With multiple rows of uniquely shaped bones, several distinct joint spaces, and a plethora of soft-tissue attachments, the horse’s hock (tarsus) plays a crucial role in equine locomotion. However, its complex anatomy can prove a diagnostic challenge for veterinarians.

“Unlike the forelimbs, lameness related to the distal extremity is less common in hind limbs, yet the hocks are a common source of pain/discomfort causing lameness,” says Pablo Espinosa-Mur, DVM, DVCs, Dipl. ACVSMR, assistant professor of diagnostic imaging at Ontario Veterinary College, in Canada. 

Horse Hock Anatomy

The hock joint is made up of three rows of bones and three distinct joints. The bulbous ends of the horse’s tibias, called the malleoli, come to rest on top of each talus, a uniquely shaped bone in the hock forming the tarsocrural joint. Each talus has two obliquely oriented (slanted or at an angle) ridges extending from the dorsal (towards the back) aspect of the bone that interlock with the malleoli and the distal (lower) intermediate ridge of the tibia.

The flat bottom of the talus articulates with the rectangular central tarsal bone, forming the talocalcaneal-centroquartal (or proximal intertarsal) joint. The bottom of the central tarsal bone overlies the small cuboidal bones, numbered one through four, forming the centrodistal joint. Finally, the small cuboidal bones articulate with the metatarsal bones (primarily the third metatarsal bone, which is the cannon bone) forming the tarsometatarsal joint.

Several soft-tissue structures such as muscles, ligaments, and tendons pass alongside the hock. This includes the digital flexor tendons coursing along the plantar aspect (toward the rear) of the joint, the extensor tendons running along the dorsal aspect, and the medial and lateral (located closer to and further away from the median plane of the horse, respectively) collateral ligaments extending from the lateral aspects of the tibia to the third metatarsal bones.

“However, there is sometimes communication between other joints as well, which means intra-articular (IA) anesthesia may not be specific,” adds Sue Dyson, MA, VetMB, PhD, an independent consultant based in the U.K. Joint communication refers to the interconnection between different joint compartments, which allows synovial fluid to flow between them.

“There are many attachments as well as deep and superficial parts both medially and laterally that have different orientations and attachments. These structures can get injured individually or together, complicating the workup,” Dyson explains.

Common Horse Hock Pathology

Dyson says common pathologies (disease or damage) causing lameness in the horse’s hock include:

1. Osteoarthritis (OA) in the distal hock joints, particularly the centrodistal (distal intertarsal) and/ or tarsometatarsal joints;
2. Osteochondrosis dissecans (loose cartilage or bone fragments in the joint) in the distal cranial (toward the horse’s head) aspect of the intermediate ridge of the tibia (DIRT lesions) or the trochleas of the talus, often combined with secondary osteoarthritis of the tarsocrural joint; and
3. Traumatic injury of the deep and superficial collateral ligaments.

“In some cases, osteoarthritis of the distal tarsal joints and desmitis (damage of a ligament and surrounding structures) of the proximal suspensory ligament (originates at the top of the cannon bone and supports the fetlock) can be seen in the same horse,” says Espinosa-Mur. “These conditions can be clinical while others can be subclinical (not visible to the eye) at the time of presentation.”

Diagnostic Analgesia in the Hock

Because the hock’s structures lie in such proximity to one another, performing nerve blocks to localize lameness to one location can be challenging for veterinarians, says Espinosa-Mur. Some of these structures share innervation or are close enough that diagnostic analgesic can diffuse and cause false results, he adds. For example, the anesthetic agent can diffuse from the tarsometatarsal joint into surrounding structures. This means the veterinarian unintentionally desensitizes the surrounding area, including the suspensory ligament.

“This is particularly true if too much local anesthetic solution is used as it will leak out from the tarsometatarsal joint and influence the suspensory ligament,” says Dyson. “Ideally, practitioners will cross-block, meaning they will block the suspensory ligament on one occasion and the tarsometatarsal joint on a separate visit.”

It is important for veterinarians to recognize that IA anesthesia will not remove bone pain, says Dyson. “So with OA that involves subchondral bone (beneath the cartilage) damage, an intra-articular block may be negative.”

Imaging the Horse’s Hock

Veterinarians can use X rays as a first-line method to evaluate the structures in the hock. When they cannot reach a clear answer using this imaging modality, advanced imaging is the next step.

To help home in on the specific source of hock lameness, Espinosa-Mur and colleagues recently published a study using ¹⁸F-sodium fluoride (¹⁸F-NaF, a radioisotope) PET for imaging the tarsus and proximal metatarsus (the upper aspect of the cannon bone near the hock). They took ¹⁸F-NaF PET images of 25 horses with lameness localized to these regions and compared them to computed tomography (CT) scans of those same 25 horses.

Espinosa-Mur says he and colleagues believe imaging modalities capable of distinguishing active injuries from chronic or incidental (found unexpectedly, typically without clinical signs or need for treatment) lesions would benefit equine practitioners, especially when evaluating the hock. In this case ¹⁸F-NaF PET provides an image that helps veterinarians detect active lesions by highlighting areas where the bone takes up the radioisotope—similar to a bone scan but in 3D.

In some cases the PET scan results correlated with those from the subjective and objective lameness evaluations. “Advanced imaging with PET could definitively help determine the (lesions) that display higher osseous (bony) turnover, which in some cases can be the ones that are causing lameness,” says Dyson.

Take-Home Message

The equine hock is a complex collection of joints, which makes diagnosing hock pain challenging. “With a good clinical examination, systematic use of nerve blocks, and good radiography and ultrasonography, many diagnoses of hock-related injury can be achieved,” says Dyson. Since the advent and wider availability of advanced imaging, including CT and PET, veterinarians can more easily diagnose hock problems and determine their clinical relevance, allowing targeted treatment options.