Bit & Bridle

Equine Carpal Hygroma

Other Names

  • Capped Knee
  • Carpal Bursitis
  • Hygroma of the Knee
  • False Bursa of the Carpus

What is Equine Carpal Hygroma?

An equine carpal hygroma is a soft, fluid-filled swelling on the front of the knee (carpus). It is caused by the formation of a false bursa — an adventitious fluid-filled sac that develops in response to repeated trauma or a single significant blow to the carpus. Unlike true bursae, which are natural anatomical structures, a hygroma is an acquired structure that the body creates to protect the area from further pressure or friction.

Carpal hygromas are distinct from other forms of knee swelling such as joint effusion (“big knee” from intraarticular fluid) or fractures. The hygroma sits directly over the front of the carpus, subcutaneously, and does not directly communicate with the carpal joints unless there has been significant tissue damage. This distinction is important because it affects both the risk associated with the swelling and the treatment approach. Most carpal hygromas do not cause lameness and are primarily a cosmetic concern, though infected hygromas are a serious complication requiring urgent treatment.

Causes

Carpal hygromas develop from repeated or acute trauma to the front of the knee:

  • Concrete or hard flooring: Horses kept on concrete or thinly bedded floors that kneel or rest with knees contacting the floor develop pressure trauma to the carpus.
  • Fence or stable injuries: Horses that paw, strike fences, or bang their knees against stable fixtures.
  • Falls: Horses that stumble and fall onto their knees sustain direct trauma that can initiate hygroma formation.
  • Jumping: Horses that repeatedly hit fences with their knees — common in young or inexperienced jumpers — may develop carpal hygromas.
  • Poorly fitted or worn boots: Ill-fitting brushing or tendon boots that create pressure over the carpal region during exercise.
  • Conformation: Horses with “knuckling over” or a tendency to hit their knees on landing may be predisposed.

Symptoms and Signs

  • Soft, fluctuant swelling: The characteristic fluid-filled swelling directly over the front of the knee. It is usually not painful unless acute or infected.
  • Varies in size: Hygromas can range from a small, golf-ball-sized swelling to a large mass covering much of the front of the knee.
  • Thickening: Chronic hygromas develop fibrous walls and become firmer over time.
  • Skin changes: Hair loss, callus formation, or skin thickening over the point of the hygroma in longstanding cases.
  • Usually non-lame: The majority of horses with carpal hygromas are not lame. Lameness, sudden increase in size, heat, or pain should raise concern about infection or joint involvement.

Diagnosis

  • Clinical examination: The location and character of the swelling are usually diagnostic. The hygroma is distinct from joint effusion, which fills the joint pouches (visible above and to the sides of the knee) rather than presenting as a single mass over the front of the knee.
  • Ultrasonography: Useful to differentiate hygroma from joint effusion, assess wall thickness, and identify any internal structure to the fluid. Can also assess whether the hygroma communicates with any of the carpal joint compartments.
  • Radiography: Useful to assess for underlying bony changes and to definitively rule out fracture — important after acute trauma. Also helps identify calcification within a chronic hygroma.
  • Aspiration: Drawing fluid from the hygroma for cytology and culture. Normal hygroma fluid is clear to straw-coloured. Cloudy, purulent, or bloodstained fluid is abnormal and suggests infection or haemorrhage.

Treatment

Management of carpal hygroma depends on the duration, size, and whether there are complications:

  • Remove the cause: Essential and the most important step. Improve bedding, remove hazards, ensure boots fit correctly, and address jumping technique issues.
  • Conservative management: Rest, cold therapy, and NSAIDs for acute cases. Many small acute hygromas will reduce significantly if the cause is removed promptly.
  • Drainage and injection: Aspiration of the hygroma fluid followed by injection of a corticosteroid can reduce swelling. This may be combined with pressure bandaging (where possible) to prevent re-accumulation. Often requires multiple treatments and only works well if the cause of trauma is eliminated.
  • Drainage with sclerosing agents: In some cases, agents such as iodine solution may be introduced into the hygroma after draining to promote scarring and collapse of the false bursa. This technique is effective in some cases but carries a risk of infection.
  • Surgical excision: Removal of the hygroma sac is the definitive treatment for chronic, large, or cosmetically troublesome hygromas. Requires general anaesthesia. Post-operative wound healing can be challenging given the location, and cosmetic results are variable. Success rates are good in experienced hands but the procedure is not without risk.
  • Infected hygroma: Septic hygromas require aggressive treatment — drainage, lavage, antibiotics, and careful monitoring for joint involvement. If the hygroma communicates with the carpal joint, this is a serious emergency.

Prevention

  • Deep, quality bedding throughout the stable — rubber matting topped with straw or shavings provides good cushioning for the knees when lying down.
  • Inspect stables and fields for hazards that could cause knee trauma — fence rails, gate hinges, sharp edges.
  • Use well-fitting knee boots during exercise and transport for horses prone to stumbling or interfering.
  • For young jumpers, focus on correct gymnastic training before asking for large fences — horses that jump through the fences rather than over them are at risk.

Prognosis

The prognosis for carpal hygroma in terms of soundness is generally good — most horses remain sound and functional. The main concern is cosmetic. Small, recent hygromas have a reasonable chance of resolution if treated early and the cause removed. Chronic, large, or fibrotic hygromas are unlikely to resolve fully without surgery, and surgical excision, whilst effective, can leave visible scarring. Infected hygromas carry a more guarded prognosis, particularly if the carpal joint becomes involved.

When to Call the Vet

Contact your vet if:

  • Your horse has a swelling on the front of the knee that is warm, painful, or associated with lameness
  • A previously stable hygroma increases suddenly in size
  • The skin over the hygroma is broken or discharging
  • Your horse has recently fallen onto its knee and the swelling is acute and significant
  • You are unsure whether the swelling involves the joint itself

Frequently Asked Questions

Is a carpal hygroma painful for the horse?

In most cases, no — chronic carpal hygromas are not painful and do not cause lameness. The horse may show mild sensitivity when the hygroma is first developing, or if it is acute. If your horse is lame or the hygroma is hot and painful, this warrants veterinary assessment to rule out infection or joint involvement, which are more serious conditions.

How is a carpal hygroma different from a bone spavin or arthritis of the knee?

A carpal hygroma is a fluid-filled swelling beneath the skin on the front of the knee — it does not involve the joint itself in uncomplicated cases. Arthritis of the carpus (carpal osteoarthritis) involves joint degeneration and typically causes lameness, reduced range of motion, and bony changes visible on radiographs. Joint effusion from carpal arthritis fills the joint pouches in a characteristic pattern different from a hygroma. A vet can differentiate between these conditions on examination and with imaging.

Can my horse still be ridden with a carpal hygroma?

In most cases, yes — a hygroma without lameness does not prevent riding. However, the underlying cause of the trauma should be addressed, and any work that involves the horse contacting its knees against obstacles should be avoided or modified while the swelling is present. If the hygroma is the result of jumping technique issues, working on gymnastic exercises and gridwork under an experienced trainer’s guidance is sensible.

Will the swelling go down without treatment?

Small, acute hygromas have the best chance of spontaneous reduction if the cause is removed promptly. Larger or more established hygromas are unlikely to resolve without intervention, and chronic fibrous hygromas will not reduce without drainage, injection, or surgery. The key is early management — addressing the swelling and its cause as soon as it is noticed gives the best outcome.