The significance of swellings on the horse’s hock *H&H Plus*

Joint problems

If a puffy lump appears on a hock joint, how worried should we be? Matt Smith MRCVS discusses the significance of swellings in this area...

Fluid-filled swellings around the hock are not uncommon. Some may be innocuous, but others represent a potentially significant injury that requires investigation and treatment.

It can be difficult for even the trained eye to discern whether a particular lump is worth worrying about, but it may be useful to ask yourself the following:

Is it a new swelling?
Has lameness developed alongside its appearance?
Is it warm, or painful to touch?
Is it large or tense?

If the answer to any point is yes, it would be prudent to consult your vet. Conversely, if it has been there as long as you can remember and doesn’t appear to be causing a problem, it may be sufficient to monitor the swelling but otherwise leave it alone unless changes occur.

A complex structure

Why do swellings arise?

The hock is complex, comprising at least six different bones, four major joints, various supporting ligaments and a number of tendons. As the muscles above the hock contract, they pull on the tendons and cause the leg to flex and extend.

To ensure a smooth, gliding action, the tendons are bathed in synovial fluid – a thick, lubricating substance found in capsules called tendon sheaths and bursae. The difference between these is purely anatomical: sheaths encircle one or more tendons, whereas bursae are interposed between two surfaces, such as tendon and bone or cartilage.

The swelling of a joint, tendon sheath or bursa occurs as a response to inflammation – spaces open up in the synovial membrane (the tissue lining the capsule), allowing more cells, protein and fluid to enter and causing visible swelling. This inflammation of the synovial membrane (known as synovitis) is a sign of an underlying problem, rather than being the problem itself.

Common causes of joint swelling include chip fractures, arthritis and the developmental condition osteochondritis dissecans (OCD), which results in loose fragments of bone or flaps of cartilage within the joint.

Tendon injuries usually cause swellings in tendon sheaths and bursae. At the milder end of the spectrum, synovitis with no underlying structural injury can occur following periods of hard work.

There is no known link with poor conformation – nor are these injuries more frequent in a particular discipline or type of horse. While most causes must be a result of over-stressing a tendon or the affected structure, their occurrence is sporadic. It’s a bit like twisting your ankle – there’s little you can do to reduce risk.

Rest and recovery

In the acute phase of injury, shortly after the swelling develops, allow the horse to rest.

Cold-hosing and anti-inflammatory treatments, such as phenylbutazone (commonly termed bute) or meloxicam, will help in the early stages, in consultation with your vet. Bandaging can bring swelling down, but the hock is a difficult area to bandage properly and skin sores are a risk.

After the acute phase has passed, typically 10 to 14 days later, the next step will depend on response to this initial treatment. A gradual return to work may be possible, but any persistence of significant swelling and lameness should be investigated further.

If the swelling has reduced significantly and the horse is comfortable, it may be appropriate to drain and medicate the remaining fluid. Most commonly this is done with a small dose of steroids, which are potent anti-inflammatory drugs. This can be successful with mild injuries, but with anything significant it is likely that the swelling will recur.

Further investigation commonly includes X-ray, to enable identification of injury to the bones, such as chip fractures, and ultrasound to examine the tendons, ligaments and other soft tissues that make up the tendon sheaths and bursae.

If these methods fail to identify the underlying cause, arthroscopic (keyhole) surgery may be appropriate. Termed tenoscopy for tendon sheaths and bursoscopy for bursae, its use in equine medicine has progressed rapidly over the past 15 years and has revealed a range of possible causes, from tears along the margin of tendons and damage to their appendages, to tearing of the outer wall of the tendon sheath or bursa.

Keyhole surgery also allows treatment, by debridement (cleaning) and removal of disrupted tissue, which has vastly improved the prognosis of injuries. Because so many more cases have now been treated this way, we also have a much better idea of the chances of obtaining a successful outcome.

Know your hock swellings

Thoroughpin: fluid swelling of the tarsal sheath, which lubricates the deep digital flexor tendon as it runs over the back of the hock on the inside. A more severe case may be caused by injury to the deep digital flexor tendon or the bone that the tendon runs over.

Action – if swelling causes a problem, draining the fluid and medication with anti-inflammatories may help. Significant lameness warrants further investigation.

Calcaneal bursitis: swelling of the deep calcaneal bursa, which acts to lubricate the superficial digital flexor tendon as it runs over the point of the hock. This usually indicates injury, typically of the superficial digital flexor tendon or its thick attachments to the point of the hock.

Action – the most severe injuries involve a slipped tendon, where one of the attachments has completely ruptured. Surgery may be necessary.

Curb: a characteristic bow-like swelling of the plantar ligament at the back of the hock, a curb is most easily viewed from the side. Other, “false” curbs include development of a bursa in the region or injury to the superficial digital flexor tendon.

Action – curbs often settle of their own accord, but leave a permanent thickening. You should consult your vet if swelling is accompanied by lameness.

Capped hock: a distinctive swelling at the point of the hock, caused by distension of the most superficial of the calcaneal bursae. A bursa at the point of the hock is variably present in horses. Swelling most commonly occurs following some kind of direct trauma, such as backing into a solid object.

Action – in most cases, nothing needs to be done. Swelling may be reduced by draining the bursa and injecting anti-inflammatories. A wound with obvious lameness indicates an infection, requiring more urgent treatment.

Bog spavin: distention of the main joint of the hock – the tarsocrural joint – appearing as excess fluid in the upper part of the hock. In young horses, and those just starting work, the most common cause is OCD. Others include injury to the soft tissues, such as the capsule or ligaments providing support on either side of the joint.

Action – mild, soft swelling of this joint in both limbs on its own is not a cause for concern. New or prominent swellings, or those with lameness, should be examined. Severe lameness may indicate a puncture wound – a veterinary emergency.

Synoviocoele: A large, fluid-filled swelling seen towards the back of the hock on both the inside and outside of the calcaneal tendon. This typically develops when fluid leaks out through a rent (tear) in the wall of the tarsal sheath and the swelling may be pushed from one side to the other.

Action – rest is required to allow inflammation to settle. If the swelling continues to enlarge, or fails to resolve, options include drainage and anti-inflammatories. Keyhole surgery allows enlargement of the rent to prevent it acting as a one-way valve.

The author: Matt Smith MRCVS is a surgeon and partner at Newmarket Equine Hospital. This referral hospital is one of the largest in Europe, treating a wide range of leisure and sport horses from throughout the UK. Matt is a Royal College of Veterinary Surgeons (RCVS) specialist in equine surgery and has a particular interest in orthopaedic problems. 01638 782020, newmarketequinehospital.com

Ref Horse & Hound; 14 May 2020

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