What Is Open Mouth View X-ray? | Bone And Spine

Last Updated on November 22, 2025

The open-mouth view x-ray or odontoid view is a specialized anteroposterior projection of the upper cervical spine designed to visualize the atlas (C1), axis (C2), and the odontoid process (dens). It provides the only plain-film visualization of the atlantoaxial complex, a region often obscured on standard cervical spine views.

It allows direct assessment of the dens, C1 ring, lateral masses, and the superior articular surfaces of C2. All these structures are critical to maintaining upper cervical stability. Subtle asymmetry or cortical disruption here can reveal significant injuries, including odontoid fractures, Jefferson (C1 burst) fractures, and atlantoaxial subluxation.

Although CT scanning has become the gold standard for evaluating cervical spine trauma, the open-mouth odontoid view remains valuable. It serves as a rapid screening tool, as a comparative baseline during follow-up, and as an essential part of systematic radiographic evaluation for suspected upper cervical injury.

It should be noted that an open-mouth view X-ray is not done routinely and is ordered when a problem is suspected in the upper cervical spine.

Contents hide 1 Relevant Anatomy 2 Indications of Open-Mouth View X-ray 3 How is an Open-Mouth View X-ray Done? 4 What is Looked For in Open-Mouth View X-ray? 4.1 Structures to Evaluate 4.2 Normal Measurements and Alignment 5 Pitfalls and Misinterpretations 6 Final Note 7 References

Relevant Anatomy

The upper cervical spine is formed by the atlas (C1), the axis (C2), and adjoining structures. The first cervical vertebra is called the atlas. It articulates with the skull base above and the second vertebra of the cervical spine or axis below.

The open-mouth (odontoid) view visualizes the atlanto-axial complex, primarily the odontoid process (dens) and the lateral masses of C1 in relation to the body and articular pillars of C2. The radiograph allows assessment of the atlanto-dens interval, symmetry of the C1 lateral masses, and integrity of the dens along its base and body.

The C1 ring, including the anterior arch, posterior arch, and lateral masses, should appear equidistant from the dens on both sides. The superior articular surfaces of C2, the C2 body, and occasionally the C2–C3 interspace may also be seen.

Indications of Open-Mouth View X-ray

The open-mouth view is essential for excluding a C1 arch or odontoid process fracture. The odontoid process is part of the second cervical vertebra that projects upward to articulate with the C1 vertebra.

Indications

• Cervical spine trauma with
  • midline tenderness
  • limited motion
  • flexion-extension injury
• Suspected odontoid fracture
  • Elderly patients after low-energy falls
  • Younger patients following high-impact trauma.
• C1 fractures, including Jefferson fractures, and assessment of transverse ligament integrity indirectly through lateral mass alignment.
• Suspected atlantoaxial instability
• Assessment of upper cervical alignment.

Although CT has higher sensitivity for fracture detection, the open-mouth view x-ray remains a practical first-line or adjunct study in environments where CT is unavailable or delayed.

How is an Open-Mouth View X-ray Done?

The position is the same as for an anteroposterior view. The patient lies in a supine position, i.e., on his back, and has to open her mouth widely. The X-ray beam is projected through the open mouth from the anterior to the posterior direction.

This X-ray requires a patient who is conscious and can cooperate. This may not occur in stressful and painful situations or in unconscious patients. Moreover, a further detailed CT or MRI of the cervical spine is almost necessary if there is doubt over the diagnosis, the view is not satisfactory, or does not match the clinical evaluation.

The shoulders should be at equal distances from the image receptor. The mid-sagittal plane is aligned perpendicular to the detector to minimize rotational artifact. This will avoid rotation. The patient is instructed to open their mouth.

The head is so positioned as to keep the lower margin of the upper incisors and the base of the skull perpendicular to the image receptor. For non-trauma patients, the erect position can be used for taking the view.

The patient must open the mouth as widely as possible to visualize the structures properly. If the patient does not open her mouth widely, the structures we intend to see may get obscured by jawbones and teeth.

A correctly obtained open-mouth projection provides an unobstructed view of the dens, C1 arches, C1–C2 lateral masses, and the atlantoaxial joint spaces, allowing reliable interpretation.

What is Looked For in Open-Mouth View X-ray?

Structures to Evaluate

The open-mouth view X-ray should be approached in a consistent sequence to avoid missing subtle injuries. The order below reflects both anatomical dependence and clinical priority:

The Odontoid Process (Dens)

Begin by examining the cortical outline of the dens. Any interruption, step, lucency, or tapering irregularity should be scrutinized. Assess:

• Vertical continuity between the dens and the body of C2. A lucent line at the base suggests a fracture.
• Small avulsion fractures or ossiculum terminale variants must be distinguished from acute injury.
• Width and symmetry of the dens relative to the C1 ring. Lateral deviation may reflect rotational displacement or ligamentous instability.

