Page 140 - Zoo Animal Learning and Training

15 Atlantoaxial Subluxation

Fred Wininger

Introduction rotary movement is permitted but ligamentous structures prevent
Atlantoaxial subluxation (AAS) is a relatively common spinal insta- dorsal/ventral flexion. It is notable that there is no intervertebral
bility disorder primarily seen in young toy‐breed dogs [1,2], but can disc between C1 and C2 and ligaments maintain the position of the
occur in larger breeds [3] and any patient when associated with a vertebrae [5].
traumatic event. It is rarely reported in cats [2]. In the young toy The odontoid process (dens) arises from the body of C2 and rests
breeds, the condition is generally associated with a combination of in the fovea of the dens (fovea dentis) on the dorsal surface of the
anatomical deformities coupled with an often supraphysiological C1 body. Interestingly, the dens is analogous to the intercentrum of
movement that leads to joint laxity and subsequent compressive C1, but during development permanently ossifies as a protuberance
cervical myelopathy. Though often challenging, reduction and sta- of C2. Occasionally, there is a malformation or dorsal notching of
bilization of these cases can be rewarding, often with complete the dens that may lead to spinal cord compression, even while in
long‐term return to function. normal reduction. Some have hypothesized that this is the result of
abnormal fusion of the proatlas, a transient area of ossification at
the apical tip of the dens. The proatlas fuses with the dens at 106
Anatomy days after birth [4,5].

Atlas and Axis Ligaments
The first cervical vertebra (C1) is also known as the atlas because of Dorsally, the joint is maintained via a dorsal atlantoaxial membrane
its transversely elongated shape. C1 articulates with the skull crani- connecting the spinous process of C2 with the vertebral arch of C1.
ally and the axis (C2) caudally. C1 is different from the typical cervi- The transverse atlantal ligament tethers between the ventral arches
cal vertebrae in that it lacks a spinous process, has elongated of C1 and maintains the dens in the fovea dentis in a retinaculum
transverse processes, and has concave articular processes that abut fashion that forms a large bursa in between. The apical ligament of
the occipital condyles of the skull. The atlantooccipital joint is also the dens is actually a remnant of the primitive notochord as are the
known as the “yes joint” as it allows for free dorsal/ventral move- nucleus pulposi of the intervertebral discs. It is composed of the
ment but is limited on lateral flexibility. solitary apical ligament that traverses through the vertebral canal of
C2 is notable because of its large and cranially protruding spinous C1 and attaches to the ventral surface of the foramen magnum and
process that is the site of attachment of the nuchal ligament in many the paired lateralized alar ligaments, which attach medial to the
species. Its transverse processes are shorter than adjacent vertebrae occipital condyles [6].
and its body notably longer. The ventral tubercle rises from the ven-
tral surface of the body and is an important structure in identifying
the joint during ventral approaches, as it is sharper than other mid- Pathophysiology
line processes of the ventral cervical spine [4]. AAS is likely a condition that is most frequently a developmental
The embryology of C1 and C2 is complex and potentially pro- anomaly exacerbated by a physiological or supraphysiological/trau-
vides insight into the mechanism of AAS. The atlas arises from matic cervical movement. Contributing developmental factors
three bony components, two neural arches that form the vertebral include aplasia or hypoplasia of the dens, nonunion of the dens with
arch and a body. The axis arises from seven total components [4]. the axis, or absence of the transverse ligament. When presenting in
C1 and C2 articulate via the caudal articular surface of C1 (fovea signalments noncharacteristic to AAS, the etiology is often purely
articularis caudalis). This is frequently known as the “no joint” as traumatic with failure of the otherwise normal ligaments [7].

Current Techniques in Canine and Feline Neurosurgery, First Edition. Edited by Andy Shores and Brigitte A. Brisson.
© 2017 John Wiley & Sons, Inc. Published 2017 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/shores/neurosurgery

141