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288
Fig. 20.2 Condylar aplasia
on the left side and hypoplasia with atlas assimilation on
the right
20 Congenital and Developmental Abnormalities
Fig. 20.3 Basiocciput hypoplasia with fused odontoid process as shown by (a) CT in sagittal reconstruction and (b) MRI in
T2-weighted images
exactly as well as its relation to neural structures. This
combination of basiocciput bulge and odontoid invagination narrowing the FOM is probably similar to that
drawn on picture by Homen in 1901 (Fig. 20.4).
20.8 Atlantooccipital Assimilation
This is probably the second most frequent pathology
of CVJ development, often accompanying other abnormalities. Atlas can be assimilated to the occiput partially or completely (Fig. 20.5). The AA joint is
overloaded and regularly subluxated. The simultaneous synostosis (Klippel-Feil) of C2-3 is common.
Fig. 20.4 Bulging of the basiocciput to the FM as seen on 3D
CT (similar to picture of Homén 1901)
20.11 Persistent Ossiculum Terminale
Fig. 20.5 Partial atlas assimilation depicted on 3D CT (the
fused posterior arch on 2D CT sagittal reconstruction called
“comma sign”)
289
[19]. The isolated atlas arch disconnections, only
rarely, are of some clinical importance. The pre-existence of arch defects, however, becomes much more
important in the case of trauma and/or surgery scheduled because of other reasons. Under these conditions,
arch defects may affect the UCS stability and without
posterior atlas arch as an anchor, the surgery might be
more challenging. It can, sometimes, be difficult to
distinguish whether the arch cleft is present as a developmental failure or as a result of fracture.
20.10 Axis Anomalies
Except for complex fusion deformities, the anomalies
of C2 are frequently isolated failures confined to odontoid process development.
20.9 Atlas Anomalies
Apart from AO assimilation, the vast majority of C1
anomalies are various clefts and/or aplasias and hypoplasias of the arch. They are often detected incidentally without other concomitant anomalies, thus not
affecting the stability. The posterior midline cleft is
most frequently detected. This so called “posterior
rachischisis” was observed in 4% of adult autopsy
specimens [34]. Anterior clefts are rare and only a few
cases of surgically treated combined anteroposterior
atlas schisis (“split atlas”) (Fig. 20.6) were reported
20.11 Persistent Ossiculum
Terminale
Ossiculum terminale called Bergman’s ossicle results
from a failure of fusion of apical ossification center of
the odontoid process. It may mimic a rare type I odontoid fracture and as a rule, does not have any influence
on UCS stability (Fig. 20.7).
Fig. 20.6 Split atlas on CT scan in axial plane Fig. 20.7 Ossiculum terminale (Bergman’s ossicle)
290
20 Congenital and Developmental Abnormalities
20.12 Odontoid Hypoplasia and Aplasia
Dysplastic changes of the odontoid process are not frequent and can vary from hypoplasia (Fig. 20.8) to total
aplasia (Fig. 20.9). Depending on the degree of hypoplasia and the relationship to transverse atlantal ligament, these changes can significantly affect the stability
of AA complex.
20.13 Os Odontoideum
Os odontoideum is defined as an ossicle with smooth
circumferential cortical margins located at the place of
odontoid process but without any osseous continuity
with the C2 body. It was first described by Giacomini
in 1886 [12].
Fig. 20.8 Odontoid process hypoplasia as a part of complex
deformity on CT scan in coronal plane
Fig. 20.9 Odontoid aplasia as seen on transoral film
It continues to be debated whether the fragment disconnection occurs prenatally as a result of a developmental failure of the odontoid or is acquired due to
perinatal or postnatal trauma.
This anomaly is very often asymptomatic and os
odontoideum is found incidentally during radiological
investigation due to other reasons. If symptomatic,
then a variety of clinical signs can be observed. The
patients may suffer from suboccipital pain and headache only; some of them, nevertheless, present with
transient or progressive signs of myelopathy.
The plain X-ray in lateral and transoral projections
is still a main diagnostic tool clearly showing the
pathology. Os odontoideum can be radiologically
classified into two anatomic variants. In the orthotopic type (Fig. 20.10), the ossicle moves together with
anterior C1 arch. In the dystopic type (Fig. 20.11), it
is fused to the basion and moves together with it, thus
possibly subluxating anteriorly to the C1 arch. The
anterior C1 arch may often appear hypertrophic and
rounded (instead of “moon” appearance). Different
shapes of odontoid ossicle were described and classified [25]. Associated bone anomalies like assimilation of atlas or C1 ring defects are observed quite
often.
Plain dynamic films of the cervical spine will reveal
AA instability if present, e.g., if the PADI is less than
14 mm and SAC 13 mm, which are considered critical
values for potential development of myelopathy. On
the other hand, there is no evidence of correlation
between the degree of instability and neurological status [32, 35, 40]. Advanced radiological techniques
were not considered important for diagnosis in the
past; availability of CT and MRI today, however,
enables a better visualization of bone anatomy including possibly associated bone abnormalities (CT). MRI
reveals a possible presence of fibrous pannus and its
relationship to the spinal cord. In reality, only MRI can
directly visualize the SAC in neutral and in critical
positions, particularly if dynamic sequences are
performed.
The AA complex is stable on dynamic films in
approximately 20% of patients with os odontoideum
[11]. Majority of cases of AA instability due to presence of os odontoideum are represented by anterior
C1 dislocations, although posterior instability was
also reported [11, 32]. However, if os odontoideum
is a part of more complex deformity, the AA subluxation can be irreducible and creates a significant
20.14 Our Preference
Fig. 20.10 Orthotopic type of
os odontoideum (fixed behind
C1 anterior arch). (a) Plain
lateral film. (b) Sagittal
tomogram. (c) Sagittal MRI
on sagittal plane of the same
patient. (d) AA instability
treated by posterior transarticular C1-2 screws ad modum
Magerl
291
compression of neural structures either directly anteriorly or posteriorly by the subluxated posterior C1
arch.
As a consequence of usually good clinical course
without any progressive neurological deficit even in
cases of unstable subluxation, as well as of unfavorable results from surgical interventions, most authors
tended to prefer conservative approach in the past [7,
35]. On the other hand, deterioration in initially mor-
phologically stable and clinically intact patients was
also described [6, 22]. Advancement in imaging technologies and namely significant improvement of surgical fixation techniques has resulted in much better
outcome after surgical AA stabilization. Klimo et al.
[22] published the largest series of 78 patients treated
with posterior transarticular fusion by Magerl’s technique with excellent results and 100% fusion rate.
Based on logical analysis of AA biomechanics, they
stated that the risk of neural injury caused by possible
trauma is unacceptable for patients with diagnosed os
odontoideum and that all of them have to be offered a
surgery.
20.14 Our Preference
There is no doubt that the patients with symptomatic
and unstable os odontoideum have to be treated surgically. We agree with Klimo et al. [22] that the risk
of destabilization due to trauma in initially stable
patients is high, particularly in population with
active lifestyle where traffic and sport injuries are
common. We, therefore, tend to offer these patients
a surgery. We prefer the “wait and see” approach
only in elderly patients with incidental finding
292
Fig. 20.11 Dystopic type of
os odontoideum (connected
to basion and moving with
the head). (a) CT scan in
sagittal plane showing the
bony connection to basion.
(b) MRI of the same patient.
(c) CT scan in sagittal plane
after transoral disconnection
and resection of os odontoideum. (d) C1-2 fusion ad
modum Goel-Harms sparing
the intact C0-1 joint
20 Congenital and Developmental Abnormalities
of os odontoideum without instability and clinical
symptoms.
If the AA dislocation is reducible and without
fibrous tissue pannus, we perform the posterior C1-2
fusion by technique of Magerl and/or Goel-Harms,
supplemented by posterior bone graft in patients with
orthotopic type of os odontoideum (Fig. 20.10). If the
fibrous tissue surrounding the ossicle creates pressure
on the spinal cord in neutral or reduced position as
seen on MRI, then we perform the posterior fusion
with transoral decompression.
