Orthopaedics 4. The back and pelvis
18.1 Anatomy of the horse’s back
18.2 Diagnostic approach to diseases of the horse’s back
18.3 Disorders of the horse’s back – conditions that may present as a back problem
18.4 Vertebral column deformities
18.7 Impingement and overriding of the dorsal spinous processes (‘kissing spines’)
18.8 Other conditions of the spine
18.10 Treatment of back conditions
18.1 Anatomy of the horse’s back
The equine thoracolumbar spine is composed of a complex arrangement of soft-tissue structures supporting a comparatively rigid vertebral column. The equine back has been likened to a ‘bow and string’ arrangement, where the ‘bow’ is the almost rigid vertebral column; the supporting muscles and ligaments act as the ‘string’, maintaining the spine under tension.
There are usually 29 vertebrae in the back (T18, L6, S5); variations in the vertebral back formula are not unusual in clinically normal horses (e.g. 17 thoracic and 7 lumbar vertebrae). The vertebrae are held in place by a series of ligaments (Figure 18.1). The supraspinous ligament is the caudal continuation of the nuchal ligament and inserts at the tip of every thoracolumbar dorsal spinous process. The main muscles of the back include the largest muscle in the body, the longissimus dorsi, and the powerful gluteal muscles (Figure 18.2).
Figure 18.2 Ligaments of the equine thoracolumbar and sacral spine. Reprinted from Jeffcott LB, Dalin G (1980) Equine Veterinary Journal 12:101–108, with kind permission from Professor Jeffcott and Wiley-Blackwell.
The neck, consisting of seven vertebrae, is most frequently associated with neurological disease and is covered in Chapter 11.
18.2 Diagnostic approach to diseases of the horse’s back
The back of the horse is defined here as the thoracolumbar (T1 to L6) vertebrae, the sacrococcygeal (S1 to Cy2) vertebrae and their associated structures, including the soft tissues. The most common presenting sign of back pain is loss of athletic performance.
History
The history is often of great help in determining whether the clinician is dealing with a genuine case of back pain, or whether factors such as poor behaviour or schooling may be the underlying problem. The history is frequently long and involved; a questionnaire helps to ensure that all relevant details are obtained. Specific aspects include:
• The type of work: the horse may not be suitable for the work expected of it; certain forms of exercise may prove more problematic, e.g. jumping.
• Temperament: owners of horses with a genuine back problem frequently report that the horse has become ill-tempered, fractious or reluctant to work.
• Acute or chronic problem: if the problem has arisen following recent trauma, details of the accident may help to localize the site of pain, although more typically no such incident will have been observed.
• Position of limbs: the horse may no longer straddle when urinating or defecating; there may be reluctance to bear more weight on one hind limb (e.g. during shoeing).
• Bucking: if the history includes episodes of bucking, the safety of any riders should be paramount, regardless of whether the signs are due to pain in the horse’s back or are behavioural; this may limit the clinical investigation, and this should be explained to the owner at the outset.
Examination at rest
Stocks are useful for this part of the investigation, and the horse must be relaxed and bearing weight evenly on all four limbs.
Conformation: General condition should be noted. Abnormal curvature of the back is assessed: i.e. lordosis (ventral deviation), kyphosis (dorsal deviation) or scoliosis (lateral deviation of spine due to spinal malformation or asymmetric muscle spasm). Conformational defects of the limbs (e.g. straight hocks characteristic of proximal suspensory disease) may suggest bilateral hind limb lameness.
• Palpate soft tissues and summits of dorsal spinous processes (DSPs) for pain, heat or swelling.
• Assess flexibility of the back by stimulation reflex extension (dipping), flexion (arching) and lateral bending. This is done by running a blunt probe along the midline of the back from the withers to the sacroiliac region, and laterally from midline. It is important to note that a high range of reflex back movements indicates a normal, pain-free horse, although these movements are often interpreted as a sign of pain. A horse with back pain braces its back rigidly and shows clear signs of resentment.
• Similarly, pressure applied upwards onto the ventral aspect of the sternum should induce a normal horse to arch its back without signs of discomfort.
• Apply pressure to both tubera coxae and tubera sacrale; pain here may indicate fracture of the ilium or a problem in the pelvic or sacroiliac region.
Examination at exercise
• The horse is walked and then trotted in-hand for lameness evaluation.
• Overt lameness or gait abnormalities should be identified; many horses with suspected back pain suffer from hind limb lameness. If present, this should be investigated first as described in other chapters.
• Positive hind limb flexion test(s) suggest lameness rather than back pain.
• Back pain often results in a reduced length of stride of the hind limbs and less hock flexion, but the same signs are seen in chronic hind limb lameness.
• Turning the horse tightly in both directions may provoke longissimus dorsi spasm due to pain caused by lateral spinal flexion.
• There may be reluctance to move backwards, with the head being raised and back muscle spasm because this maneuver may provoke spasms of the back muscles.
Riding: This should be performed – preferably by the horse’s usual rider – unless the horse is considered dangerous to ride.
Observe the process of tacking-up and mounting; some horses may dip ventrally when the girth is tightened or when they are mounted. This is, however, often an acquired habit (i.e. ‘cold back’) and usually not a sign of back pain, although it may reflect a previous episode of discomfort: for example from an ill-fitting saddle.
Assessment of gait is repeated at walk, trot and canter, on both reins.
Following a period of rest, the horse should be trotted up again; a lameness may now be apparent, or in cases of low-grade exertional rhabdomyolysis (‘tying up’) there may be stiffness and reluctance to move.
Aids to diagnosis
Clinical biochemistry: The blood concentrations of the muscle-derived enzymes creatinine kinase (CK) and aspartate transaminase (AST) are measured before, immediately following, and about 12 hours after 10–20 minutes of exercise (trotting and cantering). Significant rises (by more than 100–200%) suggest an exertional rhabdomyolysis.
Radiography (see Chapter 25): Powerful equipment (150 kVp/300 mAs) is required for a comprehensive radiographic examination of the thoracolumbar spine. Smaller portable units are capable of imaging the tips of the dorsal spinous processes (DSPs) from T6 to the cranial lumbar region, but not the vertebral bodies or dorsal intervertebral articulations, a common site of painful osteoarthritis.
Sedation (e.g. with detomidine or xylazine) should be used to reduce movement blur and reduce the restraint required to a minimum; this is important in optimizing radiation safety for the handlers. Large cassettes are used and should never be hand-held.
Typical exposure values are 80 kVp/30 mAs for the DSPs of the mid-back region.
Scintigraphy (see Chapter 25): Bone scanning using radioactive technetium labelled with methylene diphosphonate indicate areas of abnormal skeletal metabolic rate due to trauma (e.g. fracture), inflammation or infection (Figure 18.3). A gamma camera is used, and the examination can be carried out on standing, sedated horses, but images of better quality are obtained under general anaesthesia.
Ultrasonography (see Chapter 25): This technique can be used to evaluate soft-tissue structures such as the supraspinous, interspinous and dorsal sacroiliac ligaments, and is essental when directing a needle towards deep structures such as the dorsal intervertebral articulations. Its value as a routine screening tool in cases of suspected back pain, for example to detect muscle abnormalities, remains unproven.
Local analgesia: This is most useful to confirm the suspected site of pain due to DSP impingement (‘kissing spines’).
The site(s) is/are localized by radiography, prepared aseptically, and 5–10 mL of 2% lignocaine or mepivicaine injected into the interspinous space(s) using a 3.75-cm 18-gauge needle. The dorsal intervertebral (facet) joints can also be injected, although this is technically demanding due to their deeper location; ultrasound guidance is recommended.
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