Pediatric Fixed Knee Flexion Deformities 

Updated: Jul 09, 2019
Author: Peter M Stevens, MD; Chief Editor: Jeffrey D Thomson, MD 



Children with neuromuscular disorders often manifest fixed knee flexion deformity (FKFD; see the image below) as a consequence of muscle weakness or imbalance. They tend to develop a crouch gait pattern that may prove refractory to physical therapy, bracing, or spasticity management (botulinum toxin/baclofen)[1] ; for those children, surgical management may be indicated.

The lateral radiograph best demonstrates the open The lateral radiograph best demonstrates the open physes and the stigmata of chronic fixed knee flexion deformity. This patient has avulsion "fractures" of the superior pole of the patella and of the tibial tubercle.

In addition to hamstring recession for contracture, fixed deformities may require posterior capsulotomy or supracondylar osteotomy. Alternatively, or in addition to these procedures, some surgeons have resorted to external frame distraction. A more recently developed approach is to utilize guided growth of the anterior distal femur to achieve safe and gradual correction of the crouch pattern.

There may eventually be other methods of temporary physeal restraint. The allure of biodegradable implants or remote-control techniques by electronic or radiofrequency methodology will undoubtedly spur further research.


Normal knee anatomy is characterized by the following:

  • Muscle balance: quadriceps versus hamstrings
  • Straight leg raise >60 º
  • Popliteal angle (from horizontal) >60 º
  • Sagittal plane: full extension
  • Straight line between femoral cortex and tibial cortex (see the image below)
  • Open physes
  • Patella location: between the Blumensat line and physis
  • Ground reaction force passes anterior to knee's center of rotation; knee locks passively in extension
  • Posterior capsule, gastrocnemius, and hamstrings resist recurvatum
Normal sagittal alignment permits the knee to lock Normal sagittal alignment permits the knee to lock in full extension, aided by powerful quadriceps and an intact extensor mechanism. The ground reaction force passes anterior to the "center of rotation" of the knee, while the PCL, posterior capsule, hamstrings, and gastrocnemius provide a tension band effect.

FKFD is characterized by the following:

  • Weak quadriceps relative to hamstrings
  • Straight leg raise < 50º
  • Fixed flexion deformity (whether the hip is flexed or extended; see the image below)
  • Sagittal plane: knee flexed
  • Ground reaction force passes posterior to the center of rotation of the knee (cannot lock in extension)
  • With or without posterior subluxation of tibia relative to femur
  • Open physes
  • Patella alta
  • Lateral tilt, subluxation/dislocation of patella
  • Patellar fragmentation
  • Tibial tuberosity avulsion
  • Frontal plane deformity (varus/valgus)
  • Rotational deformity
Fixed knee flexion deformity. The knee is chronica Fixed knee flexion deformity. The knee is chronically bent, obligating the patient to walk with a crouch gait. The ground reaction force passes posterior to the center of rotation of the knee, where it overcomes the resistance of the weakened extensor mechanism. Secondary effects, including patella alta and fragmentation, are relatively common and painful.

Ipsilateral regional deformity is characterized by the following:

  • Hip flexion contracture/deformity
  • Ankle equinus (true vs apparent) [2] or calcaneus (calf weakness)
  • Planovalgus foot (lever arm dysfunction)


Normal sagittal alignment includes the ability to lock the knee in full extension, stabilized posteriorly by the cruciate ligaments, posterior capsule, hamstrings, and gastrocnemius. This permits the child to bear full weight without pain, instability, or fatigue, because the ground reaction force is slightly anterior to the extended knee, allowing the child to lock the knee in extension during stance.

In cerebral palsy, spastic hip flexors and hamstrings combine to flex the knee, causing the ground reaction force to pass behind it and produce a flexion moment. With compromise of the hip extensors and quadriceps, gravity and fatigue force the child into a progressive crouch gait pattern.[3, 4, 5, 6] Knee pain is a frequent complaint, which may reflect fatigue of the quadriceps, tension failure of the patellar ligament or its bony attachments, or both. Ankle or hindfoot valgus will contribute to lever arm insufficiency and further decrease the extensor moment at the knee.