In the case of dystopic form, the odontoid ossicle
connected to basion follows movement of the head. If
posterior approach alone is selected, then the OC
fusion must be performed, which means a sacrifice of
movement in the OC joint with significant reduction of
flexion-extension movement in UCS. Therefore, we
prefer a combined approach with transoral disconnection of the ossicle from the basion and its removal, and
posterior fusion of C1-2 only (Fig. 20.11).
20.15 Basilar Impression, Invagination
The terms basilar invagination, basilar impression,
cranial settling, and vertical odontoid migration have
been used in patients where the UCS is located abnormally high. However, there is still some confusion in
nomenclature of the deformity, due to mixture of
description of different pathologies.
Crockard [2] suggested to make the term ‘impression’ interchangeable with ‘invagination’ but to strictly
distinguish whether the deformity is a result of a developmental failure (primary BI) or has occurred secondarily to disease affecting the bone quality in the CVJ
(secondary BI) as was originally proposed by Saunders
[30]. Menezes et al. [27] recommended considering
reducibility of the deformity prior to surgical decision
making. Smith et al. strictly differentiated basilar
invagination as developmental anomaly from basilar
impression as acquired deformity caused by secondary
softening of the bone of skull base [33]. According to
20.15 Basilar Impression, Invagination
293
relationship of the odontoid process to FM, Goel [14]
logically reclassified basilar impressions into two
groups. In both groups, the odontoid tip is located above
Chamberlain’s and McGregor’s lines. The principal difference was that in group A, the odontoid migrated
inside the FM, thus becoming visible above McRae’s
line as well as above Wackenheim’s clival line, whereas
in group B, the whole UCS is migrated above the level
of hard palate due to skull base deformity (basiocciput)
without invagination of the odontoid inside the FM
(Fig. 20.12). The group A was first reported as a fixed
AA deformity, but later again Goel et al. recognized the
possible vertical AA instability and potential reducibility of settled AA joint (often accompanied by atlas
assimilation) allowing the reduction of the odontoid
from the FM by simple extension [16]. The previous
idea of Goel to classify only those with odontoid
appearance inside FM as basilar invagination was principally accepted also by Kovero et al. who exactly
described all possible variants of CVJ anomalies
accompanying the vertical UCS migration [23].
Patients with symptomatic neural compression are
clearly the candidates for surgery. Also, those without
symptoms but with documented morphological progression of instability should be considered for intervention. The etiology, reducibility, direction of the
compression, and status of bone growth-age are other
factors important for decision making regarding surgical tactic [27].
In reducible deformities, the traction and/or head
positioning followed by posterior fixation can be a
sufficient treatment [15]. If the deformity cannot be
reduced, different techniques for release of neural
structure and realignment of the CVJ were reported
in the literature. Historically, the posterior decompression was performed regardless of the site of compression. The adverse outcome occurred in approximately
35–40% of patients [27]. Advancement in imaging
facilities but mainly the dramatic evolution in surgical techniques and spinal implants led to more targeted
approaches. The safety of fixation techniques has also
improved. Today, the simple or extended direct anterior transoral decompression is followed by posterior
OC fusion when anterior compression is dominant
[27, 36]. If posterior fossa decompression is necessary
(Chiari malformation), it may be performed separately.
Another possibility of achieving anterior decompression is, to renew the shape of CVJ and thus indirectly
decompress the spinal cord and the brainstem. Goel
et al. [14, 16] recommended to distract AA joint from
posterior approach with intraarticularly interposed
graft or cage and to fixate the joint with screw and plate.
Abumi et al. [1] suggested to use the posterior lever
arm reduction of CVJ kyphosis by monoaxial screw
Fig. 20.12 Thirty-three year old patient with symptomatic
basilar impression after two unsuccessful transoral decompressions. (a) Sagittal 3D CT reconstruction showing basilar
impression and platybasia. (b) Sagittal 2D CT reconstruction
depicting the atlas assimilation and status after partial odontoid
resection. (c) Sagittal T2-weighted MRI clearly documenting
the neural compression with sharp cervicomedullary angle and
Chiari malformation
294
20 Congenital and Developmental Abnormalities
with U-shaped head tightened to the rod that is fixed
to occiput. Wang et al. proposed the transoral release
of AA joint without odontoidectomy prior to posterior reduction according to Abumi. Having treated 33
cases of irreducible atlantoaxial dislocations (IAAD),
the authors stated that most of them can be converted
to mobile and reducible state by this technique [39]. In
theory, the anterior plating with relordotization can be
used for this purpose as was documented in RA patients
with vertical AA subluxation and kyphosis [20, 21].
