With the emergence of advanced imaging techniques such as magnetic resonance imaging (MRI) and computed tomography (CT) in veterinary medicine, our understanding of the internal construction of the foot has improved greatly. We are no longer restricted to using the generic term “navicular disease” as a diagnosis based on radiographs (X-rays), but instead can see the plethora of structures within the hoof capsule, referred to as the podotrochlear apparatus.
The podotrochlear apparatus comprises various joints, bones and tendons, as well as key ligaments: the collateral ligaments of the distal interphalangeal joint (sometimes called the coffin joint), the distal sesamoidean impar ligament and the collateral sesamoidean ligaments.
While tendons and ligaments are similar in structure, with both being made up predominantly of bundles of type one collagen strands, or “fibrils”, they differ in that tendons attach muscle to bone and ligaments attach bone to bone. This has an effect on function, in that tendons serve to move a bone or joint, whereas ligaments serve to hold structures together and keep them stable.
Casing the joint
How do the foot ligaments operate — and what can cause them to fail?
The collateral ligaments of the distal interphalangeal joint are paired, with one on the lateral (outside) and a second on the medial (inside) aspect of the joint.
They originate on the middle bone segment, or phalanx, and run obliquely to insert on the distal phalanx (distal meaning furthest from the body).
The role of these ligaments is to provide stability to the joint, so they can become damaged if the joint slides or rotates. The ligaments can also degenerate with age — small, repetitive stresses may culminate in damage, as well as a single, large injury.
Damage to the collateral ligaments will cause a variable degree of lameness, usually in one or both forelimbs, which worsens when the horse is turned or lunged. Sometimes, there may be swelling above the coronary band. In most cases, the ligament will be damaged more distally, within the hoof capsule, so there will be no abnormalities on clinical examination. Joint instability will be evident only where the ligament is severely damaged.
Diagnosis is made after localising the lameness to the foot using a nerve block. Some more proximal lesions (those nearer to the body) may be identified on ultrasound, and occasionally a bony reaction at the ligament’s origin or insertion will be evident on X-rays. Usually, MRI or CT is required to identify collateral ligament injury accurately.
Rest is central to rehabilitation, with a minimum of two months’ box rest followed by four months’ walking in straight lines — perhaps on a water treadmill.
Other treatment options in cases with a suitable and accessible core lesion include injection of the ligament with platelet-rich plasma (PRP) or mesenchymal stem cells.
Where there is associated synovitis (inflammation of the membrane surrounding a joint) or osteoarthritis of the distal interphalangeal joint, medication of the joint with corticosteroids, hyaluronic acid or a natural anti-inflammatory called interleukin receptor antagonist protein (IRAP) can be considered.
Farriery is important in the management of these cases, with strict attention being paid to the mediolateral (side-to-side) balance of the foot. Some farriers will choose a wider shoe branch on the injured side in such cases; others prefer leverage-reduction shoes designed to relieve pressure on these structures.
Chronic changes
The most common issue with the distal sesamoidean impar ligament is chronic degenerative change as a result of ageing, often in conjunction with damage to the navicular bone, navicular bursa and deep digital flexor tendon.
Damage to the impar ligament is best investigated using the superior quality of high-field MRI. Because the ligament is quite flat, it is difficult to image on low-field (standing) MRI and may be missed on thicker CT image “slices”. Even so, standing MRI can identify damage to the bone at the insertion of the ligament, allowing diagnosis of impar ligament injury.
Ultrasound of the impar ligament through the frog is possible, although recent research has shown a wide variation in the ultrasound appearance of the ligament in sound horses. A portion of the ligament can also be seen on arthroscopy (keyhole investigation) of the navicular bursa.
Distal border fragments of the navicular bone may also be evident in association with impar ligament injury, but the clinical significance of these needs to be interpreted with caution, as this can also be found as an incidental finding in sound horses.
As with the impar ligament, injury to the collateral sesamoidean ligaments rarely occurs in isolation. Damage may be suspected when X-rays reveal new bone formation around the navicular bone at the ligament’s insertion. As this may also be found in sound horses, MRI or CT imaging is required for accurate identification.
Rest and rehab
Treatment of chronic impar and collateral sesamoidean ligament injury is similar and will depend on the degree of concurrent damage to other structures of the hoof capsule. Generally, rest is advised, except where the degenerative change to the navicular bone or distal interphalangeal joint is the primary problem, in which case a compromise must be reached.
Commonly used medications include anti-inflammatories such as phenylbutazone (bute) and bisphosphonates such as tiludronate or clodronate.
Good farriery is key, with attention being paid to foot balance. Different farriers have achieved successful results with a variety of shoes, such as natural balance, egg-bar, heart-bar and straight-bar. Elevating the heel can help to relieve pressure from the deep digital flexor tendon on the palmar (rear) aspect of the navicular bone.
Other treatment options such as shockwave therapy and the introduction of mesenchymal stem cells have been used, but scientific evidence to support their effectiveness is lacking.
Rarely, acute injury to the impar ligament, thought to be caused by trauma, has been reported. Both arthroscopy of the navicular bursa and conservative management have been tried, but the scarcity of such cases makes it impossible to give exact treatment recommendations.
Case study
A nine-year-old Irish Sport Horse mare came to us at the Bell Equine veterinary clinic for MRI examination. She had moderate left forelimb lameness when lunged on the right rein, which had first become apparent nine months previously. This lameness disappeared with a palmar digital nerve block.
Radiography of the mare’s feet revealed no abnormalities. When she was rested, she became sound, but the lameness reappeared when she returned to work. Again, a palmar digital nerve block located the lameness in her foot.
The MRI images revealed damage to the distal sesamoidean ligament, with a large cystic lesion in the distal phalanx at the insertion of the ligament (indicated by arrows). The mare is being rested, as treatment is limited when the damage is located so deep within the foot.
Ref Horse & Hound; 19 July 2018
