Suprapatellar recess (SPR). (Reprinted with permission from Philip Peng Educational Series)
The ultrasound probe is placed in long axis to the femur just on the superior border of the patella (Fig. 23.2). This will reveal the SPR (∗∗). When there is difficulty in visualizing the recess, applying pressure in the parapatellar space to squeeze the synovial fluid to the SPR may help when there is no large effusion.
Once the SPR is seen, the ultrasound probe is rotated 90° above the patella (Fig. 23.3). A 20- or 22-gauge needle is inserted from lateral to medial in-plane toward the SPR. Alternatively, the ultrasound probe is rotated 45 degrees with the cephalad end directed to the lateral side (Fig. 23.4). The rotation of the probe is to avoid needle trauma to the quadriceps tendon. The red square indicates the probe position.
Needle: 1.5-inch 25G or 3-inch 22G needle
Drugs: 5 mL of mixture of steroid and local anesthetic or viscosupplement or platelet-rich supplement
Before injection, it is always advisable to aspirate the joint fluid as much as possible. In-plane technique is used with the needle inserted from lateral to medial (Fig. 23.5).
Ultrasound improves the accuracy of knee intra-articular injection. The pool data from the literature suggested that the accuracy of landmark-guided injection is approximately 79% and is inferior to the ultrasound-guided injection even in experienced hand. Injection into the fat pad accounts for most of the inaccuracies from the landmark-guided technique.
A lot of studies were published on the efficacy of intra-articular (IA) steroid, viscosupplement, and platelet-rich plasma. In general, the duration of IA steroid is short (<3 weeks), but repeated injection recently showed acceleration of cartilage loss. Both viscosupplement and platelet-rich plasma result in longer duration of effect and in general are not very effective in advanced osteoarthritis (see Table 23.1).
Comparison of various injectate for knee
Supplementation of synovial fluid
Restore joint hemostasis
Accelerate cartilage loss
Less effect for end-stage OA
Less effect for end-stage OA
Reprinted with permission from Philip Peng Educational Series
Ultrasound-Guided Popliteal (Baker’s) Cyst Aspiration, Fenestration, and Injection
Popliteal cyst or Baker’s cyst is commonly located in the posteromedial aspect of the popliteal fossa. Technically, it is a nonmalignant, fluid-filled swelling formed by distention of the semimembranosus-gastrocnemius bursa.
A Baker’s cyst can be classified as a primary cyst if the distended semimembranosus-gastrocnemius bursa arises independently without communication to the knee joint or a secondary cyst if there is an open communication between the bursa and the knee joint cavity (Fig. 23.6). Whereas most of Baker’s cysts are secondary cysts and associated with degenerative knee joint diseases, primary cysts are less common and occur primarily in children.
Probe: Linea 6–15 MHz
Scan 1: Normal Knee
Palpate the semitendinosus (ST) tendon by slightly flex the knee. Put the ultrasound probe over the ST tendon and a “cherry on the cake” appearance with the ST tendon (arrow head) as the cherry and semimembranosus (SM) as the cake (Fig. 23.7). Deep to the SM, medial condyle (MC) is seen on the medial side and medial head of the gastrocnemius (GH) on the lateral side. The semimembranosus-gastrocnemius bursa (outlined by dotted line) appears between the tendon of the GH (∗) and SM. Because of lack of fluid in normal state, one should apply very light pressure to the ultrasound probe to reveal its presence. The cartilage is denoted by ♦.
Scan 2: Abnormal knee with Baker’s Cyst
In patient with Baker’s cyst, fluid collection can be seen between the semimembranosus and medial head of gastrocnemius (Fig. 23.8a, b). Scan carefully and mark the pedicle of Baker’s cyst.
Needles: 16G needle
Approach: Linear probe in plane medial to lateral
After obtaining a short-axis view of the semimembranosus and medial head of gastrocnemius, the sonogram should reveal the cyst and pedicle which connect to the knee joint. A 16G needle (arrows) is advanced in plane to the pedicle (∗) of the cyst deep to semimembranosus tendon trying to mark the location of the pedicle place without piercing the cyst, because it can be difficult to identify it after aspiration of the fluid (Fig. 23.9).
Then another needle (bold arrow) is inserted inside the cyst, in plane or out of plane, to perform the aspiration and fenestration (Fig. 23.10). Remember to evaluate the nature of the fluid (serous or particulate) in order to choose the appropriate size of the needle capable of fluid aspiration (18G or higher).