa Knee Rule was developed to reduce unnecessary X-rays after acute knee injury, and it is one of the better-validated decision rules in emergency medicine, with sensitivity for clinically significant fracture approaching 100 percent across validation studies. An X-ray is indicated if any one of the following is present.

• Age 55 years or older.
• Isolated tenderness of the patella, with no other bony tenderness.
• Tenderness at the head of the fibula.
• Inability to flex the knee to 90 degrees.
• Inability to bear weight for four steps, both immediately after the injury and in the assessment setting.

What each location suggests

For non-acute or persistent pain, location plus what provokes it is the most useful combination. The table lists the common causes by region; it narrows the field rather than making a diagnosis.

• Mechanical symptoms, true locking or the knee giving way, point towards a meniscal tear or a loose body rather than a soft-tissue problem.
• Morning stiffness lasting more than an hour suggests inflammatory arthritis rather than osteoarthritis, where stiffness typically eases within about 30 minutes.

Symptoms that need urgent attention

Most knee pain can wait for a routine appointment. The following cannot, and two of them are not primarily knee problems at all.

• A hot, swollen, exquisitely painful knee with fever: possible septic arthritis, which is a joint-threatening emergency.
• Calf pain and swelling with pain behind the knee: consider deep vein thrombosis, especially after immobility, surgery or long travel.
• Obvious deformity, inability to bear any weight, or a knee that felt like it dislocated.
• A large effusion developing within an hour of injury, which suggests bleeding into the joint from an ACL tear or fracture.
• Loss of sensation or pulse below the knee after injury.

Limites

• Location narrows a differential but does not diagnose. Referred pain from the hip or lumbar spine commonly presents as knee pain, particularly in children and older adults.
• The Ottawa Knee Rule applies to acute injury in adults and is a rule for whether to image, not a rule for whether an injury is serious. It has been studied in children but is applied more cautiously there.
• Several conditions coexist. Osteoarthritis and a degenerative meniscal tear frequently appear together, and imaging findings do not always explain the symptoms.
• Imaging findings are common in people with no pain at all, so a scan result has to be interpreted against the clinical picture.
• This chart does not cover post-surgical knees, inflammatory arthritis under treatment, or paediatric conditions such as slipped capital femoral epiphysis presenting as knee pain.
• Any knee that is hot, red and swollen with fever needs same-day assessment rather than a reference chart.

Questions fréquentes

What does pain on the inside of the knee mean?

Most commonly a medial meniscus tear, an MCL sprain, medial compartment osteoarthritis or pes anserine bursitis. Twisting injuries point towards the meniscus, a blow to the outside of the knee towards the MCL, and gradual onset with age towards osteoarthritis.

Do I need an X-ray after a knee injury?

The Ottawa Knee Rule indicates one if any of these apply: age 55 or over, isolated patellar tenderness, fibular head tenderness, inability to flex to 90 degrees, or inability to take four steps both at the time and during assessment. If none apply, fracture is very unlikely.

What causes pain behind the knee?

Commonly a Baker cyst, hamstring tendinopathy or a posterior meniscal tear. Importantly, pain behind the knee with calf swelling can indicate deep vein thrombosis, which needs urgent assessment rather than treatment as a knee problem.

When is knee pain an emergency?

A hot, swollen, very painful knee with fever suggests septic arthritis and needs immediate assessment. So do obvious deformity, inability to bear any weight, rapid swelling within an hour of injury, or loss of sensation or pulse below the knee.

How do I tell osteoarthritis from inflammatory arthritis?

Duration of morning stiffness is the most useful single feature. Osteoarthritis stiffness typically eases within about 30 minutes and worsens through the day with use; inflammatory arthritis stiffness commonly lasts over an hour and improves with movement.

Références

1. Bunt CW, Jonas CE, Chang JG. Knee Pain in Adults and Adolescents: The Initial Evaluation. American Family Physician.
2. American Academy of Orthopaedic Surgeons. Management of Osteoarthritis of the Knee, clinical practice guideline.
3. National Institute of Arthritis and Musculoskeletal and Skin Diseases. Osteoarthritis.
4. MedlinePlus. Knee Injuries and Disorders.