Overriding spinous processes (ORSPs) or 'kissing spines' is a condition in which the dorsal spinal processes on the vertebrae in the horses' back become too close to each other, resulting in impingement or, in more severe cases, the processes begin to override. Although it has been described to be a condition which is 'in vouge,' with suggestions that historically it wasn't as commonly seen, there is evidence to suggest that the prehistoric ancestor to the horse, the equus occidentalis, had ORSPs. Therefore, it is likely that an increase in owner/trainer awareness and improved methods of diagnosis have resulted in an increased number of horses being diagnosed with the condition. That said, not all cases of back pain are caused by ORSPs and not all horses who have the condition display symptoms, which make it difficult to accurately diagnose as the cause of pain and/or underperformance.
The occurrence of ORSPs is more than likely to be multifactorial. Genetics are thought to be a major contributor, especially as this links directly to a horse's conformation. However, other factors are also considered to be of importance. For example, exercise routine, skill and weight of rider, quality and fit of tack, abdominal and epaxial muscle strength and the way the head and neck are carried are all thought to be significant in the development of the condition.
Anatomy
The equine spine consists of 7 cervical vertebrae (the neck), 18 thoracic vertebrae (which include the withers and the part of the back riders sit on), 6 lumbar vertebrae, 5 sacral (which are fused and is the part of the spine which connects to the pelvis through the sacroiliac joint) and 15-21 caudal vertebrae (varies between individual), which form the dock. Of course, there are sometimes variations, especially in the lumbar vertebrae, where sometimes only 5 are found. There have also been cases of horses having 17 or 19 thoracic vertebrae.
Each vertebrae has a dorsal spinous process. This is the name given to the projection of bone which is found at the midline (middle point) of each vertebra. It is called a 'dorsal' spinous process because it is situated 'dorsally' on the vertebrae (think of the dorsal fin on a dolphin). The size and shape of the dorsal spinous process (DSP) is different at different points of the spine. For example, the DSPs in the cervical spine are very small (with the exception of C2 and C7, which are more prominent than the rest). In the thoracic spine, the DSPs increase in size to T4-T5, which form the withers. They then gradually reduce in size until T15-T16. From this point, they are more equal in size until L6 and S1, which are shorter.
The cervical vertebrae and the thoracic vertebrae up to T15 or T16 (differs between individuals) are positioned in a dorso-caudal direction. This means they are inclined towards the tail. T15 or T16 is different because it is orientated vertically and is called the anticlinal spinous process. From this point to S1, the processes are orientated dorso-cranially (inclined towards the head).
It is the processes between T11-T18 where ORSPs are more common, often causing pain and inflammation directly under the saddle area. Spinous impingement has been reported to occur most commonly between T13 and T18, which includes the anticlinal vertebrae. It is thought that impingement is more common at this site because the change in orientation results in a reduction in space between the processes. As the rider sits directly on top of this point, riding is considered to induce clinical signs.
Breed and Discipline
Historically, thoroughbreds, warmbloods and paint horses were suggested to be more susceptible to developing the condition. Jumping horses were also considered to be at a higher risk, whereas flat racing thoroughbreds were considered less likely to be affected. However, more recent research contradicts this view. Racing thoroughbreds, along with thoroughbred and warmblood dressage and jumping horses are all overrepresented in studies which focus on the prevalence of ORSPs. However, it is now diagnosed in a number of breeds. Therefore, it might not be a particular breed in which the condition is more prevalent but instead, the discipline in which the horse takes part influences whether or not clinical signs become apparent. For example, pain is induced by the way the horse is ridden, such as in sitting trot and the requirement for collection and lateral movement in dressage. This type of training is suggested to result in the horse over using the back, causing inflammation and pain at the impingement sites.
Statistically, fewer cases are diagnosed in ponies, leading to the view that prevalence of the condition increases with height. It has been suggested that the increase of extension forces applied across the thoracolumbar spine as the horse grows contribute to the prevalence or, as height and back length are corelated, the additional length of the horses' back reduces soft tissue stability, allowing gravitational forces to increase, which contribute to lordotic conformation which is associated with the condition. However, it is also suggested that a short back, which reduces space for the vertebrae, is an increasing risk factor.
Signs of back pain in your horse
There is not a comprehensive list of signs or symptoms that you can look for to determine whether or not your horse has ORSPs. However, there might be some clues to suggest that your horse is suffering from back pain. This doesn't mean that every horse who displays signs of back pain has ORSPs but is a good thing to be aware of because, no matter the cause, we don't want our animals to be uncomfortable.
A good starting point is knowing what is 'normal' for your horse because every horse is an individual, who has different levels of tolerance and will show their discomfort in different ways (just like humans!). Consider how your horse is to groom over the back area, their reaction to having their saddle put on or their girth fastened. Do they object to being rugged? While all of these things could be completely unrelated to back pain, they are good things to observe and monitor as they could be the first signs that something has changed.
Next think about your horse in their stable. Do they have good posture? Is their core strength good or have they lost any muscle mass or tone in the epaxial muscles of their back? Are they comfortable standing square or do they fidget? Are they loading their weight equally over each of their limbs?
