There are 2 menisci (the medial meniscus and the lateral meniscus) in each knee that serves as cartilage cushions or shock absorbers (images courtesy of the University of Georgia Educational Resources Center).

The medial meniscus is commonly torn in dogs that have complete CCL rupture. The lateral meniscus is rarely, but sometimes, torn in dogs.  An important part of treating dogs with CCL rupture is assessing the menisci and making sure they are not torn. The best way (most likely to detect tearing and minimally invasive) to assess the menisci is with arthroscopy. If they are torn, the torn portion should be removed, or in very rare instances, repaired.

The following 3 photos and video show a normal medial meniscus.

When the meniscus tears the back portion is often unstable and displaces forward between the femur and the tibia. This can sometimes (rarely) be heard by owners as a clicking or popping sound, referred to as a meniscal click. The arthroscopic video below shows a medial meniscus that is torn and displaces forward repeatedly when the tibia shifts forward due to the CCL rupture. Note that I am returning the meniscus to its normal position only to show that it will repeatedly displace, pushing it back into place won’t fix this torn meniscus. This torn portion of meniscus needs to be removed or, in rare cases, repaired.
In many cases the torn meniscus become permanently wedged between the femur and the tibia as shown below. Immediately below is an arthroscopic image of a whole back half of a medial meniscus that is torn and displaced forward sitting between the femur and tibia, similar to in the video above. This is referred to as a peripheral detachment and it needs to be removed.

Following removal of the torn portion of meniscus the remaining meniscus can be seen (within the left side of the image to the right) and the femur (in the upper portion of the image to the right) can now comfortably rest on the tibia (lower portion of image below). Note that the cartilage in this dog’s knee is still in good health and removal of the meniscus does not equate to “bone on bone” contact.