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One of the diagnoses,

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which is particularly difficult to make in the children because

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of the potential for not seeing all the bones as they have not

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completely ossified is the atlanto odontal distraction.

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And there are different measurements that can be made to

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determine whether or not there has been dislocation

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or distraction of the atlanto odontal distance.

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These include the BDI measurement or the Basion

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to the Dens Interval, the Power's Ratio,

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the CC1 I or the Condyle-C1 Interval,

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the Lateral Mass Interval,

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and some of the Basion-Axial Intervals.

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So what we're looking at here is the distance from the

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bottom portion of the clivus to the top of the dens.

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And this is the so called Basion-Dens Interval, or BDI.

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You can see that this is markedly widened.

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This is a very unstable spine.

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In fact, there's a very high mortality rate associated with when you

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have a dissociation between basically the skull base

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and the cervical spine with a widened BDI interval.

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So let's look at some of these indexes.

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So the Basion-Dens index measures from the Basion,

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which is the anterior margin of the foramen magnum

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at the inferiormost portion of the clivus to the Dens.

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On CT scan, this should be less than 8.5 mm.

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Some people just round up to 1 cm because, as you'll see,

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1 cm is kind of a good number to remember for all

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of these basic intervals. This is the Power's Ratio.

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The Power's Ratio is the ratio from the Basion to the

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spinolaminar line of the posterior arch of the C1.

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And the other measurement is from the ophistenon,

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which is the posterior margin of the foramen

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magnum to the anterior arch of C1.

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And if the ratio of the BC to OA should be less than or equal to

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one. So again, think of that one number we talked about,

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1 cm for the BDI,

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then we have the one number for the ratio.

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If it's less than that, it's normal.

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If it's greater than one,

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that's when you worry about the atlanto occipital dissociation.

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Here is the C1 interval.

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So in this case,

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we're looking at the space between the lateral mass of C1

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with the Occipital Condyle. C for condyle

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C1 for the first cervical vertebra,

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and the interval there, and the space in between

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should be less than 2.5 millimeter.

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This can be seen also on the sagittal scan where

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you're going to be measuring this distance.

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Really, I'd look and see whether there is an asymmetry from side to

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side because any asymmetry from side to side that cannot be

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accounted for the patient's head tilt may suggest that there

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has been ligamentous injury and potential for instability.

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So these are the various measurements that people use to

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determine whether or not there has

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been Atlanto occipital dissociation.

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So here is an example.

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If we look at the measurement of the C1 condyle

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to the lateral mass of the C2 vertebra,

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we want to look at this measurement also.

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This should be symmetric from side to side.

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Here we have this widening at the C1 to C2 level,

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which is the atlanto odontoid distraction.

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And on MRI scan, you will see bright signal intensity if these

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ligaments have been disrupted with abnormal signal

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there. Another example.

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Here we have the loss of the tectorial membrane.

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We have a defect in it.

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We have bright signal intensity off midline in the space here.

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And both there is widening between C1 and C2, as well

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as between the occipital condyle and the C1 vertebra.

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And this is manifested on the axial scan as this bright signal

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intensity off midline between the occipital condyle

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and C1, and between C1 and C2.