Overview
The horse normally has six lumbar vertebrae; some horses, especially Arabians, have only five. They form the skeleton of the loin, between the last thoracic vertebra and the sacrum, and support the dorsal wall of the abdomen. The equine lumbar region is rigid: its vertebrae hardly flex on one another, and dorsoventral flexion of the caudal back is concentrated at the lumbosacral joint.
Key features
- Body: shorter and wider than in the thoracic vertebrae, with a ventral crest on the first ones.
- Transverse processes: long, flat and horizontal; they give attachment to the abdominal and sublumbar muscles and form the palpable base of the loin. Those of the last vertebrae are broader and thicker.
- Intertransverse joints: between the transverse processes of L5 and L6 and between L6 and the wings of the sacrum there are synovial joints, sometimes also between L4 and L5. They are unique to the horse and add to the stiffness of the loin.
- Spinous processes: broad, of similar height, inclined cranially.
- Mamillary processes: on the cranial articular processes; they give attachment to the epaxial muscles.
Clinical relevance
- Ankylosis of the intertransverse joints: their fusion is common in adult horses and is usually an incidental finding, but osteoarthritis of these joints can cause loin pain.
- Lumbosacral puncture: cerebrospinal fluid is collected from the standing horse on the midline, between the spinous process of L6 and the sacrum; the site lies roughly at the level of the caudal edges of the tubera coxae.
- Back pain: the loin is one of the areas examined most in poorly performing horses; palpation of the epaxial muscles over the lumbar vertebrae helps to localise it.