Diagnosis of Knee Pain Due to an Acute Injury

The following suggested evaluation is based on the University of Michigan Health System Guidelines for Clinical Care “Knee Pain or Swelling: Acute or Chronic”*. The flow chart algorithms are outstanding.

For diagnosis, knee pain is divided into three categories: Knee pain due to an acute injury, knee pain with systemic symptoms (ie, fever, chills, malaise [weak or achey], night sweats),  and knee pain without systemic symptoms.

Today, I’ll go over diagnosis knee pain of an acute knee injury.

In later posts, I’ll talk about the diagnosis of (chronic) knee pain with and without systemic symptoms.

For knee pain following an acute injury we need to rule out an injury to the blood vessels that run behind the knee. We look for pallor (paleness) below the knee, for bluish discoloration below the knee, and for coolness of the leg below the knee. If any of those are present or  if the mechanism of the injury suggests the possibility of blood vessel injury** or if the doctor’s clinical suspicion is that there is the possibility of vascular injury then a vascular ultrasound, MR or CT angiogram, and/or an emergency specialty consult is indicated.

Next we look for swelling of the knee joint. If it is present, then an x-rays of the knee are indicated.
If there is no knee effusion (knee joint swelling) knee x-rays are still needed : if the patient is over 50 years of age and has disabling knee pain, or if knee movement (range of motion) is less than normal, or if it is painful regardless of the patient’s age.

If the patient is less than 50 years old, has a normal and relatively pain free range of motion and no knee swelling, then knee x-rays can be deferred. The patient is then treated with non-impact exercise (biking or swimming) and leg strengthening.

If there is joint swelling (called a joint effusion), then knee x-rays are indicated.

If a fracture or tumor is scene then the patient is referred to the specialist.

If the x-ray is normal, then the next step is to consider needle aspiration of the knee effusion (the fluid in the joint).

If the knee aspirate is bloody, then the patient should be referred to the specialist for evaluation of a possible internal derangement (meaning damage to the medial or lateral meniscus, or damage to the anterior or posterior cruciate ligaments). But note that as I have  researched other articles on acute knee hemarthrosis, it is not clear that the effusion must be drained  which means that the algorithm might not be correct(see the summary of  “Initial Management of the Sports Injured Knee”***.

If the knee aspirate is not bloody, then the joint fluid should be sent for gram stain (to look for infection), examination by polarizing light microscopy (to look for the crystals of gout or pseudogout), and white blood cell count of the joint fluid (to look evidence suggesting connective tissue diseases [like rheumatoid arthritis] or infection.

If the white blood cell count of the joint fluid is less than 2,000 white cells per cubic millimeter, the possible diagnoses include trauma and osteoarthritis. The other causes discussed in the next two paragraphs are also possible but less likely.

If the white blood cell count of the joint fluid is between 2,000 and 30,000 white cells per cubic millimeter, possible diagnoses include rheumatoid arthritis, other connective tissue diseases, acute rheumatic fever, and and joint infection. The cause could be trauma or osteoarthritis.

If the joint fluid is greater than 30,000 white cells in joint fluid, infection of the joint is suspected.

Regardless of the white cell count of the joint fluid, if infection is suspected, then blood cultures are performed and the patient is started on antibiotics. Often, a specialist will also be consulted.

So, in summary, if a person is unable to bear weight, an knee x-ray is needed. If a fracture is seen then the patient is referred to the specialist. If there is marked ligamentous laxity (instability) on the physical exam of the knee, the patient is referred to the specialist. On further reading, it is not clear that acute knee effusion (swelling) needs to be drained as a diagnostic or therapeutic maneuver is unclear***.

The American College of Radiology Guidelines for imaging evaluation of acute knee injury is worth reviewing****.

*The University of Michigan knee pain guidelines are outstanding in that they cover the evaluation of both acute and chronic knee pain. They are available at

**The mechanism of knee injury that can sometimes damage the blood vessels of the lower leg include: fall from a height, motor vehicle accident, motor vehicle striking a pedestrian, and contact sports. These can all cause dislocation of the knee which often will spontaneously reduce (go back in place before the patient gets to the emergency department).

***”Initial Management of the Sports Injured Knee” summary: “Our literature review suggests that the most common causes of an acute hemarthrosis in the sports injured knee is an injury to the anterior cruciate ligament followed by meniscus tears and medial collateral ligament injury. The Lachman test is the most sensitive clinical maneuver to diagnose an ACL tear while the pivot-shift test is the most specific. Arthroscopy is not recommended as a diagnostic tool in favor of magnetic resonance imaging in order to avoid unnecessary surgical procedures. While the literature has not demonstrated functional or clinical differences following acute reconstruction of the ACL, a well designed level one study is required before this can be routinely advocated. Additionally, the available literature can neither recommend nor discourage the use of aspiration for symptomatic relief or functional improvement in the acutely injured knee with hemarthrosis.”

****The ACR Guideline for Acute Trauma to the knee discusses the imaging evaluation of kneeand is available for download as PDF on the ACR Appropriateness Criteria List page.





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