Knee conditions can develop through excessive overuse, everyday wear and tear, or injury. There are varying degrees of knee conditions, and the most common conditions occur in athletes, or individuals who are very active. The knee is the largest joint in the body, and connects the upper leg bone (femur) and the shine bones (tibia and fibula). Ligaments, tendons and muscles, connect the knee joint. There are four main ligaments in the knee joint: anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), lateral collateral ligament (LCL), and the medial collateral ligament (MCL). The most common injury in the knee is to the ACL.