Back problems are a common cause of discomfort and poor performance in all types of horses.

Signs of back pain can be vague and variable, making it difficult to recognise the nature of the problem, and the complex anatomy of a horse’s back creates its own diagnostic challenges. The myriad of treatments offered by vets, physiotherapists and other practitioners testifies to the fact that back problems can be difficult to manage.

In this, the first of a series of three articles on back problems in horses, the anatomy of the equine back and its function in load-carrying and locomotion are examined.

This serves to identify the structures that become injured, highlighting where diagnostic and therapeutic efforts should be directed.

Subsequent articles will look at the manifestation and investigation of back pain, and its treatment.

The equine back is just like a suspension bridge, connecting the upright structures of the front and back legs. The bridge consists of 18 thoracic (or chest) vertebrae, six lumbar (or loin) vertebrae, and five sacral vertebrae that are fused together to form a single bone. These vertebrae are strapped together by strong ligaments and muscles to form a spinal column that is rigid but, just as the Forth Road Bridge on a windy day, also flexes to absorb movement.

There is no direct attachment between the spinal column and the forelimbs in horses that, unlike humans, do not have a clavicle or collar bone. Instead, the front legs are suspended from the vertebrae of the chest and neck through a system of muscles and ligaments.

It is this lack of a rigid connection that allows for great mobility in the shoulders and forearms – for example during lateral work – and explains why, when measuring the height at the withers, it is important to allow the horse to relax.

By contrast, the hindlimbs are connected to the spinal column via the attachment of the wings of the pelvis to both sides of the sacral vertebrae. These sacroiliac joints are stabilised by immensely strong sacroiliac ligaments meaning that, unlike at other joints in the horse’s body, there is little movement.

The primary purpose of the sacroiliac joints is to transfer propulsive activity from the hind limbs to the back, thereby allowing the forelimbs to reach out and create forward movement.

Each of the vertebrae that make up the spinal column, including those in the neck and tail, has a common structure. This comprises a body, a canal through which the spinal cord passes, dorsal (upwards) and transverse (sideways) processes onto which muscles and ligaments attach, and areas called facets that articulate with adjacent vertebrae and, in the chest, the ribs.

A strong elastic ligament runs from the poll at the base of the horse’s skull all the way to the sacrum, attaching onto the dorsal spinous processes of each of the vertebrae in the neck, chest, and lumbar region.

The portion of this ligament in front of the withers is called the nuchal ligament and serves to support the head and neck, helping through its elasticity the muscles that raise and carry the neck.

The portion from the withers backwards is called the supraspinous ligament, which functions to support the back giving it strength and stability.

Together with the nuchal ligament it reduces the amount of effort that muscles in the back and abdomen expend in supporting the spine, thereby allowing these muscles to play a fuller role in the propulsion and forward movement of the horse.

Muscles occupy the space between the dorsal and transverse processes, and underneath the transverse process, connecting neighbouring vertebrae and spanning the length of the spine.

Co-ordinated contraction of these muscles allows the spine to move from side to side, as well as up and down, and controls the movement of the back. Connections with the muscles in the shoulders and hind quarters mean that the back muscles also contribute to the movement of the fore and hindlimbs.

This is most obvious during collected gaits, when the muscles and ligaments work in coordination so that the back functions like a giant spring storing and releasing energy that is translated into propulsion.

Back problems arise following injury or disease to one or more of the structures of the back. In a survey of horses examined at the University of Minnesota over a five-year period, the most common cause (65%) of back pain was damage to the sacroiliac joints and associated ligaments.

Other causes included impinging dorsal spinous processes (10%), which are commonly refer to as ‘kissing spines’, injury to the supraspinous ligament (10%), muscle pain or spasm (8%) and fractures of the dorsal spinous processes in the withers region (6%).

We see the same spectrum of conditions in the UK, although the frequency of individual injuries is a little different. Although sacroiliac disease is prevalent here, it is not in my experience the most common cause of back pain.

Arthritic changes to the articulations of vertebrae were not reported in the Minnesota study, probably because of the difficulties in their diagnosis, but are increasingly recognised in sports horses of all disciplines, including harness racing.

The first question that owners of horse with sore backs usually ask is ‘how did the injury occur?’ In some cases there is a history of trauma.

Horses with fractured withers may have been cast in the stable, or become trapped under the breast bar of a horse trailer or between the partitions in a horsebox. Damage to the back muscles or sacroiliac joints can sometimes be traced back to a catastrophic fall on the cross course or in the show jumping arena.

In many cases, however, there is no ready explanation for the origins of the back pain. In many it is likely that the damage is simply the result of an athletic career, with accumulated wear and tear on the various structures of the back.

Alterations in gait, either because of the need to evade contact in a sore mouth or because of subtle lameness, can also lead to muscle stiffness and pain.

In the Minnesota study, around 20% of horses presented with back pain were found to have underlying lameness, which in all instances was traced to the stifles, hocks or lower hindlimbs.

It is an inescapable fact that, in many cases of back pain, it is the interaction between the rider and the horse that is the source of the problem.

The muscles and ligaments of the horse’s spine are designed for the animal to run, rather than to carry the weight of a rider. Horses adapt to the weight of a rider by using their head and neck as a counterbalance, so that the horse moves in an outline that is forwards and down.

This allows the horse to carry the rider’s weight and ensures that the supraspinous ligament and back muscles are free to engage in locomotion.

If the horse’s neck muscles are too weak to carry the head and neck, or if the horse’s neck is bent excessively, this balance is upset and additional strain is placed on the back.

Over-bending or hyperflexion of the neck also stretches the nuchal ligament, causing pain and damage all the way along the back. Riding with a light seat and sympathetic hands is essential, especially in young horses or those early in their training. A poor back conformation – especially a short back – will also make problems more likely.

An ill-fitting saddle is an all too frequent cause of back pain. It is important to realise that horses vary shape with age and training, as well as from season to season, and that saddles also change over time.

Just because a saddle was expertly fitted six months ago, it cannot be assumed that it is not now contributing to a problem. Damage to the tree or inadequate flocking can also result in excess movement and uneven weight distribution, leading to focal pain and inflammation.

It is the complexity of the back, together with its role in locomotion and weight-carrying, that makes it vulnerable to damage.

The challenge to all of us involved with horses – whether as an owner, rider or trainer – is to recognise the signs of back pain so that prompt investigation and appropriate treatment can be provided.