In Ultrasound Images, All That Is Black Is Not Always Fluid

We read with interest the “Images in Anesthesiology” section article entitled “Ultrasound-guided Intraarticular Knee Injection” in the September 2017 issue discussing the utility of local anesthetic injection into the suprapatellar bursa for minor arthroscopic knee surgery via an alternative position for better visualization.¹  The authors claim that placing the knee in 90° of flexion accentuates the suprapatellar bursa, thus improving the image quality and overall success of the block. However, the hypoechoic strip labeled as suprapatellar bursa in their ultrasound image could also be interpreted as hyaline cartilage overlying the femoral condyles. This usually happens when the transducer is moved more distally over the femoral condyles (fig. 1), hence the need for video clips and dynamic scanning, which will facilitate better appreciation as opposed to static images. Besides the location in the femur to aid in differentiating between the hyaline cartilage and bursa, the cartilage is uniformly regular and is not compressible, which is again difficult to assess from static images. Injecting the hyaline cartilage, or even a minor trauma from the needle, has the potential to worsen the patient’s condition as cartilage heals with fibrosis.

Quadriceps femoris is lax at full extension and becomes taut at 30° flexion, and as the angle of flexion is increased, gradually it becomes more stretched. When it is stretched, the fluid within the bursa tends to run back into the joint. In our personal experience and based on available evidence, there does not seem to be an advantage to flex the knee to 90°, as it would stretch the quadriceps tendon, whereas tightening the quadriceps muscles in 30° of flexion often aids in visualizing the suprapatellar bursa in patients with little fluid collection² (fig. 2).

The authors declare no competing interests.