Patellar (knee cap) instability results from one or more dislocations or partial dislocations (subluxations). Patella is the small piece of bone in front of the knee that slides up and down the femoral groove (groove in the femur bone) during bending and stretching movements. The ligaments on the inner and outer sides of patella hold it in the femoral groove and avoid dislocation of patella from the groove.
Any damage to these ligaments may cause patella to slip out of the groove either partially (subluxation) or completely (dislocation). This misalignment can damage the underlying soft structures such as muscles and ligaments that hold the knee cap in place. Once damaged, these soft structures are unable to keep the patella (knee cap) in position. Repeated subluxation or dislocation makes the knee unstable and the condition is called as knee instability.