Spine: os odontoideum: practice question

A Orthopaedics question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Orthopaedics · Spine: os odontoideum

Sample item

A young adult with minor trauma has radiographs showing a smooth, corticated ossicle separated from a short dens with a wide gap, and flexion-extension views show 8 mm atlantoaxial movement. What is the diagnosis and the concern?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. Os odontoideum; atlantoaxial instability with cord compression

Os odontoideum is a separate smooth ossicle above a hypoplastic dens, probably from old trauma or developmental failure of fusion. It may cause atlantoaxial instability and cord compression, as in this patient, and is treated by posterior C1-C2 fusion when unstable. A corticated margin distinguishes it from an acute fracture.

Option by option

  • A. Os odontoideum; atlantoaxial instability with cord compression (correct)

    Correct. A corticated ossicle with abnormal movement is os odontoideum, with a risk to the cord.

  • B. Normal developmental variant needing no follow-up

    Unstable movement of this degree means follow-up and often surgery are required.

  • C. Rheumatoid pannus; no instability

    Rheumatoid pannus is not the cause here and the patient has no inflammatory disease.

  • D. Acute type II odontoid fracture; no instability

    An acute fracture would have sharp, non-corticated edges and soft tissue swelling.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Orthopaedics question

What is the answer to this Orthopaedics question?

The best answer is A, Os odontoideum; atlantoaxial instability with cord compression. Os odontoideum is a separate smooth ossicle above a hypoplastic dens, probably from old trauma or developmental failure of fusion.

Why is "Normal developmental variant needing no follow-up" not the answer?

Unstable movement of this degree means follow-up and often surgery are required.

Why is "Rheumatoid pannus; no instability" not the answer?

Rheumatoid pannus is not the cause here and the patient has no inflammatory disease.

Why is "Acute type II odontoid fracture; no instability" not the answer?

An acute fracture would have sharp, non-corticated edges and soft tissue swelling.

What concept does this question test?

It tests Spine: os odontoideum in Orthopaedics. It may cause atlantoaxial instability and cord compression, as in this patient, and is treated by posterior C1-C2 fusion when unstable. A corticated margin distinguishes it from an acute fracture.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Spine: os odontoideum?

Pair each fracture with its classic complication and nerve injured. Then use the flashcards and the other Orthopaedics questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.