Kissing spines, also known as impinging dorsal spinous processes or over-riding dorsal spinous processes, may be sudden or insidious in onset or it may be provoked by a traumatic incident, such as a fall. It may become obvious as a progressive unwillingness to work or show itself in deterioration in quality of paces or performance.

The horse’s back

The horse’s back is comprised of 18 thoracic vertebrae and five or six lumbar vertebrae and the sacrum. Each vertebra has three separate joints with the adjacent vertebrae and a dorsal spinous process — a thin, bony blade that projects upwards. Attached to the top of each dorsal spinous process is a strong ligament — the supraspinous ligament. The spacing of the dorsal spinous processes is dependent on the horse’s conformation. The shorter the horse’s back, the closer the dorsal spinous processes are likely to be.

What causes the pain?

The shape and size of each dorsal spinous process varies from front to back and horse to horse. In some horses, each dorsal spinous process is more or less vertically orientated. In others, the top aspect of each dorsal spinous process may curve forwards in a beak-like shape and may actually curve over the next dorsal spinous process.

When the spine is extended (dipped) this tendency may become exaggerated. Such overriding dorsal spinous processes may be a cause of pain, or stiffness, and this may in turn give rise to either local muscle or ligament pain. This restricted movement may also cause damage elsewhere in the thoracolumbar or pelvic regions. For example, it may promote the development of osteoarthritis of the facet or the sacroiliac joints.

The dorsal spinous processes may be straight, but, despite that, may actually touch. This can cause irritation of the bone and associated pain. Alternatively, the dorsal spinous processes may overlap — overriding dorsal spinous processes — causing stiffness and pain.

Clinical signs of kissing spines

Generally, they are at their worst when the horse is ridden and often accentuated in sitting trot. The horse may resent being tacked up and may show “cold-backed” behaviour when first mounted. He may dip his back and walk off stiffly, or explode in a series of bucks and be unwilling to bend.

There may be lack of hindlimb impulsion and acceptance to working on the bit. The head carriage may become inconsistent, with a tendency to come above the bit. The horse may start to rush fences, find combination fences difficult, jump flat or stop uncharacteristically.

Careful clinical appraisal usually reveals some muscle tension when the back muscles are firmly stroked. When stimulated to arch and dip the back or to flex from side to side, the horse may hold his back stiffly or threaten to bite, kick or buck, with increased tension in the back muscles or muscle twitching.

Back pain rarely causes overt lameness, but it may result in reduced stride length in front and behind, reduced hindlimb impulsion and stiffness. When examined moving on the lunge, there may be exaggerated contraction of the back muscles. The longer the problem is present, the more likely it is that the horse will lose muscle off the top line because he is protecting his back and not using the muscles properly.

Diagnosis

Definitive diagnosis is dependent on a careful assessment of the horse, X-rays and a positive response to infiltration of local anaesthetic solution around the affected dorsal spinous processes. Treatment is influenced by the severity of clinical signs, the athletic demands placed on the horse, the number of dorsal spinous processes involved and the severity of the X-ray changes.

Treatment options

There are a number of options, including infiltration of anti-inflammatory and pain-killing drugs around the dorsal spinous processes, shock-wave therapy or surgery. Drug treatment may need to be repeated.

Shock-wave or radial pressure-wave therapies require several treatments. Surgery involves removal of part, or all, of the top of the alternate dorsal spinous processes.

This veterinary feature was first published in Horse & Hound, 29 May 2008

First published29 May 2008 – H&H