This document discusses various anatomical structures and anomalies related to the craniovertebral junction. It begins with the normal anatomy and embryology of the central pillar and ringed structures. It then describes various craniometric measurements and their clinical implications for assessing conditions like basilar invagination. Several congenital anomalies are discussed in detail, including os odontoideum, basilar impression, occipital condyle hypoplasia, atlas assimilation, and anomalies of the anterior and posterior atlantal arches.
2. ANATOMY AND EMBRYOLOGY
• Bony CVJ development is divided conceptually into 2 components:
• The ‘central pillar’- Dens, basiocciput, and C2 vertebral body
• ‘Two ringed structures’ – surrounding central pillar – foramen magnum, exocciput,
occipital condyles, opisthion and atlantal ring.
• Occipital somites (somites 1-4) and first 3 cervical somites (somites 5-7) are
involved.
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3. • 4th occipital somites – undergoes resegmentation.
• Caudal dense zone of 4th occipital somites + cranial loose zone of 1st cervical
somite = proatlas.
• Axial sclerotome of proaltas fuses with basiocciput – forms basion.
• Upper part of clivus – formed by basisphenoid.
• Sphenooccipital synchondrosis fuses – 12 yrs.
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7. ATLAS
• Ossifies from 3 centres
• Each half of post. arch with lateral mass –7 to 9 wk, unites at 3 –4years.
• Anterior arch – appears at 1 to 2 years, unites with lateral mass at 6 –8years
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9. JOINTS
• C0-C1 joint: pair of condylar synovial joint.
• Attachments:
• Anterior A-O membrane: anterior atlantal arch to anterio-inferior edge of FM.
• It is a continuous ALL above C1.
• Limits extension of A-O joint.
• Posterior A-O membrane: posterior atlantal arch to posterior outer margin of FM.
• Continuous as atlantoaxial ligament and ligementum nuchae.
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10. JOINTS
• C1-C2 joint – A complex articulation involving 3 three synovial joints. Involves:
• Paired lateral atlanto-axial joint: b/w lateral masses of C1 and C2.
• Median atlanto-dental joint: pivot-type of joint b/w dens and C1 anterior arch.
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11. CRUCIATE LIGAMENT
• Horizontal and vertical component.
• Horizontal or transverse (atlantal) ligament: from tubercles of atlas, holds dens
against the anterior arch.
• Provides stability for axial rotation of C1 on C2 and lateral bending.
• Vertical component: from posterior aspect of dens to clivus, under posterior
longitudinal ligament and tentorial membrane.
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16. Craniometric
measurement
Anatomic landmarks Normal values and
clinical implicatons
ADI From posterior border of
anterior arch of C1 to
anterior surface of dens
>3mm in adults and
>5mm in children
indicated AAD
PADI From posterior surface of
dens to anterior border of
posterior arch of C1
<13mm indicates canal
narrowing
Chamberlain’s line
(Modification: McGregor
line)
Hard palate post. plate to
opisthion
Dens >6 mm above – BI
McRae line Basion to opisthion Odontoid tip below by
>5mm.
Odontoid tip lying above
– BI
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21. Craniometric
measurement
Anatomic landmarks Normal values and
clinical implicatons
Wackenheim clivus base
line
Line drawn along clivus
and extending into upper
cervical canal
This line should fall
tangent to posterior
aspect of odontoid
Clivus canal angle The angle formed by the
Wackenheim line and a
line constructed along the
posterior surface of the
axis body and odontoid
Normal is 150-180°. If
<150° ventral cord
compression is present.
Clivus-canal angle or NTB
angle or Welcher angle
Formed between the
nasion-tuberculum and
tuberculum-basion lines
Normal: 125°-143°
Platybasia: >143°
BI: <125°
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24. Craniometric
measurement
Anatomic landmarks Normal values and
clinical implicatons
Klaus index Distance between tip of
dens and the tuberculum
torcula line. Indicates
height of post. fossa.
Normal is 40-41mm.
