AP Peg view

Long AP view

Priority 1: Lateral view checklist

Identify the odontoid peg and assess its position and anatomical relationship to the C1 vertebra. Overlapping structures (eg mastoid, ear lobes, C1 vertebra) can make this difficult. Questions 1–5 will help you to overcome this.

Ask yourself ten important questions:

1. Is the radiograph technically adequate? Ensure that the C1–C2 articulation and the superior surface of the T1 vertebra are clearly seen.

2. Have I identified the anterior arch of the C1 vertebra (the “coffee bean”)?

3. Is the anterior cortex of the odontoid peg (the Peg) closely apposed to the “coffee bean”?

4. Is the line of the anterior cortex of the Peg continuous with the anterior cortex of the body of C2? Any displacement implies a Peg fracture or fracture of C2 body.

5. Is the line of the posterior cortex of the Peg continuous with the posterior cortex of the body of C2? Any displacement or break indicates a Peg fracture.

6. Is Harris’ ring⁷ normal? A break in either the anterior or posterior margin of the ring indicates the high probability of a fracture of the Peg/body of C2 (p. 176).

7. Are the posterior arches of C1 and C2 intact?

8. Are the other vertebrae (C3–C7) intact (p. 192)?

9. Are the three main contour lines/arcs normal (p. 178)?

10. Are the pre-vertebral soft tissues normal (p. 179)?

Questions 2 and 3.

Recognising the anterior arch of C1 is key to detecting the abnormalities that may involve the C2 vertebra.

The arch looks like a small coffee bean, and is always easy to identify on the lateral view.

The gap between the Peg and the coffee bean should not exceed 3 mm in adults or 5 mm in children¹.

This is a normal lateral view.

Questions 4 and 5.

Check that the anterior cortex of the Peg is continuous with the anterior cortex of the body of C2.

Also check that the posterior cortex of the Peg is continuous with the posterior cortex of the body of C2. Any displacement or break in either of these lines indicates a Peg fracture.

Question 6. Harris’ Ring⁷. Many lateral views will show a white ring projected over the base of the Peg and over part of the body of C2 (see above). This ring may appear slightly incomplete at its inferior and/or superior aspects—that is a normal appearance. However, if either the anterior or posterior margin of the ring appears disrupted then a fracture through the base of the Peg or the body of C2 is very possible and a C2 fracture will need to be excluded (p. 188).

Question 7. Check that the posterior arches of C1 and C2 are intact.

Question 8.

Check that the vertebral bodies (C3–C7) are intact.

The vertebral bodies below C2 have a fairly uniform square or rectangular shape.

The anterior and posterior heights should be approximately the same. The front to back widths of all of the vertebrae are approximately similar.

Question 9. You can trace the three main contour lines (arcs) as follows:

Line 1: along the anterior margins of the vertebral bodies (anterior line).

Line 2: along the posterior margins of the vertebral bodies (posterior line).

Line 3: along the bases of the spinous processes (spinolaminar line).

Each line should be a smooth unbroken curve or arc. No kink. No step. Trace these lines along the full length of the cervical spine. Line 1 extends from the top of the anterior cortex of the Peg to the anterior margin of the body of T1 vertebra.

Potential Pitfall: Line 3 will sometimes show a slight step at the C2 level, particularly in children⁸.

Apply this rule: this step should not be more than 2 mm posterior to the smooth arc as it is traced upwards between C3 and C1 vertebrae.

Question 10. Are the pre-vertebral soft tissue shadows normal? The soft tissue shadow^7,9–11 anterior to the vertebral bodies has a characteristic configuration and width. Any bulge or local increase in width indicates haemorrhage and connotes an important injury.

NB: The absence of a bulge does not exclude a ligamentous or bone injury. Indeed, even with a major injury, a soft tissue bulge due to a haematoma is fairly rare.

Maximum normal width of the prevertebral soft tissues

Level	Width	Approximate % of a vertebral body (AP) width
C1–C4	7 mm	30%
C5–C7	22 mm	100%

Priority 2: AP Peg view checklist

The anatomical arrangement of the C1–C2 articulation allows extensive neck rotation whilst providing maximum stability. This stability depends on the integrity of the ligaments, particularly the C2 transverse ligament. Various other ligaments enable C1 vertebra to be held in the optimal position above the body of C2 vertebra. Any deviation from this alignment indicates either ligamentous disruption or a broken vertebra.

