There are several common causes of sudden pain in the knee joint: 1. Acute injuries to the knee joint as well as acute attacks of chronic strain and inflammation. The knee joint has a large mobility and range of motion as well as weight bearing, and is often subject to acute injuries such as bone damage, ligament strains, joint capsule lacerations, cruciate ligament injuries, and meniscal injuries, all of which can result in sudden pain in the knee joint. 2. Tumors of the knee joint, whether distal femoral or proximal tibial, generally have no obvious symptoms when the tumor is small or does not produce compression. Once the tumor grows larger or compresses the adjacent tissues, sudden pain will appear, especially if the tumor causes pathological fracture, the pain is very sudden.