When testing for a torn meniscus, a C- or horseshoe-shaped piece of cartilage in the knee, the McMurray test is used most often to diagnose posterior (along the back of the knee joint) meniscal tears. This is most likely the test your surgeon used on you.
There are two McMurray tests: the standard or conventional McMurray and the paradoxical McMurray. Both tests are performed in the same way. The knee is bent; the examiner holds the heel and twists the lower leg in and out. If there is a torn or loose meniscus, there will be a painful, snapping or clicking sensation as the movement shifts or traps the meniscal flap or fragment between the tibia (lower leg bone) and the femur (thigh bone).
The results are what make the test conventional or paradoxical. For example, the test is considered “paradoxical” when the leg is rotated internally (inward) and the medial side of the knee (side closest to the other leg) clicks. There can also be a paradoxical McMurray when the leg is rotated externally (outward) and the clicking occurs in the lateral compartment (or side) of the knee. Lateral refers to the side of the knee away from the other knee.
The results just described are considered paradoxical, meaning the opposite of what is expected. In the conventional test, the clicking occurs on the opposite side of the knee from the rotation. When the leg is rotated inward, the clicking occurs on the outside of the knee. When the knee is rotated outward, the clicking occurs on the inside of the knee.
When there is a positive McMurray test (conventional or paradoxical), it is an indication that there is a torn fragment or loose piece of meniscus. The clicking sound can occur when the meniscal piece flaps over on itself and gets stuck between the two bones. The same clicking can also occur when a torn meniscus is flapped over on itself and the test movement smoothes it back out where it belongs.
This second scenario is probably what happened to you during the test. The torn meniscus is now in place but it is likely not stable. A sudden or unexpected movement can cause the flap to come up and rub between the bones, possibly even getting torn even more. An arthroscopic exam can help confirm the diagnosis.
If there is any indication that the area is healing on its own (unlikely given what we know about this type of cartilage injury) the surgeon can leave it alone. But if the surgeon is able to see a badly frayed or damaged meniscus, she can repair it right then and protect the joint from further injury.