Foramen Magnum microanatomy based on Rhotons , useful for foramen magnum decompression procedures and CP angle exposure .

1. FORAMEN MAGNUM
ANATOMY
Dr.B Vinod
Mch Neurosurgery resident
Gandhi Medical college
Hyderabad.

2. • The foramen magnum is located in the occipital
bone,
• three parts:
1) Squamosal part located behind the foramen
magnum
2)Basal (clival) portion located anterior to the
foramen magnum;
3)Condylar part that connects the squamosal and
clival parts

3. RELATIONSHIPS
Osseous : 3 bony relations
• Occipital
• Atlas
• Axis

4. Occipital bone
• Anterior (Narrow): Clivus
• Posterior: Squamous
• Lateral:Condyles

5. • Clivus:
• Anterior to FM, thick quadrangular plate of bone
that extends forward and upward, at an angle of
about 45° from the foramen magnum.
• Superior surface concave side to side
• Separated from petrous bone of temporal by
petroclival fissure.
• Has inferior petrosal sinus on upper surface which
ends in jugular foramen .
• Inferior surface in front of the foramen magnum,
a small elevation, the pharyngeal tubercle, gives
attachment to the fibrous raphe of the pharynx.

6. Condylar
• Located lateral to the anterior half of the FM.
• Oval in shape,face downward and laterally, and have
their long axes directed forward and medially.
• Medial attachment for alar ligaments
• Above hypoglossal canal ,forward & laterally
• Posterior : condylar canals, has emmissary veins , that
connects vertebral plexus and joins sigmoid sinus
• Lateral: jugular process , connects squamous & condylar
bone
• Jugular tubercle: superior to hypoglossal canal.& medial
to petroclival fissure,caudal to tubercle courses CN
9,10,11

8. • Jugular foramen: anterior and superior to FM.
• 2 parts .
• Small anteromedial: petrous part,transmits IPS
• Large posterolateral: sigmoid part,transmits
sigmoid sinus.
• Intrajugular part. Transmits CN 9,10,11,

9. • Squamous part:
• External :
• External occipital protuberance(Inion)
convex
• 2 nucheal line
• External occipital crest, rough surface
for muscles
• Internal:
• concave & has internal protuberance .
• Superior fossa for occipital lobe
• Inferior fossa for cerebellar surface .
• Vermian fossa for vermis .

10. ATLAS

11. AXIS

12. Vertebral artery groove

13. LIGAMENTS
• Cruciform ligaments : transverse & vertical
• ALL
• PLL
• Atlanto-occipital
• Axis-occipital

14. LIGAMENTS

15. ALL
• Runs along anterior surface of VB
• From C1 to anterior sacrum
• Attached to VB and intervertebral discs along
its length
• PLL
• Along posterior surface of VB up c2
• Attaches along posterior surface of VB and
discs.

16. • Tectorial membrane :
• Upper part of PLL ,that connects C2 to clivus

17. MUSCULAR RELATIONS
• Posterior muscles
• Superficial
• Trapezius
• Sternocleidomastoid
• Deep
• Splenius capitis
• Semispinalis capitis
• Longissimus capitis
• Suboccipitalis muscle.

18. Suboccipitalis muscle
• 4 muscles in suboccipital area

19. Anterior muscles
• Longus colli (C1-T3)
• Longus capitis
(Clivus to C3 to C6 TP)
• Rectus capitus anterior
• Rectus capitus lateralis

20. NEURAL RELATIONS
• Spinal cord
• Dentate ligament
• Brainstem
• Cerebellum
• Cranial nerves
• Cervical nerve roots

21. Spinal cord
• Blends into medulla at level upper limit of
dorsal and ventral rootlets forming C1N
• Easy to identify level on ventral than dorsal
because of ventral rootlets are always present.
• Junction of SC and medulla present at rostral
end ventral C1.
• Medulla occupies FM not SC
• SC lies immediately below FM

22. Dentate ligament
• White sheeth attached to spinal cord
• Lateral projections of spinal piamater between dorsal
& ventral rootlets, forming triangle shape ligament
,that anchors the spinal cord along its length to
duramater on each side.
• At CVJ located
Anterior – between VA & C1N
ventral root
Posterior- PSA & CN 11
• Most attachments of ligament is at rostral end at level
of FM where VA pierces duramater.

24. BRAINSTEM
• Lower medulla present at C1n ventral
• Anterior medulla by medullary
pyramids faces
Anterior clivus
Anterior edge of FM
Rostral odontoid process
Lateral surface of MO by inferior olives.
Posterior MO
Superior : Midline inferior half of 4V
Lateral by inferior
cerebellar peduncles
Inferior :Gracious fasciculus and cuneatus.

