Apollo-Part Five – Equus-Soma

Apollo's Story - What Lies Beneath - Part Five

***Please be sure to read Parts One, Two, Three and Four of Apollo's Story prior to Part Five.***

Below: Cranial view of Apollo's atlas (C1). A slight asymmetry between the right and left articular foveae is indicated by the blue line and arrows.
Below: Apollo's atlas is flipped up 90º and rotated slightly to better illustrate the concavity of the cranial articular foveae. Notice too, the presence of a "notch" on each of the lateral borders. The notches are a normal feature of the atlas.

Below Left: Ventral view of the occipital bone of Apollo's skull (the mandible was removed) showing the left and right, rounded occipital condyles. Note the size difference between the two condyles as marked by the red lines.

Below right: Ventral view of Apollo's occipital bone with the atlas attached to form the atlantooccipital joint (AOJ).

Other indications of asymmetry are seen in the relative dimensions of the "foramen lacerum" (red asterisks photos below). These are paired openings through which important nerves pass out from the cranial vault (brain).

         Movement of the atlantooccipital joint is mostly dorsoventral flexion and extension. The rounded, convex shape of the paired articular condyles of the occiput conform to the concave cranial articular fovea of the atlas.
         The photos to the left and right illustrate the orientation of the atlas relative to the occiput in extension and flexion. Notice how the articular foveae of the atlas slide up and over the occipital condyles of the occiput to open (extend), then back and down to close (flex) the AO Joint.
         A closer examination of this junction revealed what appears to be a small stress fracture at the bases of both notches on the left and right articular foveae (red arrows in photo below).

Right Photo: Caudal view of the atlas. The green dots surround an area of "roughened" bone in the center of the right articular fovea.
During flexion and extension of the AO Joint in Apollo, this region passed over a raised osteophyte that developed on the surface of the right occipital condyle (blue arrow below right). Once again "suggesting" asymmetry.

Enthesophytes - form at sites where ligaments, membranes, muscles, and fascia attach to bone.
The photos above and below left show slightly different views of Apollo's occipital bone. The red arrows point to short but rather sharp enthesophytes on the dorsal border of the foramen magnum that appear to project into the dural space. The foramen magnum is the opening through which the spinal cord passes from the brain.

The photos to the immediate left and above left show mild to moderate enthosophyte formation on the external occipital protuberance. This is the site of attachment of the funicular portion of then nuchal ligament and is relatively more flat & smooth in the "normal" condition as illustrated in the photo to the right by the skull of another horse in the Learning Center.

We will discuss the significance of the osseous pathologies described here and in the previous Parts One, Two, Three and Four, in the final chapter of Apollo's Story - Part Six.

The stress fracture at the base of the notch on the left articular fovea (circled in red) is a little larger and more pronounced than the stress fracture on the right.
Below: closer view of the stress fracture shown above within the red circle.

"Enthesopathies are skeletal manifestations resulting from the [bone] remodeling stimulated by increased blood flow when muscle/tendon/ligament-bone junctions are regularly subjected to minor stress, and hypertrophy of bone is the direct result of this increased, repetitive stress."

"The[se] lesions may thus be viewed as a response to the repeated movements of the bones, causing repetitive stress on the connecting ligaments ultimately leading to their ossification."

~Bendry 2007-International Journal of Osteoarchaelogy

Equus-Soma