Ligament Tears
The knee joint relies on ligaments and surrounding muscles for stability. Any direct contact to the knee or hard muscle contraction, such as changing direction rapidly while running, can injure knee ligaments. A medial collateral ligament (MCL) tear causes pain on the inside of the knee. A lateral collateral ligament (LCL) tear causes pain on the outside of the knee. Ligament tears cause swelling over the injury site and instability.
Arthritis 
