Pediatric Fixed Knee Flexion Deformities Clinical Presentation

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