b Common causes of the injured knee Patellar dislocation occurs as the quadriceps muscles pull the patellar tendon to extend the knee, especially when the knee has a valgus stress or the extremity is suddenly abducted when the leg slips laterally If the vastus medialis fibers not keep the patella in the intercondylar groove, the patella may dislocate laterally This injury rarely occurs from direct force but more commonly from a noncontact twisting injury The patient may feel a popping sensation with intense pain and present with the knee flexed The patella will be displaced laterally, and the diagnosis is usually made based on history and examination The dislocation may be reduced before radiographs are taken Postreduction radiographs should be obtained to rule out an associated avulsion or osteochondral fracture of the patella Patellar dislocations are often recurrent