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.
Damage and degeneration
Spontaneous overstrain injuries to the DDFT are rare in the proximal limb (near the attachment) and most frequently occur within one of the regions enclosed by a tendon sheath or bursa.
The reason for this is unclear, but injuries are more common where the tendon is under compression from a bony prominence. Damage is thought to be degenerative and the tendon often contains areas of mineralisation as a result.
Injuries can affect the internal parts of the tendon (core lesions or mid-substance tears), the outer surface of the tendon (border lesions or tears), or the insertion site of the tendon on the coffin bone.
In some cases, although usually only those where there has been damage to the adjacent sheath wall, adhesions can also form, which can limit tendon movement and cause pain. Paradoxically, this can improve healing of the tendon itself, as adhesion can supply cells and vasculature to the lesion and seal it from the synovial environment.
Most DDFT injuries affect the forelimb and cause lameness, which is usually variable, unilateral and persistent. Traumatic injuries are also quite common and usually occur in the pastern region where the back of the leg comes close to the ground when bearing full weight at speed. When the DDFT is either completely severed by an external injury, or spontaneously ruptures, the characteristic sign is the toe coming off the ground when the horse bears weight on the limb and can result in partial dislocation of the coffin joint.
What to look for?
External clinical signs can vary, but the obvious clue is swelling.
A lesion within the confines of the digital sheath usually causes sheath distension, resulting in asymmetric windgalls. The degree of distension varies but is usually more marked where the injury involves the border of the tendon and is in contact with the synovial environment.
Lesions within the hoof show few external signs, although careful palpation of the two lobes of the DDFT between the bulbs of the heel can sometimes reveal pain. The horse may rest the limb when in a stable by “pointing” (placing one hoof in front of the other).
These clinical signs are far from specific, so localising lameness using nerve, joint, tendon sheath or navicular bursal “blocks” is crucial.
For injuries within the digital sheath, instilling local anaesthetic into the sheath will usually resolve lameness. This can be combined with radiographic contrast to give useful diagnostic information on the presence of a tear and the tightness of the annular ligament at the level of the fetlock.
Ultrasonography is effective in the digital sheath area, where mid-substance core lesions can be easily visualised. Border tears have been more challenging, however, although modifications to the examination procedure can improve the frequency of identifying them.
MRI has proved the most useful imaging technique for revealing lesions within the hoof, once lameness has been localised.
Which treatment is best?
As with most soft tissue injuries, the mainstay of treatment for DDFT injuries is rest and rehabilitation. Long periods of rehabilitation are necessary, often of up to a year, consisting of rest followed by a slowly ascending exercise regime.
Corrective trimming and shoeing can help lesions within the hoof, where every degree of increase in the downward angulation of the coffin bone results in a 4% reduction in loading of the deep digital flexor tendon and the navicular bone.
Provided there is adequate sole thickness, this can be achieved by trimming the sole at the toe region, preserving the heels and shortening the toe. Raising the heels with a graduated shoe or plastic wedge inserts can also help, although their long-term use is thought to be associated with heel collapse.
Lesions communicating with the synovial cavity of the tendon sheath or bursa result in inflammation (tenosynovitis or bursitis), and healing is further inhibited by the synovial fluid.
Hyaluronic acid, a key component of the lubricating properties of synovial fluid, can be injected into the tendon sheath or bursa in an attempt to limit adhesion formation. Inflammatory tendon sheaths and bursae are frequently treated with corticosteroids, although their use can delay or halt healing and should be avoided when a healing tendon injury is identified.
Injury to the external border of the tendon can result in the prolapse of damaged tendon fibres into the tendon sheath or bursa. These can remain in the synovial environment, causing inflammation and lameness. Minimally invasive surgical debridement (removal) of these fibres may be justified, using tenoscopy or bursoscopy. The lesion still needs to heal, however, which does not always occur.
Contained lesions may benefit from intra-lesional treatment with mesenchymal stem cells or platelet-rich plasma (PRP). Success rates for these treatments in DDFT injuries are largely unproven, although data for the treatment of superficial digital flexor tendon injuries with stem cells show significant benefit. At present, injecting stem cells or PRP solely into the tendon sheath does not appear to improve outcome.
DDFT injuries are serious and have a high failure rate — evaluation indicates a positive prognosis of 30-50%. Careful choice of treatment and meticulous rehabilitation can enhance the prospect of a return to performance.
Ref: Horse & Hound; 5 May 2016