What is the difference between a mid bow and a low bow besides location? Well, a low bow usually means tearing of the outside tendon (the superficial digital flexor) within the digital sheath. Why is this an important factor?

First some simple anatomy.

The digital sheath is a membrane starting above the fetlock & extending all the way to just above the navicular bone in the foot. It surrounds the 2 flexor tendons and lubricates them with synovial fuid on their journey around the twists and turns of the lower joints during flexion and extension. This means that these tendons live in a fluid environment. This fluid provides nutrients & lubrication but this fluid can also be troublesome – remember this for our later discussion on the healing process.

When there is a bit of extra fluid in the tendon sheath we see a pouch of fluid above the fetlock called a windpuff. If we look closely fluid pockets can be felt below the fetlock on the sides of the flexors and between the heels right at the back of the pastern. The reason that we don’t see fluid elsewhere is that these are the locations where horizontal fibrous ligaments cross the tendons to keep them in place. There are 3 of them with fancy names such as the palmar annular ligament of the fetlock (PAL), the proximal digital annular ligament of the pastern and the distal digital annular ligament of the foot.

Tearing of the attachments of these structures to the bone frequently shows up as a calcification and the fancy word for this is an enthesiophyte. Enthesiophytes show up on ultrasound and on x-rays.

A small amount of fluid is normal and frequently noted in the hind limbs of many working horses. But, if the horse is lame & the tendon sheath is distended with fluid then the cause needs to be investigated. A low bow is usually fairly obvious but other causes include tearing of the sheath itself, other structures within it (such as the deep digital flexor) or ligaments adjacent to the sheath. Major ligaments include the palmar ligament (formerly called the intersesamoidean) & the distal sesamoidean ligaments (known as the x-y-z’s to many horsemen). Ultrasound examination is important to help diagnose the location of the problem and help with prognosis and surgical or therapeutic planning.

Does ultrasound always locate the problem? Afraid not. Core lesions are easily identifiable but sagittal splits along the length of the tendon can easily be missed since the pieces are still lying against one another. Flexing the limb during the exam is often helpful but some cases can only be diagnosed during tenoscopy.

What is tenoscopy? A minimally invasive surgical technique similar to arthroscopy where we look directly at tendons or ligaments that are within a tendon sheath. This “gold standard” has the drawback of requiring an anesthetic but has the advantage of confirming an ultrasound diagnosis and providing therapy.

Getting split tendons to heal back together in a fluid environment is problematic.

Ever try gluing anything together that’s wet? Suturing and biological glues such as fibrin/thrombin have been tried with poor results due to the tremendous forces at play. Presently the goal at tenoscopic surgery is to resect any torn tendon that is seen floating around. This “exposed collagen” is frequently responsible for the extensive reaction in the tendon sheath. Motorized resectors are used to trim diseased tissue and decrease the adhesions which are frequent in tendon sheath problems. We frequently inject the area with bone marrow derived stem cells and platelet rich plasma (PRP) before closing the small incisions.

If the tendon or ligament injury is outside of the sheath then surgery is not indicated. These areas are typically injected with PRP with or without stem cells. Non surgical therapy or adjunctive therapy includes shock wave therapy.

What about medicating the tendon sheath? Cortisone is used frequently. When injected into the sheath it will decrease inflammation but will not help tendon healing. I recently attended the first specialty course in tenoscopy/bursoscopy at Cornell University where the following comments were made. Hyaluronic acid is extremely short lived. PRP makes the most sense for tendon injuries since it has growth factors for healing. IRAP has no track record in tendon sheaths but is a natural anti-inflammatory with some growth factors as well – a possible alternative to cortisone? Time will tell as cases are documented and research continues.

Next time we’ll discuss lameness related to a sheath behind the knee – the carpal sheath!