Working Diagnosis:
Os Odontoideum

Treatment:
The patient was deemed to be a surgical candidate. He underwent suboccipital decompression, C1 laminectomy, and C1-C2 stabilization.

Postoperative x-ray of the cervical spine, sagittal Case Photo #4 and coronal Case Photo #5 : Resection of posterior arch of C1 and bilateral posterior screws extending from C1 to C2.

Outcome:
The patient was advised to wear a cervical collar. At his two weeks post-operative follow-up with orthopedic surgery, he was advised to use the cervical collar for car use only for another two months. At his three week post-operative follow-up with neurosurgery, routine imaging of the Chiari malformation and thoracic syrinx was recommended to be repeated at six months. At his three month postoperative follow-up with orthopedic surgery, he was advised to avoid high-contact sports, but that he could start a gentle transition to non-contact or limited-contact sports in one year. His six month postoperative follow-up with orthopedic surgery was scheduled, which would include repeat flexion and extension x-rays.

Author's Comments:
Os Odontoideum is the nonunion of the dens portion to the odontoid of the C2 vertebrae, also known as “the axis”. (8) The nonunion can be associated with congenital malformations seen in some craniocervical syndromes, or solely caused by a failure of the secondary ossification center of the dens to fuse with the odontoid. (3) In addition, Os Odontoideum can also occur due to a remote fracture of the odontoid process prior to the closure of the synchondrosis of the odontoid. (1) The nonunion can predispose a person to atlantoaxial instability (AAI), which increases the chances of subluxation and dislocation of the C1-C2 vertebrae causing impingement on the proximal spinal cord. (8) In this case, the patient not only had an Os Odontoideum, but also a type 1 Chiari malformation, cervical canal stenosis with myelopathy, and a thoracic syrinx. (6) There are no clear guidelines in regard to management of asymptomatic Os Odontoideum, but upon review of expert opinion, it is recommended that high-contact sporting activities be contraindicated and athletes be educated on risks of all contact sporting activities. (4) In this case, our patient had AAI with other neurologic findings and was deemed to be a surgical candidate.

Editor's Comments:
That first cervical vertebra, “the atlas”, is uniquely shaped as a ring. The superior and inferior articulating facets of C1 arise from the lateral masses. The superior facets articulate with the occiput, whereas the inferior facets articulate with the axis. Arising from C2, the dens protrudes through C1 and articulates with the anterior portion of the ring. The stability of this articulation depends upon ligamentous attachments, primarily the transverse (cruciate) ligament, which hold the dens tightly to C1. Atlantoaxial instability describes conditions where instability in the C1-C2 articulation allows for pathologically increased ranges of motion that increase risk of spinal cord injury. Common causes of instability include ligamentous laxity (e.g. Down syndrome), malformation of the bony structures (e.g. Os Odontoideum), or damage to either (e.g. inflammatory conditions; trauma). (7) Atlantoaxial instability and Chiari malformation may occur concomitantly. Chatterjee et al. reported that 14% of patients with AAI studied also had Chiari malformation. (2) Atlantoaxial instability can be treated surgically or non-surgically. Surgical management with C1-C2 fusion can achieve good outcomes and long-term stability. (5)

References:
Arvin B, , Fournier-Gosselin MP, & Fehlings MG: Os odontoideum: etiology and surgical management.Neurosurgery 66:3 Suppl 22–31, 2010
Chatterjee S, Shivhare P, Verma SG. Chiari malformation and atlantoaxial instability: problems of co-existence. Childs Nerv Syst. 2019 Oct;35(10):1755-1761. PMID: 31302728.
Fareed Jumah, Saja Alkhdour, Shaden Mansour, Puhan He, Ali Hroub, Nimer Adeeb, Rimal Hanif, Martin M Mortazavi, R. Shane Tubbs, Anil Nanda. Os Odontoideum: A Comprehensive Clinical and Surgical Review 2017 Aug; 9(8): e1551. Published online 2017 Aug 8. doi: 10.7759/cureus.1551
France, J. C., M.D., Karsy, M., M.D., Harrop, J. S., M.D., & Dailey, A. T., M.D. (2016). Global Spine Journal. Return to Play after Cervical Spine Injuries: A Consensus of Opinion, 6(8), 792-797.
Guo et al. Is Initial Posterior Atlantoaxial Fixation and Fusion Applying Bilateral C1-2 Transarticular Screws and C1 Laminar Hooks Reliable for Acute Pediatric Atlantoaxial Instability?: A Minimal 10-Year Analysis of Outcome and Radiological Evaluation. Spine (Phila Pa 1976). 2020 Feb 15;45(4):244-249. PMID: 31568266.
Klimo Jr., P., M.D., Coon, V., M.D., & Brockmeyer, D., M.D. (2011, December 01). Incidental os odontoideum: Current management strategies. Retrieved November 08, 2020, from https://thejns.org/focus/view/journals/neurosurg-focus/31/6/2011.9.focus11227.xml
Lacy J, Bajaj J, Gillis CC. Atlantoaxial Instability. 2020 Jul 10. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan–. PMID: 30137847.
Tomlinson, Christopher BM BS, MSc SEM; Campbell, Alastair MBBCh BaO, FRCR; Hurley, Alison MB BCh, FFRRCSI; Fenton, Eoin MB BCh; Heron, Neil MBChB, BSc (Hons), MRCGP, F.FSEM, PhD. Sport Preparticipation Screening for Asymptomatic Atlantoaxial Instability in Patients With Down Syndrome, Clinical Journal of Sport Medicine: July 2020 - Volume 30 - Issue 4 - p 293-295
Guo X, Han Z, Chen Q, Yang J, Chen F, Guo Q, Lin P, Ni B. Is Initial Posterior Atlantoaxial Fixation and Fusion Applying Bilateral C1-2 Transarticular Screws and C1 Laminar Hooks Reliable for Acute Pediatric Atlantoaxial Instability?: A Minimal 10-Year Analysis of Outcome and Radiological Evaluation. Spine (Phila Pa 1976). 2020 Feb 15;45(4):244-249. doi: 10.1097/BRS.0000000000003259. PMID: 31568266.

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