Tendons can be an important source of lameness in athletic horses, but issues with the tendon's sheath–the thin connective tissue wrapped around the tendons, containing synovial fluid–shouldn't be overlooked as another potential cause of lameness.

"Diagnosing lameness originating from tendon sheaths is increasing with awareness and increased availability and use of magnetic resonance imaging (MRI)," reported Lisa A. Fortier, DVM, PhD, Dipl. ACVS, associate professor of Large Animal Surgery at Cornell University during her presentation at the 11th Congress of the World Equine Veterinary Association, held Sept. 24-27, 2009 in Guarujá, Sao Paulo, Brazil.

The three most commonly affected tendon sheaths are the:

• Digital tendon sheath, which wraps around both the superficial and the deep digital flexor tendons, and extends from just above the fetlock joint to the level of the pastern joint (proximal interphalangeal joint);

• Carpal tendon sheath, which also wraps around the superficial and deep digital flexor tendons in the back of the knee (carpus), extending several inches above and below the knee joint;

• Tarsal tendon sheath, which wraps around the superficial and deep digital flexor tendons near the back of the hock (tarsus) beginning at the top of the calcaneus (at the level of the tarsocrural joint) to a few inches below the tarsometatarsal joint.

"Diagnostic techniques most frequently used to identify tendon sheath injury or damage include ultrasonography and tenoscopy, which involves inserting an endoscope, the same instrument used for arthroscopy, into the tendon sheath to examine the sheath and the contents of the sheath including the tendons," explained Fortier