Children with spina bifida often have intrinsic weakness of the quadriceps, combined with sparing or (if tethered) spasticity of the hamstrings. This puts them at risk for the same problem of FKFD and progressive crouch gait.[6] Compounded by ankle valgus, and perhaps planovalgus of the foot, they too have lever arm dysfunction with loss of pushoff strength.

Children with arthrogryposis, pterygium syndrome, and other teratologic conditions may manifest fixed knee flexion at birth, with or without congenital lateral dislocation of the patella. They often present unique challenges for the surgeon, and treatment must be individualized accordingly. The deformity may be oblique plane, including valgus or, less commonly, varus of the knee. Concurrent dislocation or stiffness of the hip and clubfoot or vertical talus may have to be addressed as well.


A number of relatively common conditions, especially cerebral palsy and spina bifida, may lead to progressive FKFD, despite appropriate physical therapy and bracing. Congenital FKFD, with or without fixed lateral dislocation of the patella, may be evident on perinatal ultrasonography (US).


Knee flexion contractures due to hamstring contractures or quadriceps weakness are prevalent in growing children with neuromuscular conditions, such as cerebral palsy, spina bifida, and arthrogryposis. A subset of these patients will develop FKFD; the frequency of FKFD is not known. This condition is a component of bilateral, multilevel deformities that may literally be the rate-limiting step with respect to functional ambulation.


The natural history of FKFD is one of insidious progression despite bracing, therapy, and even repeated surgical intervention. The goal of treatment is to maintain standing and, it is to be hoped, walking ability while minimizing complications and encumbrance.

One must weigh the risk-to-benefit ratio of any invasive treatment, recognizing that it may need to be repeated. As with so many conditions, it is appealing to consider early intervention, before the onset of secondary problems such as patellar migration or fragmentation. With this in mind, repeated guided growth may be the pathway of least harm and maximum benefit.

In a clinical series of 18 patients with 29 cases of FKFD, the rate of correction was 1.74º per month; the highest rate noted was nearly 4º per month. Without the need for immobilization, these children have experienced rapid recovery with minimal setbacks from the surgical treatment.



History and Physical Examination

In the ambulatory patient, an obligatory crouch gait will be obvious, but it is not necessarily symmetrical. In the seated position, patella alta may be evident, along with reduced power of voluntary knee extension. The femoral condyles may be prominent with an empty sulcus, reflecting the proximal migration of the patella. There may be prominence and tenderness at either pole of the patella, over the tibial tuberosity, or both. Whereas there may be knee crepitance, an effusion is generally not present, because of the chronic nature of the problem.

In the supine position, a straight leg raise should be evaluated. If the degree of knee flexion increases as the hip is flexed (increased popliteal angle), then a concomitant hamstring contracture is likely. If there is no change in the popliteal angle with limb elevation (a bent leg raise), then fixed knee flexion deformity (FKFD) is the diagnosis.[7]

In the prone position, a torsional profile should be documented, as well as the inward-outward range, including hip rotation, and the thigh-foot axis. While the patient is prone, it is easy to look for dynamic versus fixed hip flexion deformity and rectus femoris contracture (Ely test). Also, with the hamstrings relaxed, one can recognize FKFD because the ankle/foot will not rest be resting on the table.

Fixed Knee Flexion Deformity vs Dynamic Contracture

In evaluating sagittal knee deformity, it is important to differentiate between dynamic contracture, due to tight hamstrings, and FKFD, which may or may not include tight hamstrings, depending on the etiology.

FKFD is often insidious and recalcitrant to nonoperative management. Except for teratologic etiologies such as arthrogryposis and pterygium syndrome, where deformity may develop in utero, FKFD due to cerebral palsy or spina bifida may not be problematic until the child reaches several years of age.[8, 9, 10, 11, 12, 13, 14]  Pain is a frequent problem, which may reflect patella alta and fragmentation of the patella, the tibial tubercle, or both.

