Laura Fitzharris MRCVS and Evita Busschers MRCVS delve into the details of this complex joint and the injuries that can happen in this area
The fetlock is a high-motion joint with a similar structure in both the forelimbs and the hindlimbs. The fetlock joint is the articulation between the cannon bone (third metacarpal and metatarsal bone in the forelimb and hindlimb respectively) and the long pastern bone (first phalanx), with two sesamoid bones located at the back of the joint.
There are several soft tissue structures associated with the fetlock; the joint is stabilised by collateral ligaments, located on the inside and the outside of the joint. The suspensory ligament originates at the back of the cannon bone, passing down the back of the leg before dividing into two branches which attach to the sesamoid bones.
The sesamoidean ligaments are located lower down at the back of the fetlock and pastern, and consist of five separate ligaments that attach the sesamoid bones to each other as well as to the cannon and long pastern bone.
This crucial combination of the suspensory ligament, the sesamoid bones and the sesamoidean ligaments is called the suspensory apparatus, and this structure supports the majority of the high load on the fetlock joint during the stance phase (when the leg or foot is on the ground during the weight-bearing part of the stride).
The flexor tendons are located at the back of the limb and they provide some additional support to the fetlock joint.
The fetlock is a hinge joint, meaning that the movement is predominantly in one direction – either flexion, during the swing phase of the stride when the leg is in the air, or extension, during the stance phase of the stride when the horse is weight-bearing. There is minimal twisting or side-to-side motion of the fetlock.
The high range of motion and intense loading during canter, gallop, turning and jumping make the fetlock joint susceptible to injury.
An improvement in the lameness
Fetlock injuries typically present as an unusual swelling, involving the soft tissues or joint itself. In some cases, careful physical examinations can point to the fetlock being the problem. In other cases, diagnostic anaesthesia is required, where local anaesthetic is injected into the leg to numb part of the limb. If the area where the pain is localised is desensitised, an improvement in the lameness is seen.
Once the fetlock joint has been identified as the source of the problem, further investigation in the form of diagnostic imaging can be undertaken.
Radiographs (X-rays) allow assessment of the bony structures and are often the first line of investigation. Ultrasound examination looks at the soft tissue structures, such as the tendons and ligaments. Portable X-ray and ultrasound machines mean that these can be performed out on the yard in certain situations.
In other cases, further examination in the form of either an MRI or CT scan is required to provide more detailed information in 3D, and is undertaken at specialist equine hospitals.
Injuries to the fetlock can be divided into three main areas: the soft tissues, the bones and the joint. Most injuries can be caused by a single “overload” incident which is acute in onset, or by a repetitive strain injury which is a chronic change. It is possible that multiple injuries occur in the same traumatic incident since all these structures are closely related.
Soft tissue damage can consist of injury to the suspensory ligament, sesamoidean ligaments and collateral ligaments, which is known as desmitis. Injury to these ligaments can occur in any horse, but increased athletic function puts the horse at higher risk.
Soft tissue damage can often be diagnosed with ultrasound examination, but sometimes more advanced imaging such as MRI is required.
Treatment usually involves a prolonged period of box rest and controlled exercise rehabilitation to allow healing of the damaged tissues. With some injuries, regenerative medicine can be used in addition. The prognosis depends on the severity of the injury, but some horses can return to full athletic function.
Acute single overload
Injuries to the bones mainly consist of subchondral bone damage (such as bruising of the bone close to the joint) or a fracture, resulting in either small fragments within the joint or a major fracture of the cannon bone, long pastern bone or sesamoid bones.
It is well recognised that chronic changes in the subchondral bone due to repetitive injury can change the mechanics of the bone, resulting in decreased ability of the bone to endure the forces during exercise, which can then lead to a fracture. Alternatively, a fracture can also occur due to an acute single overload.
Major fractures might require surgical fixation with screws or sometimes even a plate, and are most often seen in horses doing hard and fast exercise, such as racehorses and high-level event horses.
Smaller fragments within the joint can be the result of a fracture or a developmental disorder called osteochondritis dissecans. The first is the result of trauma, whereas the second is a disorder that is caused by abnormal development of the bone and cartilage. Fragments can be removed during arthroscopic surgery (surgery of the joint). The prognosis after fragment removal depends on how much of the joint surface is affected and the degree of joint damage that has occurred, but is often good.
Changes in the joint can happen as a consequence of another injury (such as a fracture within the joint or a soft tissue injury surrounding the joint).
Changes often start with inflammation, causing an increase in the amount of joint fluid known as synovitis. This can cause lameness and does not have to be associated with changes of the bones or soft tissues (primary synovitis). This is most often seen in young horses when the exercise level is increased, and is treated with an adjustment to the training routine and anti-inflammatory medication.
Osteoarthritis occurs when there is damage to the cartilage and bone, and is common in the fetlock joint of older horses. Osteoarthritis is a progressive, degenerative disease.
The cartilage damage cannot be reversed, but often the pain can be managed, and the disease progression slowed by adapting exercise and providing anti-inflammatory medication. There are several additional medications available to modify the disease or decrease the clinical signs associated with it.
Treating a field injury
Jack is a 12-year-old Irish Sport Horse gelding used for general riding club activities. One day Jack returned from the field lame on his right hind limb with swelling surrounding his fetlock joint.
Radiographs showed several smaller fragments within the joints and an ultrasound examination showed damage to one of the sesamoidean ligaments. It was suspected that Jack had had an acute traumatic event whereby the foot stayed on the ground while the body and limb turned, resulting in a twist at the level of the fetlock joint.
Jack had surgery to remove the fragments and underwent a period of box rest and a controlled exercise rehabilitation programme, with repeated clinical and ultrasonographical examinations every three months.
Following nine months of rest and controlled exercise rehabilitation program (and lots of hard work by his owner) Jack was sound and returned to full work.
Ref Horse & Hound; 12 November 2020
