Surgical anatomy of cranial bones

1. C
SURGICAL ANATOMY OF CRANIAL BONES
Guided by: Dr. Vidhi C. Rathi ( Reader of Dept. of OMFS)
Presented by: Gauri Bargoti

2. Contents
• Skull
• Diploic vessels
• Occipital bone
• Sphenoid bone
• Ethmoid bone
• Temporal bone
• Frontal bone
• Parietal bone
• References

3. Skull
• Made up of cranium and mandible.
• Its thickness is 5mm.
• Periosteum is deficient in regenerative osteogenic power as compared to long bones.
• The bones have 3 layes: 1) Outer laminae
2) Inner laminae
3) Middle cancellous bone: DIPLOE

4. Diploic Vessels
• The diploe is supplied by numerous diplopic arteries both on internal and external surfaces of
skull.
• These veins anastomose freely on both sides and form 5 arteries: 2 posterior temporal ; one
frontal; one anterior temporal; 1 occipital diplopic vein.
• These veins drain into sinuses and veins of scalp.

5. Cranial epidural space
• It is s potential space outside the inner periosteum, made possible by loose attachment to skull.
• Larger venous channels draining the brain lie within this.
• In addition to venous sinuses it also contains meningeal arteries and nerves.

6. Strength of attachment
• Strongest attachment of the dura to skull is in midline above superior sagittal sinus; some to the
suture and also to the branches of middle meningeal artery.
• The attachment of dura to the base of the skull is relatively strong; in anterior fossa the dura is
strongly attached to the crista galli, cribiform plate and optic canal.
• In middle fossa the attachment are to the various foramen especially the superior orbital fissure,
foramen rotundum, foramen ovale and foramen lacerum.
• In posterior region it is attached to basal portion of sphenoid bone, to the margins of foramen
magnum and jugular foramen
• It is less firmly attached to venous sinuses
• In area of loose attachment the bone flap can be easily removed and cause development of
epidural hemorrhage

7. Subdural space
• It is described as a potential space only but PENFIELD by freezing the head of dogs
demonstrated that this space is appreciable with yellow fluid that prevents intimate contact
between dura and archanoid.
• PENFIELD and NORCROSS suggested that it is the displacement of this fluid that cause post
traumatic headache.
• The origin of this fluid is unknown

8. Dural Septa
• In certain locations the dura, instead of remaining attached to inner surface of the skull project
in to form septa that partially divide the cranial cavity.
• The two most important septa are
• 1) Falx cerebri
• 2) Tentorium cerebri

9. Falx Cerebri
• Longitudinally directed septum
• Passes downward from the cranial vault between cerebral hemisphere.
• It is large sickle shaped and attaches anteriorly to crista galli and posteriorly blends with
tentorium cerebelli.
• Where it is continuous close to the midline with the dura of the vault it encloses the superior
sagittal sinus.
• In its free margin above the corpum callosum it encloses the inferior sagittal sinus.
• At its attachment to tentorium cerebelli it helps to form the wall of straight sinus.

10. Tentorium Cerebelli
• It snugly fits between the posterior portion of the two cerebral hemispheres and somewhat over
the convex portion of cerebellum.
• The line of attachment from the skull extends backward from posterior clinoid process along the
superior border of petrous portions of the two temporal bone and medially along the occipital
bone
• It anteriorly enters the straight sinus by trochlear nerve.

11. Dural Septa Venous sinus

12. Venous sinuses
• Saggital, straight and occipital sinus are unpaired sinuses lying approximately in midplane of the
body.
• Transverse, sigmoid, superior and inferior petrosal, cavernous and sphenoparietal sinus are
paired.

13. Superior sagittal sinus
• Lies in inner surface of vault of skull.
• It is triangular in cross-section
• At its anterior end where the falx is attached to crista galli it is narrow and it enlarges posteriorly.
• Anteriorly it receives communication from frontal diplopic veins and connected to nose by
emissary veins ( it cause nasal infection to be transmitted to sinus)
• The sinus is also connected to angular veins through parietal bone.
• The position of the sinus and its enlargement and occurrence of lacunae must be considered in
trephine operations.
• Ligation and resection of anterior part is more frequent since the posterior part may be
occluded due to growth of tumor.

14. Inferior Sagittal Sinus
• The inferior sagittal sinus runs in the free edge of falx cerebri, where it receives the small veins
from medial surface od the hemisphere and from the inferior part of frontal region.
• It is small throughout its course, and at the junction of falx and tentorium joins the much larger
cerebral veins.

15. Occipital Sinus
• Begin as right and left part about the margin of foramen magnum.
• They fuse above the margin of foramen magnum to form a single sinus.
• Through plexuses accompanying the hypoglossal nerves the occipital sinus communicates with
the internal jugular vein or inferior petrosal sinus outside the skull.

