Dr Peter Gillespie. BVSc MACVS.
Injuries to the suspensory ligament are a common occurrence in athletic horses. They can occur in both the fore and hind legs and have the potential to bring a horse’s competitive career to an end.
Where is the suspensory ligament and what does it do? To describe it in simple terms, it runs down behind the cannon bone between the knee and the fetlock in the fore leg and between the hock and the fetlock in the hind leg.
To be more precise, in the fore leg it originates on the distal row of carpal bones at the back of the knee and on the back of the upper part of the metacarpus (cannon bone). In the hind leg, it originates mainly on the upper metatarsus, although there are some attachments to the distal row of tarsal (hock) bones.
Two thirds of the way down the metacarpus (or metatarsus) it divides into medial and lateral branches which continue down to attach to the outside surface of the sesamoid bones at the fetlock joint. From there it continues below the fetlock as lateral and medial extensor branches which insert on the Common Digital Extensor tendon at the front of the pastern between the fetlock and the foot.
It is interesting that the suspensory ligament is actually a modified muscle. It’s anatomical equivalent in animals with more than one toe is the medial interosseous muscle. In the horse the suspensory ligament is made up of predominately tendon fibres with some residual muscle fibres. The number of muscle fibres varies between individual horses and between breeds. Standardbred horses have a higher proportion than thoroughbreds.
The suspensory ligament along with the sesamoid bones and distal sesamoidean ligaments make up what is known as the suspensory apparatus. Its function is to support the fetlock joint during the weight-bearing phase of the stride.
It is during this phase that most suspensory ligament injuries occur. Uneven loading of the limb during weight bearing is the main contributing cause helped in many cases by an uneven ground surface and poor foot balance. Overloading of the ligament leads to tearing of collagen (tendinous) fibres and the small blood vessels associated with the muscle fibres. There is bleeding within the ligament with the formation of a haematoma.
The healing process proceeds through three steps;
(1) Removal of damaged tissue by phagocytes (white blood cells).
(2) Migration of fibroblasts into the area to start producing new collagen (scar tissue).
(3) Remodelling of scar tissue.
The early scar tissue is organized in a haphazard manner. During the first 2-3 months the collagen fibres orientate themselves in a parallel alignment and slowly increase in diameter.
The repaired tissue is not as strong or as elastic as a normal ligament tissue and as such is predisposed to re injury. This is an important point when assessing the prognosis for a successful return to competition.
The inflammatory changes associated with the tearing of the collagen fibres produce the characteristic signs of heat, swelling, pain and reduced function. The term for inflammation of the suspensory ligament is desmitis. We recognize six distinct conditions.
1. Avulsion of the origin of the suspensory ligament
This condition usually affects the forelimbs. It involves a tearing of the attachment of the ligament at the back of the metacarpus. Signs can vary from an acute, severe lameness to a chronic recurring lameness that can be difficult to pinpoint. Swelling is not always obvious because the ligament at this point is surrounded on three sides by bone (cannon & two splint bones). A common feature of lameness associated with this type of injury is that it is usually worse when the horse is trotted in a circle with the injured leg to the outside. Diagnosis is usually dependent on nerve blocks, ultrasonography and radiography Treatment involves box rest for the first 4-6 weeks followed by 6-8 months paddock rest. The prognosis is good for a return to competition without recurrence of the injury.
2. Proximal suspensory desmitis
affects the suspensory in the uppermost quarter. It is a common cause of both fore and hindleg lameness. Again there is often no swelling with this condition due to the surrounding boney structures. Signs can vary greatly from a pronounced lameness to un-diagnosed poor performance. Lameness can be exacerbated by either trotting in a circle with the affected leg to the outside or by flexion of the fetlock joint. Diagnosis can be confirmed by nerve blocks and ultrasonography. The preferred treatment is a combination of box rest and controlled exercise. Box rest is necessary for the first 4-6 weeks to complete the initial phase of the healing process. Six months of controlled exercise helps the remodeling process by stimulating better alignment of the collagen fibres. The prognosis with proximal suspensory injuries varies with the age of the injury prior to diagnosis. Acute injuries are more likely to respond to treatment with a successful return to competition than are chronic injuries.
3. Desmitis of the suspensory body
The body of the suspensory is defined as the section between the upper proximal quarter and the bifurcation to the medial and lateral branches. This injury is more common in the fore legs. Swelling is generally a feature, as is pain on palpation. Lameness is usually not an early sign, in fact nine times out of ten swelling will precede any lameness. Because the body of the suspensory ligament lies close to the narrower diameter sections of the splint bones, swelling in the ligament can put pressure on these bones, causing them to fracture. For this reason it is advisable to take radiographs when this type of injury occurs. Again initial box rest and anti-inflammatory therapy followed by controlled exercise is the preferred treatment. If a splint bone is fractured it should be removed surgically. The prognosis with this type of injury is guarded.
4. Desmitis of the branch of the suspensory ligament
This is the easiest suspensory injury to diagnose because of the obvious swelling that fills the natural hollow between the ligament and the cannon bone. The swollen branch is always painful on palpation. Lameness is usually not a feature. Box rest, anti-inflammatory therapy and controlled exercise are important in the recuperative phase but the prognosis with this type of injury is poor.
5. Suspensory desmitis secondary to splints or fractured splint bones
Splints are a common occurrence in young horses. Besides being a source of pain, they can encroach on the adjacent ligament causing a focal desmitis. Often it is the less obvious splints (blind splints) that cause problems.
6. Suspensory Breakdown Injuries
Complete failure of the suspensory apparatus occurs from time to time as a high speed injury in thoroughbred racehorses and eventers. It is due to acute over-loading of the support structures of the fetlock during the weight bearing phase at the gallop. Either the suspensory ligament or the sesamoid bones break down depending on the fitness of the horse. The fitter the horse, the more likely it will break its sesamoid bones. This suggests that the suspensory ligament strengthens with training. There is an acute, severe lameness with this condition with dropping of the fetlock joint. Salvage for breeding is the only option.
Apart from avulsion injuries, the chance of a full recovery with a return to competition is generally poor. During the first 10-14 days post injury it is important the horse be confined and aggressive anti-inflammatory therapy instigated. This should consist of the early application of cold packs and bandaging combined with systemic medication. By keeping the initial inflammatory reaction to a minimum, the amount of damaged tissue that has to be later removed and remodeled is reduced.