Answer for BIR CoW 20 April 2025
Dystopic Os Odontoideum
Findings
Abnormal smooth,well corticated ossicle noted at the superior aspect of the hypoplastic C2 dens , consistent with os odontoideum. The separated part of the dens is not seen in its normal place, however, it is displaced anteriorly and superiorly and noted in alignment with clivus , in consistent with the dystopic type. Evidence of increased atlantodental interval (ADI-6.4mm) with hypoplstic C2 dens is noted. Hypoplastic C2 dens is noted compressing spinal cord at the level of atlantoaxial joint causing severe spinal canal narrowing (3.4 cm ) with T2 cord hyperintensity seen.
Diagnosis - Dystopic Os Odontoideum with Atlanto-Axial Subluxation causing severe spinal canal narrowing
Discussion
OS ODONTOIDEUM
Non union of dens with axis body A transverse radiolucent cleft separates an ossicle of variable size from the body The cause remains controversial, it can be congenital or post traumatic D/d – Ossiculum terminale.
TYPES –
ORTHOPTIC TYPE – ossicle lies in the position of normal dens and moves in unison with atlas and axis
DYSTOPIC TYPE – ossicle lies near the basion or may be fused with the occipital bone and moves in unison with the clivus. Os odontoideum is seen in Down’s syndrome, spondyloepiphyseal dysplasias and Morquio’s syndrome.
Differences between Os osontoideum and Ossiculum terminale : Os Odontoideum : - Sequela of odontoid synchondral fracture prior to union - Disruption at the cartilaginous plate between dens and body of axis - Ossicle posterior to transverse atlantal ligament - Increased risk of atlantoaxial instability Ossiculum Terminale : - Congenital nonunion of odontoid body from a terminal ossicle at the tip - Ossicle located above the transverse atlantal ligament - No atlantoaxial instability in most cases
References :
https://radiopaedia.org/cases/dystopic-os-odontoideum-with-atlantoaxial-subluxation
https://www.ncbi.nlm.nih.gov/books/NBK542297/
Findings
Abnormal smooth,well corticated ossicle noted at the superior aspect of the hypoplastic C2 dens , consistent with os odontoideum. The separated part of the dens is not seen in its normal place, however, it is displaced anteriorly and superiorly and noted in alignment with clivus , in consistent with the dystopic type. Evidence of increased atlantodental interval (ADI-6.4mm) with hypoplstic C2 dens is noted. Hypoplastic C2 dens is noted compressing spinal cord at the level of atlantoaxial joint causing severe spinal canal narrowing (3.4 cm ) with T2 cord hyperintensity seen.
Diagnosis - Dystopic Os Odontoideum with Atlanto-Axial Subluxation causing severe spinal canal narrowing
Discussion
OS ODONTOIDEUM
Non union of dens with axis body A transverse radiolucent cleft separates an ossicle of variable size from the body The cause remains controversial, it can be congenital or post traumatic D/d – Ossiculum terminale.
TYPES –
ORTHOPTIC TYPE – ossicle lies in the position of normal dens and moves in unison with atlas and axis
DYSTOPIC TYPE – ossicle lies near the basion or may be fused with the occipital bone and moves in unison with the clivus. Os odontoideum is seen in Down’s syndrome, spondyloepiphyseal dysplasias and Morquio’s syndrome.
Differences between Os osontoideum and Ossiculum terminale : Os Odontoideum : - Sequela of odontoid synchondral fracture prior to union - Disruption at the cartilaginous plate between dens and body of axis - Ossicle posterior to transverse atlantal ligament - Increased risk of atlantoaxial instability Ossiculum Terminale : - Congenital nonunion of odontoid body from a terminal ossicle at the tip - Ossicle located above the transverse atlantal ligament - No atlantoaxial instability in most cases
References :
https://radiopaedia.org/cases/dystopic-os-odontoideum-with-atlantoaxial-subluxation
https://www.ncbi.nlm.nih.gov/books/NBK542297/
Note:
We do not discourage differential diagnosis. But all the differentials must satisfy the findings noted in the case.
If you feel you have answered rightly but cannot find your name in the above list, please call 09551942599.
Did you Know?
The order in which the names appear in this winner's list is based on the time of submission. The first person to send the correct answer gets his/her name on top of the list!
We do not discourage differential diagnosis. But all the differentials must satisfy the findings noted in the case.
If you feel you have answered rightly but cannot find your name in the above list, please call 09551942599.
Did you Know?
The order in which the names appear in this winner's list is based on the time of submission. The first person to send the correct answer gets his/her name on top of the list!