Fixed Rotatory Subluxation
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On the axial scans, we make an important observation about
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whether the odontoid process is centered with
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respect to the anterior arch of C1.
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When it's not centered in association with the
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anterior arch of C1,
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more often than not,
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this is because the head is tilted.
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If the head is not tilted,
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then we worry about something called fixed rotatory
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subluxation of the C1, C2 relationship.
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In order to absolutely make this diagnosis,
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one has to see the preservation of that asymmetry
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when the patient is in neutral position,
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when the patient has the head turned to the right,
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and when the head is turned to the left.
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And that's what you're seeing here.
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This is the neutral position.
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And we notice that the space between the odontoid process and
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the lateral mass of C1 is narrower on
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the left side than on the right side.
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And this is reconstructed on the coronal plane,
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you see that there's narrowing of the space
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on the left side compared to the right side.
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When the patient turns their head to the right side,
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this narrowing is preserved. So although it's decreased,
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it still is more narrow.
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The distance from here to here, from the odontoid process
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lateral border to the anterior arch of C1, is still
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more narrow than it is on the right hand side.
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When the individual turns their head to the left
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side, you see that that narrowing is preserved.
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This is what we mean by fixed rotatory subluxation.
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It's always more narrow on the left side than it is on
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the right side. In neutral, head turn to the right,
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head turn to the left.
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This is the way we evaluate the patient radiographically.
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Most of the time, clinically, the patient has torticollis.
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That is,
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the head is cocked to one side and it just is oriented
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that way. This may be on the basis of trauma,
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but you may also see this in association with inflammatory
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processes of the nasopharynx and pharynx extending
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to the retropharyngeal space. In fact,
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there is a syndrome called Grisel syndrome,
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G-R-I-S-E-L that is associated with either this rotatory
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subluxation or actually atlantodens widening
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that can occur with Grisel syndrome.
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This is the Fielding and Hawkings classification
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of Rotatory Subluxation of C1, C2.
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And that is not just looking at the side-to-side fixation but
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also whether or not there is that widening of the atlanto odontoid
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interval with anterior displacement. And you can
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see that it is separated into those that are 3 to 5 mm
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displaced versus greater than 5 mm displaced.
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The normal distance is less than 3 mm.
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So that would be our Type I. Type II, 3-5,
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this suggests that the transverse ligament is lax
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and allowing the atlantodens interval to widen.
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And then we have the Type III with
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rotatory fixation and greater than 5mm.
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We have rotatory fixation with posterior displacement.
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This is very, very uncommon.
David M Yousem, MD, MBA
Professor of Radiology, Vice Chairman and Associate Dean
Johns Hopkins University
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