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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.
Flexion deformities can arise by different causes. Two types of flexion contracture of the knee can be distinguished: that associated with joint destruction and ankylosis and that in which joint anatomy and mobility are preserved. They can be caused by a piece of bone or other tissue getting in the way and blocking movement, knee replacement components in the wrong position, scar tissue restricting that cannot further straightening an extreme tightness in the hamstring muscles. Flexion deformities are common complications following trauma, inflammatory conditions, immobilization, burns and congenital deformities. 
A few examples in which flexion contractures of the knee can occur are: burn scars, intra-articular fractures, septic arthritis, juvenile rheumatoid arthritis, cerebral palsy and many others. 
Patients with flexion contractures often walk with a bent-knee gait. This provides increasing strain on the quadriceps and increasing strain contact forces in the patellofemoral joint. Walking distance is reduced and increased strain during bent-knee gait may lead to quadriceps weakness and earlier onset of the quadriceps fatigue.  Other symptoms of flexion contractures are anterior knee pain, a progressive crouch gait and limping while walking. Often they will make compensatory movements and a hip flexion deformity can occur accompanied by lumbar lordosis. 
Short-term and long-term changes can be distinguished. Early changes are shortening of stride gait, reduced popliteal angle and a flexed position of the knee at the initiation of the stand phase and throughout gait cycle. Changes which appear later are severe contracture of knee and hip and patella alta.
Knee flexion contractures have a lot of functional consequences such as weight-bearing activities and difficulties with bed or chair positioning.  Normal daily activities provide difficulties because they need more energy.
Physical therapy management
Depending of etiology and severity of the deformity a different therapy management is necessary. Treatment of knee flexion contractures include non-surgical and surgical methods.  In both cases physiotherapy is necessary.
Conservative treatments include physical therapy, home exercise programs, and home mechanical therapy. They are used to treat and minimize the occurrence of flexion contractures. In some cases, such as cerebral palsy, spasticity management is also necessary. 
Physical therapy consists of manual stretching exercise, prolonged stretch trough strapping on a tilt, sandbag weight over the distal femur, use of mechanical traction, passive range of motion exercise  and joint mobilization  The effectiveness of a given treatment to reduce flexion contractures is a function of the applied torque, as well as the duration and frequency of the treatment. 
For patients who have failed standard conservative treatment for 2 or more months, focused treatment protocols including physical therapy and the use of custom knee devices has been demonstrated to effectively treat flexion contractures. 
Other treatment methods are an orthesis, casting and bracing.
Some types of splits have been marketed as another method of applying low stretching forces over prolonged periods. They provide a resistance to flexion, so the knee is at rest in maximum extension. The resistance can be inflated. They are easy to apply and mobile and comfortable for the patients. 
In most of the cases splints and orthesis are used to prevent deformities or maintain range of motion after stretching but not for increasing motion. 
In the more severe cases surgical treatment such as soft-tissue release, osteotomies (removing a part of the bone), femoral shortening, hamstring lengthening an rectus transfer can be necessary.  Hamstring lengthenining is helpful to relieve excessive contractures, especially when they have a significant effect on gait. Rectus transfer may be indicated to partially reduce the spasticity of the quadriceps, especially in patients with cerebral palsy. 
- ↑ 1.0 1.1 Kwan MK, Treatment for flexion contracture of the knee during Ilizarov reconstruction of tibia with passive knee extension splint, 2004;59:39-41 (C)
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 Suksathien R., A new static progressive splint for treatment of knee and elbow flexion contractures, 2010; 93 (7): 799-804 (C)
- ↑ 3.0 3.1 3.2 damsin JP, TREATMENT OF SEVERE FLEXION DEFORMITY OF THE KNEE IN CHILDREN AND ADOLESCENTS USING THE ILIZAROV TECHNIQUE, 1995;77-B:140-4 (C)
- ↑ 4.0 4.1 4.2 4.3 4.4 Klatt J. ,Guided Growth for Fixed Knee Flexion Deformity, 2008;28:626-631 (C)
- ↑ 5.0 5.1 5.2 5.3 Timothy L., Torque Measures of Common Therapies for the Treatment of Flexion Contractures 2010; 26:328-334 (D)
- ↑ 6.0 6.1 6.2 Wheeless' Textbook of Orthopaedics (secondary)
- ↑ 7.0 7.1 steffen T., Low-Load, Prolonged Stretch in the Treatment of Knee Flexion Contractures in Nursing Home Residents, 1995; 75886-897.1 (A2)
- ↑ Mauro C, Treatment of fixed knee flexion deformity and crouch gait using distal femur extension osteotomy in cerebral palsy, 2008, 2(1): 37–43 (C)