Steels rule of thirds

Steels rule of thirds

A radiological guideline known as Steels rule of thirds is used to determine a foreign body’s approximate position within the body by analyzing how it is oriented with respect to anatomical landmarks.

This is especially important when using radiographs, or X-rays, to see the foreign body.

According to Steel’s rule of thirds:

  1. First Third: In the first third of the path of the beam, the foreign body is likely located anterior to the midpoint of the structure.
  2. Second Third: In the second third of the path of the beam, the foreign body is likely located posterior to the midpoint but anterior to the end of the structure.
  3. Last Third: In the last third of the path of the beam, the foreign body is likely located near the distal end of the structure.

This rule states that the foreign body will appear as a shadow on the X-ray film or image, with the X-ray beam being considered to travel in a straight line. It is important to keep in mind that this is merely a general guideline and may not always identify the exact location of the foreign body, especially in complex cases or when there are anatomical variations.

In clinical practice, medical professionals—radiologists in particular—use this rule in conjunction with additional data, such as patient history and additional imaging examinations, to plan a foreign body’s safe and efficient removal and to make educated conclusions regarding its possible position.

At the level of the first cervical vertebra (C1), the anteroposterior diameter is divided into thirds, allowing one-third for the dens, one-third for the spinal cord, and one-third for free space.

Because there is significant free space at this level, small degrees of C1 to C2 subluxation (3–7 mm) usually do not compromise the cord.

However, when the anterior atlantodens interval (measured from the posterior part of the anterior arch of C1 to the anterior aspect of odontoid) becomes >10 to 12 mm, all the atlantoaxial ligamentous complex has usually been destroyed, and the space available for the spinal cord is usually compromised.

Likewise, when the posterior atlantodens interval (measured from posterior aspect of the odontoid to the anterior aspect of the posterior arch of C1) is <14 mm, the spinal cord is usually compressed. 

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