Kissing spines is a colloquial name for over-riding spinous processes. The spinous processes are the vertical projections of vertebral bones in the horse’s spine, which run the length of the back and are tallest in the area of the withers.
When the spinous processes are too close to one another, they are said to be over-riding. This interference can cover a wide spectrum of presentations from touching each other when the horse’s back is extended to physical overlapping, or in long-standing severe cases, the spinous processes can actually fuse together.
Horses of any age at any stage of their career can get kissing spines. Some horses develop signs of kissing spines when they enter training, some in the middle of their training, or commonly, when they have just increased work intensity.
Kissing spines are more common in horses with short backs (short coupled horses). Similar to problems occurring in horses with the correct number of teeth in a very small jaw, overcrowding of spinous processes can happen in a horse with a short back; horses with kissing spines have the correct number of spinous processes, but are confined to a smaller space.
Any breed or type of horse can develop kissing spines, although it is very rarely encountered in ponies. Jumping and dressage horses are more commonly affected, most likely due to the demands of their discipline and how much their action is scrutinised.
Signs include being cold backed, reluctance to jump or work in an outline, as well as an exaggerated response to stimulation or palpation of the back (a sore back). Alternatively, the reverse can occur, which is little or no movement in responses to back stimulation, if the horse is guarding the thoracolumbar spine from motion that results in pain.
Some animals resent pressure to the sternum (breast bone) because this stimulates them to lift the back which may also be painful. Many horses show low grade bilateral hindlimb lameness; more severe unilateral lameness is rarely observed. Horses with kissing spines often show a decrease in hindlimb impulsion, which means they generate less power from the hindquarters.
Some affected horses resist collection, some bolt, and some even show profound behavioural changes. These clinical signs are considered as quite unspecific as they can be present with many other musculoskeletal conditions.
The first step should be a clinical examination of your horse’s back by your vet. Targeted manipulation and palpation may identify signs of back pain, decreased flexibility or guarding of the back. Many horses with long-standing back pain have a poor topline due to wasting of the large epaxial muscles, which run down the back on either side of the spine.
Next, a dynamic examination that involves examining the horse at walk and trot in-hand, and lunging on a hard surface, should be performed if it is safe to do so. This allows identification of any lameness present. The horse should then be lunged on a soft surface to allow examination at canter, which is the gait at which back pain is most easily and commonly seen. Viewing the horse ridden under saddle is an important part of this dynamic examination and should be performed in all horses with back pain if safe to do so.
Radiographs (x-rays) of the spinous processes can be relatively easy to obtain and can be done on a portable x-ray machine. However, it is not possible to image deeper structures in the back with a portable x-ray machine. X-rays represent a good screening tool and most horses with active kissing spines will have some changes on x-ray, although the severity of these radiographic changes does not necessarily correlate with the presence or severity of clinical sings associated with kissing spines.
If there are changes on the x-rays, further investigations are warranted, but it is important to remember that a horse can have abnormal spinous processes on x-rays and may not suffer from back pain. X-rays alone can therefore not be used to make a definitive diagnosis.
Your vet may suggest examination with nuclear scintigraphy (a bone scan). The disadvantage of this test is the cost, and that it can only be performed at specialist centres. However, the major advantage is that it can help to identify other conditions that affect your horse, which may alter the prognosis, compared to just having kissing spines alone.
A bone scan is a good complementary examination to x-rays because they can tell you if an area of bone is actively remodelling at that moment, whereas x-rays identify changes that could have been present for years. If a bone scan is recommended and your horse is insured, you have to obtain authorisation from your insurance company for the examination to go ahead beforehand as some insurers will not pay for a bone scan if they have not been informed.
The best way to verify the importance of any changes on a bone scan or x-ray is to desensitise (numb) the spaces between the suspected spinous processes with a local anaesthetic before and after the horse has been lunged and/or ridden (this is called a spinous process block). Once this block is in place, the horse will be re-examined or ridden again and any changes noted. As an owner, it is very useful for you to see and/or ride your horse with the spinous process block in place because you can observe (or feel if you are riding), any improvement and will have an idea of how much improvement some of the treatments could potentially provide.
