Pediatric Fixed Knee Flexion Deformities

Updated: Dec 02, 2022
  • Author: Peter M Stevens, MD; Chief Editor: Jeffrey D Thomson, MD  more...
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Practice Essentials

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.

Lateral radiograph best demonstrates open physes a Lateral radiograph best demonstrates open physes and stigmata of chronic fixed knee flexion deformity. This patient has avulsion "fractures" of superior pole of patella and of 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. [2, 3]

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 >60º
  • Popliteal angle (from horizontal) >60º
  • 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 knee to lock in Normal sagittal alignment permits knee to lock in full extension, aided by powerful quadriceps and intact extensor mechanism. Ground reaction force passes anterior to "center of rotation" of knee, while PCL, posterior capsule, hamstrings, and gastrocnemius provide 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. Knee is chronically Fixed knee flexion deformity. Knee is chronically bent, obligating patient to walk with crouch gait. Ground reaction force passes posterior to center of rotation of knee, where it overcomes resistance of 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) [4] 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. [5, 6, 7, 8] 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. [8] 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/mo; the highest rate noted was nearly 4º/mo. Without the need for immobilization, these children have experienced rapid recovery with minimal setbacks from the surgical treatment.