The quadriceps is relatively weak compared with the overpowering (and sometimes spastic) hamstrings. Whereas hamstring stretching, bracing, and weakening with botulinum toxin or baclofen have been used for dynamic imbalance, these measures have not been shown to be adequate for addressing FKFD; there may be inexorable progression of the FKFD due to growth and gravity. (See the image below.)

For flexion contracture, spasticity management (on For flexion contracture, spasticity management (onabotulinomtoxinA/phenol/baclofen) or hamstring recession may offer some improvement, but these measures cannot overcome fixed knee flexion deformity.

Even surgical lengthening of the hamstrings will only increase the dynamic range of knee extension (and potentially weaken hip extension), without affecting the fixed flexion component. One may resort to posterior capsulotomy with or without posterior cruciate ligament release, but this poses unnecessary neurovascular risks and may destabilize the knee. (See the image below.)

A posterior capsulotomy with or without PCL releas A posterior capsulotomy with or without PCL release can address fixed knee flexion deformity, albeit with some risks, including neurovascular stretch injuries. Even with prolonged bracing following cast or frame removal, recurrent deformities are common.

Once the fixed deformity surpasses 20º, braces may not be tolerated, and the child is apt lose the ability to stand and ambulate, despite attempted bracing with ankle-foot orthoses (AFOs), floor reaction braces, or even knee-ankle-foot orthoses (KAFOs). (See the image below.)

Locking KAFO may support the patient for standing Locking KAFO may support the patient for standing but cannot adequately address fixed knee flexion deformity. When the deformity exceeds 20º, braces are poorly tolerated.

The problem may not be isolated to the knee; there often is concomitant hip flexion deformity that may be either fixed or dynamic, along with pseudoequinus of the ankle. To further compound matters, there may be frontal plane knee deformities (varus or valgus) that are difficult to appreciate preoperatively, as well as torsional deformities of the long bones.

Although supracondylar femoral osteotomy may render the limb straight, this produces an obligatory recurvatum of the distal femur and unsightly prominence of the patella. (See the image below.)

This child with arthrogryposis underwent unsuccess This child with arthrogryposis underwent unsuccessful posterior capsulotomies at age 3 years.

This will alter the arc of knee motion, sacrificing flexion to gain extension. There are associated fixation challenges and neurovascular risks, which are doubled in patients with bilateral involvement. As a result of continued growth, recurrent flexion deformity is all too common, necessitating repeat osteotomy or frame distraction to regain knee extension. The protracted recovery time and associated costs of serial osteotomies make this a daunting challenge for both patients and caregivers.



Laboratory Studies

There are no particular laboratory studies that are relevant to the correction of fixed knee flexion deformity (FKFD). The only exception may be preoperative pulmonary function screening for patients in whom general anesthesia is associated with high risk.

Imaging Studies

Patients with FKFD may not be able to cooperate for meaningful full-length weightbearing anteroposterior (AP) radiographs of the lower extremities. It should be noted that if the x-ray beam is nonorthogonal to the distal femoral physis, physeal closure may incorrectly be suspected.

A close-up lateral radiograph of the knees will demonstrate whether or not the physes are open. It will demonstrate the presence or absence of posterior subluxation of the tibia relative to the femur. This view will also reflect the relative position of the patella and occasional disruption (tension failure) of the extensor mechanism in the form of patellar and/or tibial tuberosity avulsion. (See the image below.)

The lateral radiograph best demonstrates the open The lateral radiograph best demonstrates the open physes and the stigmata of chronic fixed knee flexion deformity. This patient has avulsion "fractures" of the superior pole of the patella and of the tibial tubercle.

If there is doubt about the time remaining for growth, a hand (Greulich and Pyle) or elbow (Demiglio) radiograph may be useful in estimating skeletal maturity.

Advanced imaging such as arthrography, computed tomography (CT), or magnetic resonance imaging (MRI) is not likely to be of any benefit with respect to assessing FKFD.

