These bone growths are more commonly seen in the horse in the distal caudal (lower, hind) aspect of the radius, which is the bone above the carpus or knee, but have also been diagnosed on the front of the radius, on the tibia or long bone above the hock, the calcaneus bone at the back of the hock, the upper pastern in Tbourida horses and the nasal bone.
Osteochondromas differ from other bony growths known as exostosis in their histological make-up, with osteochondromas having a cartilage cap, which is a remnant of cartilage that would normally undergo endochondral ossification to form bone. An osteochondroma will also show evidence of a bone medullary (inner core) and a cortical (outer) structure, whereas normal bone exostoses such as splints and ringbone have no cartilage and develop simply as an extension of the bone cortex. It is thought that osteochondromas are developmental rather than neoplastic in origin and occur because a small area of the cartilage in a growth plate region becomes displaced during the normal growth phase and develops into a separate area of bone growth, above the original growth plate.
Instead of the cartilage tissue completely ossifying and becoming incorporated into the shaft of the bone, this island of tissue continues to multiply and produce more cartilage that ossifies as a discrete lump and results in the formation of a tumour. These osteochondromas also differ from the small exostoses seen at the back of the distal radius that are remnants of the growth plate and remain as small spikes of bone on the physis or growth plate when the horse matures.
Whilst osteochondromas may not be painful as they develop, they can be cosmetically unsightly and often cause secondary effects to surrounding tissues as they grow, and it is this resulting damage and pain in the adjacent soft tissue that compels the owner to seek veterinary advice.
Osteochondromas on the back of the radius are frequently diagnosed after they have grown big enough to put pressure on the soft tissue structures that run down behind the knee, particularly the deep digital flexor tendon, causing inflammation that can progress to a tear in the tendon. The typical clinical presentation of an osteochondroma on the caudal radius is swelling in the carpal tendon sheath, seen as swelling on the outside of the lower forearm and on the inside and outside aspects of the upper cannon. Sometimes, there is no pain elicited when the sheath is palpated, but typically the horse resents flexion of the knee, either as a response to the increased pressure in the sheath or the pain of the osteochondroma pressing on the tendon.
Horses showing lameness can vary in severity from a mild 1/5 lameness to a more severe 4/5 lameness, depending on the degree of tearing and the horse’s individual pain tolerance. Affected horses will often trot with the affected limb abducted away from the body.
Osteochondromas located in other sites will produce clinical symptoms relating to the structures they impinge upon, for instance, an osteochondroma on the front of the radius can cause swelling in the carpi radialis sheath. The osteochondromas seen on the dorsal, proximal first phalanx (front of the top of the pastern, immediately below the fetlock joint) in Tbourida horses, cause damage to the tendons that pass in the vicinity (the common and lateral digital extensor tendons) and secondary damage to the fetlock joint and lower cannon bone. The clinical presentation of an osteochondroma compared to that of a physeal remnant exostosis are very similar, however, osteochondromas are more commonly diagnosed in young horses (2-4 years old) whereas the physeal remnant exostosis may be detected in older horses (average age 6 years) as osteochondromas tend to stop growing when the horse matures and stops growing.