V.C - H - Anatomical Varitations in the Vertebral Column - Includes the development of vertebral body & odontoid process Flashcards
What is the intervertebral disc derived from?
The IV disc is derived from the sclerotome and notochord
What part of the IV disc does the notochord form?
The notochord forms the nucleus pulposus of the IV disc
We know that the sclerotome and notochord form the IV disc What forms the vertebrae?
The sclerotome parts of the somites will form the vertebrae
Both the somites and the notochord are derived from mesoderm What is the difference in the names of mesoderm they are derived from and there relation to the neural tube?
The somite is derived from the paraxial mesoderm - on either side of the neural tube The notochord is derived form the axial mesoderm (aka the chordamesoderm) that lies along the central axis under the neural tube
Looking from superior
During week 4, where does the sclerotome migrate? What fills the space between sclerotomes after they have migrated?
During week 4, the sclerotome migrates around the neural tube and notochord to merge with the sclerotome cells on the other side of the neural tube The sclerotome descends from cranial to caudal around the notochord
Mesenchymal cells fill the space between sclerotomes
The sclerotome after migration will now undergo what process to form the vertebral body What happens during this process?
The sclerotome will now undergo resegmentation to form the vertebral body The caudal half of the sclerotome above fuses with the cranial half of the subadjacent sclerotome (sclerotome below) to form the vertebral body Note how the position of the intersegmental arteries & myotomes & segmental nerves change
What bridges the IV discs to allow the movement of the vertebral column? What happens to the notochord after resegmentation of the sclerotome?
The myotomes bridge the IV discs and allow for the movement of the vertebral column After the re-segmentation of the sclerotome, the notochord completely regresses from the vertebral body but persists and enlarges to form the nucleus pulposus
Chondrification centres form within the sclerotome mesenchyme at week 6 and cartilage replaces it. When do the primary ossification centres appear in the juvenile vertebrae and where?
3 primary ossification centres appear at week 8 One centre in the juvenile centrum just dorsal to the notochord The other primary ossification centres are in each half of the neural arch
WHITE CIRCLE IS THE NOTOCHORD
When does endochondral ossifcation begin and in which ossification centre? What is the last region to ossify?
Endochondral ossification begins around 9/10 weeks at the ossficiation centre in the centrum The spinous process is the last region to ossify
Which gene regulates the patterning of the shapes of the vertebrae?
This would be the HOX genes
Describe the formation of the vertebral body and IV disc
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What aspect of typical vertebrae does the posterior arch of the atlas and lateral masses correspond to? What arises from the lateral masses of the atlas?
The posterior arch of the atlas corresponds to the laminae of typical vertebrae * The lateral masses of the atlas correspond to the vertebral body of typical vertebrae and are the weight bearing aspect of the atlas * From the lateral masses arise the transverse processes of the atlas and therefore they are more lateral placed than in inferior vertebrae
Superior articular surfaces articulates with occipital condyles either side of foramen magnum Anterior and posterior arches form a complete ring What does the posterior arch have a wide groove for on its superior surface? Whereabouts on the posterior arch is this wide groove?
The posterior arch of the atlas has a wide groove at its base on its superior surface The vertebral artery and C1 nerve arch around the superior articular facet in this groove to reach the transverse foramen
We have discussed the formation of the vertebral body and IV discs from sclerotome and notochord NOW LETS DISCUSS ANATOMICAL VARIATIONS IN THE VERTEBRAL COLUMN Congenital absence of the posterior arch of the atlas How is this mostly diagnosed? Is it usually partial or total agenesis?
Congenital absence of the posterior arch of the atlas is asympomtatic mostly and usually an incidental finding on scans It is usally a partial agenesis (4% prevalence) and less commonly, total agenesis (0.15%)
What are the symptoms of Congenital absence of the posterior arch of the atlas? Which other conditions may be associated with congenital absence of the posterior arch of the atlas?
