Does a kneecap that shifts off the track injure your lateral tendon causing it to tear?

Patellar dislocation: You are describing a patellar (or kneecap) dislocation. It is relatively common. If you can extend your leg easily at the knee joint there is likely no major tendon tear. The stabilizing knee ligaments are not typically injured with this type of injury. However, see an orthopedist if dislocations are frequent or you have knee instability; go to ER if your leg is pale or dusky or very painful.