Interspinous ligament desmotomy surgery for horses with kissing spines

Impingement of the dorsal spinous processes, a condition known as “kissing spines”, is a common finding in horses that present with back pain.

In affected horses, the spaces between the tips of the vetebrae’s fin-like spinous processes are narrowed and eventually obliterated. Where impingement is moderate to severe, the affected area reacts by producing more bone — further reducing the space between the vertebrae and exacerbating the disease.

Although radiographic (X-ray) changes to the processes may be easily identified, making a diagnosis remains challenging. There is not necessarily a link between the changes evident on X-ray and the clinical signs — in other words, the impinging processes may not be the cause of the horse’s sore back.

If a connection can be made, however, various treatment options can be considered. Conservative therapy involves local medication of the epaxial musculature (back muscles) with corticosteroids, analgesia (pain relief) and physiotherapy. A more invasive option is a surgical procedure called an ostectomy, in which varying amounts of affected dorsal spinous processes are partially removed with a surgical saw.

This traditional “open” surgical approach can be very effective, but the operation can be technically demanding to perform. Exposing the vertebrae requires a lengthy incision along the spine, which carries associated risks of infection and potential healing problems. The supraspinous ligament, which runs along the top of the processes and must be cut to access them, is very dense. Identifying the affected areas during surgery can be difficult.

After ostectomy, the patient will require a prolonged period of recuperation before he can return to athletic work.

Keyhole surgery

A relatively new procedure for the treatment of kissing spines offers a much less invasive alternative.

Known as interspinous ligament desmotomy surgery (ISLD), the technique involves surgical division (cutting) of the interspinous ligaments that connect adjoining spinous processes. The procedure is carried out from the side, through very small incisions, leaving the supraspinous ligament intact.

The idea is that division of the ligaments enables the horse to move his back more freely and in a normal way, without pain. It has been suggested that this “re-mobilisation” of the back allows the spaces between the spinous processes to widen following surgery. The procedure can be performed with the horse standing in stocks, under sedation, and results in a far speedier recovery than with traditional kissing spines surgery.

Since ISLD was pioneered by equine surgeon Dr Richard Coomer in 2009, various surgeons have modified the technique. There has been ongoing discussion regarding how it works. One theory is that denervation (removal of the nerve supply to the affected parts of the back), which is likely to occur during the division of the interspinous ligaments, may have a part to play in relieving pain.

Fewer complications

The procedure can be performed at multiple sites on an individual, where the spinous processes are in close proximity, without affecting the prospects of a successful outcome.

Initially, a pair of stout scissors was used to open the divided ligament fibres after they were cut. Here at the Royal (Dick) School of Veterinary Studies, at the University of Edinburgh, we have developed a more refined instrument rather like a small chisel, which can be inserted into very narrow gaps.

Research by our veterinary team suggests that ISLD can be performed in horses with more severe kissing spines than was previously thought possible, although certain cases may be too severe for this minimally invasive approach.

While all surgical procedures have the potential for complications, with this procedure they are rare. Where they do occur they tend to be mild, involving short-term wound discharge or the appearance of white hairs at the incision sites.

Follow-up findings

The big question is, of course, does this new surgery work?

When ISLD is performed in horses with kissing spines that have had clinical signs localised to the affected portion of the back, long-term success rates in excess of 90% can be expected.

Recurrence of clinical signs is more often associated with new sites of impingement, adjacent to the original sites, rather than failure of the procedure.

Our team has followed up around 60 performance horses who were treated with ISLD four years ago. Findings suggest that the procedure is highly effective in the vast majority of cases.

You may be wondering what happens to the spinous processes that are in contact with each other (“kissing”), after ISLD. Well, the disease process doesn’t necessarily stop. Even if the horse has no clinical signs and appears pain-free, disease progression may continue to be evident on X-rays, which can have implications during pre-purchase examinations if screening radiographs are requested by a potential purchaser.

These radiographic changes, in the continued absence of clinical signs attributable to back pain, give credence to the argument that the innervation (nerve supply) of the interspinous spaces is altered during ISLD surgery.

The pre- and post-operative management of horses treated with this new surgery is critical to its success. The diagnosis should first be confirmed using radiographs or scintigraphy (bone scanning), along with careful veterinary examination — including observation of normal work under saddle if appropriate.

In our clinic, the initial treatment is also part of the diagnostic process: only if a horse responds favourably to medication and physiotherapy is he considered a potential candidate for surgery. What tends to happen is that both the quality and duration of response to each treatment decreases. As long as clinical signs are thought to be attributable to kissing spines, we can consider ISLD.

Following surgery, all patients will require physiotherapy. Owners and trainers are encouraged to return horses to a controlled exercise programme without delay.

There is still much work to do to determine the cause of impinging dorsal spinous processes and the long-term prognosis for affected horses. It remains unclear whether activities such as jumping contribute to kissing spines, or if carrying excess weight increases risk — or why some horses have no apparent clinical signs despite significant X-ray changes.

What we do know is that the condition is commonplace and its progression is unpredictable. There is no question that ISLD is currently the most effective treatment option we have.

Ref Horse & Hound; 4 July 2019