Introduction

Ultrasonography of the tarsus can be challenging, but it is an important part of a complete diagnostic evaluation of tarsal disease. As with other body regions, a thorough understanding of normal anatomy is essential. Comparison with magnetic resonance images, radiographs, and dissected specimens can be helpful in understanding the anatomic relationships and the precise locations and paths of the tendinous and ligamentous structures.

Excellent reviews of scanning techniques for the tarsal region have been published. Regardless of the particular approach chosen, a systematic method for evaluation of this complex structure is helpful. The structures to be examined include the tendons and ligaments, the synovial structures, the bony surfaces, and the subcutis. Comparison with the opposite limb can be extremely helpful in determining if an unusual abnormality is present or in cases of mild disease.

A linear transducer (8–12 MHz) is generally most useful, but a micro-convex transducer (7–10 MHz) can be helpful in some situations. Sedation is often necessary to ensure patient compliance and sonographer safety.

Tendons and Ligaments

The tarsal region contains many tendons and ligaments, some of which have complex attachments or insertions distant from the tarsus itself. The lateral and medial collateral ligaments both have two major components, a long superficial component and a short deeper component.

The long portion of the lateral collateral ligament originates on the caudal portion of the lateral malleolus of the distal tibia and has insertions on the calcaneus, fourth tarsal bone, and third and fourth metatarsal bone. The tripartite short portion originates on the cranial portion of the lateral malleolus and extends in a nearly transverse plane to the lateral aspect of the talus and calcaneus.

The long medial collateral ligament originates on the medial malleolus of the distal tibia and extends distally, inserting on the distal talus, central, fused first and second, and third tarsal bones, and second and third metatarsal bones. The short portion of the ligament originates on the medial malleolus and has three subsections that travel in a more transverse plane than the long portion and insert on the proximal medial talus and the sustentaculum tali.

During ultrasonographic examination, the lateral and medial collateral ligaments can be located most easily in the longitudinal plane. The short components lie in a more transverse plane than the long components. An image in the transverse plane can then be obtained by rotating the transducer 90°. In addition, some of the short portions are in partial relaxation when the horse is weightbearing, so imaging these ligaments when the horse is not fully weightbearing may aid in identification of pathology.

Desmitis of any of the collateral ligaments can occur, but is more commonly seen in the long medial collateral ligament. Horses with collateral ligament desmitis typically demonstrate moderate to severe lameness and synovial effusion. Ultrasonographic signs of desmitis are similar to those in any ligament and include increased size, decreased echogenicity, and abnormal fiber pattern (Figure 6.1). If the insertion of the ligament is involved, bony irregularity or avulsion fragments may be imaged (Figure 6.2).

The short lateral collateral ligament is invariably involved in fractures of the lateral malleolus of the distal tibia while the long lateral collateral ligament is less commonly affected (Figure 6.3). These fractures are usually caused by external trauma. Ultrasonography can be useful to assess the degree of involvement of the collateral ligaments and potential for instability of the joint.

The common calcaneal tendon/gastrocnemius tendon, superficial digital flexor tendon, and long plantar ligament are found on the plantar aspect of the tarsus. The gastrocnemius tendon inserts on the proximal surface of the tuber calcanei. The long plantar ligament originates on the plantar surface of the proximal tuber calcanei and inserts on the fourth tarsal and fourth metatarsal bones. The superficial digital flexor tendon largely passes over the surface of the tuber calcanei, but the medial and lateral aspects insert on the proximal aspect of this bone.

Soft tissue swelling of the plantar aspect of the tarsus (“curb”) may be caused by injury to the long plantar ligament or superficial digital flexor tendon. Subcutaneous edema or thickening may give a similar external appearance. Desmitis of the long plantar ligament is characterized by enlargement and hypoechogenicity of the ligament (Figure 6.4).

The deep digital flexor tendon is located on the plantaromedial aspect of the tarsus, passing over the sustentaculum tali within the tarsal sheath. The smaller medial head of the tendon is contained within its own synovial sheath and joins the main body of the tendon in the proximal metatarsal region. In the absence of soft tissue swelling of the plantaromedial aspect of the tarsus, an image of the deep digital flexor tendon can often be more easily obtained with a micro-convex probe due to its smaller footprint.

The peroneus tertius and the tendons of the cranial tibial muscle have complex insertions on dorsal aspect of the distal tarsal region. The distal peroneus tertius forms a tunnel through which the distal cranial tibial tendon emerges. The dorsal tendon of the peroneus tertius then inserts on the central and third tarsal and third metatarsal bones. The lateral tendon inserts on the fourth tarsal bone and the distolateral calcaneus and talus. The dorsal tendon of insertion of the cranial tibial muscle inserts on the third tarsal and third metatarsal bones. The cunean tendon is the medial tendon of insertion of the cranial tibial muscle. It passes medially across the central tarsal bone, inserting on the fused first and second tarsal bone, central tarsal bone, and second metatarsal bone. The tendons of insertion of the cranial tibial and peroneus tertius can be identified by following each of the tendons individually from their origins in the distal tibia.

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