Locking means that the knee will not straighten or bend fully. Loss of straightening is more common.
It can be ‘acute‘ and happen after a specific injury
‘chronic‘ where it recurs from time to time.
This usually means something is being caught or jammed in the knee. This can be a bucket handle tear of the meniscus, medial plica, loose body, cartilage flap or synovial thickening.
A posterior vertical tear of the lateral meniscus can cause the knee to lock in full extension (fully straight).
If you have advanced arthritis in your knee when the opposing bone surfaces rub on each other they can cause pain, catching or locking.
A detailed history physical examination, x-ray and/or MRI scan almost always results in an accurate diagnosis. Repeated catching and locking usually damage the joint surfaces and should be treated.
Almost all causes of locking require treatment with an arthroscopy.