Ultrasound examination of the knee is an accurate and sensitive imaging modality for many common disorders around the knee joint. The presence and grade of tendinopathies can be quickly evaluated and changes over time monitored. Injured ligaments appear swollen and of altered echogenicity and may be assessed dynamically for patency. Ultrasound can demonstrate different types of injury in the peripheral part of the meniscus although MRI is more sensitive than ultrasound for detection of meniscal lesions. Ultrasound is able to demonstrate synovial thickening and effusion in inflammatory arthropathy and erosions of the articular surface in degenerative arthritis. It can be used effectively to monitor changes in the activity of rheumatoid arthritis in response to treatment and for grading degenerative arthritis. Ultrasound examination of the knee should be undertaken using a large foot-print linear high frequency linear probe (7.5–15 MHz). Findings can be correlated with the point of maximal tenderness located with palpation and readily compared with those obtained in the contralateral joint.
10.1 Diagnostic Imaging of the Knee: Introduction
The knee may be considered as consisting of four quadrants, anterior, medial, lateral, and posterior. Ultrasound would normally be focused only one or two of these quadrants depending on the clinical diagnosis.
Imaging includes the following quadrants:
Anterior
Patellar tendon.
Tibial tubercle and deep and superficial infrapatellar bursae.
Quadriceps muscle and tendon.
Prepatellar bursa.
Suprapatellar pouch (for effusion).
Vastus medialis muscle and medial retinaculum.
Medial
Medial collateral ligament (MCL) (includes valgus stressing if indicated).
Medial tibiofemoral joint space and medial meniscus.
Pes anserine tendons and bursa (if pathological).
Lateral
Lateral collateral ligament (including varus stressing if indicated).
Iliotibial band and bursa (if pathological).
Lateral tibiofemoral joint space and meniscus.
Popliteus tendon.
Proximal tibiofibular joint.
Posterior
Semimembranosus and semitendinosus muscle and tendon.
Biceps femoris muscle and tendon.
Medial and lateral gastrocnemius muscles and tendons.
Popliteal fossa.
Popliteal artery and vein.
10.1.1 Anterior
Anterior Knee—Infrapatellar: Longitudinal Scan
The patient is positioned in supine with the knee placed in 20 to 30 degrees flexion. This puts both the patellar and quadriceps tendon under some tension allowing better visualization of these structures.
The probe is placed in the anatomical sagittal plane so that it lies over the tibial tuberosity and distal patellar tendon. The probe is then moved proximally to visualize in turn the proximal patellar tendon, the quadriceps tendon, and suprapatellar region (Fig. 10‑1 , Fig. 10‑2 , Fig. 10‑3 ).
The infrapatellar fat pad also known as Hoffa’s fat pad may be seen deep to the patellar tendon and inferior pole of the patella. This may become painful in a condition known as fat pad impingement or Hoffa’s disease. The cause is usually due to single or repetitive traumatic episodes. The inflamed fat pad becomes hypertrophied which may lead to further impingement between the tibia, femur, and inferior pole of the patella. Treatment should consist of appropriate rehabilitation and, if symptoms persist, guided injection.
The patient is positioned in supine with the knee positioned in 20 to 30 degrees flexion. This puts both the patellar and quadriceps tendon under some tension allowing better visualization of these structures.
The probe is placed in the anatomical transverse plane so that it lies over the tibial tuberosity and then moved in a proximal direction over the patellar tendon as far as the distal pole of the patella (Fig. 10‑4 , Fig. 10‑5 ).
Fig. 10.4 Transverse scan of the infrapatellar region. The probe is placed in the transverse plane to lie first over the tibial tuberosity and then moved in a proximal direction to scan the patellar tendon and finally its attachment onto the distal patella.Fig. 10.5 Transverse image of the patellar tendon. The probe is over the midsubstance of the tendon (yellow arrows). The tendon can be seen to be of good echogenicity and of normal thickness at 4 mm (caliper crosses). Hoffa’s fat pad (HFP) can be seen deep to the tendon.
Anterior Knee—Suprapatellar: Longitudinal Scan
The knee is maintained in 20 to 30 degrees of flexion and the probe is moved proximally preserving its alignment in the anatomical sagittal plane so that it lies over the quadriceps tendon and suprapatellar region (Fig. 10‑6 , Fig. 10‑7 ).
