Knee extensor mechanism is composed of the quadriceps and its tendon, patella and patellar tendon. Rupture of either the quadriceps or patellar tendon, or of the patella itself, lead to a disruption of the knee extensor mechanism. Clinical examination reveals an inability to actively extend the knee. Standard radiographs of the knee show a suprapatellar swelling of the soft tissues in case of quadriceps tendon rupture, or a displaced patellar fracture, or a patella alta in case of patellar tendon rupture. Echography and MRI confirm the diagnosis and may reveal associated injuries. In case of knee extensor mechanism rupture, early surgical reconstruction of quadriceps or patellar tendon, or patella osteosynthesis, are mandatory to achieve early functional recovery.