Decreased in BI.
Boogard’s angle Angle between McRae
line and clivus base line
Normal is 126° ± 6.
>136° - platybasia
AO Joint axis angle lines drawn parallel to the
atlantooccipital
joints to intersect at
midline
This line meet typically at
the center of odontoid
process.
Normal – 124 to 127°.
Condylar hypoplasia –
more obtuse.
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27. Craniometric
measurement
Anatomic landmarks Normal values and
clinical implicatons
Bimastoid line Between the inferior tips
of the of mastoid
processes bilaterally.
Normal – intersects
atlanto-occipital joint
Odontoid process less
than 10mm above line.
Digastric line Between bilateral
digastric groove
Normal- 10mm above
atlanto-occipital joint
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38. ODONTOID DYSGENESIS
• 4 main developmental errors are:
• Hyperplasia of primordium
• Aplasia/hypoplasia of primordium
• Disturbance of resegmentation
• Failure of midline integration of primordium
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39. • Complete odontoid agenesis – a/w collagenopathy syndrome
• Agenesis of basal segment – stumpy pivot with floating apical segment.
• Both types a/w instability
• Agenesis of apical segment – MC type.
• Treatment – C1-C2 fusion.
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C1
ODONTOID AGENESIS
43. OS ODONTOIDEUM
• Failure of basal dental and axis body ossification centers to fuse at LS.
• Non-fusion is below the level of transatlantal lig - a/w instability.
• Orthotopic and dystopic (Os avis).
• Radiographic features:
• Smooth, well-corticated ossicle at the superior ossicle of a hypoplastic dens
• around half the size of a normal dens
• a/w hypertrophied and rounded anterior arch of the atlas
• Posterior arch is hypoplastic
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46. OSSICULUM TERMINALE PERSISTENS
• d/to Non-fusion of ossification centers apical dental segment and basal dental
segment at US.
• it lies above transatlantal and alar lig. attachment – instability uncommon.
• Radiographic features:
• Small, well-corticated ossicle at the tip of the dens
• Usually in the midline
• Dens is typically normal in height
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48. OS AVIS
• d/to Apical dental segment attachment to basion – d/to abnormal
resegmentation of proatlas centrum.
• A.k.a dystopic os odontoideum.
• The dental pivot has a flat top but tall enough for the TAL and has no
instability
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49. BIFID DENS
• d/to Failure of midline integration of basal dental segment. a/w:
• Partition of basal dens segment upto LS and dislocated ossiculum terminale.
• One half of basal dens segment – attached to C2 body, other is free floating.
• Complete bifid dens - extremely rare
• Dens bicornis – only tip is bicornuate. Dental pivot is normal.
• a/w failure of midline integration of other somites.
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58. OCCIPITAL CONDYLUS TERTIUS
• d/to Hyperplasia of proatlas hypochordal bow.
• May form joint or psuedojoint with odontoid process or anterior atlantal arch.
• a/w limited mobility at C0-C1 joint.
• a/w os odontoideum.
• May need resection.
• D/d: Os avis
• C2 dental pivot is truncated and stability of atlantodental joint may be
compromised.
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63. ATLAS ASSIMILATION
• d/to non-resegmentation of proatlas.
• One of the MC CVJ anomalies
• First mobile segment b/w skull and spine has been transferred down to C1-C2.
• 3 types of fusions identified:
• Zone 1 – anterior atlantal arch assimilation
• Zone 2 – lateral processes assimilation
• Zone 3 – posterior atlantal arch assimilation
• Most of burden of movements falls on C1-C2 – develop over-stretch failure -
likely to develop C1-C2 instability.
• Treatment - aimed at stabilizing C2-C3 joint.
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70. ANOMALIES OF POSTERIOR ATLANTAL ARCH
• d/to Aplasia or hypoplasia of lateral C1 sclerotome.
• More common than anterior atlantal arch defects.
• Dens anchorage to C1 by transatlantal lig is normal.
• These patients are usually stable.
• Rarely require fusion.
