Ankle Joint and Forefoot

Anatomy and Techniques

Introduction

Ultrasound examination of the ankle and foot is one of the most common examinations in musculoskeletal ultrasound. The superficial location of most structures means that ultrasound plays an important role in the management of many patients with painful conditions of the foot. As in many other joints, symptoms guide the ultrasound approach. Patients who present with pain that is located to a single area are most often helped by ultrasound. Patients who present with more global symptoms, rather than pain localized to a particular area, require MRI for more complete assessment and in particular to assess the joint surfaces.

Many of the structures of the ankle benefit from being examined under stress. It is therefore important to be able to move the foot and the patient’s position should allow for this. This can be achieved by having the patient’s ankle and foot overhang the edge of the couch or by placing a rolled up towel underneath the distal calf so that the ankle and foot are elevated above the couch. The medial, anterior and lateral aspects of the foot are examined with the patient supine. The hindfoot is internally rotated to examine the lateral side, and externally rotated for the medial side assessment. The posterior and plantar aspects are best assessed with the patient prone.

Position 1: Medial Ankle Proximal

Imaging Goals

1. Identify tibialis posterior tendon (TPT).

2. Locate flexor hallucis and its fibroosseous tunnel.

3. Identify tibial nerve and proximal branches.

The examination begins with the probe in an axial position just above the medial malleolus (Fig. 23.1). The posteromedial margin of the tibia is easily identified and the largest and most medial of the tendons is the TPT. Deep to the TPT, a low-reflective band overlies the tibial cortex representing the hyaline cartilage of a pulley enthesis. This assists with the smooth passage of the tendon around the malleolus. The presence of cartilage at this location explains why enthesopathy can occur and why bony spur formation is a component of patients with chronic TPT disease. The tendon should have a normal internal structure comprising areas of low reflectivity, representing the tendon fibres interspaced with areas of increased reflectivity from the intervening connective tissue support structures. The most medial aspect of the tendon is in line with the medial border of the tibia.

Figure 23.1 The ultrasound transducer is positioned in the axial plane posterior to the medial malleolus, demonstrating the three medial tendons and the tibial neurovascular bundle.

Lying along the lateral margin of tibialis posterior in close proximity with it is the tendon of flexor digitorum longus (FDL). This is a much smaller tendon than TPT with a lower musculotendinous junction. The other structures in the proximal part of the tarsal tunnel include the posterior tibial artery and veins. The medial border of the posterior subtalar joint is identified deep to the flexor digitorum tendon as it passes below the sustentaculum. Immediately lateral to this is the tibial neurovascular bundle. The posterior tibial artery and its multiple surrounding veins separate flexor digitorum from the tibial nerve. The tibial nerve is a brighter, more reflective structure than the nearby tendons. Once again low-signal elements can be identified within it, representing the neural bundles, and these are separated by intensely bright components representing epineural tissue. The tibial nerve divides to form the medial and lateral plantar nerves. This branching can occur at any point and may already have occurred at the level of the medial malleolus. A smaller branch, the medial calcaneal nerve, arises directly from the tibial nerve and can be seen deep and slightly posterior as it heads towards the posterior medial corner of the os calcis. Another important branch, the inferior calcaneal nerve, arises from the lateral plantar nerve. It is sometimes referred to as the first branch lateral plantar nerve. This nerve also passes deep and somewhat posteriorly to pass around the underside of the hind foot close to the origin of the plantar fascia. It traverses the hind foot to supply abductor digiti minimi. This nerve may be impinged by plantar fasciitis, leading to atrophy of the aforementioned muscle. Compression of the nerve more proximally may also lead to a syndrome that mimics plantar fasciitis, called Baxter’s neuropathy.

Tibialis posterior is held in place by the overlying medial (or tarsal) retinaculum, which can be identified as a thick striated connective tissue structure that passes over tibialis posterior, forming the roof the tarsal tunnel. The flexor digitorum tendon and tibial neurovascular bundle are also contained within the tarsal tunnel. Medially it blends with the fascia and periosteum of the tibia.

Although flexor hallucis longus (FHL) is often considered in association with tibialis posterior and flexor digitorum, (the so-called Tom, Dick and Harry), it is in fact separated from the other two tendons and has its own fibroosseous tunnel on the dorsal aspect of the os calcis. The tendon is located by moving the probe medially and distally and using the overlying tibial nerve as a landmark for the tendon. Gentle movement of the big toe helps identify the deeply positioned FHL tendon. The groove in which it is contained is bridged by a short retinaculum creating a fibroosseous tunnel helping to guide movement of the tendon. Occasionally the retinaculum is thickened and tendon nodules may develop, creating a triggering phenomenon similar to trigger finger. If an os trigonum is present, FHL is medial to this accessory bone.

The tendon should be examined in both short and long axis with and without movement. Rotating the probe 90° once the tendon is identified and then moving the toe will serve to demonstrate that there is no adhesion or catching as the tendon passes into its fibroosseous tunnel. The proximal part of the tendon just below the musculotendinous junction is usually the first to be identified and it is not uncommon to see fluid around it in this location. This is particularly obvious if ankle joint effusion is present as the tendon sheath communicates directly with the tibiotalar joint. Generally, simple effusions are not associated with an increase in Doppler activity.

Position 2: Medial Ankle Distal

Imaging Goals

1. Locate tibialis posterior/tibiotalar/spring ligaments.

2. Find where flexor hallucis crosses flexor digitorum.

3. Identify the medial and lateral plantar nerves.

Technique

As the probe is advanced distally following tibialis posterior, the medial malleolus disappears from view. The structure which now is found deep to TPT is the tibiotalar ligament, a major component of the deltoid ligament complex (Fig. 23.2). This ligament is said to have two components, although the posterior component is more dominant and blends with the anterior component in most cases. In the relaxed position the normal internal structure of the ligament is difficult to discern due to anisotropy as the fibres are oriented at various angles to the probe. Tensing the ligament, by dorsiflexing the foot, brings the fibres into alignment and reveals the true internal structure.

Figure 23.2 There are three groups in the medial ligament complex. The deep group comprises the anterior and posterior tibiotalar ligaments (dark blue). The superficial group comprises the tibiocalcaneal, tibiospring and tibionavicular ligaments (mid blue). The principal transverse ligament is the calcaneonavicular or spring ligament (light blue).

The normal ligament is triangular or sail shaped with its narrow end superiorly attached to the medial malleolus and its wide portion distally attaching to the talus. The tibiotalar joint is deep to the ligament. If the tibialis posterior is followed a little more distally, the spring ligament appears deep to it (Fig. 23.3). The posterior part of this ligament, the component that is attached to the sustentaculum tali, is identified first as this is its widest portion. If the upper end of the probe is rotated anteriorly, the middle and anterior parts of the ligament come into view, passing around the reverse S shape of the talus towards the navicular. A small bursa, little more than a potential space, is located between the TPT and the spring ligament. This is called the gliding layer. A small quantity of fluid may be present around the tibialis posterior in this location. The other significant component of the medial ligament complex is the tibiospring ligament (Fig. 23.4), which is orientated in the coronal plane, linking the tibia to the spring ligament. The tibiocalcaneal and tibionavicular can be usually found in most individuals but with a more variable appearance.