Then move on to their ridden work. Are they performing well? Do they continue to improve overall? Of course, we all have some set backs but is the long term trajectory one of progression? If their performance is beginning to decline, when did this decline begin? Can you relate it to anything specific? Are their certain parts of their work which they dislike or try to avoid? How are the mount? Do they show any unwanted behaviours and if so, when did these begin? Are they moving symmetrically in their fore and/or hindlimbs?
Having regular appointments with your veterinary physiotherapist is also a really good way to monitor changes in your horse's overall comfort. By having the same therapist at regular intervals, they will be able to get to know what is 'normal' for your horse in both how they move and how they feel. This means they will be able to identify if something changes, in their gait, their muscle mass and tone and, most importantly, their comfort. Sometimes, these changes can be explained by a change in work load or routine, in which case, your therapist will be able to support you in achieving improvement through therapies and exercise. However, if your therapist feels that your horse might need a little more help, they will refer you to your vet.
Veterinary Diagnosis
Vets are the only ones who can diagnose ORSPs in your horse. Initially, they will probably want to see your horse move to ensure that the back pain you've noticed isn't secondary to an undiagnosed lameness issue. They will also palpate the back to evaluate reaction, muscle mass and tone. Unfortunately, this is where diagnosis can become a little tricky. Research tells us that a horse's age and breed can impact how they respond to palpation of the back, meaning that false negatives and false positives are common. Sometimes, horses can be dramatic in their response, with localised areas of muscle tension, potentially suggesting the location of a legion. However, this is still a very subjective test, which confirms the importance of having regular veterinary physiotherapy appointments for your horse so that their normal response to these tests is documented, meaning changes can be shared with your vet as significant findings. Alongside palpation, the ability to ventroflex and dorsoflex the spine can also be tested. Some horses guard against this type of flexing, which again can signify possibly pathology.
If pain is suspected during palpation assessment, radiography might then be suggested as the next tool of diagnosis. This is a non-invasive examination and enables your vet to assess the vertebra. A grading system is used to describe the severity of any ORSPs if found. This begins at Grade 1, where the space between the interspinous processes is reduced and a mild increase in opacity at the edges of the processes in present. By Grade 2, the interspinous process space is lost and opacity at the edges of the process is moderate. The grading system progresses to Grade 4 or 5, where there is increased opacity at the edges of the spinous processes, osteolysis (destruction of the bone tissue), a change in shape of the spinous processes and/or overriding.
Radiography of the spine can also help to identify other pathologies such as osteoarthritis or spondylosis. However, studies have shown that ORSPs are often seen in radiographs of clinically normal horses (not showing any signs of backpain) and therefore, caution must be taken when diagnosing ORSPs using radiographic images. That said, images which are classified as higher grade are far more closely correlated with clinical signs of back pain and are less common in horses who are presenting as 'normal.' The use of the grading system can help to determine significance.
If radiographs suggest that close spinous processes might be the cause of back pain in your horse, local anaesthetic can then be injected into the area/areas. Once this has been administered, clinical signs of back pain will then be reevaluated. If there is a clear improvement in signs after the anaethestic has been applied, it can be concluded that the ORSPs are causing the pain.
Nuclear scintigraphy images the metabolic activity of tissue. It can therefore corroborate areas of ORSPs found in radiographic images and attribute significance if metabolic activity is also found at these sites.
Treatment
Successful treatment of ORSPs is often multifactorial and includes long term careful management. Your vet will advise on the most suitable initial treatment for your horse, dependent on the severity of the ORSPs. This could include shockwave therapy, mesotherapy injections or corticosteroid injections, all of which aim to reduce local inflammation and reduce pain. In more severe cases, or when the above mentioned treatments have been unsuccessful, surgery maybe suggested. This could be surgical resection of the dorsal spinous processes to reduce impingement and again create space between the vertebra, or it could be desmotomy (ligament dissection) of the interspinous ligament. This aims to relieve tension on the nociceptive receptors located in the ligaments insertion, and therefore reduce pain.
In simple terms, the possible treatment options all have a common aim; the reduction of pain. This is because once pain has been resolved and the horse is again willing to work, exercise therapy can begin. Exercise therapy, in which the horse works freely forward in a long and low relaxed frame, has been evidenced to be one of the largest contributing factors to the successful management of the condition. Ideally, exercise should be daily, especially in early stages of rehabilitation and include baited stretches. Initially, the requirement is for the horse to stretch over the topline and protract with the hindlimbs so that they step under the trunk. Once this is achieved, ground poles and cavelletti can be introduced. This work should at first take place without a rider, to give the horse's epaxial muscles time to adequately strengthen. As with any rehabilitation program, there needs to be the correct balance of challenge for adaptation and progression to be achieved, without too much strain which could result in a set back. Again, your veterinary physiotherapist will be able to help guide you in your rehabilitation plan, as well as monitor your horse through assessment to help you evaluate progress. Horses with ORSPs will often require long term support to ensure their comfort, help maintain flexibility and reduce muscle tension in the affected areas.
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