1. Os odontoideum Antônio Santos de Araújo Jr. 1, Pedro Alberto Arlant 1, Arnaldo Salvestrini Jr. 1, Orlando Parise 2, Paulo Roberto Lazarini 3, Carlo Domenico Marrone 4, Mirella Martins Fazzito 5, Raphael R. Pratali 6, Edison Luis Dezen 6, Marcelo Bordalo Rodrigues 7. 1. Neurosurgeon, Sírio Libanês Hospital, São Paulo, Brazil; 2. Head and neck surgeon, Sírio Libanês Hospital, São Paulo, Brazil; 3. Ear, nose and throat surgeon, Sírio Libanês Hospital, São Paulo, Brazil; 4. Neurophysiologist, Ricardo Ferreira Neurophysiology Clinics, São Paulo, Brazil; 5. Neurologist, Sírio Libanês Hospital, São Paulo, Brazil; 6. Orthopedist, Sírio Libanês Hospital, São Paulo, Brazil; 7. Spine radiologist, Sírio Libanês Hospital, São Paulo, Brazil Introduction: ‘Os odontoideum’ is radiographically defined as an isolated ossicle with smooth circumferential cortical margins and no osseous continuity with the body of C2. It represents an uncommon craniovertebral junction (CVJ) abnormality that exists as a separate ossicle apart from a hypoplastic C2 dens. Anatomically, there are two types of ‘os odontoideum’: the orthotopic and the dystopic one. Orthotopic refers to an ossicle that moves with the anterior arch of C1 and that can be reduced in a normal alignment with the dens. Dystopic means an ossicle that has migrated toward the clivus and that is functionally fused to the basium, sometimes it may be even dislocated anteriorly to the C1 arch. Some authors suggest that it is congenital, while others believe that it is a result of remote trauma with a chronic nonunited fracture of the odontoid process. Biomechanics Regarding its biomechanics, craniovertebral junction (CVJ) may be divided in two motion segments, with different mechanical properties. The mechanical properties of ‘occiput-C1’ are largely determined by bony elements, while those of the C1-C2 segment are largely determined by ligamentous elements. The primary movement at Oc-C1 is flexion and extension, while the primary movement at C1-C2 is axial rotation. ‘Os odontoideum’ compromises the integrity of the odontoid-transverse ligament complex, leading consequently to atlantoaxial instability. This instability results in translation of the C1 vertebra and ‘os’ relative to C2. During flexion motion, anterolisthesis of the C1-os complex in relation to C2 may compress the spinal cord against the posterior arch of C1. During extension, posterior subluxation of the C1-os complex may compress anteriorly the spinal cord. Case report: We report a 45 year-old woman, housewife, who presented with a 2 years history of constant neck pain associated with gait disturbance and intermittent upper-extremity paresthesias. All symptoms had worsened after an unwitnessed fall to ground, just after an inadverted neck flexion. After that episode, she sustained progressive upper and lower limbs weakness. At first evaluation, neurological examination revealed a disproportionate spastic tetraparesis, with crural predominance, and obvious signs of cervical myelopathy (hyperactive deep tendon reflexes, Hoffmann’s, Babinski and Lhermitte’s signs). She was promptly submitted to plain radiographies, CT and MRI scans, and diagnosed as harboring a dystopic ‘os odontoideum’ (Figure 1). On further dynamic craniocervical radiographies, the CVJ instability was obvious. Before the first surgical procedure, the patient experienced a progression of her myelopathic symptoms. In order to alleviate the cord compression and stabilize the CVJ, it was proposed a posterior approach with suboccipital craniectomy, C1 and C2 laminectomy, duroplasty, and occipitocervical fixation, with intraoperative somatosensitive evoked potential (SSEP) monitoring. On further postoperative CT and MRI scans, posterior decompression was satisfactory, instead our concerns about anterior compression (Figure 2). One year after the first procedure, patient started to complain of recurrent right arm and left leg weakness. At this time, progressive paresis was noticed associated two months later to a swallowing problem and hoarseness. Transoral-transpharyngeal approach was then proposed under a neuronavigation protocol, crossing the soft palate, in order to perform a C1, Os and C2 osteotomy with high-speed drill (Figure 3). Postoperatively patient evolved once more assymptomatic, with complete recovering of sensorimotor deficits, with mild velopharyngeal insufficiency. Postoperative MRI scans were performed (Figure 4). Conclusion: We believe that this step-wise surgical management to ‘os odontoideum’ is feasible and reasonable. It accomplishes favorable neurological prognosis with minimal intraoperative morbidity, as shown in our case. References: 1. Araújo Jr AS, Arlant PA, Salvestrini Jr. A, et al. Os odontoideum. Neurosurgery Quarterly, Aug 2012; 22(3):174-178. 2. Klimo P Jr, Kan P, Rao G, Apfelbaum R, Brockmeyer D. Os odontoideum: presentation, diagnosis, and treatment in a series of 78 patients. J Neurosurg Spine. 2008;9(4):332-42. Figure 1: ‘Os odontoideum’ image diagnosis. (A) Sagital computed tomography (CT), bone window, showing altered CVJ with three bone segments, tubercle of C1, Os and C2; (B) Axial bone CT, revealing the relationship between the three segments; (C) Sagital T2 weighted magnetic resonance image (MRI), marked anterior and posterior spinal cord compression; (D) Sagital T2 MRI, myelomalacia secondary to severe spinal cord compression. Figure 2: Postoperative images after posterior decompression and occipitocervical fixation (OCF). (A) Lateral CT reconstruction, showing screws and rods from OCF; (B-C) Posterior CT reconstruction, revealing lateral limits from suboccipital craniectomy; (D) Sagital CT image, bone window, superior and inferior margins from decompression, including C1 and C2 laminectomy; (E) Axial bone CT scan, C1 posterior arch removal. Figure 3: (A) Intraoperative neuronavigation, real-time drill monitoring while resecting C1 tubercle; (B) Intraoperative neuronavigation, drilling at the tip of odontoid process. C1 os C2 os C1 C2 A B C D ABCD E AB Figure 4: MRI scan after transoral transpharyngeal odontoidectomy.