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.[1] They often requires an extensive rehabilitation. [4] In most of the cases flexion deformities occur bilateral.