Symptoms may include mild neck pain to neurological deficits after traumatic injury It may be associated with occipitilastion of the atlas or block vertebrae
In a sentence, explain what occipitilsation of the atlas is and what block vertebrae is?
Occipitilisation of the atlas - also known as atlanto-occipital fusion
Blocked vertebrae - when there is a failure of separation of the vertebrae and therefore no IV disc
Congenital absence of the posterior arch of the atlas can be classified form type A to type E Describe each type?
- Type A - failure of posterior midline fusion
- Type B - unilateral defect
- Type C - bilateral defect
- Type D - absence of posterior arch with preservation of posterior tubercle
- Type E - absence of posterior arch including posterior tubercle
State which type of congenital absence of the posterior arch of the atlas each image is showing from left to right
Left * Type A - failure of midline fusion
Middle * Type B - unilateral defect
Right * Type C - bilateral defect
What is it known as when there is an extra bony feature on the posterior arch of the atlas?
This is known as arcuate foramen - usually seen near the lateral masses
Arcuate foramen occurs due to what? What can it lead to compression of?
Arcuate foraemn occurs due to the ossification of the lateral edge of the posterior altanto-occipital membrane It can lead to compression of the vertebral artery
Various ways to classify: * - Partial or complete ossification * - Bilateral or unilateral As said, arcuate foramen may lead to compression of the vertebral artery When may this occur and why?
As the vertebral artery travels through the transverse foramen it normally arches around the posterior aspect of the lateral mass in the wide groove on the arch With this condition, it may travel through the arcuate foramen meaning rotation at the C1/2 joint on the contralateral side may compress & elongate the artery (and cervical nerve)
The vertebral Artery usually moves in relation to neck movements but in the presence of arcuate foramen this interferes with the movement causing arterial and cervical nerve compression at the posterior arch as well as possible vertebral artery dissection What is another name for arcuate foramen? Why may dissection of the vertebral artery occur?
Arcuate foramen is also known as ponticulus posticus Dissection of the vertebral artery may occur becuase 50% of head rotation occurs at this joint and the artery is most vulnerable to damage
What are some implications of arcuate foramen?
Vertebrobasilar insufficinecy Headaches Neck pain Vertebral artery strokes Dissection Neuralgia Loss of consciousness
What is the most cephalic ‘blocked’ vertebrae in the spine? What is the condition known as?
The most cephalic blocked vertebrae is the alanto-occipital joint Blocked vertebrae here is known as occipitilisation of the atlas It is often accompanied by congenital absence of the posterior arch of the atlas - can see type A in the pic
What is occipitilastion of the atlas also known as? What is the condition and what is it caused by? (which gene possibly)
Occipitalisation of the atlas, also known as atlantal assimilation, is the congenital fusion of the atlas and occipital bone It is caused by * Failure of segentation * Possibly to do with the HOX D-3 gene
What are the different fusion types in occipitilsation of the atlas?
Fusion types: Complete fusion - anterior arch, lateral masses and posterior arch Partial fusion - osseus discontinuity between occiput and part of the atlas
The classifications of occipitilsation of the atlas include: * Zone 1 - anterior arch * Zone 2 - lateral masses * Zone 3 - posterior arch * Combination of zones What is the most common zone for occipitilsation of the atlas to occur at?
The most common zone for occipitilsation to occur at is zone 1 - when the anterior arch of the atlas fuses with the anterior margin of the foramen magnum
Why may neurological compression occur during occipitilsation of the atlas?
Due to occipitilsation, there is a narrowing of the foramen magnum space meaning the brainstem/spinal cord may become compressed leading to neurological symptoms
Accessory ossicle of the anterior arch of the atlas is a rare, but normal anatomical variant When is it pathological? What are its implications?
It is pathological if there is a fracture but otherwise it is a normal anatomical variant Implications include - degenerative changes, neck pain and interference with rotatory movements
What is the most common type of accessory rib? What condition is this accessory rib associated with?