Occasionally, a response to medication test is performed, where the affected interspinous spaces in the back are medicated with corticosteroids (see below); the horse can improve, only to have the signs recur when this medication wears off. This test obviously takes more time to ascertain effect, but it is possible to inject medication and a local anaesthetic at the same time to enable immediate re-examination. Evaluation of the long-term response to kissing spines to local corticosteroid medication can be difficult as other treatments such as anti-inflammatories are commonly used at the same time.
There is no uniformly successful treatment for kissing spines and the prognosis is worse if the horse other orthopaedic problems.
Treatment options are divided into conservative management, and medical and surgical treatments. Often, multiple treatments from each type are required.
This includes adjusting the fit of the horse’s saddle, an altered exercise programme, and physiotherapy or a prolonged period of rest. Many owners also lower their expectations with regards to athletic performance of their horse.
It is often overlooked (and difficult to communicate), but not uncommon, that a certain type of rider is not a good match for their horse, eg a large person on a small horse/child that has outgrown their pony, which can contribute to the development of back pain.
Physiotherapy and alternative treatments, such as acupuncture, have also been shown to alleviate the muscular pain associated with kissing spines.
If your horse displays signs of back pain when ridden, your horse’s saddle should be checked by an expert. The saddle must fit properly and distribute the rider’s weight over as wide an area as possible. Special saddle pads designed for back protection are commercially available, but they may by themselves not result in the desired improvement.
Many horses respond well to the use of non-steroidal anti-inflammatory drugs, such as phenylbutazone(bute). However, like all medical treatments, these drugs just generally ease back pain rather than treating the kissing spines directly. Nevertheless, the majority of horses with kissing spines also have secondary muscular back pain and non-steroidal anti-inflammatory drugs can help in breaking the pain cycle.
Corticosteroids are the strongest anti-inflammatory drugs and can be injected directly into the affected areas of the back, similar to the process of blocking the interspinous spaces (see above), but more potent anti-inflammatories (steroids) are used instead of a local anaesthetic. Direct medication is one of the most commonly used first treatments for kissing spines because it is successful in many horses. In competition horses, it is important to take drug withdrawal times into consideration if they have been treated with corticosteroids. In some animals, steroids injected in the back can be detected for a longer time than after injection into joints. Prolonged withdrawal times may therefore be used. This limits the use of long-acting steroids to treat kissing spines, to the time out of a competition season.
Extracorporeal shockwave treatment (ESWT) falls into the medical treatment category and is effective in some horses. Extracorporeal shockwaves exert their effect on bone surfaces of kissing spines, where they are thought to have a pain-killing (analgesic) effect. Shockwave application is not painful for the horse, but the shockwave machine can be noisy and some horses need to be sedated for this treatment. ESWT can be a useful treatment alternative if drug withdrawal times do not allow direct medication. Many horses therefore receive ESWT during the competition season and direct local corticosteroid medication out of season.
Surgical treatment can involve removing the sections (resection) of affected spinous processes, which are interfering with each other. Spinous process resection is considered the most effective surgical procedure because it creates space between adjacent spinous processes and removes the source of the pain.
Interspinous ligament desmotomy is a more recently introduced surgical procedure. This surgery involves transecting (cutting) the ligament(s) between affected spinous processes. Interspinous ligament desmotomy appears to be more promising for spinous processes in close proximity to each other, rather than for spinous processes that are in actual physical contact or overlapping. No sectioning through bone is performed with this intervention. If the result of an interspinous ligament desmotomy is not satisfactory, a spinous process resection can still be performed.
Back surgery is a major surgical intervention and carries, like all surgeries, a certain risk of complications. Horses generally remain in the hospital for approximately a week, and subsequently have 6-12 weeks off before being ridden again. However, the type of procedure performed and the aftercare will vary between horse-to-horse.
Not all horses with kissing spines will return to their previous level of exercise, but the majority of horses improve substantially.
One large study of horses treated with spinous process resection found that 70% of animals went back to their previous level of work, which is encouraging.
Undiagnosed separate conditions in a horse will worsen the prognosis if they are not identified and addressed. There are many racehorses, eventers, dressage horses and pleasure horses with active kissing spines that are successfully treated, have a very good quality of life and are able to exercise to their potential.