In equine anatomy, the cannon refers to the region of the lower leg between the knee (carpus) in the front legs, or the hock (tarsus) in the hind legs, and the fetlock joint. The cannon bone — formally the third metacarpal in the front legs and the third metatarsal in the hind legs — is the principal weight-bearing bone in this section of the leg and one of the most structurally important bones in the horse’s entire body. Its strength, proportion, and soundness are central to the horse’s athletic capacity and longevity.
Anatomy of the Cannon Region
The cannon bone is a long, tubular bone that runs vertically down the lower leg. Flanking it on either side are the splint bones (the second and fourth metacarpals or metatarsals), which are much thinner and bound to the cannon by periosteum and interosseous ligaments. At the lower end, the cannon articulates with the proximal sesamoid bones and the long pastern bone to form the fetlock joint. This is a high-motion joint subject to enormous forces, particularly at speed or on landing from a jump.
Running along the back of the cannon are the tendons and ligaments that power and stabilise the lower leg: the superficial digital flexor tendon (SDFT), the deep digital flexor tendon (DDFT), and the suspensory ligament. These structures are enclosed in tendon sheaths that allow frictionless movement and are supported by the check ligaments. Any injury to this complex — a bowed tendon, a suspensory lesion, or damage to the check ligament — typically occurs within or adjacent to the cannon region.
What ‘Good Bone’ Means
In horse evaluation, the phrase “good bone” refers to the circumference of the cannon bone measured just below the knee. This measurement correlates with the bone’s capacity to support the horse’s weight and absorb the forces of work. As a general guide, a horse carrying a rider for flat work or moderate jumping needs a cannon circumference of at least eight inches; a horse in hard work or jumping at higher levels should ideally have nine inches or more. Bone measurement is assessed relative to body weight — a light-framed horse with eight inches of bone may be better supported than a heavier horse with the same measurement.
The term “light of bone” describes a horse whose cannon circumference is considered insufficient for its body mass and intended workload. Such horses are considered to carry greater risk of lower leg injuries under sustained work. When buying a horse for a particular purpose, assessing bone quantity is a standard part of the pre-purchase examination.
Common Conditions Affecting the Cannon
The cannon region is one of the most clinically significant areas in equine veterinary practice. Several conditions are specific to this area or commonly present here.
Splints are bony enlargements that develop on the splint bones, usually on the inner (medial) aspect of the front cannon. They are caused by trauma, concussion, or poor conformation and are common in young horses in early work. Once fully calcified, most splints cause no ongoing lameness, but a splint forming on the inner aspect of the knee or pressing on the suspensory ligament can be more problematic.
Periostitis (shin soreness or “sore shins”) involves inflammation of the periosteum — the fibrous covering of the cannon bone. It is particularly common in young racehorses early in their training, as the bone remodels in response to the forces of fast work. Affected horses show pain on pressure over the front of the cannon and may be mildly lame. Rest and graduated return to training are the standard management approach.
Stress fractures of the cannon can occur in horses in very hard training, particularly racehorses. They present as acute-onset lameness and require imaging to diagnose. The cannon bone’s medial condyle is a relatively common site for stress fracture in Thoroughbreds.
Tendon and suspensory injuries occur within the cannon region but technically involve the soft tissues running alongside and behind the bone rather than the bone itself. Ultrasound examination of the cannon region is a routine diagnostic procedure for any horse showing lower leg lameness or heat and swelling in the cannon area.
The Cannon in Conformation Assessment
A well-conformed cannon should be relatively short and wide, giving the impression of density and strength. Short cannon bones are considered desirable because they reduce the lever arm through which forces act on the fetlock joint, reducing injury risk. Horses with long cannons relative to their forearm length are sometimes described as “long in the cannon” — this is considered a conformation weakness that may predispose the horse to tendon and fetlock problems over time.
The cannon should also sit straight from the knee to the fetlock when viewed from the front and the side. Offset cannons — where the cannon does not run in a straight line from the knee — create uneven loading and are associated with a higher incidence of splints and other lower leg issues.
Frequently Asked Questions
What is the difference between the cannon bone and the cannon region?
The cannon bone is a specific anatomical structure — the third metacarpal or metatarsal. The cannon region describes the broader area of the lower leg encompassing the cannon bone, the splint bones, and all the soft tissue structures (tendons, ligaments, tendon sheaths) that run through this section of the leg. Veterinarians and experienced horse people use “cannon” loosely to refer to both, but the distinction matters in diagnostic contexts when pinpointing exactly which structure is involved in a problem.
How do I measure a horse’s bone?
Cannon bone measurement is taken with a soft tape around the cannon at its narrowest point, directly below the knee on a front leg. The horse should be standing squarely with its weight evenly distributed. The measurement reflects the circumference of the bone plus the tendons and soft tissue around it, so it is not a pure bone measurement, but it is a consistent and widely used guide in horse evaluation. Measurements are typically compared against published guides for different types and intended workloads, though experienced buyers develop their own calibration over time.
My horse has a lump on the cannon. What is it likely to be?
A hard, bony lump on the cannon is most commonly a splint. These develop where the splint bone meets the cannon bone, most often on the inner aspect of the front cannon. A newly forming splint may be warm, soft, and sensitive; a mature one is typically cool, hard, and pain-free. A soft, fluctuant swelling that is warm to the touch suggests tendon sheath effusion or a soft tissue reaction rather than bone. Any new swelling on the cannon should be assessed by a veterinarian, particularly if it is accompanied by heat, lameness, or a sudden change in the horse’s way of going.