An expression for what happens when you are so worried that your body shakes uncontrollably. Of course, normal shaking is a normal response to everything from stress to caffeine to cold.

but knock kneed is more then that. It is where your legs are actually shaking so much that even when you attempt to stand up your knees knock against each other uncontrollably, rebounding and then crashing against each other again. This is not just something in Scooby Doo cartoons. I always believed it was an expression, myself, until I was fifteen and I got mugged.

The evidence in my life that I am an adult is that I have first hand knowledge of what both this term and "worried sick" refer to, literally.

Knock-kneed, valgus or genu valgum, is a medical condition where the knee joint is not aligned properly, and one or both knees turn inwards, sometimes so severely that they knock together. (The opposite condition is varus which means bow-legged, where the knees turn out.)

Valgus may be either pathological or physiological. Pathological causes include vitamin D deficiency (rickets), arthritis and bone tumours.

Many children show physiological valgus between the ages of three and five, but it usually self-corrects by the age of seven. If it has not corrected itself by eight years old medical intervention may be required because the condition may eventually cause problems in the feet and/or further damage to or instability of the knee joint. An awkward gait is inevitable, the children are clumsy and fall over a lot, and may develop other psychological problems which arise from being 'different'.

Treatment

If the condition is severe (15° to 20°) a leg brace may be fitted if the patient is showing no signs of spontaneous correction.

Sometimes surgery is necessary to correct loose knee ligaments. Another type operation which may be performed is the removal of triangular wedges of bone from the femur to make the leg straight.

http://www.ccmckids.org/departments/Orthopaedics/orthoed15.htm
http://www.orthoseek.com/articles/bowlegs-kk.html

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