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73. OCCIPITAL CONDYLAR HYPOPLASIA
• d/to hypoplasia of lateral sclerotome of proatlas.
• Occipital condyles are flattened, leading to BI
• Violation of the Chamberlain line and widening of the atlanto occipital joint
axis angle.
• Upward slanting of skull base.
• The lateral masses of the atlas may be fused to the hypoplastic condyles,
further accentuating the BI.
• Limits or may even abolish movements at the A-O joint.
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Dorsoventral differentiation of somite. A Epithelial somite shows ventromedial cells (VM) destined to form the sclerotome. B Ventromedial sclerotome cells (Scl) de-epithelize from the somite and migrate towards the ventral notochord (NC). C Sclerotomal cells further subdivide into an axial cluster (Scl-A) surrounding the notochord, and lateral paired clusters (Scl-L) flanking the perichordal axial sclerotome. Dorsolateral somite retains its epithelial pattern to become the dermomyotome (DM). D The lateral sclerotome (Scl-L) forms a triangle next to the axial sclerotome. The three sides of the triangle become anlagen for the pedicle (P), neural arch (N) and the costal process (C), respectively. The dermomyotome also subdivides into the dermatome (D) and the lateral migrating myotome (M)
Severance line The severance line appears to go through the original resegmentation fronts of the adjacent somites 4 and 5, so that the final cellular separation occurs right through the junction between the basion and the apical segment of the dens, both derived from the axial portion of the proatlas, which in turn comes from a combination of the caudal half of somite 4 and the rostral half of somite 5
Atlas ossification centers. Diagram depicts the ossification centers of the atlas..
The making of any part of the vertebral column requires the successful completion of three developmental phases:
First, the mesodermal primordium has to be properly formed and, in some cases, assembled during the membranous phase;
Second, the mesodermal primordium undergoes chondrification in the cartilaginous phase; and
Finally, in the osseous phase, ossification takes place within the cartilaginous mold to complete the end product.
Transverse component of the cruciate ligament. Axial proton density CUBE image (A) shows the curved black structure, which representsthis transverse band (white arrows). Sagittal reformat of proton density CUBE sequence (B) shows the transverse band (long white arrow) between the dens (star) and the tectorial membrane (short black arrowheads) and posterior longitudinal ligament (long black arrows)
T2W MR both.
ateral margins of the sloping upper posterior margin of the dens of C2 to the lateral margins of the FM (adjacent to the occipital condyles).
Limit axial rotation and contralateral flexion.
T2W image (C) showing the various secondary stabilising ligaments in relation to odontoid and CVJ. TAL = Transverse atlantal ligament, 1: Anterior atlanto-occipital membrane, 2: ALL, 3: Tectorial membrame, 4: Posterior atlantooccipital 5. lig flavum, 6. PLL.
ADI and PADI, lateral radiograph, sagittal CT and MR T2 image.
Chamberline
Mcgregor and McRae
Weckenheim clivus base line
NTB welcher angle
Klaus index and boogard angle
AO Joint axis angle
Bimastoid line and digastric line
The term ‘ ‘ basilar invagination’ ‘ refers to a primary developmental anomaly in which the vertebral column is abnormally high and prolapsed into the skull base (2). Because the anomaly may be due to a number of causes (basiocciput hypoplasia, occipital condylehypoplasia, various atlantooccipital assimilations), it might be best to think of basilar invagination as a radiographic finding and not adiagnosis in and of itself. Also a/w neural dysgenesis, syringomyelia and chiari
The term “basilar impression’ include reserved for the secondary or acquired form of basilar invagination. It results from softening of the skull base and is uncommon,
Paget diseaseosteomalacia
Hyperparathyroidism
Osteogenesis imperfecta
Hurler syndrome
Rickets
Skull base infection
Platybasia refers to flattening of the skull base, manifested by an increase in the Welcher basal angle.
