Ultrasound of the knee
Ultrasound of the knee allows high-resolution imaging of superficial knee anatomy while simultaneously allowing dynamic evaluation of some of the tendons and ligaments. Knee ultrasound is somewhat limited compared with ultrasound examinations of other joints because the cruciate ligaments and the entirety of the meniscus are usually difficult to visualize.
There are multiple possible approaches to imaging the knee with ultrasound. A typical overall protocol is as follows 1:
The knee is flexed 20-30 degrees (flexion of the knee tightens the extensor tendons, decreasing the chance of anisotropy occurring in a lax tendon):
- Transverse and longitudinal images of the quadriceps tendon from its myotendinous junctions to its attachment on the superior patella (rectus femoris myotendinous junction is more cranial than the vastus junctions).
- Evaluate the suprapatellar and parapatellar joint recesses.
- Evaluate the femoral trochlea
- best examined in full knee flexion
- useful for examination of the trochlear cartilage
- Evaluate the patellar retinacula
- Evaluate the medial patellar articular facet (lateral facet not visible on ultrasound)
- Evaluate the patellar tendon and patellar bursa
- prepatellar bursa normally not visible
- infrapatellar bursa
- small amount of fluid in the deep infrapatellar bursa is normal
- normally no fluid in the superficial infrapatellar bursa
The knee is flexed 20-30 degrees:
- Evaluate the distal iliotibial band in long axis (located between the anterior and middle third of the lateral knee).
- Evaluate the lateral collateral ligament in long axis.
- may detect para-articular ganglia
- May see lateral meniscal pathology (e.g. meniscal cyst)
- extreme knee flexion may bring out a meniscal abnormality
The knee is flexed 20-30 degrees, with external rotation:
- Evaluate medial collateral ligament and pes anserinus tendons in long axis
- valgus stress may be useful to examine the ligament
Often examined with the patient prone and the knee extended, thereby gaining access to the dynamic fat-filled popliteal fossa:
- Evaluate the medial tendons in short axis (medial to lateral):
- Moving even more medially, evaluate the semimembranosus-gastrocnemius bursa in short axis
- a popliteal cyst (Baker's cyst) arises between these tendons
- Evaluate the popliteal neurovascular bundle and intercondylar fossa in short axis.
- Evaluate the posterolateral corner and biceps femoris in short and long axis.
- Evaluate the peroneal nerve
- start with the common peroneal nerve branching off the sciatic nerve above the knee
- follow it around the fibular head
A number of knee abnormalities can be identified on ultrasound, including: