LIGAMENTOUS  ANATOMY:

PRIMARY STABILISERS 

Fig 2: LIGAMENTOUS ANATOMY : PRIMARY STABILISERS Transverse Atlantal Ligament (TAL) Alar Ligaments

Transverse Atlantal Ligament  (TAL)

TAL is a thick band posterior to dens attached to tubercles of atlas.

Allows free gliding motion to occur over the posterior facet of the dens.

TAL effectively limits anterior translation and flexion of the atlantoaxial joint.

Alar Ligaments:

Two strong cords attached to the dorso- lateral body of the dens &  extend laterally  & rostrally.

They are ventral & cranial to the transverse ligament.

The alar ligament allows an anterior shift of C1 from 3 to 5 mm.

SECONDARY STABILISING LIGAMENTS  

Fig 3: SECONDARY STABILISING LIGAMENTS 1. Apical ligament (not seen) 2.Anterior AOmembrane 3. Anterior Longitudinal Ligament, ALL 4. Tectorial membrane 5. Posterior A-O membrane 6. Ligamentum Flavum 7. Posterior Longitudinal Ligament, PLL

1. Apical ligament
2. Anterior AOmembrane
3. Anterior Longitudinal Ligament, ALL
4. Tectorial membrane
5. Posterior A-O membrane
6. Ligamentum Flavum
7. Posterior Longitudinal Ligament, PLL

CRANIOMETRY   

Chamberlain's line   

From the posterior pole of the hard palate to the opisthion.  

Fig 4: CRANIOMETRY Chamberlain's line (red) Mc Rae Line (blue) Atlanto- Dental interval (green) Posterior Atlanto- Dental interval (pink) l

A tip of the odontoid 6.6 mm or more above this line is considered diagnostic for basilar invagination.

Mc Rae Line  

From the anterior margin of the foramen magnum to the posterior margin.

Fig 4: CRANIOMETRY Chamberlain's line (red) Mc Rae Line (blue) Atlanto- Dental interval (green) Posterior Atlanto- Dental interval (pink) l

Atlanto- Dental interval

Distance between the posterior border of the anterior arch of the atlas and the anterior border of dens.

<3mm in adults and <5mm in children. An increase in ADI is suggestive of atlantoaxial dislocation

Fig 4: CRANIOMETRY Chamberlain's line (red) Mc Rae Line (blue) Atlanto- Dental interval (green) Posterior Atlanto- Dental interval (pink) l

Posterior Atlanto- Dental interval

Measured from the dens's posterior border to the posterior tubercle's anterior border. It should not be less than 13mm.

Fig 4: CRANIOMETRY Chamberlain's line (red) Mc Rae Line (blue) Atlanto- Dental interval (green) Posterior Atlanto- Dental interval (pink) l

Wecken-Heim Clivus base line: 

 Drawn along the clivus and extrapolating it inferiorly into the upper cervical spinal canal.

It should fall tangent to the posterior aspect of the tip of the odontoid process.

If it falls too far posterior to the odontoid posterior craniocervical dislocation

If it intersects the body or base of the odontoid anterior craniocervical dislocation.

Fig 5: CRANIOMETRY Wecken-Heim Clivus baseline (blue) Clivus Canal angle (red) Welcher Basal Angle (green) Atlanto-occipital joint Axis Angle (megenta)

Clivus Canal angle:

The Wackenheim line forms the angle & a line constructed along the posterior surface of the axis body and odontoid process.

The angle ranges typically from 1500 in flexion to 1800 in extension.

Ventral spinal cord compression may occur with angles less than 150.

Fig 5: CRANIOMETRY Wecken-Heim Clivus baseline (blue) Clivus Canal angle (red) Welcher Basal Angle (green) Atlanto-occipital joint Axis Angle (megenta)

Welcher Basal Angle:

Angle formed at the intersection of the nasion tuberculum line and the tuberculum-basion line.

It should always be less than 140°. More than 140 is s/o platybasia.