Normal clival angle () measured by the NTB angle of Welcker joining the nasion (N), tuberculum (T) and basion (B). The angle should be less than 130°. Platybasia is marked by an increased NTB angle. This raises the basion and forces the foramen magnum plane (dotted line) to tilt upwards. The same upward tilt of this plane also occurs with a short clivus (normal clivus length is 4 to 4.4cm)
Severe lordotic tilting of the plane of the occipital condyle in short clivus and platybasia.. Normal clivus and opisthion are represented by dotted outlines, and orientation and plane of the occipital condyle are represented by arrow and semicircle, respectively; red for normal and black for abnormal.
The fusion and chondrification of the basioccipital and the exoccipital sclerotomes occur slightly earlier than the resegmentation of the C1–C2 sclerotomes. A severely lordotic skull base angle consequently forces the emerging upper cervical sclerotomal column to bend backwards “in sympathy”, especially its centra complex that will ultimately form the dens–axis (Fig. 39). This explains why platybasia and short clivus are frequently associated with a retroflexed and lordotic dens, that, in severe cases, points sharply and wickedly backwards into the brainstem
T2 MR: Anterior form of basilar invagination caused by an exceedingly short (<1 cm) and blunted clivus (arrow) with severe lordotic tilt of the plane of the foramen magnum, leading to a “sympathetic” lordotic bend of the dental pivot resulting in a retroflexed odontoid and basilar invagination
Combined anterior–posterior form of basilar invagination. A Sagittal CT shows extreme platybasia (NTB angle=180°), short clivus (<1.5 cm) and forward folding of the clivus–axis angle of Wackenheim (80°), causing lordotic tilt of the foramen magnum plane and plane of the occipital condyles, resulting in a retroflexed dens and severe basilar invagination. Note violation of McGregor’s, Chamberlain’s and McRae’s lines by the dens. Also, extreme invagination of the opisthion (O) and high posterior C1 arch (C1).
Sagittal MR shows distortion and compression of brainstem by both the dens and the opisthion
Platybasia and short clivus (<1.5 cm; sphenoclival synchondrosis marked by arrow) causing severe basilar invagination and mild retroflexed dens. This results in a shallow posterior fossa with a Klause index (K) of less than 1.8 cm and ectopic cerebellar tonsils (chiari 1) and cervical syringomyelia
a/w spondylometaphyseal or spondyloepiphyseal dysplasia
Fig: T1 MRI Agenesis of apical dental segment, with a slightly short dental pivot but a definite basal segment (pointed) and a lower dental synchondrosis (LS). Arrow points to anterior arch of C1. Note platybasia and Chiari I malformation
Lateral radiograph shows complete absence of odontoid process.
Midsagittal T1 MRI of brain and cervical spine. The cervicomedullary junction is atrophic.
Complete agenesis of the dens in a 10-year-old child with spondyloepiphyseal dysplasia. a CT sagittal and coronal views show no dental pivot
Although the centrum with a flat top does rise up above the “expected” level of the lower dental synchondrosis.
3-D CT reconstruction and MR show the flat top of the centrum and potential for instability
D/d:
Odontoid fracture type 2
Persistent ossiculum terminale
Os odontoideum in an 8-year-old child presented with multiple cerebellar and thalamic strokes.
b CT scans show multiple small infarcts of left cerebellar hemisphere and thalamus secondary to multiple vertebrobasilar emboli.
Cerebral angiogram shows os odontoideum with a corrugated horn-like inferior edge (arrow) with anterior C1– C2 subluxation and stretch injury to the vertebral artery.
Coronal 2D CT through plane of os odontoideum. The ossicle has defined cortical borders. There is a gap below it and a hypoplastic dens present. i Midsagittal T2-weighted MRI of CVJ. The cervicomedullary junction is draped over the superior axis body. Note the cruciate ligament in front of the axis; below the ossicle.
D/d:
Type 1 odontoid fracture.
Unstable ossiculum terminale in a 2-year-old child with intermittent quadriparesis. a Flexion (upper) and extension (lower) sagittal reconstructed CTs showing highly mobile ossiculum (O) and C1 with obvious translational subluxation.
