The patella is the circular bone in front of the knee that is part of the quadriceps muscle. It connects the quadriceps muscle to the tibia by the patellar tendon. Its important for transferring force during jumping, running, stair climbing and kicking. It needs to slide smoothly over the front of the femur.
Patellar dislocations occur when the patella slides incorrectly across the front of the femur. When the knee is flexed from a straight position, instead of the patella normally sliding straight down the front of the femur, the patella slides the wrong way down the lateral (outer big toe side) of the femur. This is usually VERY painful, and often when a person dislocates his or her patella for the first time, they often need emergency or medical staff to help straighten the knee to let the patella drop back into a normal location.
Many patients can dislocate their patella as early as in their teenage years, and even though initial treatment is usually with physiotherapy, at least 50% of patients will get repeated dislocations in the future. If the repeat dislocations and "patellar instability" is frequent or severe, sometimes surgery is needed. Depending on the severity of the knee instability, sometimes a ligament reconstruction operation - so called "medial patellofemoral ligament (MPFL)" reconstruction can be performed. In more severe cases, sometimes a bone transfer operation "tibial tubercle osteotomy" to re-align the attachment of the patellar tendon on the tibia is needed.