Other Tests

If available, gait analysis may help document the effect of FKFD on the overall gait pattern.[15, 16, 17] Furthermore, concurrent and exacerbating sagittal plane deformities such as lumbar lordosis, pelvic tilt, hip flexion, and equinus or calcaneus may be identified. Many children need simultaneous or staged multilevel, and often bilateral, surgical intervention.[15, 18]


There may be an occasional need for consultation by a geneticist or neurologist, especially for those children with suspected syndromes. There may also be indications for electromyography (EMG), nerve conduction studies, or muscle biopsy. However, the majority of patients with FKFD have well-established and chronic conditions, such as cerebral palsy, spina bifida, or arthrogryposis (amyoplasia).

Histologic Findings

Generally, histologic studies are not relevant or necessary. The correction of FKFD is the same, with or without this information. An exception would be a muscle biopsy for the workup of suspected myopathy.


The degree of FKFD is often best measured with a goniometer. One should differentiate between dynamic contracture and fixed deformity because their treatments differ. These are not mutually exclusive; a patient may have one or both.

For dynamic contractures, the generic options include hamstring stretching, physical therapy, orthoses, spasticity management (Botox/baclofen), and hamstring recession (proximal or distal).

Fixed deformities greater than 10º should be monitored, but surgical intervention is unlikely. However as the deformity surpasses 10º, progression is likely with growth, and surgical intervention should be considered.



Approach Considerations

Indications for surgery

Children with teratologic fixed knee flexion deformity (FKFD) noted in the nursery are likely to undergo surgical treatment within the first 24 months of life. Depending on the need to correct hip and foot deformities, such treatment may be multilevel or simultaneous, or it may have to be staged.

Hamstring lengthening and posterior capsulotomy may constitute the initial treatment.[19, 20, 21, 22, 23]  As these children grow older and the deformity recurs, guided growth or supracondylar femoral osteotomy may be warranted. Patellar realignment may be necessary in some patients, and postoperative bracing and physical therapy may be indicated. The goal is to gain full extension and facilitate standing and walking.

Children with neurogenic FKFD will be considered together, with the acknowledgment that spasticity management for cerebral palsy and spinal cord detethering for spina bifida may play an important role.

For younger children, hamstring stretching, physical therapy, and bracing are useful temporizing measures. However, with growth, there may be inexorable progression of a crouch gait due to the insidious evolution of FKFD. When the fixed deformity surpasses 10º, most patients start to manifest symptoms such as fatigue, pain, or brace intolerance; these symptoms escalate when the deformity exceeds 20º. The energy costs of walking increase as endurance and velocity decrease.[15, 16]


The indications and techniques for supracondylar osteotomy of the femur have been well described in standard textbooks and journals. Opinions vary as to whether internal or external fixation is preferable. In addition to the known risks of fixation failure, overcorrection or undercorrection, fracture, neurovascular compromise, skin slough, and recurrent deformity are very common.

Frame distraction

There are specific situations in which the best option is gradual distraction and extension employing an external fixator.[24]  This may best serve those patients who have neglected or teratologic deformities, such as pterygium syndrome, or have reached skeletal maturity. Despite the touted advantages, it is a costly and slow process that usually requires a posterior release and that may not be well tolerated by the patient. Unfortunately, there is an acknowledged high rate of recurrent deformity despite postoperative bracing.

Guided growth

The distal femoral physis grows fairly rapidly, even in this population. Furthermore, skeletal maturity may be delayed, extending the window of opportunity for gradually straightening the knee by means of distal anterior femoral hemiepiphysiodesis. This may be safely undertaken even in relatively young children, without causing permanent growth arrest.[24, 25]  Staples were formerly preferred for this purpose; however, the rate of correction has been improved by using a pair eight-plates (Orthofix), one on each side of the patellofemoral sulcus. (See the image below.)

This ambulatory 15-year-old boy with arthrogryposi This ambulatory 15-year-old boy with arthrogryposis has never had an osteotomy; he was managed with stapling, followed by eight-plates, when he developed a recurrence. He has full extension on the right and 7º residual fixed knee flexion deformity on the left. The eight-plates are still in situ (on the left) pending further growth.

