The menisci together convert the flat end of the tibia into a shallow socket. The mechanism involved in the locking of the knee is that a fragment of the torn meniscus can slip into a new position when the knee is flexed that it could not fit into when the knee is straight. The knee cannot then straighten because the fragment interferes with the straightening. If the fragment is small enough, or the invasion into the knee is minimal, it can be made to go back to its original position, or be reduced, more easily. The causes of a torn meniscus are numerous, including squatting and any number of athletic endeavors.
Torn menisci are common in plumbers and carpenters because of the amount of squatting that they do.
In most cases, the act of the tearing of the meniscus will be felt by the victim, not necessarily as pain but at least as something that draws one's attention to the knee. The claimant denies any incident of knee pain prior to the locking incident in 1990 on the Bombicino job. The claimant has been active in a number of sports throughout his life, including formal sports in high school and college, and informal activities since. The only occasion other than work on which he felt the knee lock occurred when he was teaching skiing. He attempted to lift up a young skier who had fallen and could not get up.