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.

Approach

There are multiple possible approaches to imaging the knee with ultrasound. A typical overall protocol is as follows ¹:

Anterior knee

The knee is flexed 20-30° (flexion of the knee tightens the extensor tendons, decreasing the chance of anisotropy occurring in a lax tendon):

1. 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)
2. evaluate the suprapatellar and parapatellar joint recesses
   • suprapatellar fat pad
   • prefemoral fat pad
     • suprapatellar recess interdigitates between, distension representing likely effusion, most sensitive region
     • extent of the medial/lateral suprapatellar recess  should also be visualized
3. evaluate the femoral trochlea
   • best examined in full knee flexion
   • useful for examination of the trochlear cartilage
4. evaluate the patellar retinacula
5. evaluate the medial patellar articular facet (lateral facet not visible on ultrasound)
6. evaluate the patellar tendon and patellar bursae
   • tendon should be constant in size and echogenicity through its course without detectable color Doppler flow
     • anisotropy may falsely indicate a change in echogenicity
     • tendon may appear enlarged just prior to insertion
   • 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

Lateral knee

The knee is flexed 20-30°:

1. evaluate the distal iliotibial band in long axis (located between the anterior and middle third of the lateral knee)
2. evaluate the lateral collateral ligament in long axis
   • may detect para-articular ganglia
3. may see lateral meniscal pathology (e.g. meniscal cyst)
   • extreme knee flexion may bring out a meniscal abnormality

Medial knee

The knee is flexed 20-30°, with external rotation:

1. evaluate medial collateral ligament and pes anserinus tendons in long axis
   • valgus stress may be useful to examine the ligament

Posterior knee

Often examined with the patient prone and the knee extended, thereby gaining access to the dynamic fat-filled popliteal fossa:

1. evaluate the medial tendons in short axis (medial to lateral)
   • sartorius
   • gracilis
   • semitendinosus
2. moving even more medially, evaluate the semimembranosus-gastrocnemius bursa in short axis
   • a popliteal cyst (Baker's cyst) arises between these tendons
3. evaluate the popliteal neurovascular bundle and intercondylar fossa in short axis
4. evaluate the posterolateral corner and biceps femoris in short and long axis
5. evaluate the peroneal nerve
   • start with the common peroneal nerve branching off the sciatic nerve above the knee
     • typically found with one investing capsule
   • follow it around the fibular head

Pathology

A number of knee abnormalities can be identified on ultrasound, including:

• patellar tendinopathies
  • overuse patellar tendinitis
• extensor mechanism injuries
  • patellar tendon tear
  • quadriceps tendon tear
• prepatellar bursitis
• infrapatellar bursitis
• popliteal cyst (Baker cyst)