Page 278 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Musculoskeletal system: 1.7b The axial skeleton – thoracolumbar region 253

VetBooks.ir 1.478 1.479

Fig. 1.479 A late middle-aged Thoroughbred with
lordosis of the back. The horse had no clinical signs of
Fig. 1.478 Transverse ultrasound image obtained back pain. (Photo courtesy Graham Munroe)
with a 13.3 MHz linear probe at L4 (arrow = top of
dorsal spinous process). The supraspinous ligament
(arrowheads) is often asymmetrically located in the
lumbar region owing to laxity at rest.

Muscles and ligaments SPINAL DEFORMITIES
Intrinsic muscles of the thoracolumbar region are
divided into those above and those below the trans- Definition/overview
verse processes: the functionally opposing epaxial Osseous abnormalities of the vertebral column are
and hypaxial muscle groups, respectively. Equine quite common, with a reported incidence of 20–36%
spinal function has been likened to the archer’s bow, of examined Thoroughbreds having deviations from
whereby the vertebrae and dorsal soft-tissue struc- the standard vertebral formula in the thoracolumbar
tures described form the bow, while the muscles or lumbosacral region. Little is known about the aeti-
and ligaments of the ventral abdominal wall form ology of such deviations. While vertebral formula
the string. The ventral abdominal muscles, such as variations are congenital by definition, other defor-
rectus abdominis, hold a huge mechanical advantage mities only become apparent during skeletal matu-
over the epaxial muscles, which allows them to ven- ration, suggesting developmental causes. Whatever
trally flex the spinal column. The ventral abdomi- their origin, only rare cases with severe deformity
nal muscles are frequently weak and dysfunctional may interfere with the athletic use of the animal.
in horses with chronic back problems, and address- Excessive curvature of the thoracolumbar spine can
ing this issue is an important part of the treatment be ventral (lordosis) (Fig. 1.479), dorsal (kyphosis)
plan of such cases. The dorsal border of the epaxial (Fig. 1.480) or lateral (scoliosis) (Fig. 1.481).
muscles, most of which is made up of the longissimus
group axially and iliocostalis abaxially, is easily visu- Aetiology/pathophysiology
alised and palpated in the back. Deep to these, the There are multiple articulating bony surfaces that
multifidus muscles sit closest to midline against the maintain the thoracolumbar vertebral column in
DSPs, providing stabilisation and proprioception. normal alignment. Severe curvatures are usually due
The psosas and iliacus muscles sit in the hypaxial to an anomaly in one or several vertebrae. Lordosis
group along with the quadratus lumborum, acting is also relatively commonly seen in geriatric animals,
to ventroflex the spine. Ligaments and adjacent fas- especially when retired from regular ridden exercise.
cial planes are richly endowed with multiple types of Broodmares may also suffer with this with increas-
sensory nerve receptor as well as bare nerve endings. ing age and repeated pregnancies.