Fig 5: CRANIOMETRY Wecken-Heim Clivus baseline (blue) Clivus Canal angle (red) Welcher Basal Angle (green) Atlanto-occipital joint Axis Angle (megenta)

Atlanto-occipital  joint Axis Angle

It is formed by lines drawn parallel to the atlanto-occipital joints, which typically intersect at the center of the odontoid process.

The average angle is 125° and becomes more obtuse in the case of occipital condylar hypoplasia.

Fig 5: CRANIOMETRY Wecken-Heim Clivus baseline (blue) Clivus Canal angle (red) Welcher Basal Angle (green) Atlanto-occipital joint Axis Angle (megenta)

                                                               PATHOLOGICAL CONDITIONS 

Fig 6: : PATHOLOGICAL CONDITIONS CLASSIFICATION

Congenital 

Anomalies of skull base

• Platybasia
  

  Fig 7: Skull Base Anomalies: Platybasia Dysplasia An abnormally obtuse basal angle, measuring 149*, is seen on the reformatted CT image (A) and on the sagittal T1W image (B). There is associated basilar invagination, occipitalisation of atlas & ventral cervicomedullary cord compression.
• Basilar Invagination
  

  Fig 8: Skull Base Anomalies: BASILAR INVAGINATION Violation of the chamberlain's line is diagnostic of BI. 2 Types Group A: Violation of Chamberlain’s line and Mc Rae line disturbance. Group B : Violation of Chamberlain’s line w/o disturbance of Mc Rae line.
• Condylar hypoplasia
  

  Fig 9: Skull Base Anomalies: NCCT Coronal image showing Condylar Hypoplasia occipital condyle: clivus- canal angle is decreased, and the underlying cervicomedullary junction severly compressed. short clivus and bowstring deformity of cervico edullary junction . Sagital T1 MRI image showing Basiocciput Hypoplasia clivus-canal: occipital condyles are underdeveloped and have a flattened appearance, leading to BI and widening of the atlantooccipital joint axis angle (>125)
• Basi-occiput dysgenesis
  

  Fig 9: Skull Base Anomalies: NCCT Coronal image showing Condylar Hypoplasia occipital condyle: clivus- canal angle is decreased, and the underlying cervicomedullary junction severly compressed. short clivus and bowstring deformity of cervico edullary junction . Sagital T1 MRI image showing Basiocciput Hypoplasia clivus-canal: occipital condyles are underdeveloped and have a flattened appearance, leading to BI and widening of the atlantooccipital joint axis angle (>125)
• Foramen magnum stenosis
  

  Fig 10: Skull Base Anomalies: Foramen magnum stenosis Sagittal T1W (A) T2W (B) MR images of a 9 mo achondroplasic child showing narrowing of the FM with compression of ervicomedullary junction. Associated prominent forehead with depressed nasal bridge and J-shaped sella are also noted.
• Clivus segmentations anomalies

Atlas malformations

Fig 11: ANOMALIES OF ATLAS NCCT Saggital (A)& Sagittal T2W MRI image (B) in a c/o klippel feil syndrome showing occipitalisation of atlas, AA dislocation and chiari I malformation. Associated syrinx is also noted.

• Bifid atlas 
• Assimilation
• Absent arches

Odontoid dysgenesis 

Aware of embryological variants of Os odontoid

Fig 13: EMBRYOLOGY OF DENS Ossification of the cartilaginous axis occurs in three chronological waves The first wave appears as a single ossification center within the axial body around 4 months of gestation. The second wave begins at 6 months of gestation as two separate ossification centers on each side of the basal dental segment. The third wave of ossification arrives within the tip (apical dens) at ~3-4 yr.

