Tendon sheaths are present in multiple locations around the body and serve to protect tendons when there is a change in the direction of pull on the tendon as it moves over mobile areas of the skeleton. There are various issues that can occur within a tendon sheath, from mild inflammation through to life-threatening infections.

Most people are familiar with the tendon sheaths that exist at the back of all four fetlocks, but other common sites are the back of the knee (carpal sheath), the front of the hock (extensor sheaths) and just above the point of the hock (lateral digital flexor sheath). Most of the time, these structures are innocuous, sitting like a sleeve around a tendon(s) and are barely visible. Once they become inflamed or injured, they fill with synovial fluid and become more obvious to the human eye.

Sometimes, even after appropriate treatment, they can remain enlarged and visible and form a blemish that remains for years. In this article we will look more closely at tendon sheaths and some of the problems horse owners can encounter with these structures.

Tendon sheaths are synovial-lined structures that form a sac around one or more tendons. The sac effectively has two surfaces that oppose each other, even though the lining is in fact one continuous structure. One part of the sac is directly attached to the tendon, whilst the other part of the lining doubles back as a second layer around the tendon, forming a cavity that is filled with synovial fluid. The area where the lining reflects back on itself effectively forms a bridge between the inside and outside layers and is referred to as the mesotendon and carries the nerves and blood supply to the tendon.

The synovial fluid that is produced lubricates the structure and allows the tendon to glide back and forth with minimal resistance. If the lining cells of the sheath are aggravated by traumatic or infectious events, they respond by producing more synovial fluid that expands the space within the sheath, causing the appearance of a balloon like structure around the tendon. In cases where an injury has opened the sheath, this balloon-like structure will not be seen, but a continuous flow of liquid will be detected around the injured area until the sheath can seal back and retain the synovial fluid within the sac.

TENOSYNOVITIS 

Tenosynovitis is the term given to an inflamed tendon sheath and can be primary (meaning the sheath is directly involved in the inflammation) or secondary (meaning it is due to injury of a structure within the sheath), infectious or non-infectious.

A primary tenosynovitis can occur from a direct impact injury to the sheath, whereas the cause of secondary tenosynovitis could include injuries or tears to the Superficial or Deep digital flexor tendons, sesamoidean ligaments or the manica flexoria within the sheath. Clinical signs in the early stages of the disease are similar, regardless of the inciting cause. These can include lameness, heat and swelling of the sheath, and pain on palpation, however, infectious cases usually exhibit a more marked lameness.

An ultrasound examination of the sheath and the internal structures should be performed in cases of active tenosynovitis to identify any lesions, as this will determine the best form of treatment, the anticipated time frame for recovery and the prognosis for the horse to return to work. In some cases where a lesion is suspected but cannot be identified with the ultrasound examination, an MRI can be performed to obtain a more accurate assessment of the soft tissue structures within the sheath.

Radiology is not commonly performed to aid in the diagnosis of tenosynovitis but is especially useful for cases with increased effusion in the carpal sheath, as one of the common causes of carpal canal tenosynovitis is the development of a bony tumour at the back of the radius called an osteochondroma. These are readily detected radiographically, and removal of the lump usually resolves the effusion.

For cases of tenosynovitis that do not resolve with conservative therapy or cases that recur when the horse returns to work, a tenoscopic examination can aid both the diagnosis and the treatment of the injury. Like an arthroscopic procedure, an endoscope is placed into the sheath so the internal structures can be directly visualised. This allows the surgeon to assess the tendons, debride any torn or frayed tendon fibres and remove thickened synovium or damaged manica flexoria if these are inciting causes.