Lateral Masses of C1 and C2

Compare the right and left lateral masses. The medial margins of the C1 lateral masses should align closely with the superior articular surfaces of C2. Any lateral overhang of C1 relative to C2 raises concern for a Jefferson (burst) fracture or transverse ligament disruption.

Differences exceeding 2 mm between sides are unlikely to be due to simple rotation alone and should prompt further assessment, typically with CT.

Atlantoaxial Joint Alignment

The atlantoaxial articulation reflects the integrity of both bony containment and ligamentous support. Evaluate joint space uniformity between C1 and C2 on each side. Loss of parallel orientation suggests rotational subluxation, often ligamentous.

If one lateral mass appears more anterior and the opposite more posterior, determine whether this correlates with the rotation of the mandible and teeth. If the rotation of the jaw matches the asymmetry, it is positional. If it does not, suspect true atlantoaxial rotatory displacement.

Occipital Condyles and Atlas

The occipital-atlas articulation is often incompletely visualized, but when seen, the joint spaces should be smooth and symmetric. Irregularity, narrowing, or ghosting of the condylar margins may indicate an occipital condyle fracture. If there is a difficulty visualizing the condyle, CT is indicated.

Normal Measurements and Alignment

Precise measurement is crucial in interpreting the open-mouth view.

C1 Lateral Mass Overhang on C2

Measure the lateral borders of the C1 lateral masses relative to the C2 superior articular surfaces. Normally, C1 and  C1 masses sit symmetrically on C2, with minimal or no overhang.

More than 2 mm asymmetry between right and left is concerning. A value greater than 7 may suggest transverse ligament rupture or a bony injury (Jefferson fracture).

Dens–Atlas Lateral Mass Spacing

Compare the space between the dens and the medial margins of each C1 lateral mass. Normally, spacing should be equal bilaterally. Asymmetric spacing if not positional, suggests rotational subluxation or ligament injury. The reason is that if C1 rotates independently of C2 (e.g., atlantoaxial rotatory fixation), the dens shifts off-center within the ring of C1.

Atlantoaxial Joint Space Orientation

Evaluate the joint surfaces of C1 and C2. Normal joint spaces appear parallel and equal in width. Non-parallel joint space suggests rotational subluxation, or facet surface impaction from vertical compression injuries.

Pitfalls and Misinterpretations

Obscured Structures

The upper incisors, hard palate, or skull base may project over the dens, hiding fractures or mimicking irregular margins. If the dens is not completely visualized, the study is not diagnostic.

False vs True Rotation

Asymmetric appearance of the C1 lateral masses can be caused simply by head rotation. If jaw rotation equals lateral mass asymmetry, the finding is positional. If asymmetry persists despite a midline jaw, suspect true atlantoaxial rotatory subluxation.

Apparent Lateral Mass Overhang

The C1 lateral masses may appear laterally displaced when the head is rotated, even without structural injury. Check whether only one side appears overhanging. A true Jefferson fracture tends to show bilateral overhang as the burst causes the arch to become bigger.

Asymmetric but unilateral displacement strongly suggests a positional artifact. True instability rarely displaces just one side.

Inadequate View

A normal-appearing open-mouth view is not adequate if:

• The dens tip is not seen
• The lateral masses are partially obscured
• The patient cannot open the mouth adequately
• The patient is in pain or uncooperative

If the open-mouth view does not clearly show the entire dens and both C1 lateral masses, it should be treated as non-diagnostic.

Final Note

Compared with CT, plain radiographs detect only about 50–70% of upper cervical fractures. The open-mouth view X-ray still has practical value as a rapid screening tool when CT access is limited, but it is easily rendered nondiagnostic by minor positioning errors, frequently misses nondisplaced Type II odontoid fractures, and does not reliably show occipital condyle injuries.

Therefore, in the presence of a high-energy mechanism, neurological deficit, an inadequate or inconclusive film, or a confirmed C1/C2 fracture needing better definition, CT should be obtained without delay.

MRI has become commonly available and is a better primary investigation for this purpose, especially in soft tissue injury and associated neurological deficit.

References

• Johnson MJ, Lucas GL. Cervical spine evaluation: efficacy of open-mouth odontoid view for nontraumatic radiography. Radiology. 1993 Oct;189(1):247-50. [PubMed]
• Hart J. A brief history of the anteroposterior open-mouth radiograph. J Manipulative Physiol Ther. 2004 Oct;27(8):e13. [PubMed]
• Hubbard TA, Pickar JG, Lawrence DJ. Radiographic analysis of the anterior to posterior open mouth (APOM) cervical spine view: frequency of atlas transverse process overlap of the inferior tip of the mastoid process. J Manipulative Physiol Ther. 2012 Jul;35(6):477-85. [PubMed]