Ask yourself three important questions:

Question 1.

We teach that a more relevant name for the Peg view is:

“The lining up of the lateral masses view”.

Normal anatomy of the C1–C2 articulation. Looking down from above.

If the lateral masses do not align vertically, then there could be several possible explanations.

First consider subluxation due to ligament rupture.

Second, consider fracture of the body of C1; either isolated to a single lateral mass or to a burst fracture of C1 (a Jefferson fracture).

Finally, consider a developmental variation or just simple rotation of a normal neck (see p. 182), as shown on this AP Peg view.

Question 2: Frequent pitfall.

Slight rotation of the neck may cause the space on each side of the Peg to appear unequal. However, if the lateral masses of C1 and C2 remain normally aligned then the asymmetry can be attributed to rotation rather than indicating damage to the transverse ligament (p. 191).

Question 2: Pitfall.

Occasionally, asymmetric alignment of a lateral mass will be present; ie the edges of the adjacent C1/C2 lateral masses do not line up perfectly. This might suggest vertebral subluxation. However, non-pathological causes for this finding do occur: either some slight positional rotation of the neck, or a developmental variation in the size of the C1 and C2 lateral masses^1,12–14. In most instances it is fairly easy to decide whether an offset on one side of the C1–C2 articulation is simply developmental… just check whether there is any offset on the other side. If the lateral masses line up normally on the other side then developmental asymmetry is the likely explanation.

Question 3: Pitfall—the Mach effect.

It is very common to see a thin black line crossing the base of the Peg or the top of the Peg. This is an optical illusion¹⁵ resulting from the overlapping shadows of superimposed structures. This line is known as a Mach band or a Mach effect. Be aware of this line.

But, be careful…

The inexperienced observer should always seek advice before too readily dismissing any line in this area as a Mach artefact.

Another artefact.

The apparent vertical fracture through the Peg is an artefact. It is caused by the gap between two of the teeth (incisors).

Priority 3: Long AP view checklist

The lateral and Peg views are the most useful images. The diagnostic return from the long AP view, in terms of abnormalities detected, will be considerably less. In addition, it is easy for the unwary to read too much into the AP view.

Ask yourself two questions:

A warning: if a space is more than 50% wider than the space immediately above or below, this is highly suggestive of an anterior cervical dislocation¹⁶. In practice this observation is most useful in the severely injured patient whose shoulders have obscured some of the vertebrae on the lateral radiograph¹⁷. It can provide an important warning that the neck must be managed very carefully until a lateral view or CT has accurately defined the alignment of the vertebrae.

The normal radiograph shows the spinous processes in a straight line.

The C4 and C5 spinous processes are bifid and bifid processes are a very common occurrence.

The drawings illustrate how an anterior dislocation can result in a large gap between the spinous processes at the affected level.

Question 1: Pitfall.

Sometimes a bifid spinous process (a normal variant) might suggest that the processes are not in a straight line.

This normal radiograph shows bifid processes at all levels.

Question 2: Pitfall.

If muscle spasm causes the neck to be held in flexion then the 50% rule does not apply.

Whenever the 50% rule (p. 184) is broken you must check the lateral view again in order to see whether or not neck flexion or a dislocation is the explanation for the increase in space on the AP radiograph.

Injuries involving the body or the posterior elements of C2

C2 Hangman’s fracture⁸.

A bilateral fracture through the pars interarticularis of C2 vertebra. Unstable injury caused by hyperextension. Historically consequent on hanging. Now typically results from a motor vehicle accident with (eg) the forehead striking the dashboard. In this patient there is also a fracture of the body of C3.

Oblique fracture through the body of the C2 vertebra.

Sometimes this causes the anterior-posterior width of the body of C2 to be increased¹⁸ (the so-called “Fat C2“ sign).

C2 Hangman’s fracture with anterior displacement of C2 on C3 vertebra.

C2 subluxation due to rupture of the transverse ligament

The transverse ligament viewed from above. (a) = normal, (b) = ruptured.

