Age / Sex : 43 / M

Condylus tertius

Discussion

Findings:

1. C-spine Lateral view with dynamic study

: underlying cervical kyphosis

: Dynamic instability of C1-2 on flexion and extension view

: no definitely visualized odontoid process of C2 

2. C-spine CT sagittal view

: Abnormal bony process connected with end of the clivus 

3. C-spine MR sagittal view (T2WI)

: Condylus tertius, pseudo-joint formation with odontoid process and C1 anterior

Arch

     : Cord atrophy with chronic compressive myelopathy due to dynamic instability at

C1-2   

Differential Diagnosis: none 

Diagnosis:  Condylus tertius 

Discussion:

Condylus Tertius :

• - The third occipital condyle (condylus tertius or median occipital condyle) = Ossified remnant of the hypochordal bow of the 4^th occipital sclerotome (the proatlas) 


• - Incidence : 0.47%

Attached to the basion

Pseudojoint with the odontoid process or the anterior arch of the atlas

Instability and compression of important surrounding neurovascular structures.

May cause myelopathy

Imaging findings

• - CT : (saggital) Well-circumscribed bony process attached to the basion or unattached between the occipital bone and the anterior arch of the atlas or the tip of the dens


• - MR : helpful to evaluate synovial joints and spinal cord 

References:

• - Radiographics 1994;14:255–277


• - Childs Nerv Syst 2011;27:523–564


• - Skeletal Radiol 2014; 43:535–539