Nevertheless, to our best knowledge, this method has
been used in only one case of developmental IAAD so
far [41].
20.16 Our Preference
In summary, the experience with congenital CVJ
anomalies is limited, particularly in countries where
systematic prenatal screening is established, or where
congenital anomalies are detected and treated early
before they become decompensated and irreducible. A
lack of continuous experience may result in a large
variability of treatment modalities among different
countries and surgeons. Also, our experience is limited
by the small number of patients treated, almost all of
them being adults.
To make the nomenclature more transparent, we
use the term “basilar invagination” when the odontoid
occupies the FM (Goel’s group A), and the term “basilar impression” when the entire UCS is migrated
upward with flattening of skull base but where the
odontoid is not inside the FM (Goel’s group B). We
use both these terms for developmental anomalies
regardless of their time of occurrence and etiology.
“Platybasia” and “vertical odontoid migration” are
descriptive terms, only commenting the anatomical
situation and/or its change in time. We use the term
“cranial settling” in RA patients, usually meaning not
only vertical migration of the odontoid process but also
telescoping (downward dislocation) of atlas on axis.
In fact, any intracranial position of the odontoid is
possible only if structures holding the head weight
above C2 (including the lateral masses of C2) are missing, underdeveloped or destroyed by disease.
We offer surgery to those patients with basilar anomalies who suffer from pain and/or neurological deficit provided there is an appropriate
morphological background explaining their symptoms. Asymptomatic patients are treated in case
of threatening or progressing compression of neural structures due to instability and/or deformity as
detected by radiological investigation.
In those cases where some degree of reducibility
can be expected we start the treatment with traction for
a few days and if favorable reduction is achieved and
verified by MRI, then the posterior fusion is performed.
If the reduction is not good enough or not achievable at
all, then we prefer to decompress and stabilize. We
perform the decompression always from the side of
compression, which means that we most often start
with anterior simple or extended (Fig. 20.13) transoral
approach. The anterior AA joint release is attempted in
order to make the deformity reducible. If extensive
bone removal diminishes the load-bearing capacity of
the anterior spine, we occasionally support the anterior
column with mesh cage fixated to the clivus cranially
and anchored to the first vertebral body enabling strong
enough support to cage, caudally (usually, C3)
(Fig. 20.14). The posterior OC fusion is then added as
a single session surgery with or without posterior fossa
decompression and C1 laminectomy, depending on the
presence of Chiari malformation. When turning the
patient to prone position, a halo-vest could be of advantage; however, even with a free-hand manipulation,
monitoring of somatosensory and motoric evoked
potentials (EP) should be mandatory. Turning the
patient during the surgery is the most dangerous part of
the procedure and without electrophysiological monitoring, a possible injury to the spinal cord would occur
unnoticed. Angular reduction, if necessary for cervical
spine alignment and sagittal balance, is performed
manually with the head fixed in a halo ring and, again,
under EP monitoring. Alternatively, posterior distraction or angular correction of the AA joint can be
achieved by means of C2 pedicle screw levers or distraction forceps.
20.16 Our Preference
295
Fig. 20.13 The same patient as in Fig. 20.12, anterior transoro-
maxillar approach. (a) Extent of maxillotomy. (b) Both side
Crockard deep retropharyngeal distractors. (c) Cosmetic result
of extended maxillotomy
Fig. 20.14 The same patient as in Fig. 20.12, result of extended
anterior decompression, anterior mesh cage support, posterior C1
laminectomy and posterior fossa decompression with OC instrumented fusion. (a) Plain laterogram. (b) MRI documenting the
extent of decompression and change of cervicomedullary angle
296
20 Congenital and Developmental Abnormalities
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