Atlanto-axial rotary fixation is a disorder of C1-C2 causing impairment in neck rotation. The anterior facet of C1 is fixed on the facet of C2. It may be associated with dislocation of the lateral mass of C1 on C2.
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Terminology
Antlanto-axial rotary fixation is the preferred term as fixation occurs usually within the normal range of motion of the joint and subluxation or dislocation are not always present 8. Further, a normal range of motion includes atlantoaxial rotation up to 79 degrees, asymmetry of the lateral masses of C1 and the dens, and up to 85% loss of contact of the facet joints 9,10.
Epidemiology
Atlanto-axial rotary fixation is often associated with high-energy traumas, with up to 80% of cases occurring in those under 13 years old 6,8.
Pathology
Etiology
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congenital
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arthritides
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acquired
trauma
surgery
upper respiratory tract infection
sternomastoid muscle spasm secondary to a local process such as lymphadenitis or a tumor
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spontaneous
idiopathic
Mechanism
There are several ways in which a fixation can occur:
anteroposterior subluxation
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atlanto-axial rotatory fixation (AARF) is characterized into four different types according to the Fielding and Hawkins classification 3,8:
type I: the atlas is rotated on the odontoid with no anterior displacement (most common)
type II: the atlas is rotated on one lateral articular process with 3 to 5 mm of anterior displacement, indicating transverse ligament injury
type III: comprises rotation of the atlas on both lateral articular processes with anterior displacement greater than 5 mm, indicating transverse and alar ligament injury
type IV: rotation and posterior displacement of the atlas
vertical subluxation
lateral subluxation 4
Radiographic features
Plain radiograph
In a non-traumatic setting, flexion and extension views may be performed. The expected distance between the anterior arch of C1 and the dens in the fully flexed position should be <3 mm in an adult (5 mm is the upper limit of normal in a child) 5.
Also, the anterior translocation of the atlas causes the posterior arch of C1 to become anterior to the spinolaminar line 7.
In a vertical subluxation, the dens is often above the McGregor line by over 8 mm in men and 9.7 mm in women.
CT
On CT, C1 and the head are rotated out of alignment around a vertical axis. If this is a fixed defect, C2 is rotated in conjunction with C1.
Differential diagnosis
Possible differential considerations on imaging include:
normal, noting that in normal motion there is a wide range of atlantoaxial rotation, asymmetry of the lateral masses of C1 and the dens, and loss of facet articular surface contact 9,10
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