(See also Overview of Sprains and Other Soft-Tissue Injuries Overview of Sprains and Other Soft-Tissue Injuries Sprains are tears in ligaments; strains are tears in muscles. Tears (ruptures) may also occur in tendons. In addition to sprains, strains, and tendon injuries, musculoskeletal injuries include... read more .)
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
Quadriceps tendon tears
Patellar tendon tears
Tibial tubercle fractures
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
Use of certain medications (eg, fluoroquinolones, corticosteroids)
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.
Symptoms and Signs of Knee Extensor Mechanism Injuries
The affected area is painful and swollen.
Patients with complete tendon tears cannot stand, do a straight leg raise while lying on their back, or extend their knee while seated.
Long-term complications (eg, loss of motion, weakness) are common.
Diagnosis of Knee Extensor Mechanism Injuries
Examination of the knee can suggest which structure is injured:
Quadriceps tendon tear: The patella is palpably displaced inferiorly (patella baja).
Patella tendon tear: The patella is displaced superiorly (patella alta).
Transverse patellar fracture: There is often a palpable gap between the two bone fragments.
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.
Pearls & Pitfalls
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.
Treatment of Knee Extensor Mechanism Injuries
Treatment of knee extensor mechanism injuries is surgical repair.