A recently published article in the Journal of Equine Veterinary Science suggests our knowledge of equine anatomy may be based on outdated findings and needs to be revised.
The paper centres around significant variations found in the nuchal ligament lamellae, which do not match conventional anatomic descriptions.
Australian researchers Sharon May-Davis and Dr Janeen Kleine presented their findings, and explained the possible affect on horse’s health and performance at the recent Bowker Lectures in Merrijig, Victoria.
In great demand as a lecturer, both in Australia and abroad, Sharon May-Davis is a respected research scientist and equine therapist, particularly passionate about researching gross anatomy and how it impacts on equine performance.
The study’s co-author, Dr Janeen Kleine, is a paediatric and obstetric osteopath who mostly works with human patients, but has become increasingly interested in equine anatomy and applying her knowledge to the horse.
May-Davis began this fascinating and engaging presentation by defining the nuchal ligament - a two-part ligament. The top part is known as the funicular cord, a very tough rope-like ligament that runs along the top of the neck, connecting the horse’s poll to the withers, where it joins to the supraspinous ligament that runs along the top of the spine, all the way to the tail. The second part of the nuchal ligament is known as the lamellae, a triangular sheet-like ligament that splices into the funicular cord, and spreads down to attach to and support the cervical vertebrae.
According to the anatomy books that, to date, have served as study reference to veterinarians and practitioners around the world, the nuchal ligament lamellae attaches from C2 to C6 or C2 to C7. However, in the researchers’ experience, the attachments to the last two vertebrae in the neck (C6 and C7) are completely absent, with some specimens also presenting with weak and feeble attachments to C5.
Function and dysfunction
The peer-reviewed paper reports on 35 dissections performed over a two-year period. However, May-Davis had noticed the same in every one of hundreds of previous dissection specimens. “I was told it was incidental so I had not questioned it before,” May-Davis said. “It was only when I started to work with Dr Janeen Kleine that we decided to formally document and publish this mysterious ‘missing’ lamellae.”
As May-Davis explained to the conference delegates, the nuchal ligament’s structure and function can be visualised as a suspension bridge. The funicular cord serves as the main cable, the lamellae form the suspension cables and the bridge’s deck is made up by the cervical vertebrae (Note for illustration: anchors are the skull and supraspinous processes; the pylons are the legs).
If we refer to this suspension bridge comparison, we can see that, as well as suspending the vertebrae to counteract the effect of gravity, the lamellae also helps to control twisting (axial rotation) of each individual vertebrae except C1, adding stability.
Structurally, it makes sense that all the vertebrae should be firmly supported as is depicted in the anatomy books. So, why are two and, in some cases three, vertebrae not fully supported in so many specimens? Is this normal anatomy, or is it compromising stability and normal function?
An osteopathic perspective
Dr Kleine took the stage to explain why this absent lamellae warrants investigation: “Thinking about how the lamellae attaches and knowing what we know about how the neck moves, the loss of suspensory struts has to have an impact on performance and may be compromising other structures. As an osteopath, I am particularly interested in how the horse’s body will attempt to compensate for this lack of support. Is this absent lamellae altering or impairing function in the lower cervical neck in every horse or is this normal anatomy?”
Dr Kleine summarised for the delegates the principles of osteopathy, a manual medicine practice with a strong concept of somatic (body) dysfunction, i.e. impaired or altered function of related components of the body, the bones, the joints, and myofascial tissue and structures, and their related vascular, lymphatic and neurological systems, as well as the organs (viscera).
From an osteopath’s perspective, the body (all that is contained under the skin) is a unit and possesses self-regulatory mechanisms. When faced with an imbalance, it will attempt to compensate and find homeostasis. “Structure governs function and vice versa,” said Dr Kleine. “So, if we have changes in the structure of the neck, how is this affecting function? An altered function of the neck will have an effect further down the limb, back along the spine and forward to the head.”
Dr Kleine remarked there has been quite a lot of study into segmental motion in the equine neck.
The greatest range of lateral bending is firstly, C7-T1 and secondly, C6-C7; flexion and extension again the articulations from C5-T1 with axial rotation greatest at C1-C2 (due to joint specificity), and C6-C7, C7-T1 second and third respectively. In both researchers’ opinions, the large range of movement may be due to the lack of lamellae, what is not as clear to them is whether it is a result of the missing lamellae or it is contributing.
Effect of pressure
Kleine also spoke about the effect that gravity and loading from the forelimb may potentially have on these less-supported vertebrae. “Researchers are certainly reporting cervical joint disease (arthritic changes) in this area of the neck with the increasing age of the horse.”
Increased movement in this area will, over time, cause wear and tear to these joints, and potentially pressure to the spinal cord and to the nerves as they leave this part of the spine. Spinal cord pressure in this area may cause nerve changes in the hindquarters, whereas pressure to the nerves as they leave the spine can compromise the brachial plexus (the network of nerves that run from the spinal cord to the forelimbs). Therefore, these effects of pressure may lead to changes in locomotion activity in both the hind and the forelimb.
