Approach to knee pain: Clinical sciences

Knee pain is a common musculoskeletal symptom that can be caused by damage to any of the ligamentous, muscular, or bony structures comprising the joint. Most often, knee pain is associated with trauma and conditions such as fractures and ligament or meniscus injuries.

On the other hand, non-traumatic causes can be classified based on the presence of joint effusion. No joint effusion is common in conditions, such as patellofemoral pain syndrome, prepatellar bursitis, and osteoarthritis; while joint effusion is typically seen in septic arthritis, crystal-induced arthropathy, and non-infectious inflammatory arthritis.

If a patient presents with knee pain, the first step is to perform a focused history and physical examination. Start by assessing the patient’s history of trauma preceding the onset of knee pain.

If the patient reports trauma to the knee, consider traumatic causes of knee pain and assess their risk for fracture using the Ottawa knee rules.

Ottawa knee rules include the age of 55 or older, point tenderness at the fibular head, isolated point tenderness of the patella, inability to flex the knee to 90 degrees, or inability to bear weight on the affected knee.

If the patient does not have any of these characteristics, your patient has a low risk for fracture, so no further evaluation is needed.

On the flip side, if the patient meets any of the criteria, then they have a high risk of fracture, so you should order x-rays. If an x-ray is performed, it may reveal a fracture, for example a fracture of the tibia, patella, or other surrounding bones.

Let’s take a look when x-ray findings are normal. If the x-ray findings show no fracture, consider ligamentous or meniscal injury. The mechanism of injury and physical exam can help distinguish between injuries of the anterior cruciate, posterior cruciate, and collateral ligaments.

First, let’s start with the anterior cruciate ligament injury or ACL injury for short. Let’s say the patient's knee pain developed after suddenly pivoting the knee with the foot firmly planted on the ground. Also, the patient may report an audible pop during the injury, followed by knee instability and a rapidly developing effusion.

Additionally, the physical exam reveals a positive anterior drawer test. You can perform the anterior drawer test by having the patient lie supine with the affected knee flexed and the foot flat on the table. Sit directly in front of the leg and try to draw the tibia forward. If the tibia can be pulled too far forward, the anterior drawer test is positive, which confirms the diagnosis of ACL injury.

Ok, now let’s take a look at the posterior cruciate ligament injury or PCL injury for short. In this case, history typically reveals knee pain after a high-impact trauma to the anterior tibia, like during a motor vehicle accident, or after a hyperflexion injury; while the physical exam reveals a positive posterior drawer test.

You perform the posterior drawer test by having the patient lie supine with the knee flexed and the foot flat on the table. Sit directly in front of the leg, and try to draw the tibia backward. If the tibia can be pushed too far back, you have a positive posterior drawer test, which confirms the PCL injury.

Alright, moving on to collateral ligament injuries. Your patient will typically report a knee injury from a valgus or varus force as well as medial or lateral joint pain. Physical exam reveals point tenderness along the joint line and laxity or gapping with valgus or varus stress testing.

Perform valgus and varus stress testing by asking the patient to lie with the leg extended. Place one hand on the femur and one hand on the tibia, and apply oppositional pressure, moving the tibia medially and the femur laterally, and vice versa. If there is excessive movement or laxity, that’s a positive test, which indicates lateral or medial collateral ligament injury.

Finally, don’t forget the meniscal tear as a cause of knee pain! History usually reveals knee pain that develops after a twisting injury, and the patient often experiences episodes in which the affected knee locks or gives way. Additionally, important physical exam findings include joint line tenderness and a positive McMurray test.

Perform the McMurray test by having the patient lie supine with the affected knee flexed. Place one hand across the knee joint line, hold the patient's foot with the other. Then, extend the patient’s knee while internally rotating the tibia and applying varus stress. This motion tests the lateral meniscus. Next, extend the patient’s knee while externally rotating the tibia and applying valgus stress. If the maneuver reproduces the patient's pain, or there’s locking or clicking in the joint, it’s a positive test, and indicates a meniscal injury.

Now let’s go all the way back and take a look at individuals with no history of trauma preceding the knee pain. In these patients, you should consider non-traumatic causes and assess whether or not knee effusion is present. You can do this using the bulge or stroke test.

Perform the test by asking the patient to lie supine with the knee extended. Next, “milk” the knee by sweeping fluid from the medial aspect of the knee, over the top of the patella, and into the lateral compartment. Then, apply pressure laterally. If the skin on the medial side of the knee bulges as fluid moves back into the medial compartment, then the test is positive for effusion.

But, if the test is negative and there is no effusion, assess the location of the patient’s knee pain. If the pain is in the anterior knee, you should consider patellofemoral pain syndrome and prepatellar bursitis.