Ruptured ligament: Evidence from the Peg view. Check the margins of the lateral masses of C1 and C2. If all four lateral masses do not line up in a normal manner then a rupture of the ligament is probable. A Jefferson fracture (p. 186) is an alternative possibility.

Ruptured ligament: Evidence from the lateral view.

The space between the anterior arch of C1 (the “coffee bean”) and the anterior aspect of the Peg must not exceed 3 mm in adults.

In this patient the space is approximately 7 mm. C2 has subluxed posteriorly because the transverse ligament has ruptured.

Fractures C3–C7

Including spinous process fracture, vertebral body compression fracture, and hyperflexion teardrop fracture.

The cardinal rule: The lateral radiograph must always, always, include a well visualised superior surface of the T1 vertebra. Many errors^1,3,5 occur because the top of T1 has not been included on the radiograph. Check the lateral view as described on p. 174.

C7 spinous process fracture. This is evidence of major violence. Important ligaments are likely to be torn.

C7 vertebral body compression fracture (arrow). Consequent on a flexion injury.

C6 hyperflexion teardrop fracture.

Consequent on extreme flexion and axial loading. An unstable injury.

Subluxations/dislocations C3–C7

Anterior subluxation.

A flexion-rotation injury. Recognition is usually easy. Detected when there is disruption—at any level—of the three arcs described on page 178. Sometimes it is a single arc that appears most out of line.

Subluxed mid-cervical vertebra with arc disruption.

Unilateral facet joint dislocation.

Consequent on a distraction–flexion force with a rotational element. Commonly overlooked on radiographs.

Look for:

▪ AP view: spinous processes out of line.

▪ Lateral view: 10–20% forward subluxation (at the C6/C7 level in this patient).

Then:

▪ Obtain the opinion of an experienced observer.

▪ Confirmation of this very precise diagnosis requires the judgement of an expert.

Explaining unilateral facet joint dislocation

The facet of one vertebra has jumped forwards off the facet of the vertebra below. The jump occurs on one side only.

The rotational displacement at the facet joint causes the spinous processes to be out of line on the AP view.

The jumping off of one facet (arrow) from the facet below (arrowhead) results in the forward subluxation on the lateral view.

On the long AP view

Developmental variants

A vertebra (arrow) may appear slightly narrow anteriorly with loss of the normal square or rectangular outline. This can mimic a compression fracture.

Sometimes this narrowed appearance is due to old trauma. Occasionally it is due to persistence of the normal but slightly wedged shape that is often present during adolescence¹⁹.

Anterior opacity

A small calcified opacity anterior to a vertebral body can be mistaken for an avulsed fracture fragment. Sometimes it is simply a remnant of an old secondary ossification centre¹⁹ (left). An age related osteophyte can also produce a similar appearance (right). When such an opacity is detected, an experienced observer should review the radiographs.

Spasm related—delayed instability

Following trauma, severe pain and spasm may make it difficult to exclude a significant injury to the posterior ligament complex. Muscle spasm can hold the neck in an anatomical position and mask ligamentous rupture. Instability may only become evident after a few days when the spasm has reduced.

It is therefore important that any patient with severe pain and spasm who appears fit for discharge and is put in a collar should be asked to attend again in a few days for further evaluation. Lateral views in flexion and extension might form part of that evaluation. These additional radiographs must be taken under close clinical supervision. If they remain at all equivocal, an MRI examination should be obtained in order to exclude a ligamentous injury.

Age related changes

Age related degenerative changes are very common over the age of 40. Distinguishing between changes due to cervical spondylosis and those resulting from an acute injury is not always easy.

The age related appearances shown below are frequently present in the middle-aged and the elderly.

Anterior subluxation of a vertebra secondary to facet joint degenerative change (at several levels on this radiograph). There is no simple way of distinguishing this from traumatic subluxation. In most cases, correlation of the clinical symptoms and signs with the site of the radiographic abnormality will provide reassurance. In some cases an injury should be assumed until an experienced observer has reviewed the radiographs²⁰.

A step in the anterior vertebral line. The step is due to a protruding anterior osteophyte. It might be misinterpreted as indicating vertebral subluxation.