The Equine Tarsus (Hock)

Anatomy

Five joints ● Six bones ● Three rows

Medial view of tarsal joint – showing the 1st and 2nd fused tarsal bones

TARSAL BONE FUNCTION: undergo axial compression and tension alongside torsional loading during locomotion.

DISTAL TARSAL BONES: function to absorb shock and neutralise twisting forces.

Central tarsal bone absorbs most of the stresses.

Articulation

The movement of the tarsus is linked to the movement of the stifle; they work in co-ordination due to the reciprocal mechanism (two opposing sets of muscles). Example: when the tarsus is flexed, the stifle also flexes simultaneously. This reciprocal mechanism is important for counteraction and absorption of concussive forces.

The tarsus is a ginglymus (capable of unidirectional movement) that is also able to absorb direct shock.

The trochlear (part of the talus bone) is important for the articulation of the tarsal joint. It articulates with the distal tibia (which is moulded so it sits over the trochlear ridges). When the joint is flexed, the distal limb is pulled slightly to one side as the trochlear ridges slant outwards. This is how the hindlimb hooves avoid hitting the abdomen; if the tarsus was a hinge joint, this would happen.

The rows of tarsal bones should have minimal movement (limited to slight gliding), and to ensure this there is a system of collateral and dorsal ligaments. The collateral ligaments are arranged in a fan shape to allow for the joint to return to its neutral position. The amount of ligaments means that fractures and laxations are not common.

The tibiotarsal joint is the joint of highest motion, accounting for 90% of the range of motion. The three lower joint below are responsible for the remaining 10% of the range of motion.

Ideal Conformation

Smooth and without obvious swellings
Tarsus should be higher than the carpus
Left and right tarsal joints = symmetrical
Tarsal bones should be substantial; too light a structure increases the likelihood of injury.
When observing from behind, the whole limb should appear straight and the tarsus should lie in this straight line. There should be no major angulation inward and outwards.
Metatarsal bone should be near perpendicular to the ground when observing from the side.
Angle of the tarsal joint should not be too wide (post-legged/greater than 165 degrees) or too angled (sickle hocked/less than 155 degrees).
The range of hock angles is quite wide – a variety of angles can be present without causing lameness, it is the extreme ends of these angles that can pose a threat to soundness. Slight adaptations to angle can also benefit the horse; ie. increased tarsal angulation in dressage horses is beneficial to allow for collection potential.
The positioning of the pelvis, femur and tibia all require analysing as they can affect the location of the tarsus.
Deviation from the ideal conformation will place additional stress on tendons and ligaments.

Dysfunctions

Osteoarthritis ● Synovitis ● Osteochondrosis ● Osteitis ● Fractures (most commonly to exposed areas of tarsus such as sustentaculum tali/trochlear ridges) ● Luxation ● Ligament and tendon conditions ● Inflammation (capped hock, thorough pin, bog spavin)

The most common area for injury and damage (degeneration) is in the lower two rows of the flattened tarsal bones.

Tarsal dysfunction may cause or be resulting from a secondary or primary issue that is separate to the original issue.

Bone Spavin

Fluid in the tarsus joint; commonly on the medial aspect of the joint.
Clinical Signs: lameness (slight or absent), inflammation
Aetiology: poor conformation, degeneration

Bone Spavin Fact Sheet

Fracture to Calcaneus

Often open and can be complicated by osteomyelitis, septic calcanea bursitis, sequestration and chronic drainage.
Aetiology: kicks, external trauma
Clinical signs: severe lameness; diffuse swelling around the wound; increased filling of the talocrural joint and/or tendon sheath.