A flexion deformity of the knee is the inability to fully straighten the knee. A synonym for it is flexion contracture. Normal active range of motion of the knee is 0° extension and 140° flexion. In people with a flexion deformity one of them or both are reduced. It develops as a result of failure of knee flexors to lengthen in tandem with the bone, especially when there is inadequate physical therapy to provide active and passive mobilization of the affected joint. They often requires an extensive rehabilitation.  In most of the cases flexion deformities occur bilateral.