Locking is when the knee suddenly gets caught in a bent position, and it is impossible to straighten it without assistance.
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. An unstable tibio-tibular 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.
Self-Reduction of Displaced Bucket-Handle Medial Meniscal Tear in a 71-Year-Old Patient. Ciminero ML, Huntley SR, Ghasem AD and Pitcher JD. Geriatr Orthop Surg Rehabil. 2015 Dec; 6(4): 334–337.
Knee locked in flexion: incarcerated semitendinosus tendon around a proximal tibial osteochondroma. Andrews K, Rowland A and Tank J. J Surg Case Rep. 2019 Feb; 2019(2): rjy346.