Page 96 - Adams and Stashak's Lameness in Horses, 7th Edition

62 Chapter 1

slightly craniad. The vertebral arches tend to overlap between adjacent spinous processes. In the thoracic
dorsally, except at the L5–L6 and L6–S1 interspaces region, intercapital ligaments pass transversely between
VetBooks.ir clinically accessible. The cranial and caudal articular the intervertebral discs. Articulations between articular
the heads of contralateral ribs over the dorsal aspects of
where the larger interarcuate spaces are much larger and
processes on vertebral arches are true synovial joints. In
processes articulate in an approximation of the sagittal
plane, an orientation that allows for a very slight degree the cervical region, these constitute broad plates, ori­
of flexion and extension of the vertebral column but ented in a nearly horizontal plane to permit significant
prevents lateral flexion. The transverse processes of the lateral bending. Articular facets on the cranial articular
lumbar vertebrae are large and blade‐like. They project processes face dorsomediad, while the complementary
laterad. The caudal aspect of the fifth transverse process facets on the caudal articular processes face ventrolat­
articulates with the cranial aspect of the sixth. The cau­ erad. Synovial joints also exist between the transverse
dal aspect of the sixth transverse process features a large processes of the fifth and sixth lumbar vertebrae and
concave facet through which it articulates with the between the transverse processes of the sixth lumbar
sacrum. vertebra and the wings of the sacrum.

Sacrum Sacroiliac Region
The equine sacrum is a single bone formed through The axial skeleton and appendicular skeleton of the
fusion of embryologically distinct sacral vertebrae, gen­ hindlimb are united at the sacroiliac joint (Figure 1.54).
erally five of these, with four, six, and seven sacral verte­ This planar joint is created by the auricular face of the
brae also being reported. Fusion is usually complete by wings of the sacrum, which are oriented dorsolaterad,
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5 years of age. The sacrum is triangular and gently curv­ and the auricular face of the wings of the ilia, which
ing so as to present a slightly concave ventral aspect. angle ventromediad. This joint is histologically synovial
Intervertebral foramina are transformed by the fusion of but is capable of only extremely limited gliding move­
adjacent vertebrae into a row of four dorsal sacral ment. Its principle purpose is most likely absorption of
foramina and four ventral sacral foramina, through some of the concussive forces transmitted through the
which pass dorsal and ventral branches, respectively, of appendicular skeleton to the vertebral column. The joint
the sacral spinal nerves. The spinous processes remain capsule is close‐fitting and is substantially reinforced by
individually distinct and incline slightly caudad. The sec­
ond through fifth spinous processes end in slight enlarge­
ments that are not uncommonly bifid. Tuber Dorsal part of dorsal
The first sacral vertebra gives rise to the wings of the sacrale sacroiliac ligamet
sacrum. Their articular surfaces face dorsolateral to Sacrum
articulate with the auricular surface of the ilium. The
ventral aspect of the first sacral vertebra is slightly
rounded, forming the promontory of the sacrum, the Lateral part of
point from which the conjugate diameter (the dorsal– dorsal sacroiliac
ventral distance) of the pelvic canal is measured. ligament
llium Sacrosciatic
Caudal Vertebrae ligament
Although there is considerable individual variation,
the average horse has 18 caudal vertebrae. Only the first
three or so have vertebral arches, the remaining being
represented by cylindrical bodies only. The first caudal Tuber
vertebra is not uncommonly fused with the sacrum, coxae Dorsal sacroiliac
especially in old horses. Tuber ligament Interosseous
sacrale ligament
Vertebral Articulations
Excluding the atlantoaxial joint (a pivot joint), the
joints of the vertebral column all permit small amounts
of flexion, extension, lateral flexion, and limited rota­
tion. These movements are especially limited through Ilium
thoracic and lumbar regions, but the cervical vertebral Ventral
column is capable of more extensive movement. Sacroiliac sacroiliac
Intervertebral discs of fibrocartilage are interposed joint Sacrum ligament
between adjacent vertebral bodies. Further stabilization
is provided to the vertebral column by (1) the continu­
ous dorsal and ventral longitudinal ligaments on their
respective surfaces of the vertebral bodies; (2) a supras­
pinous ligament that passes along the dorsal aspect of
the spinous processes of thoracic, lumbar, and sacral
vertebrae; and (3) interspinal ligaments that pass Figure 1.54. Sacroiliac joint. Lateral (top) and cranial (bottom) views.