Fig 14: EMBRYOLOGICAL VARIANTS OF DENS: Coronal and Saggital NCCT images show Dens bicornis

 

• Odontoid Aplasia
  

  Fig 18: ANOMALIES OF DENS: OODONTOID APLASIA Sagittal T1W (A), T2W (C) & coronal T2W (B) MR images showing aplastic dens. Additional findings include platybasia, short clivus and syrinx involving the cervical spinal cord.
• Os odontoideum  
  

  Fig 16: ANOMALIES OF DENS: OS OODONTOIDEUM Sagittal MRI (A,B) & CT (C) images : showing rounded bony fragment lying above and anterior to the base of dens. Dens is hypoplasticstic, smooth and well corticated and Anterior arch is hypertrophied and rounded differentiating the condition from fracture. Additional findings on MR include marked ligament thickening, spinal canal narrowing with cord compression and myelomalacic changes
  

  Fig 17: TYPE 2 FRACTURE verses OS-ODONTOIDEUM
• Os Avis 
  

  Fig 15: EMBRYOLOGICAL VARIANTS OF DENS: PERSISTENT OS-TERMINALE & OS- AVIS
• Persistent Os-terminale
  

  Fig 15: EMBRYOLOGICAL VARIANTS OF DENS: PERSISTENT OS-TERMINALE & OS- AVIS

Atlanto-axial dislocation

Fig 19: ATLANTO-AXIAL DISLOCATION Increased ADI > 3mm in adults > 5mm

Acquired 

1. Traumatic
2. Degenerative
   

   Fig 20: DEGENERATIVE CHANGES AT CVJ Secondary degenerative changes in patients with multiple segmentation anomalies (A to C): Reduced ADI with sclerosis and osteophytes formation. The fluid signal is noted in the predental space, which normally contains fat/soft tissue. Also seen is Group B Basilar invagination. Blue : Chamberlain’s Line & Yellow :Mc Rae Line.
3. Inflammatory: 

• Infectious: TB
  

  Fig 24: Tuberculosis: Sagittal reformatted CT (A & B) images show areas of osseous destruction involving the axis and C3 with associated prevertebral collection. Sagittal T2W (C) and sagittal post contrast T1W (D) MR images show rim enhancing prevertebral collection with epidural extension causing cord compression.
• Noninfectious:

RA & JRA

Fig 21: INFLAMMATORY: RHEUMATOID ARTHRITIS MRI Saggital T2 & T1 Images show Peridental pannus and prominent retrodental soft tissue. Thickening of ligaments (> 3mm) Saggital NCCT Image shows that the hook-like appearance of the odontoid is due to erosions by the pannus.

CPPD

Fig 22: CPPD CRYSTAL DEPOSITION DISEASE Axial, coronal, and sagittal NCCT images show Type II pathological # of odontoid with erosions, sclerosis, osteophytes & narrowing of Atlanta-dental interval. Calcification of ligaments(A) and hypointense retro-odontoid mass is seen on T2W MRI images.

Ankylosing Spondylitis  

Fig 23: Ankylosing Spondylitis Reformatted Sagittal CT (A & C ) axial CT (D) and sagittal MRI (B) in a 34-year-old HLA-B 27 positive male reveals atlantoaxial dislocation , ligament calcification, fusion of zygoapophyseal joints and secondary atlanto-ocipital assimilation. Reformatted coronal CT images (E & F) shows b/l sacroiliac joint and b/l hip joint ankylosis

Reiters

      4. Neoplastic

• Primary
  

  Fig 25: TUMOURS INVOLVING CVJ 1.Foramen Magnum meningioma: Sagittal T2(A),T1(B) & Post GAD (C) images showing well defined dural based homogenous enhancing mass, isointense on T1 & iso to hyperintese on T2 seen along anterior foramen magnum causing compression on CMJ 2. Multiple Myeloma: Axial & reformatted sagittal CT images (A & B) showing well defined lytic lesion involving lower part of odontoid. Multiple other well defined lytic bony lesions were present in skull and spine.
• Metastasis
  

  Fig 25: TUMOURS INVOLVING CVJ 1.Foramen Magnum meningioma: Sagittal T2(A),T1(B) & Post GAD (C) images showing well defined dural based homogenous enhancing mass, isointense on T1 & iso to hyperintese on T2 seen along anterior foramen magnum causing compression on CMJ 2. Multiple Myeloma: Axial & reformatted sagittal CT images (A & B) showing well defined lytic lesion involving lower part of odontoid. Multiple other well defined lytic bony lesions were present in skull and spine.