Abstract
We report ossiculum terminale persistens associated with atlantoaxial instability in a child less than 12 years of age. Static and dynamic X-rays, thin-cut computed tomography with sagittal and coronal reconstructions, and magnetic resonance imaging of the cervical spine showed atlantoaxial instability and an “orthotopic” ossiculum terminale persistens. This pathologic state was differentiated from the primary ossification center at the tip of the odontoid, which normally is not expected to fuse with the body of the odontoid until the age of 12 years. The patient was taken to the operating room for a posterior instrumented fusion of C1 and C2. The patient has done well in short- and long-term follow-up.There have been only a few case reports and small case series regarding atlantoaxial instability, requiring surgical intervention, from ossiculum terminale persistens. Most have presented later in life or in association with Down syndrome. Furthermore, most cases have been of the “dystopic” variant. The terms - “orthotopic” and “dystopic” anatomic variants - have usually been reserved to describe os odontoideum. However, we introduce these terms in describing ossiculum terminale persistens and show a rare case of “orthotopic” ossiculum terminale persistens associated with atlantoaxial instability in a pediatric patient less than the age of 12.
Highlights
History and presentation This 10-year-old Caucasian boy from the USA with no history of Down’s syndrome and no past medical history presented to our institution after a fall while skateboarding on a ramp
We report ossiculum terminale persistens associated with atlantoaxial instability in a child less than 12 years of age
As the apical ossicle remained in anatomic position during dynamic X-rays, the abnormality was consistent with an orthotopic ossiculum terminale persistens
Summary
Embryology The axis ossifies from five primary ossification centers two forming the body of the odontoid process, one for the body of the vertebra, and one each for each side of the posterior neural arch - and one secondary ossification center, giving rise to the tip of the odontoid. The apex of the odontoid process develops from a separate ossification center called the ossiculum terminale or apical odontoid epiphysis, derived from the most caudal occipital sclerotome. Coronal CT in this patient clearly demonstrated a terminal ossicle with intact and sclerotic cortices and nonfusion of this epiphysis This helped differentiate true pathologic ossiculum terminale persistens from a normal apical primary ossification center that has not yet fused to the odontoid peg in a child less than 12 years of age. In some patients with ossiculum terminale persistens, good spinal stability is demonstrated because bone fusion has developed between the ossicle and odontoid process [9]. Cases of ossiculum terminale persistens and atlantoaxial instability with good anatomic alignment of the odontoid tip and odontoid proper orthotopic ossiculum terminale persistens - may be considered a subtype of this disease entity.
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