Os odontoideum.
Antônio Santos de Araújo Jr.1, Pedro Alberto Arlant1, Arnaldo Salvestrini Jr.1,
Orlando Parise2, Paulo Roberto Lazarini3, Carlo Domenico Marrone4, Mirella
Martins Fazzito5, Raphael R. Pratali6, Edison Luis Dezen6, Marcelo Bordalo
Rodrigues7.
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
Abstract
Introduction: ‘Os odontoideum’ is a rare craniovertebral junction (CVJ) disease that
presents itself as a dystopic ossicle from a hypoplastic C2 dens, contributing to a craniocervical
instability.
Objective and Methods: We report a 45-year old woman harboring a dystopic ‘os
odontoideum’,
who
sustained
a
progressive
neurological
deterioration
with
marked
circumferential spinal cord compression. She was submitted to a prior posterior suboccipital
craniectomy with occipitocervical fixation, followed by an anterior transoral transpharyngeal
odontoidectomy.
Results and Discussion: Both procedures were performed uneventfully, and the
patient had a remarkable improvement afterwards. We believe that our step-wise surgical
management to ‘os odontoideum’ is feasible and accomplishes excellent outcomes.
Key Words: Os odontoideum, craniocervical instability, craniovertebral junction
diseases.
Antônio Santos de Araújo Júnior
Correspondence Address:
Rua Peixoto Gomide, 515, cj 96, Cerqueira César, São Paulo, São Paulo, Brazil. CEP 01409-001.
Phone/Fax: 05511-32890411 Cel: 05511-84567404
Email: dr.antonioaraujojr@gmail.com; Website: araujoefazzito.com.br
Introduction
‘Os odontoideum’ was first described by Giacomini in 18861. ‘Os
odontoideum’ is radiographically defined as an isolated ossicle with smooth
circumferential cortical margins and no osseous continuity with the body of C2 2.
It represents an uncommon craniovertebral junction (CVJ) abnormality that
exists as a separate ossicle apart from a hypoplastic C2 dens 3.
Anatomically, there are two types of ‘os odontoideum’: the orthotopic and
the dystopic one 4. 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 4.
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 4.
Since ‘os odontoideum’ is a rare lesion its pathogenesis is controversial.
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 2.
Some evidences support both etiologies. According to the congenital
hypothesis, the incomplete ossification across the vestigial intervertebral disc
separating the odontoid process from the body of the axis (a cartilage known as
a ‘neurocentral synchondrosis’) would be responsible for the formation of the
ossicle 4. A familial case of ‘os odontoideum’ with an autosomal dominant
pattern of inheritance5, and a report of identical twins sisters with ‘os
odontoideum’ 6 support this theory.
Proponents of the traumatic hypothesis suggest an unrecognized
odontoid type II fracture followed by avascular necrosis and osseous
remodeling as the cause of os odontoideum 4. This remodeling makes ‘os
odontoideum’ indistinguishable from an old nonunion type II odontoid fracture
2,4.
This theory is sustained by the fact that previous history of trauma is
observed in a majority of patients.
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 7.
The primary movement at Oc-C1 is flexion and extension, while the
primary movement at C1-C2 is axial rotation. Rotational movement is limited by
C1-C2 articulation, the contralateral alar ligament, the capsular ligaments, and
last but not the least by the ipsilateral transverse ligament 7. By its time,
translational motion is prevented by the transverse ligament trapping the intact
odontoid process within the anterior arch of C1 2.
‘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 2. 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.
Therefore, the instability produced by os odontoideum is biomechanically
identical to the atlantoaxial instability seen in a Type II odontoid fracture 2.
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. According to her, 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.
C1
Os
C2
Os
C1
C2
B
A
C
D
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.
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. During
this procedure an attempted occiput to C1-C2 transarticular screws fixation was
not feasible, because the SSEP alterations after alignment. In view of it, occiput
to C2-C3 lateral mass screws fixation was then performed with intraoperatively
gains in SSEP.
