Why idoes my kneecap keep subluxing? when i stand and try to straighten my left leg my knee gets painfully stuck that i cant bend or straighten. i am talking excruciating pain. what could be wrong?

You have a problem with muscles/tendons keeping your knee cap in place. There are several that do this. Only a careful exam can give a clue to the problem. Show to your Dr.

This suggests that the ligaments and tendons attaching to that knee are weak/lose and compromised by Degeneration ( wear and tear) of the surrounding bones aka Osteoarthritis. An Orthopedic Consult is indicated for appropriate management.

This can be your patella (knee cap) subluxating or dislocating. It is common and can be helped with bracing, physical therapy, or surgery. It is also possible to get catching/locking in your knee from a meniscus tear or arthritis. Seen an orthopedic surgeon.