Primary cases of tenosynovitis can be treated conservatively with ice, bandaging and anti-inflammatories, but those that are secondary will require resolution of the inciting cause to alleviate symptoms. Treatment with corticosteroids and hyaluronic acid injected into the sheath can be performed for cases of non-infectious tenosynovitis, but should be done with care as in my experience, once the initial anti-inflammatory effects of the cortisone have worn off, the swelling in the sheath can return and be larger than pre-injection, particularly if an attempt to drain the fluid from the sheath at the time of injection has occurred. Cortisone into the sheath may also retard the healing of damaged tendons and should never be used if the sheath or its contents are infected. If an infection is suspected, taking a sample of sheath fluid, referred to as synoviocentesis, can enable the white cell count and the protein level to be assessed, and the presence of an infection confirmed or dismissed.

“They can become rideable again.”

WIND GALLS

Wind galls, or wind puffs, are commonly encountered in horses of all disciplines. For the most part, these are blemishes that cause no lameness or issue with performance but can be difficult to reduce in size and are unsightly to look at. They are generally the result of a previous irritation or inflammation of the sheath that has occurred at some point in the horse’s past, causing an increase in the synovial fluid within the sheath that has stretched the lining beyond its normal state.

The initial cause of irritation to the sheath may not be detected but can include factors such as working young horses, especially overweight ones, on hard or very irregular surfaces, poor conformation, or bad shoeing that cause an overstretching of the sheath. Once this stretching occurs the sheath rarely returns to its original shape, leaving an enlarged sac that we commonly call a wind gall. It is easy to think of it as though the sheath was a balloon that has never been blown up — it is small and tight — but once the balloon has been blown up with air and stretched out, it rarely returns to size, even with all the air removed.

Thoroughpin is another example of distension of a tendon sheath, in this case, of the sheath around the lateral digital flexor tendon at the back of the hock. Depending on the inciting cause of the thoroughpin, it can cause lameness or can simply form a blemish in a similar way to digital sheath wind galls. For those cases of wind galls where the sheath intermittently swells and then settles, an underlying pathology within the sheath, resulting in a chronic, low-grade tenosynovitis, should be suspected.

INFECTION

Infection of the tendon sheath can be a life-threatening ailment, as it can be with any other infected synovial-lined structures such as joints or bursas. When bacteria are introduced into the sheath, usually due to a wound, they can proliferate quite readily due to the nutrient-rich synovial fluid, which provides an ideal environment for bacteria to thrive. Once the infection becomes established in the sheath, it can be particularly hard to eliminate as the bacteria can sit deep within the synovial lining or within fibrin strains and be protected from the normal body processes that remove foreign substances. Even by flushing the sheath with large amounts of fluids, the bacteria can remain lodged and protected and can continue to proliferate.

For this reason, many infected sheaths undergo tenoscopy as part of their treatment, allowing large areas of synovium and pockets of fibrin to be removed so the bacteria are exposed and can be physically removed in the process. Other methods of treatment include injecting medication directly into the sheath, using antibiotic infusion pumps or performing regional limb perfusion (RLP). RLP is a procedure whereby a tourniquet is applied to the limb, above the infected site, and antibiotic is injected into the vein below the tourniquet. The tourniquet effectively holds the blood in the region of the infection, allowing high concentrations of the antibiotic to penetrate the infected area and fight the infection. Often these procedures are required to be performed multiple times to overcome the infection and are performed in conjunction with prolonged administration of systemic antibiotics.

Following resolution of many of these tendon sheath infections in the limbs, the horse remains lame to some degree for many months afterward. The lameness usually responds to low doses of anti-inflammatory medication given periodically through this recovery, and then quite suddenly the lameness appears to resolve and the horse remains sound. Therefore, it is important to persevere with many of these lame horses once the infection has been cured, as they can become rideable again.

One of the complicating sequela of tenosynovitis, particularly in infectious cases, is the formation of adhesions and secondary thickenings of surrounding structures that restrict the smooth flow of the tendon through the sheath. Whilst these can be treated conservatively or medically, they often require further surgery to resolve the issue.

Although rare, there have been cases of tumours developing within tendon sheaths and these can usually be detected as an abnormality using ultrasonography or in difficult cases an MRI. A biopsy may be required to confirm the diagnosis and surgical removal of the tumour is usually necessary to resolve the inflammation. EQ

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