Tenosynovitis of the tarsal sheath (thoroughpin)
The tarsal sheath is a synovial fluid-filled structure that runs down the hock. Tenosynovitis, inflammation of the tendon sheath, can be caused by trauma, infection or, most commonly, be idiopathic (i.e. no obvious cause). Inflammation and subsequent swelling of this structure is known as a thoroughpin.
Signs
All cases will have an obvious enlargement of the sheath usually most visible towards the top of the hock. Lameness can vary from a non-weight bearing lameness with infection to no lameness at all with the idiopathic form.
Diagnosis
Infectious tenosynovitis is usually diagnosed by analyzing a sample of the synovial fluid. Ultrasound scanning is often useful in finding the underlying cause. X-rays should be taken to investigate potential traumatic causes especially if the horse is lame.
Treatment
There is no treatment required for the idiopathic form. Sheath infection is best treated by flushing the sheath surgically, usually via arthroscopy (key hole surgery), combined with antibiotic therapy. The affected limb should be bandaged tightly and the horse placed on box rest.
Traumatic tenosynovitis is initially treated with rest and anti-inflammatory drugs such as phenylbutazone (‘bute’). Agents are often injected directly into the sheath such as corticosteroids and hyaluronic acid to reduce the inflammation.