Anatomy clinical correlates: Knee

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Anatomy clinical correlates: Knee

Prerequisite basic sciences

Prerequisite basic sciences


Anatomy clinical correlates: Knee

USMLE® Step 1 questions

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USMLE® Step 1 style questions USMLE

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A 47-year-old man presents to the emergency department with left lower extremity pain and swelling. The patient was outside jogging when he suddenly felt a “pop” with associated pain behind the left knee. The left lower extremity has become progressively more swollen and painful over the past few hours. The patient recently traveled to the United States from London. Past medical history is notable for hypertension. Vital signs are within normal limits. Bilateral lower extremity pulses are normal and equal. Physical examination reveals a swollen and tender calf and ecchymosis at the popliteal region. Anterior and posterior drawer tests are negative. An ultrasound of the left calf is shown below. Which of the following is the most likely diagnosis?
Image reproduced from Wikimedia Commons


The knee is one of the most complex joints in the human body, and along with the rest of the lower limb there are numerous ligamentous, muscular, and bony structures that are prone to injury. Oftentimes we can injure many of these structures at the same time, but injury to even one of these structures can affect how we walk, dance, or exercise. This video will go over all the relevant anatomy you ‘kneed’ to know in order to understand the clinical conditions affecting the knee.

First up, let’s discuss the Q angle - where “Q” stands for quadriceps. The Q angle is the angle measured between the femur and the tibia.

This angle is created by the femur’s diagonal placement within the thigh and by the tibia’s vertical placement in the leg. This angle is typically below 20 degrees and on average is higher in biologic females, and a normal Q angle allows the weight supported by the knee joint to be centered through the middle of the knee, in the knee’s intercondylar region.

When the Q angle increases over the normal range, it can lead to genu valgum or knock knees. With genu valgum, the increase in Q angle shifts the weight bearing center to the lateral compartment of the knee, which increases the quadriceps lateral pull and causes the medial collateral ligament to overstretch. This can cause joint misalignment and a predisposition for articular degeneration in the lateral compartment of the knee and subsequent gait abnormalities.

To remember the knock knee appearance of genu valgum, think of the ‘g’ as standing for ‘gum’ sticking the knees together!

Alternatively, when the Q angle is below normal range, a genu varum alignment can occur where the legs angulate away from the midline and create a bow legged appearance. This results in the weight bearing center being shifted through the medial compartment of the knee, stretching the lateral capsule lateral collateral ligament. So this time, there’s joint misalignment and a predisposition to articular degeneration in the medial compartment of the knee and subsequent gait abnormalities.


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