Popping/ cracking joint is very common. If there is no inflammation or pain, it is considered to be physiological. There are many possible causes/hypotheses for physiologic popping, but the main causes are as follows: (1) The distance between adjacent bones increases (e.g., pulling on the knee joint) and a new air bubble forms in the joint cavity and ruptures. A schematic diagram is shown below. Source: http://1.bp.blogspot.com/ (2) Under normal conditions, the joint cavity maintains negative pressure, and the abundant synovial folds (synovium) create small localized cavities. When the joint moves, some of the negative pressure cavities disappear, resulting in a sound. Note the red synovial folds shown in the figure below; source: The Asahi Shimbun (3) The ligament changes its shape and course during sudden and violent flexion and extension movements, resulting in a sound [2]. Note the change in the course of the periprosthetic tendon and its relative relationship to the bone when the knee is flexed and extended in the figure below; source: ACLSolutions- ACL injury, knee pain and ACL knee surgery explained. (4) The joint cavity is rich in joint fluid, so when the ligament is in contact with the bone and the bone, synovial adhesions form between the two. (4) The joint cavity is rich in joint fluid, so when the ligament is in contact with the bone and the bone is in contact with the bone, synovial adhesions (Adhesion) are formed between the two. When the joint is moved, this adhesion disappears and a sound is produced.