Once in a while leg will give out no reason can't extend it i have to keep it bent feels like will crumble and break if i try to use no prior trauma?

Meniscal injury: Patients with untreated meniscal tears can present weeks after the injury complaining of popping, locking, catching, and the knee "giving out," or patients may simply report a vague sense that the knee is not moving properly. This feeling of instability is related to the proprioceptive misinformation that occurs when a meniscal fragment floats between the two articular surfaces.