Ossiculum Terminale

 May be dystopic or orthotopic



• Soft tissue pannus suggests atlantoaxial instability

• Myelomalacia, brainstem compression in rare cases




Top Differential Diagnoses




• Os odontoideum

• Type I or II odontoid fracture

• Normal unfused odontoid tip synchondrosis

• Degenerative remodeling of odontoid process


Pathology




• Terminal ossicle normally appears by 3 years of age, fuses with odontoid body by 12 years of age

• Terminal ossicle located above transverse ligament
image Atlantoaxial instability less common than with os odontoideum

image Unstable ossiculum terminale usually dystopic


Clinical Issues




• Usually asymptomatic

• Uncommonly, neck pain or myelopathy related to CVJ instability


Diagnostic Checklist




• Consider ossiculum terminale in patients > 12 years with persistent terminal ossicle at odontoid tip

• Evaluate for atlantoaxial instability if dystopic ossicle, excessive soft tissue pannus, or trisomy 21 patient

image
(Left) Sagittal bone CT demonstrates a small ossiculum terminale image located adjacent to the odontoid tip. The ossicle is in orthotopic position, and CVJ alignment is normal.


image
(Right) Coronal bone CT confirms orthotopic placement of a small orthotopic ossiculum terminale image. Note that there is minimal flattening of the odontoid tip adjacent to the ossicle, supporting classification as a persistent terminal odontoid ossicle.

image
(Left) Sagittal bone CT reveals a small dystopic ossiculum terminale image positioned between the odontoid tip and clivus. The odontoid tip has remodeled.


image
(Right) Sagittal bone CT shows subluxation of the anterior C1 ring image relative to the nearly normal-sized odontoid process image. There is a large dystopic ossiculum terminale image. Although debatable, the nearly normal size of the odontoid process favors classification as an ossiculum terminale rather than os odontoideum.


TERMINOLOGY


Synonyms




• Ossiculum terminale persistens


Definitions




• Persistence of unfused odontoid tip ossification center into adulthood


IMAGING


General Features




• Best diagnostic clue
image Separate ossicle with intact cortical margin positioned above normal-sized odontoid process in adolescent or adult

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Oct 5, 2016 | Posted by in NEUROLOGICAL IMAGING | Comments Off on Ossiculum Terminale

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