The horse’s limbs have evolved as bony columns with palmarly (back of the leg) situated tendons that act as springs to store energy from each stride. As a result the horse has very long flexor tendons, of which there are two — the superficial digital flexor tendon, which has most of the spring-like function, and the deep digital flexor tendon, which serves more of a positional and supportive role for the foot (see diagram, below).
The deep digital flexor tendon (DDFT) is in excess of 50cm in length in the adult horse, attaching to muscle above the carpus (knee) in the forelimb and the hock in the hindlimb. It runs down the back of the leg, beneath the superficial digital flexor tendon, around the fetlock and inserts into the back of the coffin bone inside the hoof.
Where it changes direction around a number of joints that flex and extend, the tendon is under compression and is surrounded by structures known as tendon sheaths or bursae. These contain synovial fluid, ensuring that movement is almost frictionless. Tendon tissue in these regions resembles cartilage, containing significantly greater amounts of pressure-resisting proteins with fewer blood vessels.
These adaptations result in limitations to the normal healing mechanisms and so make treating injuries of this structure challenging.