Most common type of accessory rib is the lumbar rib It is usually asymptomatic Lumbar rib is associated with lumbarisation - when the first sacral segement is loose and behaves like another lumbar vertebrae (will discuss later)
The other type of accessory rib is the cervical rib It is only seen in 0.5% of the population and is only symptomatic in 10% What percentage is bilateral? Who is this condition more common in? What type of symptoms usually occur?
50-80% of the cases of a cervical rib are bilateral The condition is more common in females Symptoms are usually neurological but sometimes can be arterial
When the cervical rib or fibrous extensions extend down to T1 rib, what may it compress that causes thoracic outlet syndrome?
It may compress neurological structures travelling from upper thorax to axilla and less commonly arterial structures
Lumbosacral Transitional Anomalies are what we shall discuss next This basically means sacralisation and lumbarisation What is the difference and which is more common?
Sacralisation is more common - it is when the 5th lumbar vertebrae fuses with the sacrum meaning the lumbosacral junction is at L4-S1
Lumbarisation is when the 1st sacral vertebrae shows signs of lumbar configuration so the lumbosacral junction is L6-S1
What is this and which type?
This is sacralisation of the L5 vertebrae - left side is completed fused with adjacent sacral ala Right side is incompletely fused As it has a mixture of Type III and Type II - this is a Type IV sacralisation of L5 vertebrae
What are some implications of lumbosacral transitional anomalies?
Intervertebral disc herniation as the disc is significantly narrowed which may enable it to protrude Backache
How common is spina bifida? Is it commoner in males or females? What are they two types?
Spina bifida affects 1 in 1000 babies It affects females > males Two types SPina bifida cystica - meningocele (80%) and myelomeningocele Spina bifida occulta
What are the symptoms between each
Occulta - usually asympotmatic, may see a tufft of hair overlying the skin or a bbirthmark Cystica meningocele - small protrusion of skin in the back due to membranes herniating Cystic myleomingocele - spinal cord and meninges herniated - can have severe neuroligcal deficit below the lesion and maybe hydrocephalus due to blockage of CSF flow
Spina bifida occulta in 27 year old man who was involved in a car crash – Spina bifida occulta in 66 YEAR OLD MAN WITH LUNG CANCER – unfused L1 spinous process (arrows) and left L1 lamina *stars and dysplastic left L1-2 facet joint (arrow head) WHAT IS BUTTERFLY VERTEBRAE?
Butterfly vertebrae is a vertebral abnormality resulting from failure of fusion of the lateral halves of the vertebral body causing a saggital cleft due to persistent notochord tissue between them Failure of both chondrification centres to fuse in the midline results in butterfly vertebrae
What weeks of development does butterfly vertebra occur between? What may this condition be confused with?
Butterfly vertebrae is a symmetrical fusion defect occurring between 3-6 weeks This condition may be confused with a vertebral fracture
How do the normal ossification centres for the vertebrae develop? Ie what surrounds the notochord and neural tube during early development WHat are the chondrification centres at the 6 th week? When do these form primary ossification centres?
- During early development, the notochord and developing neural tube become surrounded by the sclertome
- During the 6th week, as early as 6 chondrification centres appear with one centre for each lateral half of the centrum
- By the 8th week, these centres have joined to form the single ossification centre of the centrum, the ossification centres of both sides of the neural arch (and a centre for each costal facet)
The incidence of hemivertebra is estimated to be 0.5-1.0 cases per 1000 births, and is more prevalent in females. The mechanism is understood but the cause is not. It has been proposed that hemivertebra results from an abnormal distribution of intersegmental arteries in the developing vertebral column. What is the most common symptoms of the scoliosis in hemivertebrae?
The most common symptoms would be pain
Which vertebrae are fused in the picture? What can happen to mechanical stress in blocked vertebrae?