Fig. 10.6 Longitudinal image of the suprapatellar region of the knee. The quadriceps tendon (yellow arrows) can be seen to have an intact fibrillar pattern. The loss of echogenicity within the tendon (white arrowhead) toward its insertion onto the proximal patella (P) is an example of anisotropy and does not represent pathology. Deep to the quadriceps tendon and proximal to the patella is the superior patella fat pad (white star). The anterior cortex of the femur can be seen to the bottom left of the image with the prefemoral fat lying above this (yellow star). The dark area superior to the superior fat pad represents the suprapatellar pouch (curved arrow).Fig. 10.7 Longitudinal image of the medial patella retinaculum. From imaging the quadriceps tendon, the probe may be moved into a sagittal oblique orientation to lie over the medial femoral condyle (MFC), the medial patella retinaculum (white arrows), and vastus medialis obliquus (VMO).
Anterior Knee—Suprapatellar: Transverse Scan
The knee is maintained in 20 to 30 degrees of flexion and the probe is moved proximally over the suprapatellar region in the anatomical transverse plane so that it lies over the quadriceps tendon and suprapatellar region.
Maintaining the probe in the transverse plan and flexing the knee allows for visualization of the femoral trochlea (Fig. 10‑8 , Fig. 10‑9 , Fig. 10‑10 ).
Fig. 10.11 Longitudinal image of a patellar tendon. The knee has been flexed to place stress through the tendon. However, the tendon still demonstrates a “wavy-like” appearance within its lower third (yellow arrows). This appearance is suggestive of either partial or complete rupture as the tendon lacks firm anchorage in such cases. In this image the tendon has lost its normal fibrillar pattern within its deeper portion toward its attachment on the patella (white arrowhead). These findings are in keeping with a partial tear of the deeper portion of the tendon. P, patella; TT, tibial tuberosity.Fig. 10.12 Longitudinal image of the infrapatellar region of the knee demonstrating a complete loss of the normal fibrillar pattern of the patella tendon in keeping with complete rupture. The proximal and distal stumps of the tendon can be seen (white arrowheads). The double headed yellow arrow demonstrates a considerable gap between the two stumps. P, patella; TT, tibial tuberosity.Fig. 10.13 MRI scan of the sagittal view of the same knee as in Fig. 10‑12 . The image also demonstrates a complete loss of the normal fibrillar pattern of the patella tendon. In addition, a large intra-articular effusion is seen.Fig. 10.14 Longitudinal image of the patella and infrapatellar region of the knee. The patella tendon (yellow arrows) appears intact, however, a hybrid anechoic/low echo swelling (white arrowheads) is seen overlying the lower half of the patella and upper third of the patellar tendon (note the posterior acoustic enhancement). The findings are indicative of a prepatellar bursitis. HF, Hoffa’s fat pad; P, patella; TT, tibial tuberosity.Fig. 10.15 Longitudinal image of the infrapatellar region of the knee in a patient who had sustained a fall onto the knee 10 days prior to examination. The patella tendon (yellow arrows) appears intact, however, there is a large low echo swelling measuring approximately 9 cm in length (white crosses) superficial to the patellar tendon within the subcutaneous tissue. With Power Doppler, there was no associated increase in vascularity. These findings are indicative of a subcutaneous posttraumatic hematoma. HF, Hoffa’s fat pad; P, patella; TT, tibial tuberosity; white arrowheads, prepatellar bursitis; white crosses, subcutaneous hematoma.Fig. 10.16 Longitudinal image of the proximal patella tendon. The rectangular box indicates Power Doppler imaging. The image demonstrates some thickening of the proximal patella tendon and with Power Doppler in place, there is a clear neovascularity throughout the tendon. These findings indicate a proximal patellar tendinopathy, commonly termed “jumper’s knee.” HF, Hoffa’s fat pad; P, patella; yellow arrows, patella tendon.Fig. 10.17 Longitudinal image of the patella and patella tendon of the knee. The patella tendon (yellow arrows) appears intact. Large calcified foci may be seen at the distal point of the patella measuring approximately 1 cm in longitudinal length (yellow crosses). There is some thickening of the patella tendon around the foci. Findings indicate a possible old Sinding-Larsen-Johansson disease. HF, Hoffa’s fat pad; P, patella; TT, tibial tuberosity; yellow crosses, calcified foci.Fig. 10.18 Transverse image of the patella tendon (yellow ellipse). The tendon is of good echogenicity and appears intact other than a spherical low echo foci within the lateral aspect measuring approximately 5 mm in transverse diameter (yellow crosses). Findings are suggestive of an intrasubstance tear/cystic degeneration.Fig. 10.19 Longitudinal image of the patella tendon outlined in Fig. 10‑18 . The low echo foci (yellow crosses) can be seen to extend through the tendon approximately 2.8 cm. Yellow crosses, intrasubstance foci suggestive of a partial tear or degenerative cystic lesion.
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