Long Digital Extensor (LDE) Tenosynovitis
The tendon of the long digital extensormuscle is the primary structure used to extend the horse's hind limb. It extends the length of the dorsal tarsus (hock) and metatarsal (hind cannon bone) to eventually insert on the extensor process of the third phalanx (coffin bone) in the foot. The long digital extensor (LDE) tendon is joined by the tendon of the lateral digital extensor muscle below the level of the hock joint.
The LDE tendon is held in position along the front of the hock by three retinaculi, which are transverse bands of fibrous tissue (highlighted with red arrows in the diagram above).
Like all tendons coursing over or under joints, the LDE tendon is housed within a synovial sheath. The sheath functions as a "lubricating sleeve" that allows for easier movement of the tendon during flexion and extension of the limb. The sheath contains synovial fluid identical to that found in joints. The fluid lubricates the tendon as it slides within the sheath.
Occasionally, horses will hyperextend the LDE tendon. This usually occurs as a result of forward buckling of the limb at the level of the fetlock joint (which then often contacts the ground surface). This type of buckling is most frequently attributed to upward patellar fixation and/or distal patellar hyperextension in the horse.
Transient "catching" of the stifle followed by an unpredicted release of the patella (i.e. knee cap) triggers hindlimb buckling. Many riders describe the feeling of hindlimb buckling as "though the horse is falling through a trap door".
We therefore observe a strong correlation between horses that have sustained LDE hyperextension and those with a history of biomechanical stifle interference. Concurrent evidence of a wound or scar along the dorsal aspect (front) of the fetlock joint is especially suggestive of a connection between the two.
As you might guess, hyperextension (i.e. excessive stretching) of the LDE tendon triggers an inflammatory response within its sheath. This, in turn, results in extra synovial fluid accumulation (termed "effusion"). Effusion within the sheath is the primary indicator of tenosynovitis, which denotes inflammation associated with the tendon and/or its sheath housing.
Horses with LDE tenosynovitis display characteristic swelling along the dorsal aspect (front) of the hock.
In most cases, those regions of the sheath not restricted by the fibrous retinaculi exhibit the majority of gross swelling. Each retinaculum pushes extra fluid above and below its margins, resulting in the creation of several synovial fluid "pockets" along the length of the sheath. The specific locations of the retinacular bands are visible as small transverse "indentations" (indicated by red arrows in the photograph below) which separate the enlarged segments of the sheath.
LDE tenosynovitis rarely causes lameness and is typically considered to be a cosmetic issue.
Nevertheless, inflammation present within the LDE sheath tends to "linger" once it develops. In rare cases, the swelling resolves spontaneously; in the vast majority of cases, however, the swelling persists until it is treated.
Treatment usually involves drainage of the fluid via needle aspiration and subsequent injection with a combination of synthetic hyaluronan, atropine and/or corticosteroid. The cost to perform this procedure (including medications and sedation) is approximately $250.
Recurrence rate following treatment is approximately 50%.
The relatively high rate of recurrence coupled with the fact that affected horses are rarely lame prompt most clients to bypass treatment and "live with" the swelling.