The equine neck is a highly complex structure, capable of supporting a huge range of head and body positions. The movement of the neck is controlled by a number of muscles, some acting as passive stabilisers, others playing a key role in movement and control. The most important passive stabiliser is the nuchal ligament. A well-functioning nuchal ligament will allow the horse to move and position his neck without requiring excessive energy. When it is not working properly the result can be an inflammatory condition called cranial nuchal bursitis.
The condition results in an accumulation of pressure at the site where the nuchal ligament abuts on the occipital bone (atlas and axis) of the skull. When the horse performs certain head and neck movements, such as hyperflexion of the neck in a dressage technique called ‘rollkur’, an excessive build-up of pressure can occur at this point. The horse will then adapt a posture where the poll is higher than normal, in an effort to decrease pressure at the area. Unfortunately this can also lead to the development of a condition known as atlanto-axial subluxation, which can cause pain and other problems.
Often, this is a very painful problem for the horse. If left untreated, the horses will become averse to this head and neck position and will not perform well when in it. This can have a profound effect on the performance of the horse, and is therefore a very serious issue that needs to be investigated and managed effectively.
Anatomy of the horse’s nuchal ligament
The nuchal ligament is a two part ligament that consists of the funicular cord and the lamellae. The funicular cord is a tough rope-like ligament that runs from the horse’s poll to the withers, and joins to the supraspinous ligament that extends up along the top of the spine. The lamellae is a triangular sheet-like ligament that splices into the funicular cord and then spreads down to attach to and support the cervical vertebrae.
Interestingly, the omohyoideus muscle, which separates the external jugular vein and carotid artery in the cranial neck region, is located within the lamellar portion of the nuchal ligament. It is thought that this muscle may be a key contributor to the development of atlanto-axial subluxation.
The funicular portion of the nuchal ligament has attachments to all but the last two cervical vertebrae in the neck (C6 and C7). The lamellar portion has attachments to C2 and to a large extent C3. Traditionally, the nuchal ligament has been described as having a strong connection between the dens of these two vertebrae, and the omohyoideus, but it has recently been discovered that this is incorrect. This is due to the fact that the omohyoideus has a one, or two-sided absence of its own dens at these locations, and this means that the omohyoideus cannot function as it should to stabilise the base of the neck. The exact reason for this anomaly has not been determined but it could be due to genetic malformation or an abnormality of the attachment to the transverse processes of the cervical vertebrae.