Spine: Steel rule of thirds: 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: Steel rule of thirds

Sample item

On an axial CT through C1, the Steel rule of thirds divides the space within the ring of the atlas. What are the three parts?

AThe spinal cord, the vertebral artery, and the duraBThe anterior arch, the dens, and the posterior archCThe occipital condyles, the dens, and the cordDThe odontoid process, the spinal cord, and free space

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

The answer and the reasoning

D. The odontoid process, the spinal cord, and free space

Within the atlas ring, one third is occupied by the dens, one third by the spinal cord and one third is free space. This free space gives some room for cord safety if the atlas displaces. Rupture of the transverse ligament reduces the free space and risks compression of the cord.

Option by option

A. The spinal cord, the vertebral artery, and the dura
The vertebral artery lies in the foramen transversarium and not in the canal.
B. The anterior arch, the dens, and the posterior arch
The arches form the ring and are not the contents.
C. The occipital condyles, the dens, and the cord
The occipital condyles articulate above the atlas and are not within the ring.
D. The odontoid process, the spinal cord, and free space (correct)
Correct. Dens, cord and free space each occupy one third.

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.

Questions about this Orthopaedics question

What is the answer to this Orthopaedics question?

The best answer is D, The odontoid process, the spinal cord, and free space. Within the atlas ring, one third is occupied by the dens, one third by the spinal cord and one third is free space.

Why is "The spinal cord, the vertebral artery, and the dura" not the answer?

The vertebral artery lies in the foramen transversarium and not in the canal.

Why is "The anterior arch, the dens, and the posterior arch" not the answer?

The arches form the ring and are not the contents.

Why is "The occipital condyles, the dens, and the cord" not the answer?

The occipital condyles articulate above the atlas and are not within the ring.

What concept does this question test?

It tests Spine: Steel rule of thirds in Orthopaedics. This free space gives some room for cord safety if the atlas displaces. Rupture of the transverse ligament reduces the free space and risks compression of the cord.

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: Steel rule of thirds?

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.