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Let's review some examples of the different types of odontoid

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fractures on sagittal and coronal reconstructed CT scan.

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So that fracture, which involves the tip of the odontoid process,

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maybe even off center,

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is the Type I Odontoid fracture seen on the sagittal

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and the axial scan, just the top of the odontoid process.

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Type II is at the base of the odontoid process,

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and it doesn't extend into the vertebral body.

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And you see that on the sagittal and coronal reconstructions

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of this example of Type II Odontoid process.

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The classic Type III Odontoid process,

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you see the fracture line extending into the vertebral

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body going down more inferiorly on the sagittal scan.

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You see also there's abnormal angulation here,

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and that extension into the body represents the

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between Type III from Type II Odontoid fractures.

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Here is a Type III odontoid fracture where you see that

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there is extension into the inferior portion of the C2

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body. Here it is angulated, inferiorly and anteriorly,

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not just involving the odontoid process,

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but getting into the body.

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And not only that,

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but you see a little bit of an offset right

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here representing the lowest

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most portion of this fracture and extension even

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into the posterior portion of the arch here.

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This is the plane of the fracture into the vertebral body

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that you're seeing representing the lowest portion

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of the Type III Odontoid fracture on axial scan.

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We're actually scanning right through this level,

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and you can see it's a side by side,

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right to left transverse fracture fragment.

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We want to contrast the Type I

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odontoid fracture of the tip of the odontoid

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process from the Os odontoideum.

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How do we distinguish an Os odontoideum from

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a fracture of the tip of the odontoid process?

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Well, you notice that this is a well defined,

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well corticated piece of bone that is

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associated with the C1 vertebra.

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And there is this gap between the lower odontoid process and

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the upper odontoid process. This is a congenital abnormality,

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or some people think that it's an early in development

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separation of the Os odontoideum from the rest

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of the odontoid process. This, however,

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despite it being thought to be a congenital abnormality,

2:50

may be an unstable process.

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You can see instability of the atlantoaxial joint, again,

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between C1 and C2, which

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can place the spinal cord at significant risk.

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And usually what happens here is that the clinicians may do

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flexion extension views to see whether there

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is some anterior to posterior instability.

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And that may be because the Os odontoideum is sticking with

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the C1 vertebra as a functional

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unit rather than the Os

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odontoideum sticking with the lower

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C2 as a functional unit.

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So Orthotopic maintains its normal relationship with the

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clivus and moves in unison with the atlas and axis.

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Whereas, Dystopic Os lies in close proximity to the skull

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base and moves as an extension of the clivus rather

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than as an extension of C1 or C2.

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So these are normal variations or variants that should

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be reported because of the potential for instability.

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And this is particularly true

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in individuals who have had a trauma to the cervical spine.

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On this MRI scan, knows that the patient has high signal

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intensity in the spinal cord because it has been damaged,

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because this is an unstable Os odontoideum.