“We also know from studies that when there is increased loading from the funicular cord higher on the neck, we will see insertional desmopathies (disease at the point where the ligament joins the bone) in C2,” continued Dr. Kleine. “And I suspect that this may be the result of taking a load that is not supported further down.”
“As an osteopath, understanding more of the picture helps us understand what is under our hands. Horses will use their limbs to minimise stress in any given area - probably because the limbs have a greater degree of movement. An altered muscle recruitment pattern in a limb will biomechanically alter the flight of the limb’s arc and, therefore, the movement pattern will be altered.”
How can we help?
The presentation became very relevant to all practitioners when May-Davis changed the rein to present ways in which therapists and horse owners could help horses compensate for this apparent lack of support and stability.
“If we have a structural compromise,” said May-Davis, “Then maybe we can help horses by developing the muscles involved in intersegmental rotation. These are not going to be those long superficial muscles of the neck; the brachiocephalic and splenius aren’t going to hold the cervical vertebrae in place. I am talking about deeper, ventral muscles and, particularly, the longus colli.”
The longus colli is a muscle that is slung like a hammock between the vertebrae from C1 to T5, and supports the base of the neck by stabilising, fixating, flexing and rotating the vertebrae. It is a postural or cybernetic muscle that is very rich in proprioceptive innervation (i.e. it responds immediately to external forces).
In cybernetic muscles, a single motor neuron services between 20 and 30 myofibrils (the fibres that contract and make up the muscle), whereas in the large ‘gymnastic’ muscles, there is one motor neurone every 1,000 myofibrils. “If we can influence [cybernetic] muscle memory for just a few minutes a day,” continued May-Davis, “we can change muscle very quickly and the effect can be long-lasting.
Nothing will bring the nuchal ligament lamellae back, but if we focus on strengthening the multi bundled longus colli, we can help support the cervical and thoracic vertebrae from a ventral (underside) perspective. A simple way to achieve this is by allowing horses to browse - a natural behaviour in the wild.”
Previous issues of Horses and People have reported on browsing in horses, and the research being conducted into foraging behaviour by equine nutritionist and regular contributor Mariette van den Berg, also a presenter at the Bowker Lectures.
In Australia, where access to pastures containing different varieties of trees and shrubs is fairly common, owners have reported their horses don’t just graze the grasses below, but also stretch up to browse on higher trees and shrubs.
May-Davis soon realised the connection between the study’s findings and a technique she had been experimenting with since 2003 of feeding horses hay high. “That’s when the jackpot hit,” she said. Placing a hay net at between eye and poll height (depending on the horse’s flexibility) is one way we can mimic the browsing posture seen in feral horses. May-Davis showed photographs of semi-feral herds of Konik horses browsing under the shade of big trees to explain the head-high browsing posture is, in fact, activating all the deep ventral muscles.
“There’s a discussion that horses shouldn’t eat high, that they are grazers,” said May-Davis. “But, the trouble is that in a natural setting horses will browse, sometimes up to 20 minutes or longer, and what I’ve always found pretty pertinent is how square they are all standing while they browse (as opposed to having one leg forward and the other back when they are grazing).
"The browsing posture engages and strengthens the ventral muscles of the caudal cervical and cranial thoracic vertebrae, particularly the longis colli, and the ventral intersegmental muscles between C1 and T5. These are all little stabilisers. They are cybernetic muscles that ballerinas use to remain in posture.”
“The results of hay high have been exciting for me and I have not seen any negative side effects; well none apart from a miniature companion pony who could not reach the hay.”
Next month, I will report on another fascinating Bowker Lecture that centred on laterality by Dr Kerry Ridgway, an internationally renowned lecturer in integrative veterinary and conventional medicine that has specialised in equine performance issues and postural rehabilitation.
About the lecturers:
Sharon May-Davis, BAppSc, MAppSc. Sharon May-Davis began riding at age four and stopped competitive riding in her late 20’s. Soon afterwards, she began her academic journey that included the building of equine skeletons and research. Her innovative therapy practice saw the State and Australian Champions in seven different disciplines, of which she had a part in the selection process in three. Now, Sharon is primarily an equine scientist and lecturer in Australia and overseas, with published works in the Journal of Equine Veterinary Science, Australian Veterinary Journal and Centre for Veterinary Education.
Dr Janeen Kleine, BClinSc, BOstSc, MOstSc. Dr Janeen Kleine has become increasingly interested in equine anatomy, and the application of osteopathic principles and techniques to horses. This has led to her dissecting horses to enhance her understanding of their anatomy and biomechanics. Janeen is now venturing into the world of treating and attempting to understand the musculoskeletal system of horses, and sees a strong correlation between her equine and paediatric human patients.