Ankle Joint Ultrasound: Protocol, Ligaments, Tendons, Effusion

The Role of the Protocol in the AIUM Standard

AIUM Practice Parameter for the Performance of a Musculoskeletal Ultrasound Examination, 2023 sets the general standard for performing musculoskeletal ultrasound: the examination should be clinically directed, cover relevant anatomical structures, include dynamic assessment, targeted visualization of the pain area, and documentation of images sufficient for interpretation. For the ankle joint, this translates conveniently into a zonal protocol: anterior section, lateral complex, medial complex, and posterior section.

AIUM does not set universal numerical thresholds for tendon thickness, effusion volume, or the degree of ligament damage in the ankle. Therefore, descriptive features are preferred in the protocol: fiber continuity, echogenicity, contour, thickening, local tenderness under the probe, effusion, synovial proliferation, hypervascularization on Doppler, dynamic instability, and comparison with the opposite side.

Transducer, Positioning, and Documentation

A high-frequency linear transducer is used; frequency and focus are selected to confidently visualize superficial ligaments, tendon sheaths, and the cortical contour of bony landmarks. Gel or a pad helps avoid compression of small effusions and superficial soft tissue structures.

The patient's position is changed according to the area of interest: supine for anterior and medial sections, with internal or external foot rotation for lateral and medial ligaments, prone or sitting for the posterior section. Longitudinal and transverse sections of pathological structures, dynamic frames in tendon subluxation or instability, Doppler images in synovitis, tenosynovitis, and enthesopathy are documented.

Ankle Scanning Map

Anterior Section: Joint Effusion and Extensors

The anterior approach begins with a longitudinal section along the axis of the tibia and talus. The anterior recess is assessed for anechoic or hypoechoic content, synovial thickening, compressibility, and Doppler signal. A small amount of fluid may be positionally and clinically insignificant; asymmetry, tenderness, synovial hyperemia, and association with trauma or inflammatory arthritis are more important.

The anterior group tendons are evaluated in short and long axes: anterior tibial muscle, extensor hallucis longus, extensor digitorum longus. Pathological signs: loss of fibrillar pattern, local thickening, hypoechoic splits, fluid in the sheath, tenderness on sonopalpation. In suspected anterior impingement, osteophytes, synovial tissue, and conflict during dorsiflexion are documented.

Lateral Complex: Ligaments and Peroneal Tendons

The lateral section is key in inversion injury. The anterior talofibular ligament is visualized between the anterior edge of the lateral malleolus and the neck of the talus. Continuity, fibrousness, thickening, hypoechogenicity, perifocal edema, and tenderness are described. In complete rupture, fiber defect, retraction of ends, hematoma, and pathological opening in dynamic testing may occur.

The calcaneofibular ligament is sought deeper and caudally, considering anisotropy and overlap by peroneal tendons. The posterior talofibular ligament is assessed in posterolateral pain or suspected more severe complex injury. Peroneal tendons are scanned behind the lateral malleolus and distally: it is important to see both tendons in one channel, exclude tenosynovitis, longitudinal split tear, tendinopathy, and dynamic subluxation during active eversion or dorsiflexion.

Medial Section: Deltoid Ligament and Tarsal Tunnel

Medial pain after trauma requires assessment of the deltoid ligament. Ultrasound signs of damage are similar to lateral ligaments: thickening, decreased echogenicity, fiber disruption, local fluid, and tenderness. In suspected instability, the ultrasound result is described in conjunction with clinical testing and X-ray or MRI data if available.

The tendon of the posterior tibial muscle is the main medial tendon landmark. It is traced behind the medial malleolus and distally to the navicular bone; tenosynovitis, tendinopathy, partial defects, enthesopathic changes, and peritendinous edema are documented. Nearby, the tendons of the flexor digitorum longus and flexor hallucis longus, as well as the soft tissues of the tarsal tunnel, are assessed if symptoms correspond.

Posterior Section and Effusion

The posterior recess of the ankle joint is assessed in posterior pain, movement blockade, suspected posterior impingement, or intra-articular effusion. Fluid may be better detected with foot position change and minimal probe pressure. Intra-articular effusion should be distinguished from fluid in the tendon sheath of the flexor hallucis longus and from retrocalcaneal bursitis.

The Achilles tendon is described if it is part of the clinical request or the pain area is shifted to the calcaneal tuberosity, but the ankle ultrasound protocol should not be limited only to the Achilles tendon. In the posterior section, the os trigonum syndrome, synovitis, tenosynovitis of the flexor hallucis longus, and signs of enthesopathy are also important.

How to Formulate a Conclusion

The conclusion should answer the clinical question: is there a ligament rupture, tenosynovitis, tendon subluxation, intra-articular effusion, or signs of active inflammation. It is advisable to specify the side, exact structure, localization, nature of the damage, presence of dynamic instability, Doppler activity, and associated soft tissue changes.

Correct formulation: "Ultrasound signs of anterior talofibular ligament damage with partial fiber disruption, perifocal edema; small effusion in the anterior recess; no signs of peroneal tendon subluxation on dynamic testing." It is incorrect to limit to the phrase "ligament sprain" without specifying the particular ligament and ultrasound criteria.

Method Limitations

Ultrasound effectively shows superficial ligaments, tendons, sheaths, bursae, effusion, and dynamic phenomena. Limitations are related to deeply located intra-articular structures, bone marrow edema, chondral damage, and complex post-traumatic anatomy. In such situations, ultrasound should be considered as part of the diagnostic pathway, not as a replacement for all imaging methods.