Deep Digital Flexor Tenotomy As a Treatment for Chronic Laminitis

The coffin bone is the primary bone within the horse's foot. The hoof capsule encases this bone like a body in a coffin (hence the name coffin bone). The laminae within the foot are the soft tissue structures that firmly attach the coffin bone to the hoof wall. Laminitis, in its simplest terms, is inflammation of the laminae that attach the coffin bone to the hoof capsule. This inflammation decreases blood flow to the area of the laminar attachments. Local alterations in blood flow may be the result of a variety of systemic illnesses, such as grain overload, colic, retained placenta, etc. Whatever the source of the disease, laminitis usually causes crippling pain in horses and is potentially devastating to horse owners.

In some instances, the laminar attachments become so compromised that the coffin bone and hoof capsule actually separate from each other. If the normal pull of the deep digital flexor tendon exceeds the strength of the remaining laminar attachments, the bone may rotate downward away from the hoof wall. Veterinarians consider the disease chronic if rotation occurs or if the condition lasts for more than several days.

Most treatments for laminitis focus on improving blood flow to the foot, alleviating the pain associated with this condition, halting disease progression, and re-establishing a functional relationship between the coffin bone and hoof wall. Veterinarians often use vaso-dilating agents such as acepromazine, isoxsuprine, pentoxyphyline, and nitroglycerin in hopes of improving blood flow. Phenylbutazone (Bute) commonly relieves pain and decreases inflammation in laminitic horses. A variety of recommended shoeing and trimming techniques attempts to decrease the amount of tension on the coffin bone and redistribute pressure on the hoof's weight-bearing surfaces.

In spite of extensive research, numerous approaches to treating horses with laminitis are sometimes frustrating and unrewarding. A multi-factorial condition, laminitis involves several body systems and prevents a single treatment regimen from becoming universally accepted or effective. When the disease does not respond favorably to conventional therapies, owners often turn to euthanasia to end the intractable pain of this condition.

The purpose of the authors' current study, presented at the AAEP's 44th annual meeting in Baltimore, Md., was to evaluate a surgical option for laminitic horses that failed to respond to standard treatments. Deep digital flexor tenotomy is a surgical alternative that transects the deep digital flexor tendon in the mid-cannon bone region. This major tendon runs down the back of the horse's leg and attaches to the back of the coffin bone. Transection removes one of the main forces responsible for rotation of the coffin bone in laminitis. This surgery may also alleviate a source of pain by decreasing pull on the bone and inflamed laminae. Under local anesthesia, the horse stands during this quick and minimally complicated procedure.

This retrospective study evaluated the procedure's effectiveness by obtaining follow-up information on all horses with laminitis treated by deep digital flexor tenotomy over a 10-year or more period. Thirty-seven cases met the criteria for inclusion in the study. Seventy-seven percent of the horses that had the operation were still alive after six months, and 60% survived at least two years. The results are substantial, considering the severity of the laminitis and extreme lameness of all the study horses prior to surgery. These horses had not responded to established medical treatments and many were facing euthanasia. The high percentage of owner satisfaction with the surgery seems to indicate the improved quality of life for these horses. Seventy-three percent of owners said they would have the procedure repeated in similar circumstances, 17% would not, and 10% were unsure. The study also found that the severity of lameness and amount of rotation evident on pre-surgical X rays did not influence the outcome of the operation.

Previous studies have yielded conflicting results for the procedure. Some indicate the procedure is less effective early in the disease's process during the acute stages of laminitis. Others show a lack of response to any kind of treatment in cases where the coffin bone sinks in the hoof capsule rather than rotating. When the bone sinks, the extensive loss of laminar attachments contributes to the disease's severity.

Horse owners should consider the cost of aftercare when contemplating tenotomy of the deep digital flexor. The actual operation is relatively inexpensive because the surgery does not require general anesthesia. Expenses accumulate from the essential corrective trimming and shoeing performed by a qualified farrier at four-week intervals. In rare instances, the horse's toe will flip up following tendon transection due to the loss of the supporting structure. This condition necessitates a heel extension on the affected foot.

The severed tendon heals with scar tissue, leaving an enlarged thickening on the leg where the tissue forms. Some horses may become sound enough to resume light work, but most achieve pasture soundness and are best suited for breeding purposes. Although a universally effective treatment for laminitis is unlikely, tenotomy of the deep digital flexor tendon may benefit selected horses with chronic laminitis which continues to deteriorate despite intensive medical therapy.

Timothy G. Eastman, DVM, MPVM, and Clifford M. Honnas, DVM, Diplomate ACVS

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