The Ultrasonographic Approach of the Equine Tarsus: a Protocol for a Systematical Approach

Introduction
Injuries of the equine tarsus are frequent causes of hind- limb lameness. In the past ultrasonography (US) has been proven to be useful for the diagnosis of many tarsal injuries. However, performing an US examination of the equine tarsus is demanding and requires a systematic approach and an adequate knowledge of the cross- sectional anatomy of this joint.
The aim of this presentation is :

• To describe and illustrate the US anatomy of the equine tarsus.
• To demonstrate the according reference US images.
• To propose a standardised US protocol for the examination of the equine tarsus.

Cross-sectional anatomy equine tarsus: reference images
A practical four-step-tour is proposed as a protocol for a complete US evaluation of the equine tarsus. The dorsal aspect (step I), the medial aspect (step II), the lateral aspect (step III) and the plantar aspect of the joint (step IV) are evaluated subsequently with the leg weight- bearing.
The US examination is performed with a linear 7.5-12 MHztransducer.Everystepisperformedin2perpendicular planes, however, the horizontal approach is always the refference approach.

Step I: Dorsal approach of the tarsus

1. Medial ridge: palpate the medial ridge of the talus (MR), place the probe upon the MR in a horizontal plane and scan the MR transversally. Constantly tilt the head of the transducer and direct the ultrasound beam perpendicularly to the surface of the subchondral bone. Rotate the probe 90° to evaluate the ridge longitudinally.
2. Lateral ridge: starting from the MR with the transducer in a horizontal plane, move laterally till you encounter the lateral ridge (LR). The same protocol for US evaluation is used as for the MR.
3. Cochlea: the sulcus of the talus is located in between the MR and LR. Moving proximally in a horizontal plane over the sulcus of the talus, the cranial part of the intermediate ridge of the tibia is evaluated.
4. EDLO tendon: dorsomedial to the LR, the oval shaped long digital extensor tendon (EDLO) and its tendon sheath are evaluated.
5. PT and TC: dorsolateral to the MR, the peroneus tertius (PT) tendon and the muscle and tendon of the tibialis cranialis (TC) muscle are evaluated. Moving from proximal to distal, the PT tendon typically creates two lobes, located at both sides of the TC muscle, splitting distally into two separate branches. The TC is identified proximally by a) its muscle part that surrounds its own tendon and b) its location plantar to the PT. Moving distally the muscle part becomes smaller and the tendon pentrates to the surface through the bifurcation of the PT. The insertion of TC also consists of a lateral and medial branch.

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