Completely bifid dens. a Note complete lack of midline integration of basal dental segment down to lower dental synchondrosis and an unfused and forward dislocated apical dens, suggesting midline integration abnormality interferes with growth and fusion of adjacent upper dental synchondrosis.
Flexion–extension CT shows C1–C2 instability
Sagittal MR shows severe cord compression with flexion. Arrow points to small ossiculum
State of ossification of the dens of a 4-year-old child. The tip of the basal dental segment is bicornuate from bilateral secondary ossification centres. Small density above this represents early third wave of ossification within the apical dental segment. Note lower dental synchondrosis (LS) in the coronal and sagittal views
Dens bicornis in a 7-year-old child. Lower synchondrosis has closed. Dens pivot is of normal height, suggesting the bifid tip is of the apical segment
Axial CT shows the gap in the lower clivus (arrow). Right: Sagittal CT shows the odontoid process is far anterior to its usual position below the clivus. Fusion of the C2, C3 and C4 centra is also seen
CT 3-D rendering of the skull base. Left: View from the back shows widely bifid basiocciput and an oval defect (arrow) higher in the clivus. The posterior C1 arch is deficient. Right: view from the front shows the odontoid is far forward from the bifid clivus (mostly covered by the dens), and the anterior C1 arch is also bifid. Note upper clival defect (arrow). Incom post C1 arch incomplete posterior C1 arch; Incom ant C1 arch incomplete anterior C1 arch
Pre-basioccipital arch (thin arrows), a U-shaped bony valance on the ventral lip of the anterior foramen magnum rim. This results from complete preservation of the hypochordal bow of the proatlas. remains solid anchor for the TAL; instability is unlikely. Although some third condyles are short and protrude along the contour of the clivus or even curl forward away from the brainstem
Third occipital condyle or condylus tertius (thin arrow) attached to the ventral surface of the clival tip and fused to the anterior atlantal arch (thick arrow). On MRI, the condyle appears to form a synovial joint with an ossiculum terminale that is unfused to the basal dens. The third occipital condyle represents midline hyperplasia of the proatlas hypochordal bow
Unilateral hyperplasia of the occipital condyle (arrow) with cervicomedullary distortion. This represents hyperplasia of the exoccipital (lateral) sclerotome of the proatlas
C1 assimilation or occipitalization. a Assimilation of the anterior atlantal arch (zone 1 assimilation). b Assimilation of the lateral masses (zone 2 assimilation). c Posterior arch (zone 3) assimilation
Complete aplasia of C1 hypochordal bow—complete aplasia of C1 anterior arch. a CT and 3-D reconstruction shows no anterior atlantal arch or insertion tubercles for the TAL. The two anterior stunted dental hemi-os (arrow) are unfused to the C2 centrum. Posterior arch of C1 is also unfused.
Extreme C1–C2 flexion instability and severe cord compression
Severe hypoplasia of C1 hypochordal bow—small anterior C1 arch. a Match-head size anterior C1 arch (thick arrow) associated with complete agenesis of the dens (thin arrow), a result of concomitant C1 centrum aplasia.
C1–C2 instability with cord compression
Complete agenesis of posterior atlantal arch and bifid anterior atlantal arch. Normal TAL insertion tubercles (arrows). There is no C1–C2 instability
Unilateral aplasia of the right posterior arch of C1
Coronal CT scan reveals widening of the atlantooccipital joint axis angle (dotted line). The joints are extrapolated in this case, as the lateral C-i masses are fused to the hypoplastic occipital condyles and seem to abut the jugular tubercles
Condylar hypoplasia. (a) Coronal T1 MR image shows the extreme upward slope to the skull base in the medial direction (arrows) . (b) Axial CT scan obtained at the level of the ossicles demonstrates the sphenoid sinus (5), clivus (C), anterior arch of the atlas (dot), and odontoid process (0). It is markedly abnormal that all of these structures are visible on the same section.