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EQUINE VETERINARY EDUCATION / AE / SEPTEMBER 2018
Epaxial musculature of spine
Protractors of foreleg
Abdominal viscera
Retractors of hindleg
Sternum and abdominal muscles
Retractors of foreleg
Protractors of hindleg
Fig 2: Diagrammatic representation of the construction of the horse’s body axis (Jeffcott 1979a).
been identified as having a flexing effect, but primarily affect the pelvic and lumbosacral regions (Van Weeren 2009). The epaxial muscles consisting of the strong longissimus
dorsi and the smaller multifidus muscles lie dorsal to the transverse processes of the vertebral bodies (Kidd 2009). Research performed by Robert et al. (2001) analysed the effect of treadmill speed on the kinematics of the horse’s back. With an increase in the activity of the muscles in the horse’s trunk, there was a restriction in back movement and a stabilisation of the spine. This is reflected in the work done by Hyyti€
longissimus dorsi muscles have a high proportion of fast twitch fibres, suggesting a predominantly locomotory role. The
multifidus muscle contains a mixture of fast and slow twitch fibres, indicating both a stabilising and locomotory role. When activated, the multifidus muscle resists the flexion caused by the abdominal muscles, thereby increasing stabilisation or stiffness of the spine (McGowan et al. 2007). The multifidus is also believed to be involved in proprioception and postural control (Stubbs et al. 2006). Using the string and bow concept, the vertebral column
with its associated musculature is maintained as a relatively rigid structure with less than 10° of flexion at each intervertebral joint. There is even less ROM in the extension of the spine. Lateral flexibility is greater in the mid-thoracic spine, with up to 10° of movement, than in the lumbar spine, where there is only 3–5° of lateral flexibility (Townsend et al. 1983). Limb movements also affect the horse’sTL spine. Retraction of the forelimb and protraction of the hindlimb have a flexing affect on the back (Van Weeren 2009). The head and neck of the horse also factor into the spinal kinematics as lowering of the head will cause tension in the nuchal ligament and forward rotation of the cranial
thoracic vertebrae (Berner et al. 2012). This action will also flex the TL spine, just as raising the head will result in tensing of the back muscles, causing reduced ROM (Rhodin et al. 2005).
© 2016 EVJ Ltd In 1980, Jeffcott quantified the types of pathology seen in
ainen et al. (2014) in determining the muscle fibre types in the deep epaxial muscles of the horse. The
the back of the horse by evaluating the records of 443 cases referred for back problems to the Equine Research Station of the Animal Health Trust, Suffolk, UK. He found that the two most common causes of back pain were soft tissue damage (38.8%) and bony lesions (38.6%). The most common bony lesion was overriding dorsal spinous processes (DSPs). The most common soft tissue condition was muscle strain, with sacroiliac strain also prevalent (Jeffcott et al. 1985). Since Jeffcott completed this work in 1980, ultrasonography and scintigraphy have improved the ability to identify more spinal issues and it would be interesting to repeat this study today, using current diagnostic techniques. Stubbs et al. (2010) examined 22 racehorses that were subjected to euthanasia for problems other than back conditions. All horses had mild to moderate osseous pathology in the areas of the spine examined. In 17/22 horses, active new bone formation was noted, indicating severe spinal pathology. The prevalence of severe osseous pathology was greatest in the lumbar area, but all horses had some degree of bony pathology in their thoracolumbar spine. Improved understanding of functional anatomy has
enabled recognition of aberrant movement patterns in the horse. It is important to realise that there are many more causes of pathology than there are methods for horses to alter their way of moving (Van Weeren 2009). Detecting changes in a horse’s pattern of movement can be difficult. There are, however, many other factors to consider when determining the level of TL pain in a horse.
Equine thoracolumbar pain
Pain occurs when noxious stimuli affect a tissue or organ and the unmyelinated nociceptor nerve fibres are stimulated. Nociceptive fibres are specific high threshold sensory nerves programmed to respond only to noxious stimuli (Woolf 2004). These fibres enter the spinal cord and travel to the brain via the lateral spinothalamic tract. The nerve fibres end in the
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