If a child cannot straighten their knee, do not keep pushing it to see whether it will unlock. First establish how the problem began: after a fall, during sport or without an obvious injury. Pain, swelling and the ability to put weight on the leg also matter.
Holding the knee bent is a symptom, not a diagnosis. The child may be protecting a painful joint, or something may be obstructing movement. It is not always possible to tell the difference at home.
When not to wait for a routine appointment
An urgent assessment is needed if the child can no longer walk or bear weight, if the knee suddenly remains locked after an injury, or if pain and swelling are substantial. A hot, swollen knee, particularly with fever or general illness, should be assessed without delay: joint infection is one of the conditions that must be excluded. The absence of fever alone does not rule it out. [1–3]
After an injury, being unable to lift the leg while keeping the knee straight also needs prompt assessment. The problem may involve the structures that actively straighten the knee, rather than something catching inside the joint. [1]
A milder problem still needs attention if it persists, keeps returning or changes the way the child walks. There is no need to wait for severe pain.
Is pain stopping the movement, or is the knee mechanically locked?
After a twisting injury, swelling and muscle spasm can make even a simple movement difficult. The child protects the knee by keeping it slightly bent.
Mechanical locking is different: something inside the joint obstructs movement. A meniscal tear or a loose fragment can cause this. Certain fractures around the knee can also prevent full extension. The angle at which the child holds the leg does not identify the cause. [1]
Forcing the knee or repeatedly trying to produce a click does not establish a diagnosis and may increase pain. The distinction requires examination, sometimes with further investigations.
Discoid meniscus: one possibility, not an automatic diagnosis
Some children are born with a lateral meniscus that is broader than usual. This is called a discoid meniscus. Many have no symptoms. Others may develop clicking, swelling, pain or episodes when the knee will not fully straighten. [4,5]
A click alone does not mean that the meniscus is torn or that surgery is needed. Finding a discoid meniscus on MRI must also be interpreted alongside symptoms and examination. An unusual meniscal shape that causes no problems is not, by itself, a reason to operate. [4,5]
Treatment, when it is needed, is discussed separately in the article on discoid meniscus and meniscal preservation. The question here comes earlier: why has the knee lost movement? Not every joint noise comes from a meniscal problem.
What if there has been no injury?
No history of injury does not make the symptom unimportant. Joint inflammation can cause swelling and make straightening difficult. Morning stiffness or persistent swelling should be discussed with the child's paediatrician. Severe pain, warmth, fever or inability to walk require urgent assessment. [2,3]
The examination should not stop at the knee. In a limping child, pain felt around the knee can sometimes come from the hip, which is why neighbouring joints also need to be examined. [2]
Which investigations might be needed?
Examination establishes when the problem began, identifies swelling and tenderness, and assesses movement without forcing it. After an injury, X-rays may be needed to assess the bones, but they do not directly show every possible meniscal injury. MRI is requested when the clinical question concerns internal structures or symptoms suggest an injury that needs further investigation. [1,4]
There is no need to arrange an MRI before every appointment. Equally, a normal X-ray does not settle the issue if the knee remains locked or the child still cannot bear weight. [1,2]
While waiting to be assessed, avoid sport and attempts to force the leg straight. Allowing a comfortable position is not the same as choosing an immobilisation method at home: any support needs to suit the underlying problem.
If my child can walk, can I stop worrying?
Being able to walk is useful information, but it does not explain a persistent loss of extension. If normal movement does not return or the problem recurs, an assessment is needed even without marked pain.
Does a locked knee always need surgery?
No. The cause needs to be identified first. Pain-limited movement and an injury physically obstructing the joint are different problems. Observation, physiotherapy or surgery cannot be chosen from the symptom alone.
Author: Dr Daniele Priano, paediatric orthopaedic surgeon.
This information does not replace an individual medical assessment. For sudden locking, inability to walk or possible signs of infection, do not wait for an online reply.
Sources
- Royal Children's Hospital Melbourne. Acute Knee Injuries — Emergency Department.
- Royal Children's Hospital Melbourne. The limping or non-weight bearing child.
- Royal Children's Hospital Melbourne. The acutely swollen joint.
- American Academy of Orthopaedic Surgeons, OrthoInfo. Discoid Meniscus.
- Pediatric Orthopaedic Society of North America, OrthoKids. Discoid Meniscus.