Contraindications for surgery

There are few contraindications for surgical correction of FKFD.

Contraindications for osteotomy include the following:

  • Nonambulatory status
  • Stiff or unstable knee
  • Severe osteopenia (fixation issues)
  • Flaccid paraplegia

Contraindications for guided growth include the following:

  • Closed physes
  • Stiff or unstable knee

Medical Therapy

Medical therapy for FKFD usually consists of onabotulinumtoxinA injections in the hamstrings or baclofen administered orally or through an intrathecal pump (for cerebral palsy).[26] This is only useful for the dynamic component of crouch gait; it may be an adjunct to osteotomy or guided growth. The patient is often working with a physical therapist on hamstring stretching, quadriceps strengthening, and gait training. This approach is suitable for younger patients, but after the age of 10 years, deformities are likely to progress despite concerted efforts to the contrary.

Surgical Therapy

Posterior capsulotomy/hamstring recession

This relatively invasive soft-tissue procedure poses some risks to the posterior neurovascular structures and requires immobilization with braces, casts, or frames. (See the image below.)

For flexion contracture, spasticity management (on For flexion contracture, spasticity management (onabotulinomtoxinA/phenol/baclofen) or hamstring recession may offer some improvement, but these measures cannot overcome fixed knee flexion deformity.


Supracondylar extension osteotomy of the femora has a long track record and is the default approach for many surgeons. Unfortunately, there are associated drawbacks, not the least of which is recurrence with growth, thus mitigating the temporary benefit of this maximally invasive treatment. (See the images below.) The varied techniques, tricks, results, and complications have been well described in standard textbooks and journals.

Starting at age 4 years, this patient subsequently Starting at age 4 years, this patient subsequently underwent bilateral extension osteotomies 4 times, with recurrence each time as expected. Perhaps this sequence could have been abbreviated with guided growth, which, even if repeated, requires no casts or delay in weight bearing.
Part 1 of 5. This 5-year-old boy presented with a Part 1 of 5. This 5-year-old boy presented with a congenital knee flexion deformity. His only prior surgery was a Symes disarticulation for fibular absence and a rigid teratologic foot deformity. He was ambulatory in a prosthesis.
Part 2 of 5. This patient underwent a supracondyla Part 2 of 5. This patient underwent a supracondylar extension osteotomy of the femur.
Part 3 of 5. Because of a relatively rapid recurre Part 3 of 5. Because of a relatively rapid recurrence of fixed knee flexion deformity, this patient underwent anterior stapling of the femur; unfortunately, the staples migrated, but the physis is still open.
Part 4 of 5. The staples in this patient were retr Part 4 of 5. The staples in this patient were retrieved and replaced with a pair of eight-plates.
Part 5 of 5. If we could turn back the clock, perh Part 5 of 5. If we could turn back the clock, perhaps guided growth would have been sufficient to correct the problem in this patient, without an osteotomy or cast. The effective gain in limb length would occur gradually, without risk to the neurovascular structures.

Frame distraction

With or without soft-tissue release, some authors favor frame distraction as a means of gradual correction of FKFD. However, the bilateral nature of these problems makes this method relatively expensive and unwieldy. Furthermore, even with protracted bracing, recurrence is relatively common.

This girl born with a teratologic knee flexion def This girl born with a teratologic knee flexion deformity and absent quadriceps had previous posterior capsulotomy, supracondylar osteotomy, and attempted stapling. Subsequently, she had a spatial frame applied to gradually extend the ankylosed knee; however, she fell and sustained a Salter I fracture of the proximal tibia.

Guided growth

Guided growth is a newer approach that consists of anterior hemiepiphysiodesis of the distal femora.[27] Staples were originally used, but it became apparent that some children were relatively small for the Blount staples.[25] These rigid devices would occasionally migrate or permit relatively slow correction.

Using a pair of eight-plates as an alternative has resolved the problem of hardware migration and enabled more rapid correction. The titanium plates, though intracapsular, are nonarticular, being placed medial and lateral to the patellofemoral sulcus. Thus, they are well tolerated, even by young children. (See the images below.)