16. Transverse and Sigmoid Sinus
• Transverse sinus lies in dura at attachment of tentorium cerebelli to occipital bone.
• The sigmoid sinus then runs downward in the posterior cranial fossa, grooving the petrous part
of temporal bone and slightly forward in floor of fossa to reach jugular foramen which it makes
the exit.
• The transverse sinus receives the superior cerebellar veins and may receive inferior cerebrall
ones.
• Both the transverse sinus are typically large.
• The sigmoid sinus after passing through foramen jugular outside of which it expand to form
superior bulb of internal jugular vein.

17. Superior petrosal sinuses
• Leaves the cavernous sinus on either side of sella turcica.
• It usually passes above the mouth of trigeminal cave.
• It receives inferior and superior cerebellar veins and veins from brain stem, including one that
runs in close approximation to root of trigeminal nerve in posterior cranial fossa.

18. Inferior petrosal sinus
• It originate from cavernous sinus parallel to superior sinus.
• It receives veins from lower surface of cerebellum, brain stem and also from internal ear.
• As it passes through foramen jugular, it usually passes laterally between the ninth and tenth
cranial nerve to reach internal jugular vein.

19. Cavernous Sinus
• The cavernous sinus are large venous sinus on each side of sphenoid bone.
• These are broken up by much heavy channels , much broken by heavy trabeculae and hence
blood supply is much slowed.
• Laterally each cavernous sinus receives the sphenoparietal sinus, which communicates with
middle meningeal vein.
• The two sinuses also communicate with each other through an ill defined small channels, the
basilar plexus.

20. Basilar plexus
• The basilar plexus lies in the dura mater of the posterior cranial fossa over the clivus
• It receives small veins from brain stem and cerebellum.

21. Sphenoparietal and other sinus
• The sphenoparietal sinuses arises from one of the meningeal arteries usually the middle
meningeal artery and runs downward on lesser wing of sphenoid bone to empty into the
corresponding cavernous sinus.
• It is in this sinus that the anterior temporal diplopic vein usually end.
• The petrosquamous sinus that may be sometimes present is the most constant.

22. Average Diameter of vessels
• The average values of the vessels are as follows:
• Superior sagittal sinus: 20 sq mm
Straight Sinus: 15 sq mm
Occipital sinus: 7 sq mm
Right lateral sinus: 30 sq mm
Left lateral sinus: 24 sq mm

23. Bones Of Skull

24. Occipital bone
• Develops by four endochondral process and one membranous process
• It has following part:
• 1) Four paired cartilaginous bone of squama called PLANAL NUCHAL.
• 2) Unpaired Basilar part
• 3) Paired lateral part
• These parts surround a large elliptical cavity called foramen occipital magnum.

26. Sphenoid Bone
• The bone has a body and three paired processes
• Body continues anteriorly from basilar part anteriorly to nasal cavity.
• Body is divided into
• 1) Anterior aspect : Presphenoid
• 2) Posterior aspect: Basosphenoid
• These parts fuse after birth.

28. Frontal Bone
• The squama of frontal bone form the vertical, anterior wall of cranial vault, the paired horizontal,
orbital portions from the greater part of roofs of orbit.
• The frontal bone develop as a paired bone, the two halves of which are separated by a frontal
suture, which is still present at birth.
• Normally the suture closes during the second year of life but may persist in a small percentage
of individuals as metopic suture.

29. Anterior Aspect Inferior Aspect

30. Ethmoid Bone
• The unpaired ethmoid fits into the ethmoid notch of frontal bone so that its
cribiform plate forms the middle part of floor of anterior cranial fossa.
• In midline a perpendicular plate joins the horizontal cribiform plates.
• The conchal plate of ethmoid bone is incompletely divided by horizontal slit into an
upper and lower part.
• The lateral border of cribiform plate connect with the inner plate of orbital part of
frontal bone.
• The intracranial part of vertical plate of ethmoid bone, crista galli, is lower in its
posterior part and higher in anterior part.

32. Temporal Bone
• It develops from the fusion of three elements that can be separated at birth.
• The temporal bone gives attachment to eardrum and tympanic membrane and styloid
process is added to it later on.
• Early in the life petrosal bone, squama, and tympanic bone remain fused with one
another while the styloid process may remain independent for sometime.
• The shape of temporal bone is different in children as compared to adult
• The tympanic bone is represented as C-shaped bone ring that gives attachment to
tympanic membrane.

33. Lateral Aspect Inferior Aspect

34. Parietal Bone
• It is a quadrangular cup-shaped bone.
• The outer surface is smooth and has its highest convexity slightly below
the center.
• The anterior border, which is at right angle to saggital border, is in
contact with frontal bone in coronal suture.
• The posterior border, almost parallel to anterior forms the lambdoid
suture with occipital squama.
• In front of the squamous border the parietal bone is in contact with
greater wing of sphenoid bone.
• Behind the squamous border the parietal bone is united with the
mastoid notch temporal bone.

35. External Aspect Internal Aspect

36. References
• Sicher and Dubrul’s Oral Anatomy
• Anatomy for surgeons: Volume 1 by W. Henry Hollinshead