Capped Joints
Also Known As: Acquired Bursae — Acquired Pseudobursitis — Capped Hocks — Cunean Bursitis — Capped Elbows — Olecranon Bursitis — Shoe Boil — Bursitis — Pseudobursitis — Hygroma — Carpal Hygroma
Introduction
Capped joints are fluid filled swellings which occur over the dorsal surfaces of the carpi, points of the elbows or plantar surfaces of the hocks. They can be unilateral or bilateral.
They almost always originate from traumatic injury, resulting in formation of an acquired subcutaneous bursa where no such physiological structure exists. The cells lining the bursa then start to produce a fluid not unlike synovial fluid. The tendon sheaths of the extensor carpi radialis, common digital extensor, may also be involved and these cases carry a poorer prognosis than uncomplicated ones.
The main concern with most acquired bursae is cosmetic.
Distribution
Affects horses worldwide.
Signalment
Horses getting up and down on hard ground are more prone to development of capped joints, particularly carpal hygromas. They can also be produced by repetitive pawing and striking of a hard surface such as wall or stable door. These behaviours can also cause capped hocks.
Draft breeds are more prone to capped elbows. These are caused by the shoe of the affected limb striking and traumatising the point of the olecranon when in motion or when lying down. Thus, this type of capped joint is also seen in horses with exaggerated motion or poor conformation.
Clinical Signs
Swellings over the joints may take on various shapes but most feel palpably like fluid filled structures. Capped elbows form a characteristically mobile swelling over the point of the olecranon tuberosity. Capped hocks form fluctuant swellings at the proximal limits of the calcaneal tuberosity.
Chronic bursae have thicker walls than acute ones and may form fibrous bands and septa within their cavities.
Swellings are usually cold and generally not painful; hence lameness is rarely a feature, although there are exceptions to this. If the bursa becomes very large, it may physically interfere with motion of the joint.
If bursae rupture, fibrous tissue and often exuberant granulation tissue form. Discharging sinuses may feature if secondary infection is present.
Diagnosis
Drainage of the fluid with a hypodermic needle can be performed and the contents sent for cytological analysis. Fluid is usually serous if acute and strongly resembles synovial fluid if more chronic in nature. It may be more viscous than synovial fluid and contain mucin plugs, but resemble it in other constituents.
Contrast radiography should be used if there is any concern about involvement of joints or tendon sheaths or about possible fistula formation.
Ultrasound examination may help to rule out other causes of the swelling. It will also detect the septation and cavitation within chronic bursae.
Treatment
Most cases respond to drainage and elastic bandaging, often accompanied by corticosteroid injections into the swelling. Extreme care should be taken when performing these procedures so as not to introduce infection into a region so close to joint spaces and tendon sheaths as consequences can be disastrous.
The skin distension and thickening often does not completely resolve and therefore may leave a permanent visible abnormality which horse owners should be informed about.
Cases that do not respond are usually of a more chronic nature and can be drained by making a larger vertical incision into the most distal aspect of the swelling and then debriding the inner surface of the swelling, manually or chemically with iodine tincture. A penrose drain should also be placed and covered with sterile bandaging for 10-14 days before removal by which time adhesions should have formed.
Very severe and long standing hygromas can be surgically resected en-bloc under general anaesthesia.
Control
Prevention of behaviour and movements that cause and aggravate acquired bursae is key but often very difficult. Stable bandaging and rest at appropriate times may help.
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References
Adams, O. R., Stashak, T. S (2002) Adams’ Lameness in Horses 5th Ed. Lippincott, Williams & Wilkins, Philadelphia, USA.
Stashak, T. S., Hill, C (1995) A Practical Guide to Lameness in Horses. Lippincott, Williams & Wilkins, Philadelphia, USA.
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