Everything Horse magazine February 2014 - Page 42

The Horse From The Inside written by Eiddwen Fletcher The Importance of Ligaments The horse’s skeleton provides stability and protection to the body, connecting the bones and cartilage of the body are bands of fibrous tissue known as ligaments, ligaments are essential to provide support and strength to the joints of the body, they are positioned around the joints to prevent movement outside the natural range of motion, preventing over stretching of the tendons and damage to the bones and cartilage. At moveable joints the ligaments form a capsular sac which contains synovial fluid and prevents the articulating bones from rubbing upon one another. Ligaments are made up of dense bundles of collagenous fibres which are extremely strong and relatively inelastic. An extracellular matrix of elastin fibres provides the ligaments with some flexibility, but this is often limited and if put under extreme stress a ligament will rupture. Blood flow to the internal ligament cells is limited; resulting in slow healing and repair functions, the scar tissue that forms is made up of much smaller collagen fibres with altered cell connections and flaws in the elastin matrix, retaining only 10-20% of the normal viscoelastic properties of the original ligament fibres. Ultimately the ligament becomes less able to maintain a load and less able to absorb energy before failure. As riders we all appreciate the joy we get from horses, yet we rarely stop to wonder how they achieve the amazing variety of movements we ask them to perform. Whether you’re a happy hacker, showjumper, jockey or dressage rider; you are constantly reliant on your horse’s Anatomy and Physiology. In this series of articles you will learn how the horse functions and how this influences his ability to carry you as a rider and perform as a competition animal. In the ridden horse there are a number of ligaments that are of vital importance to performance: The main ligament of the neck is known as the nuchal ligament, this elastic ligament attaches at the occipital bone at the back of the horse’s skull and across the ridge of the first two cervical vertebrae of the neck, it spans the crest of the neck and attaches at the beginning of the withers where it merges with the supraspinous ligament of the back. The remaining cervical vertebrae of the neck are attached to the nuchal ligament via two fan-like sheets of elastic ligament which functions to restrain the movement of the cervical vertebrae and support the weight of the head – similar to a suspension bridge. While keeping the vertebrae in place the nuchal ligament also assists the upper neck muscles by storing energy during movement. When the horse’s head is raised the nuchal ligament becomes slack and unstrained, when the horse’s head is stretched down and forwards the nuchal ligament is put in traction which loads the elastic matrix with energy, creating elastic recall to assist the muscle to lift the head back to its resting position. This energy storage reduces the strain placed upon the muscles during locomotion by up to 55%. The nuchal ligament also has a huge influence on the support of the horse’s back. When the horse is ridden in a ‘correct outline’ (on the bit) the cervical vertebrae are flexed and the nuchal ligament is placed under tension which passes to the supraspinous ligament that runs along the midline of the horse’s back and covers the tips of the spinous processes. When under tension the supraspinous ligament provides support to the spine allowing the muscles of the back to contribute more energy to propulsion rather than support Continuing down the back to the hind quarters we reach the sacroiliac joint which is of huge importance to the equine athlete. The end portion of the spine is known as the sacrum and consists of 5 fused 42 Everything Horse UK Magazine • February 2014 • Issue 5 vertebrae, on either side of this lies the front most wing of the pelvis (ilium) which is held in place by ligaments. The joining of these areas is known as the sacroiliac joint which holds the pelvis tightly to the spine allowing little movement to occur and providing stability to the hind quarters. Two sets of ligaments hold the sacroiliac joint together and these ligaments are often subject to strain injury, it is suspected that 50% of horses with back injuries present with sacroiliac pain, unfortunately this joint is positioned deep within the gluteal muscles of the hind quarters making diagnosis difficult. Signs of sacroiliac strain include: asymmetry of the hindquarters, poor performance (more evident at slower paces), intermittent hind limb lameness, toe-drag and tendency to plait with hind limbs, lack of impulsion, disunited canter, difficulty to collect, refusal to jump, sensitivity at top of hind quarters and loins and general signs of discomfort during ridden exercise. Causes of sacroiliac strain can range from traumatic injury to conformational weakness but modern training techniques may also play a role. Commonly in performance horses we require that the canter is sustained for long periods, this asymmetric gait combined with repetitive movements such as jumping, will ultimately place uneven strain on the body resulting in weakness and injury. In performance horses the limbs are the most common area for ligament injury to occur and the worst offender has to be the suspensory liga ment! The primary function of the suspensory ligament is to prevent excessive extension of the fetlock joint during weight bearing. This ligament lies along the back of the cannon bone where it splits into two branches which pass either side of the fetlock and pass to the front of the pastern where they join the extensor tendon. The suspensory ligament is part of the suspensory Issue 5 • February 2014 • Everything Horse UK Magazine apparatus which prevent the fetlock joint collapsing during standing and locomotion and acts as a spring which stores energy when the fetlock is extended and returns energy to the limb when the fetlock flexes. In addition the suspensory apparatus is a critical part of the ‘stay apparatus’ which allows the horse to stand at rest for long periods with barely any muscular effort. So if the suspensory ligament is so vital to the horse both during locomotion and inactivity, why is it so commonly subject to injury? As previously discussed ligaments are relatively inelastic, when a ligament becomes over loaded it is unable to stretch, resulting in tearing or rupture of the fibres. During strenuous exercise such as galloping or jumping; excessive strain is placed on the flexor muscles of the limb causing them to fatigue, combined with extreme forces, the fetlock is forced to over extend resulting in the supportive structures being overloaded and subject to injury. The signs of suspensory ligament injury can vary depending on the area of the ligament damaged; slight or intermittent lameness may present but often in the early stages a horse can continue to function in spite of the injury resulting in further damage and progressive thickening of the ligament. In more acute cases heat and swelling may be evident and the area may become sore to touch. It is essential that any intermittent lameness or loss of performance be seen by your vet immediately to rule out possible ligament damage, if left untreated the ligament fibres will continue to tear creating further injury and reducing the chance of recovery. Although ligaments are able to produce new connective tissue, the scar tissue is significantly weaker and it is important to limit further damage as much as possible. In severe cases thickening of the suspensory ligament can result in pressure upon the structures of the limb and may result in splint bone fractures. 43