Extension of the knee involves the quadriceps muscles, which are attached to the patella by the quadriceps tendon; the patella is connected to the tibial tubercle by the patellar tendon. Forced flexion at the knee with a contracted quadriceps muscle can damage these structures. Injuries include
In healthy people, significant force is required to injure these structures; normal tendons are strong enough that the patella often fractures transversely before a tendon tears. However, people with certain conditions are at risk of tendon tears. These conditions include
In these at-risk people, the injury can result from minor trauma (eg, when descending stairs). The quadriceps tendon is injured more often than the patellar tendon, particularly in older people.
Examination of the knee can suggest which structure is injured:
However, swelling in the area can be significant and mask these findings so that the injury may be misinterpreted as a ligamentous knee joint injury with hemarthrosis. If patients have knee swelling and pain after an injury, clinicians ask patients to sit and try to extend their injured leg to test active knee extension or to lie on their back and raise the injured leg, keeping the leg straight.
Routine knee x-rays are taken. Patella alta and patella baja can be seen on knee x-rays. X-rays often show displacement or fracture of the patella but may appear normal. MRI confirms the diagnosis.