Postoperatively, the patient was symptoms-free, with regained motor and
sensory functioning, and a normal gait. She gradually recovered normal daily
activities, with full extremities coordination, despite some limitation of neck
mobility.
On further postoperative CT and MRI scans, posterior decompression
was satisfactory, instead our concerns about anterior compression (Figure 2).
B
A
C
D
E
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.
One year after the first procedure, this 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.
At hospital admission, neurological examination revealed severe motor
impairment, tetraparesis with strength grade 0/V in right arm, 4/5 in left arm, 4/5
in right leg, and 3/5 left leg, tetraparesthesia and lower cranial nerves signs.
Transoral-transpharyngeal
approach
was
then
proposed
under
a
neuronavigation protocol (Figure 3).
A multidisciplinary surgical team was conveyed to handle this unusual
surgical problem. Laryngeal fibroscopy intubation was performed. Monitoring
devices
were
connected,
in
followed
by
tracheostomy.
Transoral-
transpharyngeal approach, crossing the soft palate, was accomplished by our
ENT surgeon. Neurosurgeons, under microscopic magnification, performed a
C1, Os and C2 osteotomy with high-speed drill (Medtronic Midas Rex, Fort
Worth, Texas, U.S.A.) under real-time neuronavigation (BrainLab VectorVision
Neuronavigation System, Heimstetten, Germany). Bone drilling was advanced
up to the duramater level. Internal cortical bone from C2 was removed with 1
mm Kerrison rongeur up to reestablishment of normal duramater pulsation. Just
after, SSEP from all limbs were normalized.
A
B
C
Figure
3:
(A)
Preoperative
neuronavigation: C1 tubercle and
odontoid process are targeted;
(B)Intraoperative neuronavigation,
real-time drill monitoring while
resecting
C1
Intraoperative
tubercle;
(C)
neuronavigation,
drilling at the tip of odontoid
process.
Postoperatively patient evolved once more assymptomatic, with complete
recovering of sensorimotor deficits, with mild velopharyngeal insufficiency, nasal
regurgitation and nasal quality of the voice during speech. Tracheostomy was
closed after three days. Nasoenteric feeding tube was removed by the 5th
postoperative day, with normal swallowing. Postoperative MRI scans were
performed (Figure 4). Patient was discharged by the 8th postoperative day.
A
B
C
Figure 4: MRI scan after transoral transpharyngeal odontoidectomy. (A-B)
Sagital T2 weighted MRI, showing extension from the C1, Os and C2 osteotomy;
(C) Anterior decompression accomplished, without much lateral bone removal.
Discussion
‘Os odontoideum’ is a rare craniocervical disease, with harmful
consequences over the craniovertebral junction. Patients can present with
whole gamut of neurological symptoms 4. It is reported that even minor trauma
can result in severe neurological injury and may result in sudden death 2.
Standard plain radiographies are sufficient to diagnose this condition, but
the advent of MRI has improved its precision, revealing in details the spinal cord
compromise.
Assymptomatic patients should be followed closely, while every patient
with unstable or compressive disease should be offered surgery 4. Surgical
indication should be tailored according to the patient’s status, and respecting
the surgical team experience.
We believe that circumferential spinal cord compression by ‘os
odontoideum’ can be first managed via the posterior route (suboccipital
craniectomy, C1 and C2 laminectomy, and duroplasty), with C1-C2 or
occipitocervical fixation in the same surgical stage. Most neurosurgeons are
very familiar with this approach, with minimal morbidity and high successful
rates of complete fusion.
After the first procedure, closely follow-up is mandatory to identify
impending neurological deterioration. Since symptoms progress despite
posterior decompression, anterior approach should not be postponed. A
multidisciplinary surgical team, with neurosurgeons and ENT- head and neck
surgeons, is indispensable to best perform the transoral transpharyngeal
approach to the upper cervical spine.
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.
Despite its potential severity, ‘os odontoideum’ is a treatable disease.
Surgery may allow patients to return early to normal daily activities, with
excellent outcomes.
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