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Buyer Beware of Racehorses with Long Pasterns
The Horsemen''''''''s Journal - Winter 2009

by Toby Raymond

A horse’s pastern plays a vital role in its ability to absorb the concussive forces of every foot fall. As the front leg lands, it causes the elbow and knee to lock in place, calling on the fetlock and pastern to absorb the resulting shock. Working in tandem, they then disperse that energy among the bones, ligaments and tendons.

In a perfect world, the slope of a horse’s pastern would always match the angle of the hoof after it is trimmed to keep the bones – including the coffin bone, suspensory apparatus and joints – in proper alignment. Ideally, this would be viewed anywhere from a 45 to 55 degree angle on the front leg and a slightly shorter and more upright angle, about 49 to 59 degrees in back.

With that said, unfortunately it is not a perfect world, and horses whose conformation includes long, sloping pasterns may be prone to a number of challenges during their racing careers, among them sesamoid fractures. Seen as approximately three-quarters the length of the cannon bone, this pastern type has the disadvantage of being weaker, even as it tends to better assist the shock-absorption ability of the leg. While the tendons and ligaments that extend down the back of the leg and continue under the back of the fetlock joint, attaching either to the pastern bones or coffin bone, enable the fetlock to endure the weight of each foot fall, pasterns that are too long or sloping are not adequately able to support the fetlock.

Also located at the back of the fetlock joint, the sesamoid bones act as a lever over which tendons and ligaments pass. Inclined toward hyper-extension to the point where they may even touch the ground, the result of such stress on a continued basis – primarily when the horse tires at the end of a race – may be the precursor to severe soft tissue damage, sesamoiditis, and/or sesamoid fractures.

However, due to stringent conditioning regimes and advancements made in veterinary diagnostics, racers with long pasterns tend to have an added advantage when an injury does occur, such as with a proximal sesamoid fracture. However, with the complex relationship between other bones and the supporting bones, tendons and ligaments that comprise the pastern joint, the story does not always have a happy ending and, more to the point, can spell a catastrophic disaster. Even the best case scenario often renders the ankle too weak to withstand the rigors of strenuous exercise because of poor blood supply to the area, coupled with repetitive tensile force. As a result, recovery is usually slow, with only a fibrous union serving as the stabilizing force as opposed to actual bone.

There are six types of sesamoid bone fractures, and their location, size, and the severity of the detachment from the suspensory ligament can determine the prognosis:

  • Apical fractures involve the top one-third of the sesamoid bone. If the fragments are viewed as less than one-third of the bone, and if there is no suspensory damage, the prognosis is generally positive for recovery, and even for a possible return to racing.
  • Midbody sesamoid fractures affect the middle third of the sesamoid bone and are apt to have a poor prognosis due to the inadequate blood supply and/or detached fragments, which result from continuous tensile force caused by the suspensory apparatus.
  • Basilar sesamoid fractures, which occur at the base of the bone, also have a poor prognosis based principally on the strength of their attachment to the lower portion of the distal sesamoidean ligaments. For instance, if they encompass the articular width at the bottom of the bone, or are large in size, displaced, or are in many pieces (comminuted), they are not considered candidates for surgery, essentially ending a racing career.
  • Abaxial sesamoid fractures involve the outside border where the branch of the suspensory ligament attaches and typically result from sesamoiditis. If they are small and do not compromise joint function, they can be managed with stall rest that may or may not include corrective shoeing. However, if the joint is involved but the fragment is still small, treatment may involve inserting a lag screw and/or performing a bone graft.
  • Axial sesamoid fractures are vertical in nature and typically occur in conjunction with a condylar fracture of the cannon bone. Surgical repair begins with reconstruction of the condylar fracture (i.e. fusing the ankle joint by applying stainless-steel plates, screws, and wires), followed by attending to the two fracture fragments of the sesamoid. The prognosis for a full recovery in these cases is questionable.
  • Comminuted sesamoid fractures also have a poor recovery rate or may even be life threatening, as they may signal a traumatic rupture of the suspensory apparatus leading to catastrophic breakdowns.

Deep footing is thought to set the stage for sesamoid fractures, particularly when combined with fatigue of the flexor muscles, seen primarily at the end of exercise/race. It is also thought, according to Stashack DVM, MS, (1995), author of Horseowner’s Guide to Lameness, Media, PA: Williams & Wilkins, that “... sesamoid fractures are usually seen in foals or racehorses at the track and are usually on the front legs, and because sesamoid fractures are usually associated with young horses, it may be safely assumed that bone strength and development play a major part.”

Uneven track surfaces, improper shoeing (chiefly when the toe is left long and the heel is trimmed too low), and toe grabs may also be contributing factors, especially when added to conformation abnormalities.

Treatment

As with any class of fracture, the horse should be evaluated for additional musculoskeletal injuries before a specific treatment is recommended. Evaluation protocols may include the use of radiographs to help identify enlarged vascular channels within the sesamoid bone, particularly if sesamoiditis is suspected. Likewise, nuclear scintigraphy is used to further determine the extent to which sesamoiditis and associated arthritic changes have taken place.

Once determined, treatment options will vary depending on the size, location, and severity of the injury. For instance, for the horse that is confined to its stall, corrective shoeing might assist in the healing process. By using wedge pads, which are intended to reduce the flexion of the long pastern joint by increasing the bearing on the toe, they will create a more upright stance to alleviate tension on the fetlock.

For small apical sesamoid fragments, arthroscopic surgery is shown to have a good success rate. The procedure involves the insertion of the arthroscopic instrument through small holes in the joint; fragments are elevated and then removed.

Smaller fractures also can be treated by screw fixation. A process in which screws are surgically inserted into the bone to hold the fragments together, in many cases the horse treated with screw fixation may be able to return to racing. Conversely, in severe cases, such as distal condylar fracture in company with an axial sesamoid fracture, this treatment can also be performed successfully, but a return to racing may be doubtful.

In addition, bone grafts are sometimes used to help repair fractures. A procedure in which bone is taken from another area, typically from the hip, the bone graft is then implanted into the area being repaired.

Another option, extracorporeal shock wave therapy (ESWT), has emerged as a new technology designed to stimulate healing for horses with musculoskeletal problems, soft-tissue injuries, and bone injuries, such as sesamoid fractures. Taking only minutes of treatment per session, ESWT is a non-invasive procedure, is portable, and is able to be used on a standing horse. The unit is comprised of a machine attached to an energy source and fastened to a hand-held wand that generates high-intensity shock or pressure waves, which pulse to a specific site within the injured tissue, resulting in an analgesic effect and a reduction in inflammation.

In the end, regardless of the conformation type, prevention is always the best cure. Support wraps during exercise and standing bandages later on, coupled with routinely monitoring the legs for heat and swelling after exercise, since actual lameness may not occur until later, may forestall an injury. Applying ice and compression may also be warranted, as well as judiciously administering anti-inflammatory medication.

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