SET T0026 -
Emergency Department

Abnormal. Diagnosis: Blow Out fracture of the left orbit.

• The fracture itself - or a fracture fragment - is not seen (it is rarely detectable on plain radiographs with this injury).
• The evidence for a fracture on this radiograph in this particular case is: " the Black Eyebrow " sign; ie the crescent of black above the globe. This crescent is free air within the orbit.
• Where has the air come from? From a sinus . . . either via a blow out fracture of the orbital floor, or via a blow out fracture of the medial wall of the orbit (ie the lateral wall of the ethmoid sinus). These plates of bone are very thin and are vulnerable to a sudden increase in pressure within the orbit.
• In this patient the punch to the face involved the globe and resulted in a sudden rise in intraorbital pressure.

Blow Out fracture details:

See : Accident & Emergency Radiology : A Survival Guide (3rd edition) : Pages 64-67

The lateral radiograph is abnormal - in several respects.

Observations:

• Increase in the atlanto-axial distance [ie the anterior margin of the odontoid peg is more than 3mm posterior to the anterior aspect of the atlas (C1) vertebra].
• Abnormal spino-laminar line at the C1 - C2 level. Indicating posterior subluxation of C2 on C1 vertebra.
• The alignment of the margins of the odontoid peg with the anterior and posterior margins of the body of C2 is most worrying - because there is a very slight step in both positions. In other words - suspect that there is also a minimally displaced fracture of the base of the odontoid peg.
• (Much more subtle - note the soft tissue swelling anterior to the C1 and C2 Vertebrae; this is a haematoma).
• Severe degenerative change at all levels below the C2 vertebra.

Conclusions:

Two important abnormalities. a) Rupture of the transverse ligament with posterior subluxation of C2 on C1; b) a minimally displaced fracture of the odontoid peg.

Suggested reading: See : Accident & Emergency Radiology : A Survival Guide (3rd edition) : Page 172

If you have any difficulty in assessing the C1 / C2 articulation then it will be helpful if you start by identifying the so called "Coffee Bean" shadow. This is described on page 172.

Intra-articular fracture at the base of the 1st metacarpal.

• This is not a simple avulsion fracture.
• This is a Bennett's fracture ;potentially / invariably an unstable injury.

Why is a Bennett's fracture unstable? In the reference below, the medical artist, Philip Wilson, has created drawings which show the positions and influence of the normal ligaments and tendons at the base of the 1st metacarpal. Philip Wilson's artworks illustrate very clearly why instability occurs.

See : Accident & Emergency Radiology : A Survival Guide (3rd edition) : Page 153

 Type I Hangman's fracture with vertebral subluxation.

• Longitudinal (ie vertical) fracture line is seen between the pars interarticularis and the posterior body of the C2 vertebra. This fracture is commonly termed a "Hangman's" fracture.
• Widened C2 / C3 disc space.
• The spino-laminar line at the C2 / C3 level is markedly out of line - the C2 spinous process is displaced posteriorly.

Important Note: the top of the T1 vertebra is not shown on the lateral radiograph. This can be technically difficult in some patients and also with some neck injuries. Hence the cardinal rule . . . "  the top of T1 vertebra must be shown in all patients who have sustained a neck injury and are referred for C-spine radiography. If the top of T1 is not shown - then additional imaging is mandatory ".

There are several good reasons for this important rule: (1) if the top of T1 vertebra is not visualised - it is frequently difficult to see and visualise adequately the C6 & C7 vertebrae; (2) more than one fracture of the cervical spine is a fairly common occurrence. Thus, the entire C-spine must be visualised and assessed.

Frequently - when the top of T1 vertebra is not shown - it is necessary to evaluate and clear the lower part of the cervical spine by obtaining a CT examination.

Suggested reading:  See : Accident & Emergency Radiology : A Survival Guide (3rd edition) : Page 192