Ultrasound of the Musculoskeletal System
Authors: Bianchi, Stefano, Martinoli, Carlo
Normal Ultrasound Findings and Scanning Technique: Anterior Knee
As already stated, the anterior aspect of the knee is best examined with the patient supine. A knee flexion of approximately 20°–30° obtained by placing a small pillow beneath the popliteal space stretches the extensor mechanism and avoids possible artifacts related to anisotropy which are secondary to the concave profile these tendons assume in full extension (Bianchi et al. 1994). In this position, the anterior aspect of the knee is examined starting from cranial to caudal. The suprapatellar, juxtapatellar and infrapatellar regions are imaged in succession (Fig. 12).
Fig. 12a,b. Anterior aspect of the knee. a Schematic drawing with b corresponding extended-field-of-view longitudinal 12– 5 MHz US image over the anterior aspect of the knee demonstrates the extent of the suprapatellar (A), iuxtapatellar (B) and infrapatellar (C) regions. Due to their superficial location, the main structures of the extensor mechanism of the knee, the quadriceps tendon (Qt), the patella and the patellar tendon (Pt) are well depicted with US. Fem, femur; Tib, tibia.
Relevant anatomic structures in the suprapatellar region that are amenable to US examination are: the quadriceps tendon, the suprapatellar synovial recess, the suprapatellar fat pad, the prefemoral fat, the distal femoral metaphysis and the trochlea. Longitudinal US images obtained in the midline with the probe placed with its distal edge on the patella are able to display the quadriceps tendon. If the quadriceps tendon is stretched and the US beam is perpendicular to its long axis, it shows a hyperechoic fibrillar appearance (Fig. 13a,b).
Fig. 13a–d. Quadriceps tendon: scanning technique. a,b Schematic drawings show the US study of the quadriceps tendon a with the knee flexed at approximately 30° and b extended. Both the quadriceps and the patellar tendons are stretched (arrow) and assume a straight course when the knee is flexed by placing a pillow under the popliteal space. c,d Corresponding long-axis 12–5 MHz US images of the quadriceps tendon (Qt) obtained with flexed (c) and extended knee (d). In c, the tendon tends to assume a perpendicular course relative to the US beam and, therefore, is characterized by a hyperechoic structure with a discrete fibrillar echotexture (white arrows). Note that the distal tendinous portion can be accurately evaluated until its insertion into the upper pole of the patella. d With full knee extension, the distal portion (black arrows) of the quadriceps tendon (white arrows, Qt) assumes a falsely hypoechoic appearance as a result of anisotropy because the tendon is loose. Fem, femur; P, patella.