Locking of the knee is when it suddenly gets caught in a bent position, and it is impossible to straighten it without assistance.
Sudden locking of the knee - the person can bend the knee but not straighten it.
Causes of knee locking
Knee locking may be due to a loose body or a torn piece of meniscus catching in the joint, most often a 'bucket-handle' tear of the meniscus. A bucket-handle tear is a torn rim of the knee meniscus, which can flap over like the handle of a bucket, and catch intermittently in the joint. This causes intermittent sudeen pain and an inability to straighten the knee fully. Once the 'handle' flips back, the symptoms resolve, until the next episode.
An unstable tibio-fibular joint may also rarely be a cause of locking or pseudo-locking. Very rarely a locked knee may have a more sinister cause, such as a tumour obstructing the hamstrings.
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See also -
eBook - Knee Meniscus Injury
This ebook discusses the various types of meniscus injuries and their consequences.