25. Cerebellum
• Rests above the posterior and lateral edge of FM
• Tonsils- above and posterior edge
• Biventral and lobules-sits above and lateral
• Vermis lower part.
Cranial nerve
Spinal accessory nerve only nerve pass through FM

26. Cervical nerve roots
• C1N: suboccipital.N
• Dorsal and ventral root
• Dorsal root has medial and lateral
branches,supplies skin and muscles of posterior
region of neck.
• C1 leaves Vertebral canal between occipital bone
and atlas.
• Dorsal root courses between posterior arch of
atlas and VA ,reaches suboccipital traingle and
innervate muscles of this traingle and scalp .

27. • C1 ventral ramus courses between VA and
posterior arch of atlas , pass lateral to lateral
mass of C1 ,supplies Rectus capitis lateralis .

28. • C2N:
• Between posterior arch of
atlas and lamina of axis.
• Dorsal root has lateral and
medial branches
• Medial branch forms Greater
occipital nerve,supply
semispinalis capitis .
• Ascends with occipital artery
,supply scalp till vertex and
back ear.
• C2 ventral courses between
arch & TP of atlas ,axis .

29. ARTERIAL RELATIONS
• Vertebral artery
• PSA
• PICA
• ASA
• Meningeal artery.

30. VERTEBRAL ARTERY
• Starts at subclavian artery.
• Ascends through upper six cervical vertebra
• Enters duramater behind occipital condyles
• Into FM to front of MO
• Join Basilar artery at ponto medullary junction
.

31. • Has extradural & intradural
• Extradural VA:
• 1st segment ,origin to Transverse foramen of
C6
• 2nd : C6 to C1 infront of cervical roots
• 3rd : C1 to duramater entrance .

32. • VA after passing TP of C1 lies medial to rectus capitis
lateralis
• Pass medially below C1 lateral mass & A-O joint ,in
groove of posterior arch of C1 lies in floor of suboccipital
triangle.
• C1N pass through duramater of VA lower
surface,between VA and posterior arch of C1
• Terminal extradural gives branches
1. Posterior meningeal A
2. PSA
3. Br. To cervical muscles
4. PICA infrequently

33. Intradural VA
• 4th segment
• Origin :inferior to lateral edge of
FM
• Duramater is thick and funnel
shape,4-6 mm length approx.
• C1 exit & PSA enter spinal canal
through this funnel.
• Initial intradural segment of VA
pass superior to C1N,and
anterior to PSA, dentate
ligament & CN 11

34. Anterior spinal artery
• Origin anterior medial VA near basilar artery
• Descends through FM on anterior surface of
medulla & Spinal cord.
• Supplies :
1. Pryamids and dissucations
2. Medial lemniscus
3. Inter olivary bundles
4. Cn 12
5. Posterior longitudinal fasciculus

35. POSTERIOR SPINAL ARTERY
• Origin : outside duramater postero-medial of
VA
• Branches at medulla oblongata into ascending
and descending branches
• Ascending (Dura part):supplies,
1. Gracilis cuneatus
2. Cn11
3. Choroid plexus of foramen magendie

36. • Descending branch : supply
• Spinal cord posteriorly
• Anastomose with ascending radicular artery of
dorsal

37. PICA
• Largest
branch of VA,
• Usually fron
intradural
part
• Tonsomedulla
ry Pica which
forms caudal
loop in
relation to
FM

38. MENINGEAL ARTERY
1. Anterior meningeal
2. Posterior meningeal
3. Meningeal br.of pharyngeal & occipital
4. Dorsal meningeal br.of meningohypophyseal
of ICA

39. • Anterior meningeal artery:
• Origin immedialty above T.foramen of C3 medial
VA
• Ascends up C2-C1 region
• At dens apex joins opp side
• Supplies :
1. Clivus
2. Anterior part of FM
3. Sc
4. Axis, odontoid ,AA joint

40. Posterior meningeal artery
• Origin posterior superior of VA
• Course along lateral mass pf C1
• Enters FM
• Ascends near falx cerebelli
• Divides near torcula into branches
• Terminates posterior part of tentorium and
cerebral falx & posterior fossa dura ,
• Anastomose with ascending pharyngeal A.

41. Ascending pharyngeal artery
• Branch of ECA
• Two Branches that
1. Anastomose to dorsal meningeal A via
hypoglossal canal
2. With meningeal br of occipital & post
meningeal A via jugular foramen

42. Venous relations

43. Surgical approaches

44. Thank you