Left pic - fusion of the C4/5 and C6/7 vertebrae Right pic - fusion of the C4/5/6 vertebrae Block vertebra eputs a greater biomechanical stress on the adjacent vertebrae - in particular the inferior adjacent vertebrae Can lead to premature IV disc degneration
A bifid C7 spinous is very rare but clinically important, why is this? The bifid spinous process of C7 anatomical variation can be partial or completely bifid. How is this so?
The long monofid spinous process of C7 can be used as an anatomical landmark on Xrays and during posterior cervical surgery Partial bifid process - 2 distinct tubercles at the end of the spinous process Completely bifid process - distinct cleft or bifurcation of the spinous process
Spinous process deviation What is this and where is it most common? It is commonly associated with other degenerative conditions such as what?
This is where the spinous process deviated from the midsaggital line and it is most commonly found in the lumbar region It is commonly associated with other degenerative conditions such as IV disc prolapse, Lumbar stenosis, degenerative spondylolisthesis
At ~2 years, a small ossific nodule, the ‘ossiculum terminale’, appears in the cartilage plug that fills the apical cleft When does the ossiculum terminale fuse with the dens?
The ossiculum temrinale fuses with the dens around 12 years of age Debate as to whether it is primary or secondary ossification
The formation of a bony fragment due to unsuccessful fusion of the odontoid process and the vertebral body of C2. WHAT IS THIS KNOWN AS AND WHAT DOES IT RESULT IN?
The formation of a bony fragment due to unsuccessful fusion of the odontoid process and the vertebral body of C2 is known as Os Odontoideum This results in instability and reduced joint mobility at the atlanto-axial joint
What type of fracture does the os odontoideum look like? The incidence is hard to determine as it is often asymptomatic and therefore unreported. Why must os odontoideum be treated if incidentally diagnosed?
Os odontoideum may appear to look like a Type II fracture of the dens If diagnosed it must be treated as as migration of the bony fragment may cause compression of the spinal nerves, spinal cord or canal at C1
What is the anatomical variation known as when the ossiculum terminale fails to fuse with the odontoid process? What fracture does this often mimic? The incidence is hard to determine as it is an asymptomatic and stable configuration so often goes unreported.
The anatomical variation when the ossiciulum terminale fails to fuse with the odontoid process is known as a PERSISTENT OSSICULUM TERMINALE
It often mimics a type I odontoid process
How can a persistent ossiculum terminale be distinguished from a type I odontoid fracture?
Persistent ossiculum terminale will be determined by a V shaped cartilaginous cleft between the ossiculum temrinale and the odontoid process
Which vertebral level is image on the left showing the sesamoid ossicles forming in the nuchal ligament? Why is surgical excision never an option?
Image on the left shows the sesamoid ossicles forming at C4/5 vertebral levels Surgical excision is never an option as injury to the nuchal ligament can result in cervical spine instability
During development, the neural arches develop from primary ossification centres. What do the articular facets develop from?
The articular facets develop from secondary ossification centres which appear at the beginning of puberty and fuse with the neural arches Image from radiopedia: Ossification centres of the vertebral column
What is formed if there is failure of the inferior articular facets with the neural arches?
If there is failure of fusion, separate bony fragments are formed known as Oppenheimer’s ossicles Unbroken arrow points to anatomical variation Broken arrow points to normal Can see the inferior articular facet not connecting with arch
Are oppenheimers ossicles usually unilateral or bilateral? Where does it most commonly occur? Which sex is it more common in? What symptoms is it associated with?
In 80% of cases oppenhiemers ossicles are bilateral and it most commonly occurs in the lumbar region, specifically L2 and L3 It is 6 times more common in males Associated with lower back pain due to spinal cord compression
A single T10 vertebra was found to have 2 transverse processes on the left side. No records accompanied the specimen so no implications of this variation are known. What is this known as?
DUPLICATED TRANSVERSE PROCESS Top arrow = superior articular surface Bottom arrow = spinous process A single right transverse process was present but had broken off.
