Introduction

Asymmetry of the lateral masses of C1 with respect to the odontoid process is commonly seen on the AP C1/C2 image. The difficulty with this finding in trauma patients is to know what to do next. The finding is very commonly found to be a normal variant or idiopathic. It does not however seem appropriate to ignore the finding in trauma patients.

Research

Odontoid-lateral mass asymmetry in minimally symptomatic patients: a normal variant. European Journal of Orthopaedic Surgery & Traumatology. Volume 13, Number 2 / June, 2003

	This patient presented to the ED with neck pain following a sports injury. The lateral cervical spine image appears to be normal. The prevertebral soft tissues are within normal limits (arrowed). Hard cervical collar artifact noted
	The AP cervical spine appears to be normal. Hard cervical collar artifact noted
	The first attempt at an odontoid view demonstrates significant odontoid-lateral mass asymmetry.
	The asymmetry persists in the head-to-the-left LPO oblique
	The asymmetry is also unchanged in the head-to-the-right RPO oblique.
	CT imaging of the patient's cervical spine was undertaken 24hours after the initial plain films. There is no evidence of odontoid-lateral mass asymmetry.
	The CT coronal images similarly demonstrate no significant asymmetry of the odontoid-lateral masses.

Case 1 Discussion

You could speculate as to the cause of the apparent odontoid-lateral mass asymmetry on the initial plain films. I would suggest that it was not purely projectional- three slightly different AP projections demonstrated the same asymmetry. Was it caused by the soft tissue injury that the patient received during the trauma? Was it associated with the wearing of the cervical hard collar? Was it a combination of a number of factors? Importantly, the asymmetry was not apparent on CT scan imaging 24 hours after the initial trauma. This case does not support the notion of asymmetry as a normal variant- the asymmetry was not evident 24 hours after the injury.

It is noteworthy that the head rotation employed was minimal and may not have been sufficient to produce appreciable difference in the lateral mass-dens distances.

	This AP view of the odontoid peg demonstrates asymmetry of the lateral masses of C1 with respect to the dens. This asymmetry is evident despite the fact that the patient's skull appears to be in a true AP position. Note that the upper central incisor teeth are symmetrically distributed with respect to the dens. To gain additional confidence that the appearance is physiological rather than pathological, oblique projections can be of assistance.
	RPO Oblique This is an RPO oblique. Note that the space between the central incisor teeth (white line) is no longer superimposed over the midline (black line).
	LPO Oblique This is an LPO oblique. Note that the space between the central incisor teeth (white line) is no longer superimposed over the midline (black line)

Case 2 Discussion

The LPO position in particular shows evidence of symmetry of the C1 lateral masses with respect to the dens and with respect to the body of C2. This additional information needs to be considered in the context of:

• clinical signs (e.g. paresthesia, head control, pain)
• mechanism of injury (e.g. high speed MVA)
• evidence of injury demonstrated on the other cervical spine projections
• soft tissue signs of injury.

	This patient presented to the ED following a sports injury. There is no clear evidence of a cervical injury.
	Bilateral offsets of the lateral masses of C1
	Incomplete fusion of the anterior and posterior arches of C1 spina bifida occulta of the neural arch of c1

These cases demonstrate a variety of possible findings associated with odontoid-lateral mass asymmetry. It is useful for the radiographer to be aware of the significance of this appearance and the possible imaging options and diagnostic outcomes