Guided growth permits one to address the fixed kne Guided growth permits one to address the fixed knee flexion deformity at or close to the level of the CORA (center of rotational axis of deformity). This is efficient and prevents the need for translocation, such as is required in an osteotomy. The gradual correction poses no risk to the neurovascular structures.
With the C-arm in the lateral, horizontal position With the C-arm in the lateral, horizontal position, the physis is localized. A Keith needle is placed in the physis, and two 1.6-mm guide pins are inserted: one medial and one lateral to the sulcus. The cannulated 4.5-mm screws are then inserted. They need not be parallel, but they should not transgress the physis, joint, or posterior cortex.
For fixed knee flexion deformity, an eight-plate i For fixed knee flexion deformity, an eight-plate is placed on either side of the patellofemoral sulcus, through a small arthrotomy. Though intracapsular, the plates are nonarticular; synovitis has not been observed.
The efficacy of floor reaction braces is compromis The efficacy of floor reaction braces is compromised in the presence of fixed knee flexion deformity. However, they may be continued following guided growth, pending correction, whereupon bracing may be unnecessary, provided the quadriceps are sufficiently strong.

If one elects to employ guided growth, it is important to ascertain whether the distal femoral physes are open and whether there is, ideally, 12 months or more of predicted growth remaining. The decision to undertake concomitant multilevel reconstructive procedures, including hamstring recession for dynamic contracture, is left to the discretion of the surgeon.

Note, however, that as FKFD gradually corrects, there may be beneficial effects upon the hip, spine, and ankle. Therefore, it may be wise to await full knee extension and address residual deformities at the time of eight-plate removal.

The key aspects of the procedure may be summarized as follows:

  • Supine position with knee flexed on bolster
  • Tourniquet control
  • Image intensifier: parked in cross-table, horizontal position
  • Two incisions (3 cm long), medial and lateral to the patella
  • Open capsule and synovium
  • Keith or similar needle (sequentially) placed into the anteromedial and anterolateral physis
  • Application of an eight-plate (eg, Orthofix), usually 16 mm
  • Introduction of 1.6 guide pins, first epiphyseal and then metaphyseal - Pins need not be parallel but should avoid the physis and joint
  • Predrilling of cortex (to a depth of 5 mm)
  • Insertion of the 4.5-mm titanium, self-tapping, cannulated screws (24 or 32 mm in size)
  • Soft dressing

Postoperative Care

Typically, treatment is provided either on an outpatient basis or with an overnight stay. Immediate range of motion and weightbearing are encouraged. Bracing and physical therapy are resumed as indicated.

One study found that in children with cerebral palsy, the pain pump is effective in postoperative pain management after lower extremity orthopedic procedures.[28]


Complications of posterior capsulotomy include the following:

  • Neurovascular damage
  • Undercorrection or overcorrection (posterior cruciate ligament release)
  • Recurrent deformity

Complications of supracondylar osteotomy include the following:

  • Neurovascular damage
  • Loss of fixation
  • Undercorrection or overcorrection - 2º varus or valgus
  • Pathologic fracture
  • Recurrent deformity

Complications of frame distraction include the following:

  • Pin tract problems
  • Infection
  • Recurrent deformity

Complications of guided growth include the following:

  • Undercorrection, if not enough growth remains
  • Overcorrection, if the patient is lost to follow-up
  • Recurrent deformity
  • Premature growth arrest will not occur if the periosteum is protected

When compared to the other options, guided growth has far fewer risks and complications and is more cost-effective. The procedure is well tolerated, and the recovery is rapid. It may be repeated as necessary and is readily combined with other procedures as indicated.

Long-Term Monitoring

The patient should be seen at 3-month intervals to measure FKFD and assess gait. Functional limb length and stride length will improve as the knee straightens.

Remove plates if and when the knee is fully extended (avoid recurvatum). Reinsert plates as needed if FKFD recurs with growth.

Follow the patient until skeletal maturity isn reached.